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  • 1.
    Berg, Marcus
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Karlstad University, Sweden.
    Buchberger, Igor
    Enander, Jonas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sjörs, Stefan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Growth histories in bimetric massive gravity2012In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 12, article id 021Article in journal (Refereed)
    Abstract [en]

    We perform cosmological perturbation theory in Hassan-Rosen bimetric gravity for general homogeneous and isotropic backgrounds. In the de Sitter approximation, we obtain decoupled sets of massless and massive scalar gravitational fluctuations. Matter perturbations then evolve like in Einstein gravity. We perturb the future de Sitter regime by the ratio of matter to dark energy, producing quasi-de Sitter space. In this more general setting the massive and massless fluctuations mix. We argue that in the quasi-de Sitter regime, the growth of structure in bimetric gravity differs from that of Einstein gravity.

  • 2.
    Blomqvist, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Enander, Jonas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Constraining dark energy fluctuations with supernova correlations2010In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, Vol. 10, p. 018-Article in journal (Refereed)
    Abstract [en]

    We investigate constraints on dark energy fluctuations using type Ia supernovae. If dark energy is not in the form of a cosmological constant, that is if the equation of state is not equal to -1, we expect not only temporal, but also spatial variations in the energy density. Such fluctuations would cause local variations in the universal expansion rate and directional dependences in the redshift-distance relation. We present a scheme for relating a power spectrum of dark energy fluctuations to an angular covariance function of standard candle magnitude fluctuations. The predictions for a phenomenological model of dark energy fluctuations are compared to observational data in the form of the measured angular covariance of Hubble diagram magnitude residuals for type Ia supernovae in the Union2 compilation. The observational result is consistent with zero dark energy fluctuations. However, due to the limitations in statistics, current data still allow for quite general dark energy fluctuations as long as they are in the linear regime.

  • 3.
    Blomqvist, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Supernovae as seen by off-center observers in a local void2010In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, Vol. 05, p. 006-Article in journal (Refereed)
    Abstract [en]

    Inhomogeneous universe models have been proposed as an alternative explanation for the apparent acceleration of the cosmic expansion that does not require dark energy. In the simplest class of inhomogeneous models, we live within a large, spherically symmetric void. Several studies have shown that such a model can be made consistent with many observations, in particular the redshift-luminosity distance relation for type Ia supernovae, provided that the void is of Gpc size and that we live close to the center. Such a scenario challenges the Copernican principle that we do not occupy a special place in the universe. We use the first-year Sloan Digital Sky Survey-II supernova search data set as well as the Constitution supernova data set to put constraints on the observer position in void models, using the fact that off-center observers will observe an anisotropic universe. We first show that a spherically symmetric void can give good fits to the supernova data for an on-center observer, but that the two data sets prefer very different voids. We then continue to show that the observer can be displaced at least fifteen percent of the void scale radius from the center and still give an acceptable fit to the supernova data. When combined with the observed dipole anisotropy of the cosmic microwave background however, we find that the data compells the observer to be located within about one percent of the void scale radius. Based on these results, we conclude that considerable fine-tuning of our position within the void is needed tofit the supernova data, strongly disfavouring the model from a Copernican principle point of view.

  • 4.
    Blomqvist, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Noboli, Serena
    Stockholm University, Faculty of Science, Department of Physics.
    Probing dark energy inhomogeneities with supernovae2008In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, Vol. 06, p. 027-Article in journal (Refereed)
    Abstract [en]

    We discuss the possibility of identifying anisotropic and/or inhomogeneous cosmological models using type Ia supernova data. A search for correlations in current type Ia peak magnitudes over a large range of angular scales yields a null result. However, the same analysis limited to supernovae at low redshift shows a feeble anticorrelation at the 2σ level at angular scales θ≈40°. Upcoming data from, e.g., the SNLS (Supernova Legacy Survey) and the SDSS-II (SDSS: Sloan Digital Sky Survey) supernova searches will improve our limits on the size of—or possibly detect—possible correlations also at high redshift at the per cent level in the near future. With data from the proposed SNAP (SuperNova Acceleration Probe) satellite, we will be able to detect the induced correlations from gravitational lensing on type Ia peak magnitudes on scales less than a degree.

  • 5. Bull, Philip
    et al.
    Akrami, Yashar
    Adamek, Julian
    Baker, Tessa
    Bellini, Emilio
    Jimenez, Jose Beltran
    Bentivegna, Eloisa
    Camera, Stefano
    Clesse, Sebastien
    Davis, Jonathan H.
    Di Dio, Enea
    Enander, Jonas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Heavens, Alan
    Heisenberg, Lavinia
    Hu, Bin
    Llinares, Claudio
    Maartens, Roy
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Nadathur, Seshadri
    Noller, Johannes
    Pasechnik, Roman
    Pawlowski, Marcel S.
    Pereira, Thiago S.
    Quartin, Miguel
    Ricciardone, Angelo
    Riemer-Sørensen, Signe
    Rinaldi, Massimiliano
    Sakstein, Jeremy
    Saltas, Ippocratis D.
    Salzano, Vincenzo
    Sawicki, Ignacy
    Solomon, Adam R.
    Spolyar, Douglas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Starkman, Glenn D.
    Steer, Daniele
    Tereno, Ismael
    Verde, Licia
    Villaescusa-Navarro, Francisco
    von Strauss, Mikael
    Winther, Hans A.
    Beyond Lambda CDM: Problems, solutions, and the road ahead2016In: Physics of the Dark Universe, ISSN 0953-8585, E-ISSN 2212-6864, Vol. 12, p. 56-99Article in journal (Refereed)
    Abstract [en]

    Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, Lambda CDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of Lambda CDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.

  • 6. Davis, Tamara M.
    et al.
    Hui, Lam
    Frieman, Joshua A.
    Haugbølle, Troels
    Kessler, Richard
    Sinclair, Benjamin
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bassett, Bruce
    Marriner, John
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Nichol, Robert C.
    Richmond, Michael W.
    Sako, Masao
    Schneider, Donald P.
    Smith, Mathew
    The Effect of Peculiar Velocities on Supernova Cosmology2011In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 741, p. 67-Article in journal (Refereed)
    Abstract [en]

    We analyze the effect that peculiar velocities have on the cosmological inferences we make using luminosity distance indicators, such as Type Ia supernovae. In particular we study the corrections required to account for (1) our own motion, (2) correlations in galaxy motions, and (3) a possible local under- or overdensity. For all of these effects we present a case study showing the impact on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). Correcting supernova (SN) redshifts for the cosmic microwave background (CMB) dipole slightly overcorrects nearby SNe that share some of our local motion. We show that while neglecting the CMB dipole would cause a shift in the derived equation of state of Δw ~ 0.04 (at fixed Ω m ), the additional local-motion correction is currently negligible (Δw <~ 0.01). We then demonstrate a covariance-matrix approach to statistically account for correlated peculiar velocities. This down-weights nearby SNe and effectively acts as a graduated version of the usual sharp low-redshift cut. Neglecting coherent velocities in the current sample causes a systematic shift of Δw ~ 0.02. This will therefore have to be considered carefully when future surveys aim for percent-level accuracy and we recommend our statistical approach to down-weighting peculiar velocities as a more robust option than a sharp low-redshift cut.

