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  • 1. Akrami, Yashar
    et al.
    Kühnel, Florian
    Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC). Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Sandstad, Marit
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Uncertainties in primordial black-hole constraints on the primordial power spectrum2018Inngår i: Physics of the Dark Universe, ISSN 0953-8585, E-ISSN 2212-6864, Vol. 19, s. 124-128Artikkel i tidsskrift (Fagfellevurdert)
    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
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Sandstad, Marit
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Primordial black holes as dark matter2016Inngår i: Physical Review D, ISSN 2470-0010, Vol. 94, nr 8, artikkel-id 083504Artikkel i tidsskrift (Fagfellevurdert)
    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. Casadio, Roberto
    et al.
    Kühnel, Florian
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Orlandi, Alessio
    Consistent cosmic microwave background spectra from quantum depletion2015Inngår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, nr 9, artikkel-id 002Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Following a new quantum cosmological model proposed by Dvali and Gomez, we quantitatively investigate possible modifications to the Hubble parameter and following corrections to the cosmic microwave background spectrum. In this model, scalar and tensor perturbations are generated by the quantum depletion of the background inflaton and graviton condensate respectively. We show how the inflaton mass affects the power spectra and the tensor-to-scalar ratio. Masses approaching the Planck scale would lead to strong deviations, while standard spectra are recovered for an inflaton mass much smaller than the Planck mass.

  • 4.
    Kühnel, Florian
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Bose-Einstein condensates with derivative and long-range interactions as set-ups for analog black holes2014Inngår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 90, nr 8, artikkel-id 084024Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    General types of Bose-Einstein condensates are considered. The formation of black-hole analogues is examined for both short-and long-range interactions for arbitrary spatial dimensions greater than 2. The former case includes nonlinear derivative terms plus an inevitable external potential, while the latter one consists solely of gravitylike self-interactions for which intrinsic formation of analogue Schwarzschild-type background space-times is possible. The corresponding geometries are studied, and it is shown how they can be made stable. Their Hawking temperature is estimated, and it is found that in certain setups it can be significantly increased, thus providing better detectability.

  • 5.
    Kühnel, Florian
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Thoughts on the vacuum energy in the quantum N-portrait2015Inngår i: Modern Physics Letters A, ISSN 0217-7323, E-ISSN 1793-6632, Vol. 30, nr 36, artikkel-id 1550197Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An application of the quantum N-portrait to the Universe is discussed, wherein the spacetime geometry is understood as a Bose-Einstein condensate of N soft gravitons. If near or at the critical point of a quantum phase transition, indications are found that the vacuum energy is partly suppressed by 1/N, as being due to quanta not in the condensate state. Time evolution decreases this suppression, which might have implications for cosmic expansion.

  • 6.
    Kühnel, Florian
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC). KTH Royal Institute of Technology, Sweden.
    Freese, Katherine
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC). University of Michigan, USA.
    Constraints on primordial black holes with extended mass functions2017Inngår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, nr 8, artikkel-id 083508Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Constraints on primordial black holes in the range 10(-18) M circle dot to 10(3) M circle dot are reevaluated for a general class of extended mass functions. Whereas previous work has assumed that PBHs are produced with one single mass, instead there is expected to be a range of masses even in the case of production from a single mechanism; constraints therefore change from previous literature. Although tightly constrained in the majority of cases, it is shown that, even under conservative assumptions, primordial black holes in the mass range 10(-10) M circle dot to 10(-8) M circle dot could still constitute the entirety of the dark matter. This stresses both the importance for a comprehensive reevaluation of all respective constraints that have previously been evaluated only for a monochromatic mass function and the need to obtain more constraints in the allowed mass range.

