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  • 1. Alonso, D.
    et al.
    Bellini, E.
    Ferreira, P. G.
    Zumalacárregui, Miguel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Berkeley Center for Cosmological Physics and University of California at Berkeley, USA.
    Observational future of cosmological scalar-tensor theories2017In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, no 6, article id 063502Article in journal (Refereed)
    Abstract [en]

    The next generation of surveys will greatly improve our knowledge of cosmological gravity. In this paper we focus on how Stage IV photometric redshift surveys, including weak lensing and multiple tracers of the matter distribution and radio experiments combined with measurements of the cosmic microwave background will lead to precision constraints on deviations from general relativity. We use a broad subclass of Horndeski scalar-tensor theories to forecast the accuracy with which we will be able to determine these deviations and their degeneracies with other cosmological parameters. Our analysis includes relativistic effects, does not rely on the quasistatic evolution and makes conservative assumptions about the effect of screening on small scales. We define a figure of merit for cosmological tests of gravity and show how the combination of different types of surveys, probing different length scales and redshifts, can be used to pin down constraints on the gravitational physics to better than a few percent, roughly an order of magnitude better than present probes. Future cosmological experiments will be able to constrain he Brans-Dicke parameter at a level comparable to Solar System and astrophysical tests.

  • 2.
    Bettoni, Dario
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Ezquiaga, Jose Maria
    Hinterbichler, Kurt
    Zumalacárregui, Miguel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Berkeley Center for Cosmological Physics, LBNL and University of California at Berkeley, USA.
    Speed of gravitational waves and the fate of scalar-tensor gravity2017In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, no 8, article id 084029Article in journal (Refereed)
    Abstract [en]

    The direct detection of gravitational waves (GWs) is an invaluable new tool to probe gravity and the nature of cosmic acceleration. A large class of scalar-tensor theories predicts that GWs propagate with velocity different than the speed of light, a difference that can be O(1) for many models of dark energy. We determine the conditions behind the anomalous GW speed, namely, that the scalar field spontaneously breaks Lorentz invariance and couples to the metric perturbations via the Weyl tensor. If these conditions are realized in nature, the delay between GW and electromagnetic signals from distant events will run beyond human time scales, making it impossible to measure the speed of GWs using neutron star mergers or other violent events. We present a robust strategy to exclude or confirm an anomalous speed of GWs using eclipsing binary systems, the electromagnetic phase of which can be exquisitely determined. The white dwarf binary J0651 + 2844 is a known example of such a system that can be used to probe deviations in the GW speed as small as cg/ c - 1 greater than or similar to 2 x 10(-12) when LISA comes online. This test will either eliminate many contender models for cosmic acceleration or wreck a fundamental pillar of general relativity.

  • 3. Ezquiaga, Jose Maria
    et al.
    Garcia-Bellido, Juan
    Zumalacárregui, Miguel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Towards the most general scalar-tensor theories of gravity: A unified approach in the language of differential forms2016In: Physical Review D, ISSN 2470-0010, Vol. 94, no 2, article id 024005Article in journal (Refereed)
    Abstract [en]

    We use a description based on differential forms to systematically explore the space of scalar-tensor theories of gravity. Within this formalism, we propose a basis for the scalar sector at the lowest order in derivatives of the field and in any number of dimensions. This minimal basis is used to construct a finite and closed set of Lagrangians describing general scalar-tensor theories invariant under local Lorentz transformations in a pseudo-Riemannian manifold, which contains ten physically distinct elements in four spacetime dimensions. Subsequently, we compute their corresponding equations of motion and find which combinations are at most second order in derivatives in four as well as an arbitrary number of dimensions. By studying the possible exact forms (total derivatives) and algebraic relations between the basis components, we discover that there are only four Lagrangian combinations producing second-order equations, which can be associated with Horndeski's theory. In this process, we identify a new second-order Lagrangian, named kinetic Gauss-Bonnet, that was not previously considered in the literature. However, we show that its dynamics is already contained in Horndeski's theory. Finally, we provide a full classification of the relations between different second-order theories. This allows us to clarify, for instance, the connection between different covariantizations of Galileons theory. In conclusion, our formulation affords great computational simplicity with a systematic structure. As a first step, we focus on theories with second-order equations of motion. However, this new formalism aims to facilitate advances towards unveiling the most general scalar-tensor theories.

  • 4. Ezquiaga, Jose María
    et al.
    Zumalacárregui, Miguel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). LBNL, USA; University of California at Berkeley, USA; Université Paris Saclay, France.
    Dark Energy After GW170817: Dead Ends and the Road Ahead2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 25, article id 251304Article in journal (Refereed)
    Abstract [en]

    Multimessenger gravitational-wave (GW) astronomy has commenced with the detection of the binary neutron star merger GW170817 and its associated electromagnetic counterparts. The almost coincident observation of both signals places an exquisite bound on the GW speed vertical bar c(g)/c-1 vertical bar <= 5 x 10(-1)6. We use this result to probe the nature of dark energy (DE), showing that a large class of scalar-tensor theories and DE models are highly disfavored. As an example we consider the covariant Galileon, a cosmologically viable, well motivated gravity theory which predicts a variable GW speed at low redshift. Our results eliminate any late-universe application of these models, as well as their Horndeski and most of their beyond Horndeski generalizations. Three alternatives (and their combinations) emerge as the only possible scalar-tensor DE models: (1) restricting Horndeski's action to its simplest terms, (2) applying a conformal transformation which preserves the causal structure, and (3) compensating the different terms that modify the GW speed (to be robust, the compensation has to be independent on the background on which GWs propagate). Our conclusions extend to any other gravity theory predicting varying c(g) such as Einstein-Aether, Ho. rava gravity, Generalized Proca, tensor-vector-scalar gravity (TEVES), and other MOND-like gravities.