  • 7. Davis, Tamara
    et al.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy.
    Becker, A. C.
    Blondin, Stéphane
    Challis, P
    Clocchiatti, A
    Filippenko, Alexei
    Foley, Ryan
    Garnavich, P. M.
    Jha, Saurabh
    Krisciunas, K
    Kirshner, Robert
    Leibundgut, Bruno
    Li, W
    Matheson, T
    Miknaitis, G
    Pignata, G
    Rest, A
    Riess, Adam
    Schmidt, B. P.
    Smith, R. C.
    Spyromilio, J
    Stubbs, C. W.
    Suntzeff, N. B.
    Tonry, John
    Wood-Vasey, W. M.
    Zenteno, A
    Scrutinizing Exotic Cosmological Models Using ESSENCE Supernova Data Combined with Other Cosmological Probes2007In: The Astrophysical Journal, Vol. 666, no 2, p. 716-725Article in journal (Refereed)
    Abstract [en]

    The first cosmological results from the ESSENCE supernova survey (Wood-Vasey and coworkers) are extended to a wider range of cosmological models including dynamical dark energy and nonstandard cosmological models. We fold in a greater number of external data sets such as the recent Higher-z release of high-redshift supernovae (Riess and coworkers), as well as several complementary cosmological probes. Model comparison statistics such as the Bayesian and Akaike information criteria are applied to gauge the worth of models. These statistics favor models that give a good fit with fewer parameters. Based on this analysis, the preferred cosmological model is the flat cosmological constant model, where the expansion history of the universe can be adequately described with only one free parameter describing the energy content of the universe. Among the more exotic models that provide good fits to the data, we note a preference for models whose best-fit parameters reduce them to the cosmological constant model.

  • 8.
    Dhawan, Suhail
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    The effect of inhomogeneities on dark energy constraints2018In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 7, article id 024Article in journal (Refereed)
    Abstract [en]

    Constraints on models of the late time acceleration of the universe assume the cosmological principle of homogeneity and isotropy on large scales. However, small scale inhomogeneities can alter observational and dynamical relations, affecting the inferred cosmological parameters. For precision constraints on the properties of dark energy, it is important to assess the potential systematic effects arising from these inhomogeneities. In this study, we use the Type Ia supernova magnitude-redshift relation to constrain the inhomogeneities as described by the Dyer-Roeder distance relation and the effect they have on the dark energy equation of state (w), together with priors derived from the most recent results of the measurements of the power spectrum of the Cosmic Microwave Background and Baryon Acoustic Oscillations. We find that the parameter describing the inhomogeneities (eta) is weakly correlated with w. The best fit values w = -0.933 +/- 0.065 and eta = 0.61 +/- 0.37 are consistent with homogeneity at < 2 sigma level. Assuming homogeneity (eta = 1), we find w = -0.961 +/- 0.055, indicating only a small change in w. For a time-dependent dark energy equation of state, w(0) = -0.951 +/- 0.112 and w(a) = 0.059 +/- 0.418, to be compared with w(0) = -0.983 +/- 0.127 and w(a) = 0.07 +/- 0.432 in the homogeneous case, which is also a very small change. We do not obtain constraints on the fraction of dark matter in compact objects, f(p), at the 95% C.L. with conservative corrections to the distance formalism. Future supernova surveys will improve the constraints on eta, and hence, f(p), by a factor of similar to 10.

  • 9.
    Dhawan, Suhail
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Amanullah, Rahman
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Feindt, Ulrich
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Narrowing down the possible explanations of cosmic acceleration with geometric probes2017In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 7, article id 040Article in journal (Refereed)
    Abstract [en]

    Recent re-calibration of the Type Ia supernova (SNe Ia) magnitude-redshift relation combined with cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) data have provided excellent constraints on the standard cosmological model. Here, we examine particular classes of alternative cosmologies, motivated by various physical mechanisms, e.g. scalar fields, modified gravity and phase transitions to test their consistency with observations of SNe Ia and the ratio of the angular diameter distances from the CMB and BAO. Using a model selection criterion for a relative comparison of the models (the Bayes Factor), we find moderate to strong evidence that the data prefer flat Lambda CDM over models invoking a thawing behaviour of the quintessence scalar field. However, some exotic models like the growing neutrino mass cosmology and vacuum metamorphosis still present acceptable evidence values. The bimetric gravity model with only the linear interaction term as well as a simplified Galileon model can be ruled out by the combination of SNe Ia and CMB/BAO datasets whereas the model with linear and quadratic interaction terms has a comparable evidence value to standard Lambda CDM. Thawing models are found to have significantly poorer evidence compared to flat Lambda CDM cosmology under the assumption that the CMB compressed likelihood provides an adequate description for these non-standard cosmologies. We also present estimates for constraints from future data and find that geometric probes from oncoming surveys can put severe limits on non-standard cosmological models.

  • 10.
    Enander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Adam R., Solomon
    Yashar, Akrami
    Edvard, Mörtsell
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cosmic expansion histories in massive bigravity with symmetric matter coupling2015In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 1, article id 006Article in journal (Refereed)
    Abstract [en]

    We study the cosmic expansion history of massive bigravity with a viable matter coupling which treats both metrics on equal footing. We derive the Friedmann equation for the effective metric through which matter couples to the two metrics, and study its solutions. For certain parameter choices, the background cosmology is identical to that of LCDM. More general parameters yield dynamical dark energy, which can still be in agreement with observations of the expansion history. We study specific parameter choices of interest, including minimal models, maximally-symmetric models, and a candidate partially-massless theory.

  • 11.
    Enander, Jonas
    et al.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Akrami, Yashar
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Renneby, Malin
    Solomon, Adam R.
    Integrated Sachs-Wolfe effect in massive bigravityManuscript (preprint) (Other academic)
    Abstract [en]

    We study the Integrated Sachs-Wolfe (ISW) effect in ghost-free, massive bigravity, where only one metric couples to matter. We focus on the infinite-branch bigravity (IBB) model which exhibits viable cosmic expansion histories and stable linear perturbations, while the cosmological constant is set to zero and the late-time accelerated expansion of the Universe is due solely to the gravitational interaction terms. The ISW contribution to the CMB auto-correlation power spectrum is predicted, as well as the cross-correlation between the CMB temperature anisotropies and the large-scale structure. We use ISW amplitudes as observed in the WMAP 9-year temperature data together with galaxy and AGN data provided by the WISE mission, in order to compare the theoretical predictions to the observations. The ISW amplitudes in IBB are found to be larger than the corresponding ones in the standard LCDM model by roughly a factor of 1.5, but are still consistent with the observations.