  • 7.
    Kühnel, Florian
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Rampf, Cornelius
    Astrophysical Bose-Einstein condensates and superradiance2014Inngår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 90, nr 10, s. 103526-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate gravitational analogue models to describe slowly rotating objects (e. g., dark-matter halos, or boson stars) in terms of Bose-Einstein condensates, trapped in their own gravitational potentials. We begin with a modified Gross-Pitaevskii equation, and show that the resulting background equations of motion are stable, as long as the rotational component is treated as a small perturbation. The dynamics of the fluctuations of the velocity potential are effectively governed by the Klein-Gordon equation of an Eulerian metric, where we derive the latter by the use of a relativistic Lagrangian extrapolation. Superradiant scattering on such objects is studied. We derive conditions for its occurrence and estimate its strength. Our investigations might give an observational handle to phenomenologically constrain Bose-Einstein condensates.

  • 8.
    Kühnel, Florian
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Rampf, Cornelius
    Sandstad, Marit
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Effects of critical collapse on primordial black-hole mass spectra2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 2, artikkel-id 93Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 9.
    Kühnel, Florian
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC). Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Sandstad, Marit
    Baryon-number conservation in Bose-Einstein condensate black holes2015Inngår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 92, nr 12, artikkel-id 124028Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Primordial black holes are studied in the Bose-Einstein condensate description of space-time. The question of baryon-number conservation is investigated with emphasis on the possible formation of bound states of the system's remaining captured baryons. This leads to distinct predictions both for the formation time, which for the naively natural assumptions is shown to lie between 10(-12) s and 10(12) s after the big bang, and for the remnant's mass, yielding approximately 3 x 10(23) kg in the same scheme. The consequences for astrophysically formed black holes are also considered.

  • 10.
    Kühnel, Florian
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Sandstad, Marit
    Corpuscular consideration of eternal inflation2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 10, artikkel-id 505Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We review the paradigm of eternal inflation in the light of the recently proposed corpuscular picture of space-time. Comparing the strength of the average fluctuation of the field up its potential with that of quantum depletion, we show that the latter can be dominant. We then study the full respective distributions in order to show that the fraction of the space-time which has an increasing potential is always below the eternal-inflation threshold. We prove that for monomial potentials eternal inflaton is excluded. This is likely to hold for other models as well.

  • 11.
    Kühnel, Florian
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Sandstad, Marit
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Ellipsoidal collapse and primordial black hole formation2016Inngår i: Physical Review D, ISSN 2470-0010, Vol. 94, nr 6, artikkel-id 063514Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 12.
    Kühnel, Florian
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Sundborg, Bo
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Decay of graviton condensates and their generalizations in arbitrary dimensions2014Inngår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 90, nr 6, s. 064025-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Classicalons are self-bound classical field configurations, which include black holes in general relativity. In quantum theory, they are described by condensates of many soft quanta. In this work, their decay properties are studied in arbitrary dimensions. It is found that generically the decays of other classicalons are enhanced compared to pure graviton condensates, i.e. black holes. The evaporation of higher dimensional graviton condensates turns out to match Hawking radiation solely due to nonlinearites captured by the classicalon picture. Although less stable than black holes, all self-bound condensates are shown to be stable in the limit of large mass. Like for black holes, the effective coupling always scales as the inverse of the number of constituents, indicating that these systems are at critical points of quantum phase transitions. Consequences for cosmology, astro-and collider physics are briefly discussed.

  • 13.
    Kühnel, Florian
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Sundborg, Bo
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    High-energy gravitational scattering and Bose-Einstein condensates of gravitons2014Inngår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 12, s. 016-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Quantum black holes are difficult to describe. We consider two seemingly divergent approaches, high-energy scattering and the proposal to regard black holes as Bose-Einstein condensates of gravitons, and establish a connection between them. Results from the eikonal approximation of high-energy scattering are reconsidered and processed further by a saddle-point approximation. The dominant contribution to the scattering amplitude comes from a ladder diagram with the exchange of N gravitons, and the number of gravitons follows a Poisson distribution. This approximation supports the picture of a graviton Bose-Einstein condensate with an extent equal to the Schwarzschild radius, which grows with N in a way determined by the saddle point. The approach permits calculations of 1/N corrections from the fluctuations around the saddle points and we comment on these. Scattering methods might be useful probes of quantum black holes, especially when interpreted in terms of condensates.

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