  • 5. Könnig, Frank
    et al.
    Nersisyan, Henrik
    Akrami, Yashar
    Amendola, Luca
    Zumalacárregui, Miguel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ruprecht-Karls-Universität Heidelberg, Germany.
    A spectre is haunting the cosmos: quantum stability of massive gravity with ghosts2016In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 11, article id 118Article in journal (Refereed)
    Abstract [en]

    Many theories of modified gravity with higher order derivatives are usually ignored because of serious problems that appear due to an additional ghost degree of freedom. Most dangerously, it causes an immediate decay of the vacuum. However, breaking Lorentz invariance can cure such abominable behavior. By analyzing a model that describes a massive graviton together with a remaining Boulware-Deser ghost mode we show that even ghostly theories of modified gravity can yield models that are viable at both classical and quantum levels and, therefore, they should not generally be ruled out. Furthermore, we identify the most dangerous quantum scattering process that has the main impact on the decay time and find differences to simple theories that only describe an ordinary scalar field and a ghost. Additionally, constraints on the parameters of the theory including some upper bounds on the Lorentz-breaking cutoff scale are presented. In particular, for a simple theory of massive gravity we find that a breaking of Lorentz invariance is allowed to happen even at scales above the Planck mass. Finally, we discuss the relevance to other theories of modified gravity.

  • 6. María Ezquiaga, Jose
    et al.
    García-Bellido, Juan
    Zumalacárregui, Miguel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Berkeley Center for Cosmological Physics and University of California at Berkeley, USA.
    Field redefinitions in theories beyond Einstein gravity using the language of differential forms2017In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, no 8, article id 084039Article in journal (Refereed)
    Abstract [en]

    We study the role of field redefinitions in general scalar-tensor theories. In particular, we first focus on the class of field redefinitions linear in the spin-2 field and involving derivatives of the spin-0 mode, generically known as disformal transformations. We start by defining the action of a disformal transformation in the tangent space. Then, we take advantage of the great economy of means of the language of differential forms to compute the full transformation of Horndeski's theory under general disformal transformations. We obtain that Horndeski's action maps onto itself modulo a reduced set of non-Horndeski Lagrangians. These new Lagrangians are found to be invariant under disformal transformation that depend only in the first derivatives of the scalar. Moreover, these combinations of Lagrangians precisely appear when expressing in our basis the constraints of the recently proposed extended scalar-tensor theories. These results allow us to classify the different orbits of scalar-tensor theories invariant under particular disformal transformations, namely, the special disformal, kinetic disformal, and disformal Horndeski orbits. In addition, we consider generalizations of this framework. We find that there are possible well-defined extended disformal transformations that have not been considered in the literature. However, they generically cannot link Horndeski theory with extended scalar-tensor theories. Finally, we study further generalizations in which extra fields with different spin are included. These field redefinitions can be used to connect different gravity theories such as multiscalar-tensor theories, generalized Proca theories, and bigravity. We discuss how the formalism of differential forms could be useful for future developments in these lines.

  • 7.
    Renk, Janina
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Ruprecht-Karls-Universität Heidelberg, Germany.
    Zumalacárregui, Miguel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ruprecht-Karls-Universität Heidelberg, Germany.
    Montanari, Francesco
    Gravity at the horizon: on relativistic effects, CMB-LSS correlations and ultra-large scales in Horndeski's theory2016In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 7, article id 040Article in journal (Refereed)
    Abstract [en]

    We address the impact of consistent modifications of gravity on the largest observable scales, focusing on relativistic effects in galaxy number counts and the cross-correlation between the matter large scale structure (LSS) distribution and the cosmic microwave background (CMB). Our analysis applies to a very broad class of general scalar-tensor theories encoded in the Horndeski Lagrangian and is fully consistent on linear scales, retaining the full dynamics of the scalar field and not assuming quasi-static evolution. As particular examples we consider self-accelerating Covariant Galileons, Brans-Dicke theory and parameterizations based on the effective field theory of dark energy, using the hi_class code to address the impact of these models on relativistic corrections to LSS observables. We find that especially effects which involve integrals along the line of sight (lensing convergence, time delay and the integrated Sachs-Wolfe effect- ISW) can be considerably modified, and even lead to O(1000%) deviations from General Relativity in the case of the ISW effect for Galileon models, for which standard probes such as the growth function only vary by O(10%). These effects become dominant when correlating galaxy number counts at different redshifts and can lead to similar to 50% deviations in the total signal that might be observable by future LSS surveys. Because of their integrated nature, these deep-redshift cross-correlations are sensitive to modifications of gravity even when probing eras much before dark energy domination. We further isolate the ISW effect using the cross-correlation between LSS and CMB temperature anisotropies and use current data to further constrain Horndeski models. Forthcoming large-volume galaxy surveys using multiple-tracers will search for all these effects, opening a new window to probe gravity and cosmic acceleration at the largest scales available in our universe.