  • 12.
    Enander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Akrami, Yashar
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Renneby, Malin
    Solomon, Adam R.
    Integrated Sachs-Wolfe effect in massive bigravity2015In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 91, no 8, article id 084046Article in journal (Refereed)
    Abstract [en]

    We study the integrated Sachs-Wolfe (ISW) effect in ghost-free, massive bigravity. We focus on the infinite-branch bigravity (IBB) model which exhibits viable cosmic expansion histories and stable linear perturbations, while the cosmological constant is set to zero and the late-time accelerated expansion of the Universe is due solely to the gravitational interaction terms. The ISW contribution to the CMB auto-correlation power spectrum is predicted, as well as the cross-correlation between the CMB temperature anisotropies and large-scale structure. We use ISW amplitudes as inferred from the WMAP 9-year temperature data together with galaxy and AGN data provided by the WISE mission in order to compare the theoretical predictions to the observations. The ISW amplitudes in IBB are found to be larger than the corresponding ones in the standard Lambda CDM model by roughly a factor of 1.5, but are still consistent with the observations.

  • 13.
    Enander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    On stars, galaxies and black holes in massive bigravity2015In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 11, article id 023Article in journal (Refereed)
    Abstract [en]

    In this paper we study the phenomenology of stars and galaxies in massive bigravity. We give parameter conditions for the existence of viable star solutions when the radius of the star is much smaller than the Compton wavelength of the graviton. If these parameter conditions are not met, we constrain the ratio between the coupling constants of the two metrics, in order to give viable conditions for e. g. neutron stars. For galaxies, we put constraints on both the Compton wavelength of the graviton and the conformal factor and coupling constants of the two metrics. The relationship between black holes and stars, and whether the former can be formed from the latter, is discussed. We argue that the different asymptotic structure of stars and black holes makes it unlikely that black holes form from the gravitational collapse of stars in massive bigravity.

  • 14.
    Enander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    On the use of black hole binaries as probes of local dark energy properties2010In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 683, no 1, p. 7-10Article in journal (Refereed)
    Abstract [en]

    Accretion of dark energy onto black holes will take place when dark energy is not a cosmological constant. It has been proposed that the time evolution of the mass of the black holes in binary systems due to dark energy accretion could be detectable by gravitational radiation. This would make it possible to use observations of black hole binaries to measure local dark energy properties, e.g., to determine the sign of 1+w where w is the dark energy equation of state. In this Letter we show that such measurements are unfeasible due to the low accretion rates.

  • 15.
    Enander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Strong lensing constraints on bimetric massive gravity2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 10, p. 031-Article in journal (Refereed)
    Abstract [en]

    We derive dynamical and gravitational lensing properties of local sources in the Hassan-Rosen bimetric gravity theory. Observations of elliptical galaxies rule out values of the effective length-scale of the theory, in units of the Hubble radius, in the interval 10(-6). lambda(g) = rH less than or similar to 10(-3), unless the proportionality constant between the metrics at the background level is far from unity, in which case general relativity is effectively restored for local sources. In order to have background solutions resembling the concordance cosmological model, without fine-tuning of the parameters of the model, we are restricted to the upper interval, or lambda(g) /gamma H similar to 1, for which the Vainshtein mechanism is expected to restore general relativity for local sources. Except for a limited range of parameter values, the Hassan-Rosen theory is thus consistent with the observed lensing and dynamical properties of elliptical galaxies.

  • 16.
    Goobar, Ariel
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Amanullah, Rahman
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Kulkarni, S. R.
    Nugent, P. E.
    Johansson, Joel
    Steidel, C.
    Law, D.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Quimby, Robert
    Blagorodnova, N.
    Brandeker, A.
    Cao, Y.
    Cooray, A.
    Ferretti, Raphael
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fremling, Christoffer
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hangard, Laura
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kasliwai, M.
    Kupfer, T.
    Lunnan, R.
    Masci, F.
    Miller, A. A.
    Nayyeri, H.
    Neill, J. D.
    Ofek, E. O.
    Papadogiannakis, Seméli
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Petrushevska, Tanja
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ravi, V.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sullivan, M.
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Walters, R.
    Wilson, D.
    Yan, L.
    Yaron, O.
    iPTF16geu: A multiply imaged, gravitationally lensed type Ia supernova2017In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 356, no 6335, p. 291-295Article in journal (Refereed)
    Abstract [en]

    We report the discovery of a multiply imaged, gravitationally lensed type Ia supernova, iPTF16geu (SN 2016geu), at redshift z = 0.409. This phenomenon was identified because the light from the stellar explosion was magnified more than 50 times by the curvature of space around matter in an intervening galaxy. We used high-spatial-resolution observations to resolve four images of the lensed supernova, approximately 0.3 arc seconds from the center of the foreground galaxy. The observations probe a physical scale of ~1 kiloparsec, smaller than is typical in other studies of extragalactic gravitational lensing. The large magnification and symmetric image configuration imply close alignment between the lines of sight to the supernova and to the lens. The relative magnifications of the four images provide evidence for substructures in the lensing galaxy.

  • 17.
    Goobar, Ariel
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Paech, Kerstin
    Stockholm University, Faculty of Science, Department of Physics.
    Stanishev, Vallery
    Stockholm University, Faculty of Science, Department of Physics.
    Amanullah, Rahman
    Stockholm University, Faculty of Science, Department of Physics.
    Dahlén, T.
    Jönsson, Jakob
    Stockholm University, Faculty of Science, Department of Physics.
    Kneib, J. P.
    Lidman, Chris
    Stockholm University, Faculty of Science, Department of Physics.
    Limousin, M.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Nobili, Serena
    Stockholm University, Faculty of Science, Department of Physics.
    Richard, J.
    Riehm, Teresa
    Stockholm University, Faculty of Science, Department of Astronomy.
    von Strauss, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Near-IR search for lensed supernovae behind galaxy clusters. II. First detection and future prospects2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 507, no 1, p. 71-83Article in journal (Refereed)
    Abstract [en]

    Aims: Powerful gravitational telescopes in the form of massive galaxy clusters can be used to enhance the light collecting power over a limited field of view by about an order of magnitude in flux. This effect is exploited here to increase the depth of a survey for lensed supernovae at near-IR wavelengths.

    Methods: We present a pilot supernova search programme conducted with the ISAAC camera at VLT. Lensed galaxies behind the massive clusters A1689, A1835, and AC114 were observed for a total of 20 h divided into 2, 3, and 4 epochs respectively, separated by approximately one month to a limiting magnitude J ≲ 24 (Vega). Image subtractions including another 20 h worth of archival ISAAC/VLT data were used to search for transients with lightcurve properties consistent with redshifted supernovae, both in the new and reference data.

    Results: The feasibility of finding lensed supernovae in our survey was investigated using synthetic lightcurves of supernovae and several models of the volumetric type Ia and core-collapse supernova rates as a function of redshift. We also estimate the number of supernova discoveries expected from the inferred star-formation rate in the observed galaxies. The methods consistently predict a Poisson mean value for the expected number of supernovae in the survey of between N_SN = 0.8 and 1.6 for all supernova types, evenly distributed between core collapse and type Ia supernovae. One transient object was found behind A1689, 0.5 arcsec from a galaxy with photometric redshift z_gal = 0.6 ± 0.15. The lightcurve and colors of the transient are consistent with being a reddened type IIP supernova at z_SN = 0.59. The lensing model predicts 1.4 mag of magnification at the location of the transient, without which this object would not have been detected in the near-IR ground-based search described in this paper (unlensed magnitude J ~ 25). We perform a feasibility study of the potential for lensed supernovae discoveries with larger and deeper surveys and conclude that the use of gravitational telescopes is a very exciting path for new discoveries. For example, a monthly rolling supernova search of a single very massive cluster with the HAWK-I camera at VLT would yield ≳ 10 lensed supernova lightcurves per year, where type Ia supernovae would constitute about half of the expected sample. Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme ID 079.A-0192 and ID 081.A-0734.

