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  • 1. Bambi, Cosimo
    et al.
    Freese, Katherine
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Michigan, USA.
    Vagnozzi, Sunny
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Cambridge, United Kingdom.
    Visinelli, Luca
    Testing the rotational nature of the supermassive object M87*from the circularity and size of its first image2019In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 100, no 4, article id 044057Article in journal (Refereed)
    Abstract [en]

    The Event Horizon Telescope (EHT) collaboration has recently released the first image of a black hole (BH), opening a new window onto tests of general relativity in the strong field regime. In this paper, we derive constraints on the nature of M87* (the supermassive object at the center of the galaxy M87), exploiting the fact that its shadow appears to be highly circular, and using measurements of its angular size. We first consider the simple case where M87* is assumed to be a Kerr BH. We find that the inferred circularity of M87* excludes Kerr BHs with observation angle theta(obs) greater than or similar to 45 degrees for dimensionless rotational parameter 0.95 less than or similar to a(*) <= 1 whereas the observation angle is unbounded for a(*) less than or similar to 0.9. We then consider the possibility that M87* might be a superspinar, i.e., an object described by the Kerr solution and spinning so fast that it violates the Kerr bound by having vertical bar a(*)vertical bar > 1. We find that, within certain regions of parameter space, the inferred circularity and size of the shadow of M87* do not exclude the possibility that this object might be a superspinar.

  • 2. Di Valentino, Eleonora
    et al.
    Ferreira, Ricardo Z.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Visinelli, Luca
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Uppsala University, Sweden; University of Amsterdam, The Netherlands.
    Danielsson, Ulf
    Late time transitions in the quintessence field and the H-0 tension2019In: Physics of the Dark Universe, ISSN 0953-8585, E-ISSN 2212-6864, Vol. 26, article id 100385Article in journal (Refereed)
    Abstract [en]

    We consider a quintessence field which transitions from a matter-like to a cosmological constant behaviour between recombination and the present time. We aim at easing the tension in the measurement of the present Hubble rate, and we assess the Lambda CDM model properly enlarged to include our quintessence field against cosmological observations. The model does not address the scope we proposed. This result allows us to exclude a class of quintessential models as a solution to the tension in the Hubble constant measurements. 

  • 3.
    Kinney, William H.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). University at Buffalo, United States of America.
    Vagnozzi, Sunny
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Visinelli, Luca
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Uppsala, Sweden.
    The zoo plot meets the swampland: mutual (in)consistency of single-field inflation, string conjectures, and cosmological data2019In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 36, no 11, article id 117001Article in journal (Refereed)
    Abstract [en]

    We consider single-field inflation in light of string-motivated 'swampland' conjectures suggesting that effective scalar field theories with a consistent UV completion must have field excursion Delta(phi) less than or similar to M-Pl, in combination with a sufficiently steep potential, MPlV phi/V greater than or similar to O(1). Here, we show that the swampland conjectures are inconsistent with existing observational constraints on single-field inflation. Focusing on the observationally favoured class of concave potentials, we map the allowed swampland region onto the n(S)-r 'zoo plot' of inflationary models, and find that consistency with the Planck satellite and BICEP2/Keck Array requires MPlV phi/V less than or similar to 0.1 and -0.02 less than or similar to M-Pl(2)/V-phi phi < 0, in strong tension with swampland conjectures. Extension to non-canonical models such as DBI Inflation does not significantly weaken the bound.

  • 4. Tenkanen, Tommi
    et al.
    Visinelli, Luca
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Uppsala University, Sweden; University of Amsterdam, The Netherlands.
    Axion dark matter from Higgs inflation with an intermediate H-*2019In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 8, article id 033Article in journal (Refereed)
    Abstract [en]

    In order to accommodate the QCD axion as the dark matter (DM) in a model in which the Peccei-Quinn (PQ) symmetry is broken before the end of inflation, a relatively low scale of inflation has to be invoked in order to avoid bounds from DM isocurvature fluctuations, H-* less than or similar to O(10(9)) GeV. We construct a simple model in which the Standard Model Higgs field is non-minimally coupled to Palatini gravity and acts as the inflaton, leading to a scale of inflation H-* similar to 10(8) GeV. When the energy scale at which the PQ symmetry breaks is much larger than the scale of inflation, we find that in this scenario the required axion mass for which the axion constitutes all DM is m(0) less than or similar to 0.05 mu eV for a quartic Higgs self-coupling lambda phi= 0.1, which correspond to the PQ breaking scale v(sigma) greater than or similar to 10(14) GeV and tensor-to-scalar ratio r similar to 10(-12). Future experiments sensitive to the relevant QCD axion mass scale can therefore shed light on the physics of the Universe before the end of inflation.

  • 5.
    Visinelli, Luca
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Uppsala University, Sweden.
    Vagnozzi, Sunny
    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). University of Cambridge, UK.
    Danielsson, Ulf
    Revisiting a Negative Cosmological Constant from Low-Redshift Data2019In: Symmetry, ISSN 2073-8994, E-ISSN 2073-8994, Vol. 11, no 8, article id 1035Article in journal (Refereed)
    Abstract [en]

    Persisting tensions between high-redshift and low-redshift cosmological observations suggest the dark energy sector of the Universe might be more complex than the positive cosmological constant of the ?CDM model. Motivated by string theory, wherein symmetry considerations make consistent AdS backgrounds (i.e., maximally-symmetric spacetimes with a negative cosmological constant) ubiquitous, we explore a scenario where the dark energy sector consists of two components: a negative cosmological constant, with a dark energy component with equation of state w phi on top. We test the consistency of the model against low-redshift baryon acoustic oscillation and Type Ia supernovae distance measurements, assessing two alternative choices of distance anchors: the sound horizon at baryon drag determined by the Planck collaboration and the Hubble constant determined by the SH0ES program. We find no evidence for a negative cosmological constant and mild indications for an effective phantom dark energy component on top. A model comparison analysis reveals that the ?CDM model is favoured over our negative cosmological constant model. While our results are inconclusive, should low-redshift tensions persist with future data, it would be worth reconsidering and further refining our toy negative cosmological constant model by considering realistic string constructions.

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