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  • 1. Akrami, Yashar
    et al.
    Kühnel, Florian
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Uncertainties in primordial black-hole constraints on the primordial power spectrum2018In: Physics of the Dark Universe, ISSN 0953-8585, E-ISSN 2212-6864, Vol. 19, p. 124-128Article in journal (Refereed)
    Abstract [en]

    The existence (and abundance) of primordial black holes (PBHs) is governed by the power spectrum of primordial perturbations generated during inflation. So far no PBHs have been observed, and instead, increasingly stringent bounds on their existence at different scales have been obtained. Up until recently, this has been exploited in attempts to constrain parts of the inflationary power spectrum that are unconstrained by cosmological observations. We first point out that the simple translation of the PBH non-observation bounds into constraints on the primordial power spectrum is inaccurate as it fails to include realistic aspects of PBH formation and evolution. We then demonstrate, by studying two examples of uncertainties from the effects of critical and non-spherical collapse, that even though they may seem small, they have important implications for the usefulness of the constraints. In particular, we point out that the uncertainty induced by non-spherical collapse may be much larger than the difference between particular bounds from PBH non-observations and the general maximum cap stemming from the condition Omega <= 1 on the dark-matter density in the form of PBHs. We therefore make the cautious suggestion of applying only the overall maximum dark-matter constraint to models of early Universe, as this requirement seems to currently provide a more reliable constraint, which better reflects our current lack of detailed knowledge of PBH formation. These, and other effects, such as merging, clustering and accretion, may also loosen constraints from non-observations of other primordial compact objects such as ultra-compact minihalos of dark matter.

  • 2. Carr, Bernard
    et al.
    Kühnel, Florian
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Primordial black holes as dark matter2016In: Physical Review D, ISSN 2470-0010, Vol. 94, no 8, article id 083504Article in journal (Refereed)
    Abstract [en]

    The possibility that the dark matter comprises primordial black holes (PBHs) is considered, with particular emphasis on the currently allowed mass windows at 10(16)-10(17) g, 10(20)-10(24) g and 1-10(3)M(circle dot) The Planck mass relics of smaller evaporating PBHs are also considered. All relevant constraints (lensing, dynamical, large-scale structure and accretion) are reviewed and various effects necessary for a precise calculation of the PBH abundance (non-Gaussianity, nonsphericity, critical collapse and merging) are accounted for. It is difficult to put all the dark matter in PBHs if their mass function is monochromatic but this is still possible if the mass function is extended, as expected in many scenarios. A novel procedure for confronting observational constraints with an extended PBH mass spectrum is therefore introduced. This applies for arbitrary constraints and a wide range of PBH formation models and allows us to identify which model-independent conclusions can be drawn from constraints over all mass ranges. We focus particularly on PBHs generated by inflation, pointing out which effects in the formation process influence the mapping from the inflationary power spectrum to the PBH mass function. We then apply our scheme to two specific inflationary models in which PBHs provide the dark matter. The possibility that the dark matter is in intermediate-mass PBHs of 1-10(3)M(circle dot) is of special interest in view of the recent detection of black-hole mergers by LIGO. The possibility of Planck relics is also intriguing but virtually untestable.

  • 3. Golovnev, Alexey
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Oslo, Norway.
    Effectively nonlocal metric-affine gravity2016In: Physical Review D, ISSN 2470-0010, Vol. 93, no 6, article id 064081Article in journal (Refereed)
    Abstract [en]

    In metric-affine theories of gravity such as the C-theories, the spacetime connection is associated to a metric that is nontrivially related to the physical metric. In this article, such theories are rewritten in terms of a single metric, and it is shown that they can be recast as effectively nonlocal gravity. With some assumptions, known ghost-free theories with nonsingular and cosmologically interesting properties may be recovered. Relations between different formulations are analyzed at both perturbative and nonperturbative levels, taking carefully into account subtleties with boundary conditions in the presence of integral operators in the action, and equivalences between theories related by nonlocal redefinitions of the fields are verified at the level of equations of motion. This suggests a possible geometrical interpretation of nonlocal gravity as an emergent property of non-Riemannian spacetime structure.

  • 4. Golovnev, Alexey
    et al.
    Koivisto, Tomi
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    On the covariance of teleparallel gravity theories2017In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 34, no 14, article id 145013Article in journal (Refereed)
    Abstract [en]

    The basics of teleparallel gravity and its extensions are reviewed with particular emphasis on the problem of the Lorentz-breaking choice of connection in pure-tetrad versions of the theories. Various possible ways to covariantise such models are discussed. A by-product is a new form of f (T) field equations.

  • 5.
    Kühnel, Florian
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Rampf, Cornelius
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Effects of critical collapse on primordial black-hole mass spectra2016In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, no 2, article id 93Article in journal (Refereed)
    Abstract [en]

    Certain inflationary models as well as realisations of phase transitions in the early Universe predict the formation of primordial black holes. For most mass ranges, the fraction of matter in the form of primordial black holes is limited by many different observations on various scales. Primordial black holes are assumed to be formed when overdensities that cross the horizon have Schwarzschild radii larger than the horizon. Traditionally it was therefore assumed that primordial black-holemasses were equal to the horizon mass at their time of formation. However, detailed calculations of their collapse show that primordial black holes formed at each point in time should rather form a spectrum of different masses, obeying critical scaling. Though this has been known for more than 15 years, the effect of this scaling behaviour is largely ignored when considering predictions for primordial black-hole mass spectra. In this paper we consider the critical collapse scaling for a variety of models which produce primordial black holes, and find that it generally leads to a shift, broadening and an overall decrease of the mass contained in primordial black holes. This effect is model and parameter dependent and cannot be contained by a constant rescaling of the spectrum; it can become important and should be taken into account when comparing to observational constraints.

  • 6.
    Kühnel, Florian
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sandstad, Marit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Ellipsoidal collapse and primordial black hole formation2016In: Physical Review D, ISSN 2470-0010, Vol. 94, no 6, article id 063514Article in journal (Refereed)
    Abstract [en]

    We reinvestigate gravitational ellipsoidal collapse with special focus on its impact on primordial black hole formation. We start with the results for the collapse threshold in the case of ellipsoidal collapse in the context of structure formation, and show heuristically that a similar functional form for the threshold should be expected in the case of primordial black hole formation. For a generic model we apply this to demonstrate that the abundance and energy density of the produced primordial black holes can be significantly decreased when the nonsphericity of the overdensities is taken into account. Further numerical study of this phenomenon may be crucial in order to produce reliable predictions for primordial black hole abundances from inflationary models.

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