  • 18.
    Gunnarsson, Christofer
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Dahlen, Tomas
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics.
    Jönsson, Jakob
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Corrections for gravitational lensing of supernovae: better than average?2006In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 640, p. 417-427Article in journal (Refereed)
    Abstract [en]

    We investigate the possibility of correcting for magnification due to gravitational lensing of standard candlesources, such as Type Ia supernovae. Our method uses the observed properties of the foreground galaxies along thelines of sight to each source, and the accuracy of the lensing correction depends on the quality and depth of theseobservations as well as the uncertainties in translating the observed luminosities to the matter distribution in thelensing galaxies. The current work is limited to cases in which the matter density is dominated by the individualgalaxy halos. However, it is straightforward to generalize the method to also include gravitational lensing fromcluster scale halos.We show that the dispersion due to lensing for a standard candle source at z ¼1:5 can be reducedfrom about 7% toP3%; i.e., the magnification correction is useful in reducing the scatter in the Type Ia Hubble diagram,especially at high redshifts where the required long exposure times make it hard to reach large statistics andthe dispersion due to lensing becomes comparable to the intrinsic Type Ia scatter.

  • 19. Jönsson, J.
    et al.
    Dahlén, T.
    Hook, I.
    Goobar, A.
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, E.
    Stockholm University, Faculty of Science, Department of Physics.
    Weighing dark matter haloes with gravitationally lensed supernovae2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 402, no 1, p. 526-536Article in journal (Refereed)
    Abstract [en]

    High-redshift Type Ia supernovae (SNe Ia) are likely to be gravitationally lensed by dark matter haloes of galaxies in the foreground. Since SNe Ia have very small dispersion after light-curve shape and colour corrections, their brightness can be used to measure properties of the dark matter haloes via gravitational magnification. We use observations of galaxies and SNe Ia within the Great Observatories Origins Deep Survey (GOODS) to measure the relation between galaxy luminosity and dark matter halo mass. The relation we investigate is a scaling law between velocity dispersion and galaxy luminosity in the B band: σ = σ*(L/L*)η, where L* = 1010h-2Lsolar. The best-fitting values to this relation are σ* = 136 kms-1 and η = 0.27. We find σ* <~ 190 kms-1 at the 95 per cent confidence level. This method provides an independent cross-check of measurements of dark matter halo properties from galaxy-galaxy lensing studies. Our results agree with the galaxy-galaxy lensing results, but have much larger uncertainties. The GOODS sample of SNe Ia is relatively small (we include 24 SNe) and the results therefore depend on individual SNe Ia. We have investigated a number of potential systematic effects. Light-curve fitting, which affects the inferred brightness of the SNe Ia, appears to be the most important one. Results obtained using different light-curve fitting procedures differ at the 68.3 per cent confidence level.

  • 20. Jönsson, J.
    et al.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy.
    Reducing the gravitational lensing scatter of type Ia supernovae without introducing any extra bias2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 493, no 1, p. 331-337Article in journal (Refereed)
    Abstract [en]

    Aims: Magnification and de-magnification due to gravitational lensing will contribute to the brightness scatter of type Ia supernovae (SNe Ia). The purpose of this paper is to investigate the possibility of decreasing this scatter by correcting individual SNe Ia using observations of galaxies in the foreground, without introducing any extra bias. Methods: We simulate a large number of SN Ia lines of sight populated by galaxies. For each line of sight the true magnification factor and an estimate thereof are calculated. The estimated magnification factor corresponds to what an observer would infer from a survey like the SNLS. Using the simulated data we investigate the possibility of estimating the magnification of individual supernovae with enough precision to be able to correct their brightness for gravitational lensing with negligible bias. Results: Our simulations show that the bias arising from gravitational lensing corrections of individual SNe Ia is negligible for current and next generation surveys and that the scatter from lensing can be reduced by approximately a factor of 2. The total scatter in the SN Ia magnitudes could be reduced by 4% for an intrinsic dispersion of 0.13 mag. For an intrinsic dispersion of 0.09 mag, which may be feasible for future surveys, the total scatter could be reduced by 6%. This will reduce the errors on cosmological parameters derived from supernova data by 4-8%. The prospect of correcting for lensing is thus very good.

  • 21.
    Jönsson, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Dahlén, Tomas
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics.
    Gunnarsson, Christofer
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Lensing magnification of supernovae in the GOODS-fields2006In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 639, no 2, p. 991-998Article in journal (Refereed)
    Abstract [en]

    Gravitational lensing of high-redshift supernovae is potentially an important source of uncertainty when cosmological parameters are being derived from the measured brightness of Type Ia supernovae, especially in deep surveys with scarce statistics. Photometric and spectroscopic measurements of foreground galaxies along the lines of sight of 33 supernovae discovered with the Hubble Space Telescope, both core-collapse and Type Ia, are used to model the magnification probability distributions of the sources. Modelling galaxy halos with SIS or NFW profiles and using M/L scaling laws provided by the Faber-Jackson and Tully-Fisher relations, we find clear evidence for supernovae with lensing (de)magnification. However, the magnification distribution of the Type Ia supernovae used to determine cosmological distances matches very well the expectations for an unbiased sample; i.e., their mean magnification factor is consistent with unity. Our results show that the lensing distortions of the supernova brightness can be well understood for the GOODS sample and that correcting for this effect has a negligible impact on the derived cosmological parameters.

  • 22.
    Jönsson, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Dahlén, Tomas
    Stockholm University, Faculty of Science, Department of Physics.
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, Edvard
    Department of Astronomy.
    Riess, Adam
    Tentative detection of the gravitational magnification of Type Ia supernovae2007In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, Vol. 2007, no 6, p. 002-Article in journal (Refereed)
    Abstract [en]

    The flux from distant Type Ia supernovae (SNe) is likely to be amplified or de-amplified by gravitational lensing due to matter distributions along the line of sight. A gravitationally lensed supernova would appear brighter or fainter than the average SN at a particular redshift. We estimate the magnification of 26 SNe in the GOODS fields and search for a correlation with the residual magnitudes of the SNe. The residual magnitude, i.e. the difference between observed and average magnitude predicted by the 'concordance model' of the Universe, indicates the deviation in flux from the average SN. The linear correlation coefficient for this sample is r = 0.29. For a similar, but uncorrelated sample, the probability of obtaining a correlation coefficient equal to or higher than this value is ~10%, i.e. a tentative detection of lensing at ~90% confidence level. Although the evidence for a correlation is weak, our result is in accordance with what could be expected given the small size of the sample.