  • 8.
    Renk, Janina
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zumalacárregui, Miguel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). LBL and University of California at Berkeley, USA.
    Montanari, Francesco
    Barreira, Alexandre
    Galileon gravity in light of ISW, CMB, BAO and H-0 data2017In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 10, article id 020Article in journal (Refereed)
    Abstract [en]

    Cosmological models with Galileon gravity are an alternative to the standard ACDM paradigm with testable predictions at the level of its self-accelerating solutions for the expansion history, as well as large-scale structure formation. Here, we place constraints on the full parameter space of these models using data from the cosmic microwave background (CMB) (including lensing), baryonic acoustic oscillations (BAO) and the Integrated Sachs Wolfe (ISW) effect. We pay special attention to the ISW effect for which we use the cross spectra, C-l(Tg), of CMB temperature maps and foreground galaxies from the WISE survey. The sign of C-l(Tg) is set by the time evolution of the lensing potential in the redshift range of the galaxy sample: it is positive if the potential decays (like in ACDM), negative if it deepens. We constrain three subsets of Galileon gravity separately known as the Cubic, Quartic and Quintic Galileons. The cubic Galileon model predicts a negative C-l(Tg) and exhibits a 7.8 sigma tension with the data, which effectively rules it out. For the quartic and quintic models the ISW data also rule out a significant portion of the parameter space but permit regions where the goodness-of-fit is comparable to ACDM. The data prefers a non zero sum of the neutrino masses (Sigma m(v) approximate to 0.5eV) with similar to 5 sigma significance in these models. The best-fitting models have values of Ho consistent with local determinations, thereby avoiding the tension that exists in ACDM. We also identify and discuss a similar to 2 sigma tension that Galileon gravity exhibits with recent BAO measurements. Our analysis shows overall that Galileon cosmologies cannot be ruled out by current data but future lensing, BAO and ISW data hold strong potential to do so.

  • 9.
    Zumalacárregui, Miguel
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of California at Berkeley, USA; Ruprecht-Karls-Universität Heidelberg, Germany.
    Bellini, Emilio
    Sawicki, Ignacy
    Lesgourgues, Julien
    Ferreira, Pedro G.
    hi_class: Horndeski in the Cosmic Linear Anisotropy Solving System2017In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 8, article id 019Article in journal (Refereed)
    Abstract [en]

    We present the public version of hi_class (www.hiclass-code.net), an extension of the Boltzmann code CLASS to a broad ensemble of modifications to general relativity. In particular, hi_class can calculate predictions for models based on Horndeski's theory, which is the most general scalar-tensor theory described by second-order equations of motion and encompasses any perfect-fluid dark energy, quintessence, Brans-Dicke, f(R) and covariant Galileon models. hi_class has been thoroughly tested and can be readily used to understand the impact of alternative theories of gravity on linear structure formation as well as for cosmological parameter extraction.

  • 10.
    Zumalacárregui, Miguel
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of California at Berkeley, USA; Université Paris Saclay, France.
    Seljak, Uros
    Limits on Stellar-Mass Compact Objects as Dark Matter from Gravitational Lensing of Type Ia Supernovae2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 14, article id 141101Article in journal (Refereed)
    Abstract [en]

    The nature of dark matter (DM) remains unknown despite very precise knowledge of its abundance in the Universe. An alternative to new elementary particles postulates DM as made of macroscopic compact halo objects (MACHO) such as black holes formed in the very early Universe. Stellar-mass primordial black holes (PBHs) are subject to less robust constraints than other mass ranges and might be connected to gravitational-wave signals detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO). New methods are therefore necessary to constrain the viability of compact objects as a DM candidate. Here we report bounds on the abundance of compact objects from gravitational lensing of type Ia supernovae (SNe). Current SNe data sets constrain compact objects to represent less than 35.2% (Joint Lightcurve Analysis) and 37.2% (Union 2.1) of the total matter content in the Universe, at 95% confidence level. The results are valid for masses larger than similar to 0.01 M-circle dot (solar masses), limited by the size SNe relative to the lens Einstein radius. We demonstrate the mass range of the constraints by computing magnification probabilities for realistic SNe sizes and different values of the PBH mass. Our bounds are sensitive to the total abundance of compact objects with M greater than or similar to 0.01 M-circle dot and complementary to other observational tests. These results are robust against cosmological parameters, outlier rejection, correlated noise, and selection bias. PBHs and other MACHOs are therefore ruled out as the dominant form of DM for objects associated to LIGO gravitational wave detections. These bounds constrain early-Universe models that predict stellar-mass PBH production and strengthen the case for lighter forms of DM, including new elementary particles.

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