  • 23.
    Jönsson, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, Edvard
    Department of Astronomy.
    Tuning Gravitationally Lensed Standard Sirens2007In: The Astrophysical Journal, Vol. 658, no 1, p. 52-59Article in journal (Refereed)
    Abstract [en]

    Gravitational waves emitted by chirping supermassive black hole binaries could in principle be used to obtain very accurate distance determinations. Provided they have an electromagnetic counterpart from which the redshift can be determined, these standard sirens could be used to build a high-redshift Hubble diagram. Errors in the distance measurements will most likely be dominated by gravitational lensing. We show that the (de)magnification due to inhomogeneous foreground matter will increase the scatter in the measured distances by a factor of ~10. We propose to use optical and IR data of the foreground galaxies to minimize the degradation from weak lensing. We find that the net effect of correcting the estimated distances for lensing is comparable to increasing the sample size by a factor of 3 when using the data to constrain cosmological parameters.

  • 24. Kessler, Richard
    et al.
    Becker, Andrew C.
    Cinabro, David
    Vanderplas, Jake
    Frieman, Joshua A.
    Marriner, John
    Davis, Tamara M.
    Dilday, Benjamin
    Holtzman, Jon
    Jha, Saurabh W.
    Lampeitl, Hubert
    Sako, Masao
    Smith, Mathew
    Zheng, Chen
    Nichol, Robert C.
    Bassett, Bruce
    Bender, Ralf
    Depoy, Darren L.
    Doi, Mamoru
    Elson, Ed
    Filippenko, Alexei V.
    Foley, Ryan J.
    Garnavich, Peter M.
    Hopp, Ulrich
    Ihara, Yutaka
    Ketzeback, William
    Kollatschny, W.
    Konishi, Kohki
    Marshall, Jennifer L.
    Mc Millan, Russet J.
    Miknaitis, Gajus
    Morokuma, Tomoki
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Pan, Kaike
    Prieto, Jose Luis
    Richmond, Michael W.
    Riess, Adam G.
    Romani, Roger
    Schneider, Donald P.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Takanashi, Naohiro
    Tokita, Kouichi
    van der Heyden, Kurt
    Wheeler, J. C.
    Yasuda, Naoki
    York, Donald
    First-Year Sloan Digital Sky Survey-II Supernova Results: Hubble Diagram and Cosmological Parameters2009In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 185, no 1, p. 32-84Article in journal (Refereed)
    Abstract [en]

    We present measurements of the Hubble diagram for 103 Type Ia supernovae (SNe) with redshifts 0.04 < z < 0.42, discovered during the first season (Fall 2005) of the Sloan Digital Sky Survey-II (SDSS-II) Supernova Survey. These data fill in the redshift "desert" between low- and high-redshift SN Ia surveys. Within the framework of the MLCS2K2 light-curve fitting method, we use the SDSS-II SN sample to infer the mean reddening parameter for host galaxies, RV = 2.18 ± 0.14stat ± 0.48syst, and find that the intrinsic distribution of host-galaxy extinction is well fitted by an exponential function, P(AV ) = exp(-AV /τV), with τV = 0.334 ± 0.088 mag. We combine the SDSS-II measurements with new distance estimates for published SN data from the ESSENCE survey, the Supernova Legacy Survey (SNLS), the Hubble Space Telescope (HST), and a compilation of Nearby SN Ia measurements. A new feature in our analysis is the use of detailed Monte Carlo simulations of all surveys to account for selection biases, including those from spectroscopic targeting. Combining the SN Hubble diagram with measurements of baryon acoustic oscillations from the SDSS Luminous Red Galaxy sample and with cosmic microwave background temperature anisotropy measurements from the Wilkinson Microwave Anisotropy Probe, we estimate the cosmological parameters w and ΩM, assuming a spatially flat cosmological model (FwCDM) with constant dark energy equation of state parameter, w. We also consider constraints upon ΩM and ΩΛ for a cosmological constant model (ΛCDM) with w = -1 and non-zero spatial curvature. For the FwCDM model and the combined sample of 288 SNe Ia, we find w = -0.76 ± 0.07(stat) ± 0.11(syst), ΩM = 0.307 ± 0.019(stat) ± 0.023(syst) using MLCS2K2 and w = -0.96 ± 0.06(stat) ± 0.12(syst), ΩM = 0.265 ± 0.016(stat) ± 0.025(syst) using the SALT-II fitter. We trace the discrepancy between these results to a difference in the rest-frame UV model combined with a different luminosity correction from color variations; these differences mostly affect the distance estimates for the SNLS and HST SNe. We present detailed discussions of systematic errors for both light-curve methods and find that they both show data-model discrepancies in rest-frame U band. For the SALT-II approach, we also see strong evidence for redshift-dependence of the color-luminosity parameter (β). Restricting the analysis to the 136 SNe Ia in the Nearby+SDSS-II samples, we find much better agreement between the two analysis methods but with larger uncertainties: w = -0.92 ± 0.13(stat)+0.10 -0.33(syst) for MLCS2K2 and w = -0.92 ± 0.11(stat)+0.07 -0.15 (syst) for SALT-II.

  • 25.
    Kocic, Mikica
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Högås, Marcus
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Torsello, Francesco
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    On Birkhoff's theorem in ghost-free bimetric theory2017In: Article in journal (Refereed)
    Abstract [en]

    We consider the Hassan-Rosen bimetric field equations in vacuum when the two metrics share a single common null direction in a spherically symmetric configuration. By solving these equations, we obtain a class of exact solutions of the generalized Vaidya type parametrized by an arbitrary function. Besides not being asymptotically flat, the found solutions are nonstationary admitting only three global spacelike Killing vector fields which are the generators of spatial rotations. Hence, these are spherically symmetric bimetric vacuum solutions with the minimal number of isometries. The absence of staticity formally disproves an analogue statement to Birkhoff's theorem in the ghost-free bimetric theory which would state that a spherically symmetric solution is necessarily static in empty space.

  • 26.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Calibrating Milky Way dust extinction using cosmological sources2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 550, p. A80-Article in journal (Refereed)
    Abstract [en]

    We constrain the light extinction properties of Milky Way dust. We investigated the correlations between dust column density as inferred from infrared data and the observed colours of celestial objects at cosmological distances with low levels of colour dispersion. Results derived using colours of quasars, brightest central galaxies, and luminous red galaxies are broadly consistent, indicating a proportionality constant between the reddening E(B - V) = A(B) - A(V) and the dust column density D-T (given in units of MJy/sr) of p = E(B - V)/D-T similar to 0.02 and a reddening parameter R-V = Lambda(V)/E(B - V) similar to 3 with fractional uncertainties of approximately 10%. The data do not provide any evidence for spatial variations in the dust properties, except for a possible hint of scatter in the dust extinction properties at the longest optical wavelengths.

  • 27.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Cosmological histories from the Friedmann equation: the Universe as a particle2016In: European journal of physics, ISSN 0143-0807, E-ISSN 1361-6404, Vol. 37, no 5, article id 055603Article in journal (Refereed)
    Abstract [en]

    In this note, we discuss how possible expansion histories of the Universe can be inferred in a simple way, for arbitrary energy contents. No new physical results are obtained, but the goal is rather to discuss an alternative way of writing the Friedmann equation in order to facilitate an intuitive understanding of the possible solutions; for students and researchers alike. As has been noted in passing by others, this specific form of the Friedmann equation allows us to view the universal expansion as a particle rolling along a frictionless track. Specific examples depicted include the current concordance cosmological model as well as a stable static universal model.

  • 28.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cosmological histories in bimetric gravity: a graphical approach2017In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 2, article id 051Article in journal (Refereed)
    Abstract [en]

    The bimetric generalization of general relativity has been proven to be able to give an accelerated background expansion consistent with observations. Apart from the energy densities coupling to one or both of the metrics, the expansion will depend on the cosmological constant contribution to each of them, as well as the three parameters describing the interaction between the two metrics. Even for fixed values of these parameters can several possible solutions, so called branches, exist. Different branches can give similar background expansion histories for the observable metric, but may have different properties regarding, for example, the existence of ghosts and the rate of structure growth. In this paper, we outline a method to find viable solution branches for arbitrary parameter values. We show how possible expansion histories in bimetric gravity can be inferred qualitatively, by picturing the ratio of the scale factors of the two metrics as the spatial coordinate of a particle rolling along a frictionless track. A particularly interesting example discussed is a specific set of parameter values, where a cosmological dark matter background is mimicked without introducing ghost modes into the theory.

  • 29.
    Mörtsell, Edvard
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Clarkson, Chris
    Model independent constraints on the cosmological expansion rate2009In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, Vol. 01, p. 044-Article in journal (Refereed)
    Abstract [en]

    We investigate what current cosmological data tells us about the cosmological expansion rate in a model independent way. Specifically, we study if the expansion was decelerating at high redshifts and is accelerating now, without referring to any model for the energy content of the universe, nor to any specific theory of gravity. This differs from most studies of the expansion rate which, e.g., assumes some underlying parameterised model for the dark energy component of the universe. To accomplish this, we have devised a new method to probe the expansion rate without relying on such assumptions. Using only supernova data, we conclude that there is little doubt that the universe has been accelerating at late times. However, contrary to some previous claims, we can not determine if the universe was previously decelerating. For a variety of methods used for constraining the expansion history of the universe, acceleration is detected from supernovae alone at >5σ, regardless of the curvature of the universe. Specifically, using a Taylor expansion of the scale factor, acceleration today is detected at >12σ. If we also include the ratio of the scale of the baryon acoustic oscillations as imprinted in the cosmic microwave background and in the large scale distribution of galaxies, it is evident from the data that the expansion decelerated at high redshifts, but only with the assumption of a flat or negatively curved universe.

  • 30.
    Mörtsell, Edvard
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Dhawan, Suhail
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Does the Hubble constant tension call for new physics?2018In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 9, article id 025Article in journal (Refereed)
    Abstract [en]

    The Lambda Cold Dark Matter model (Lambda CDM) represents the current standard model in cosmology. Within this, there is a tension between the value of the Hubble constant, H-0, inferred from local distance indicators and the angular scale of fluctuations in the Cosmic Microwave Background (CMB). In terms of Bayseian evidence, we investigate whether the tension is significant enough to warrant new physics in the form of modifying or adding energy components to the standard cosmological model. We find that late time dark energy explanations are not favoured by data whereas a pre-CMB decoupling extra dark energy component has a positive, although not substantial, Bayesian evidence. A constant equation of state of the additional early energy density is constrained to 0.086(-0.03)(+0:04). Although this value deviates significantly from 1/3, valid for dark radiation, the latter is favoured based on the Bayesian evidence. If the tension persists, future estimates of H-0 at the 1% level will be able to decisively determine which of the proposed explanations is favoured.

  • 31.
    Mörtsell, Edvard
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Enander, Jonas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Scalar instabilities in bimetric gravity: the Vainshtein mechanism and structure formation2015In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 10, article id 044Article in journal (Refereed)
    Abstract [en]

    We investigate the observational consequences of scalar instabilities in bimetric theory, under the assumption that the Vainshtein mechanism restores general relativity within a certain distance from gravitational sources. We argue that early time instabilities have a negligible impact on observed structures. Assuming that the instabilities affect sub-horizon density fluctuations, we constrain the redshift, z(i), below which instabilities are ruled out. For the minimal beta(1)-model, observational constraints are close to the theoretical expectations of z(i) approximate to 0.5, potentially allowing the model to be ruled in or out with a more detailed study, possibly including secondary cosmic microwave background constraints.

  • 32.
    Riehm, T.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, E.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mortsell, E.
    Stockholm University, Faculty of Science, Department of Physics.
    Wiik, K.
    Detecting CDM substructure via gravitational millilensing2008In: Identification of dark matter 2008, 2008, p. 54-Conference paper (Refereed)
    Abstract [en]

    Not Available

  • 33.
    Riehm, Teresa
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Amanullah, Rahman
    Stockholm University, Faculty of Science, Department of Physics.
    Dahlén, Thomas
    Jönsson, Jakob
    Stockholm University, Faculty of Science, Department of Physics.
    Limousin, Marceau
    Paech, Kerstin
    Richard, Johan
    Near-IR search for lensed supernovae behind galaxy clusters: III. Implications for cluster modeling and cosmologyArticle in journal (Refereed)
  • 34.
    Riehm, Teresa
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Wiik, K.
    Strong Lensing by Subhalos in the Dwarf-galaxy-mass Range. II. Detection Probabilities2009In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 700, no 2, p. 1552-1558Article in journal (Refereed)
    Abstract [en]

    The dark halo substructures predicted by current cold dark matter simulations may in principle be detectable through strong-lensing image-splitting of quasars on small angular scales (0.01 arcsec or below). Here, we estimate the overall probabilities for lensing by substructures in a host halo closely aligned to the line of sight to a background quasar. Under the assumption that the quasar can be approximated as a point source, the optical depth for strong gravitational lensing by subhalos typically turns out to be very small (τ < 0.01), contrary to previous claims. We therefore conclude that it is currently not feasible to use this strategy to put the simulation predictions for the dark matter subhalo population to the test. However, if one assumes the source to be spatially extended, as is the case for a quasar observed at radio wavelengths, there is a reasonable probability for witnessing substructure lensing effects even at rather large projected distances from the host galaxy, provided that the angular resolution is sufficient. While multiply imaged, radio-loud quasars would be the best targets for unambiguously detecting dark matter subhalos, even singly imaged radio quasars might be useful for setting upper limits on the abundance and central surface mass density of subhalos.

  • 35.
    Sjörs, Stefan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Spherically Symmetric Solutions in Massive Gravity and Constraints from GalaxiesManuscript (preprint) (Other academic)
    Abstract [en]

    In this paper, analytical solutions describing static and spherically symmetric sources in the decoupling limit of massive gravity are derived. We analyze the model parameter range and specify when a Vainshtein mechanism is possible. Furthermore, we use gravitational lensing and velocity dispersion data from galaxies to put constraints on the mass scale of the graviton. The result for the inverse graviton mass scale lambda_g = h/(2pi)/(c m_g), in units of the Hubble radius r_H=c/H_0, is of the order lambda_g/r_H > 0.01-0.02 at 95% confidence level.

  • 36.
    Sjörs, Stefan
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Spherically symmetric solutions in massive gravity and constraints from galaxies2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 2, p. 080-Article in journal (Refereed)
    Abstract [en]

    In this paper, analytical solutions describing static and spherically symmetric sources in the decoupling limit of massive gravity are derived. We analyze the model parameter range and specify when a Vainshtein mechanism is possible. Furthermore, we use gravitational lensing and velocity dispersion data from galaxies to put constraints on the mass scale of the graviton. The result for the inverse graviton mass scale lambda(g) = h/(cm(g)), in units of the Hubble radius r(H) = c/H-0, is of the order lambda(g)/r(H) greater than or similar to 0.01 - 0.02 at 95% confidence level.

  • 37.
    Sollerman, J.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mörtsell, E.
    Stockholm University, Faculty of Science, Department of Physics.
    Davis, T. M.
    Blomqvist, M.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bassett, B.
    Becker, A. C.
    Cinabro, D.
    Filippenko, A. V.
    Foley, R. J.
    Frieman, J.
    Garnavich, P.
    Lampeitl, H.
    Marriner, J.
    Miquel, R.
    Nichol, R. C.
    Richmond, M. W.
    Sako, M.
    Schneider, D. P.
    Smith, M.
    Vanderplas, J. T.
    Wheeler, J. C.
    First-Year Sloan Digital Sky Survey-II (SDSS-II) Supernova Results: Constraints on Nonstandard Cosmological Models2009In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 703, no 2, p. 1374-1385Article in journal (Refereed)
    Abstract [en]

    We use the new Type Ia supernovae discovered by the Sloan Digital Sky Survey-II supernova survey, together with additional supernova data sets as well as observations of the cosmic microwave background and baryon acoustic oscillations to constrain cosmological models. This complements the standard cosmology analysis presented by Kessler et al. in that we discuss and rank a number of the most popular nonstandard cosmology scenarios. When this combined data set is analyzed using the MLCS2k2 light-curve fitter, we find that more exotic models for cosmic acceleration provide a better fit to the data than the ΛCDM model. For example, the flat Dvali-Gabadadze-Porrati model is ranked higher by our information-criteria (IC) tests than the standard model with a flat universe and a cosmological constant. When the supernova data set is instead analyzed using the SALT-II light-curve fitter, the standard cosmological-constant model fares best. This investigation of how sensitive cosmological model selection is to assumptions about, and within, the light-curve fitters thereby highlights the need for an improved understanding of these unresolved systematic effects. Our investigation also includes inhomogeneous Lemaître-Tolman-Bondi (LTB) models. While our LTB models can be made to fit the supernova data as well as any other model, the extra parameters they require are not supported by our IC analysis. Finally, we explore more model-independent ways to investigate the cosmic expansion based on this new data set.

  • 38. Solomon, Adam R.
    et al.
    Enander, Jonas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Akrami, Yashar
    Koivisto, Tomi S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Könnig, Frank
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cosmological viability of massive gravity with generalized matter coupling2015In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 4, article id 027Article in journal (Refereed)
    Abstract [en]

    There is a no-go theorem forbidding flat and closed FLRW solutions in massive gravity on a flat reference metric, while open solutions are unstable. Recently it was shown that this no-go theorem can be overcome if at least some matter couples to a hybrid metric composed of both the dynamical and the fixed reference metric. We show that this is not compatible with the standard description of cosmological sources in terms of effective perfect fluids, and the predictions of the theory become sensitive either to the detailed field-theoretical modelling of the matter content or to the presence of additional dark degrees of freedom. This is a serious practical complication. Furthermore, we demonstrate that viable cosmological background evolution with a perfect fluid appears to require the presence of fields with highly contrived properties. This could be improved if the equivalence principle is broken by coupling only some of the fields to the composite metric, but viable self-accelerating solutions due only to the massive graviton are difficult to obtain. These problems can be avoided by giving the reference metric dynamics.

  • 39.
    Stanishev, Vallery
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics.
    Paech, Kerstin
    Stockholm University, Faculty of Science, Department of Physics.
    Amanullah, Rahman
    Stockholm University, Faculty of Science, Department of Physics.
    Dahlén, T.
    Jönsson, Jakob
    Stockholm University, Faculty of Science, Department of Physics.
    Kneib, J. P.
    Lidman, Chris
    Stockholm University, Faculty of Science, Department of Physics.
    Limousin, M.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Nobili, Serena
    Stockholm University, Faculty of Science, Department of Physics.
    Richard, J.
    Riehm, Teresa
    Stockholm University, Faculty of Science, Department of Astronomy.
    von Strauss, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Near-IR search for lensed supernovae behind galaxy clusters. I. Observations and transient detection efficiency2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 507, no 1, p. 61-69Article in journal (Refereed)
    Abstract [en]

    Context: Massive galaxy clusters at intermediate redshift can magnify the flux of distant background sources by several magnitudes.

    Aims: We exploit this effect to search for lensed distant supernovae that may otherwise be too faint to be detected.

    Methods: A supernova search was conducted at near infrared wavelengths using the ISAAC instrument at the VLT. The massive galaxy clusters Abell 1689, Abell 1835, and AC114 were observed for a total of 20 h to search for supernovae in gravitationally magnified background galaxies. The observations were split into individual epochs of 2 h of exposure time, separated by approximately one month. Image-subtraction techniques were used to search for transient objects with light curve properties consistent with supernovae, both in our new and archival ISAAC/VLT data. The limiting magnitude of the individual epochs was estimated by adding artificial stars to the subtracted images. Most of the epochs reach 90% detection efficiency at SZ(J) ≃ 23.8-24.0 mag (Vega).

    Results: Two transient objects, both in archival images of Abell 1689 and AC114, were detected. The transient in AC114 coincides - within the position uncertainty - with an X-ray source and is likely to be a variable AGN at the cluster redshift. The transient in Abell 1689 was found at SZ = 23.24 mag, ~0.5´´away from a galaxy with photometric redshift z_gal = 0.6 ± 0.15. The light curves and the colors of the transient are consistent with a reddened type IIP supernova at redshift z = 0.59 ± 0.05. The lensing model of Abell 1689 predicts ~1.4 mag of magnification at the position of the transient, making it the most magnified supernova ever found and only the second supernova found behind a galaxy cluster. Conclusions: Our pilot survey has demonstrated the feasibility to find distant gravitationally magnified supernovae behind massive galaxy clusters. One likely supernova was found behind Abell 1689, in accordance with the expectations for this survey, as shown in an accompanying analysis paper. Based on observations made with ESO telescopes at the Paranal Observatory under program IDs 079.A-0192 and 081.A-0734.

  • 40. Sundell, Peter
    et al.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Vilja, Iiro
    Can a void mimic the Lambda in Lambda CDM?2015In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 8, article id 037Article in journal (Refereed)
    Abstract [en]

    We investigate Lematre-Tolman-Bondi (LTB) models, whose early time evolution and bang time are homogeneous and the distance-redshift relation and local Hubble parameter are inherited from the Lambda CDM model. We show that the obtained LTB models and the Lambda CDM model predict different relative local expansion rates and that the Hubble functions of the models diverge increasingly with redshift. The LTB models show tension between low redshift baryon acoustic oscillation and supernova observations and including Lyman-alpha forest or cosmic microwave background observations only accentuates the better fit of the Lambda CDM model compared to the LTB model. The result indicates that additional degrees of freedom are needed to explain the observations, for example by renouncing spherical symmetry, homogeneous bang time, negligible effects of pressure, or the early time homogeneity assumption.

  • 41.
    Torsello, Francesco
    et al.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Kocic, Mikica
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Högås, Marcus
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    On spacetime symmetries and topology in bimetric theoriesIn: Article in journal (Refereed)
    Abstract [en]

    We explore spacetime symmetries and topologies of the two metric sectors in Hassan-Rosen bimetric theory. We show that, in vacuum, the two sectors can either share or have separate spacetime symmetries. If stress-energy tensors are present, a third case can arise, with different spacetime symmetries within the same sector. This raises the question of the best definition of spacetime symmetry in Hassan-Rosen bimetric theory. We emphasize the possibility of imposing ansatzes and looking for solutions having different Killing vector fields or different isometries in the two sectors, which has gained little attention so far. We also point out that the topology of spacetime imposes a constraint on possible metric combinations.

  • 42.
    Torsello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kocic, Mikica
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Högås, Marcus
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Spacetime symmetries and topology in bimetric relativity2018In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 97, no 8, article id 084022Article in journal (Refereed)
    Abstract [en]

    We explore spacetime symmetries and topologies of the two metric sectors in Hassan-Rosen bimetric theory. We show that, in vacuum, the two sectors can either share or have separate spacetime symmetries. If stress-energy tensors are present, a third case can arise, with different spacetime symmetries within the same sector. This raises the question of the best definition of spacetime symmetry in Hassan-Rosen bimetric theory. We emphasize the possibility of imposing ansatzes and looking for solutions having different Killing vector fields or different isometries in the two sectors, which has gained little attention so far. We also point out that the topology of spacetime imposes a constraint on possible metric combinations.

  • 43.
    Torsello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kocic, Mikica
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Classification and asymptotic structure of black holes in bimetric theory2017In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 96, no 6, article id 064003Article in journal (Refereed)
    Abstract [en]

    We study general properties of static and spherically symmetric bidiagonal black holes in Hassan-Rosen bimetric theory by means of a new method. In particular, we explore the behavior of the black hole solutions both at the common Killing horizon and at the large radii. The former study was never done before and leads to a new classification for black holes within the bidiagonal ansatz. The latter study shows that, among the great variety of the black hole solutions, the only solutions converging to Minkowski, anti-de Sitter, and de Sitter spacetimes at large radii are those of general relativity, i.e., the Schwarzschild, Schwarzschild-anti-de Sitter and Schwarzschild-de Sitter solutions. Moreover, we present a proposition, whose validity is not limited to black hole solutions, which establishes the relation between the curvature singularities of the two metrics and the invertibility of their interaction potential.

  • 44.
    von Strauss, Mikael
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schmidt-May, Angnis
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Enander, Jonas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hassan, S. Fawad
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cosmological solutions in bimetric gravity and their observational tests2012In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 3, p. 042-Article in journal (Refereed)
    Abstract [en]

    We obtain the general cosmological evolution equations for a classically consistent theory of bimetric gravity. Their analytic solutions are demonstrated to generically allow for a cosmic evolution starting out from a matter dominated FLRW universe and relaxing towards a de Sitter (anti-de Sitter) phase at late cosmic time. In particular, we examine a subclass of models which contain solutions that are able to reproduce the expansion history of the cosmic concordance model inspite of the nonlinear couplings of the two metrics. This is demonstrated explicitly by fitting these models to observational data from Type Ia supernovae, Cosmic Microwave Background and Baryon Acoustic Oscillations. In the appendix we comment on the relation to massive gravity.

  • 45.
    Östman, L.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Goobar, A.
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, E.
    Stockholm University, Faculty of Science, Department of Physics.
    Looking at Quasars Through Galaxies2006In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 450, no 3, p. 971-977Article in journal (Refereed)
    Abstract [en]

    Observations of quasars (QSOs) shining through or close to galaxies offer a way to probe the properties of the foreground matter through dust extinction and gravitational lensing. In this paper the feasibility of measuring the dust extinction properties is investigated using the backlitting of QSOs. We test our method to search for QSOs affected by intervening extinction, by matching the coordinates in the SDSS QSO DR3 catalogue with the New York University Value-Added Galaxy Catalog. In total, 164 QSO-galaxy pairs were found with a distance of less than 30 kpc between the galaxy centre and the QSO line-of-sight at the galaxy redshift. Investigating the QSO colours with multiband SDSS photometry, two pairs with galaxy redshifts z < 0.08 were found to be particularly interesting in that the QSOs show evidence of heavy Galactic type extinction with at very large optical radii in the foreground spiral galaxies. With the available data, it remains inconclusive whether the two pairs can be explained as statistical colour outliers, by host extinction or if they provide evidence of dust in the outskirts of spiral galaxies. Deeper galaxy catalogues and/or higher resolution follow-up QSO spectra would help resolve this problem. We also analyse five QSOs reported in the literature with spectroscopic absorption features originating from an intervening system. These systems are at higher redshifts than the other two and we find in most cases significantly lower best fit values of RV. The wide range of preferred values of RV found, although affected by substantial uncertainties, already indicates that the dust properties in other galaxies may be different from the Milky Way. Furthermore, the available data suggest a possible evolution in the dust properties with redshift, with lower RV at high z.

  • 46.
    Östman, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Extinction properties of lensing galaxies2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 485, no 2, p. 403-415Article in journal (Refereed)
  • 47.
    Östman, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Limiting the dimming of distant type Ia supernovae2005In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 2, p. 5-Article in journal (Refereed)
    Abstract [en]

    Distant supernovae have been observed to be fainter than what is expected in a matter dominated universe. The most likely explanation is that the universe is dominated by an energy component with negative pressure, namely dark energy. However, there are several astrophysical processes that could, in principle, affect the measurements and in order to be able to take advantage of the growing supernova statistics, the control of systematic effects is crucial. We discuss two of these; extinction due to intergalactic grey dust and dimming due to photon-axion oscillations and show how their effect on supernova observations can be constrained using observed quasar colours and spectra. For a wide range of intergalactic dust models, we are able to rule out any dimming larger than 0.2 magnitudes for a type Ia supernova at z = 1. The corresponding limit for intergalactic Milky Way type dust is 0.03 magnitudes. For the more speculative model of photons mixing with axions, we find that the effect is independent of photon energy for certain combinations of parameter values and a dimming as large as 0.6 magnitudes cannot be ruled out. These effects can have profound implications for the possibility of constraining dark energy properties using supernova observations.

1 - 47 of 47
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