Florian Kuhnel

Primordial Black Holes as Dark Matter

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- ...

Constraints on Primordial Black Holes with Extended Mass Functions

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 ...

High-energy gravitational scattering and Bose-Einstein condensates of gravitons

A bstractQuantum 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.

Decay of Graviton Condensates and their Generalizations in Arbitrary Dimensions

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.

Instability of certain bimetric and massive-gravity theories

Stability about cosmological background solutions to the bi-metric Hassan-Rosen theory is studied. The results of this analysis are presented, and it is shown that a large class of cosmological backgrounds is classically unstable. This sets serious doubts on the physical viability of the Hassan-Rosen theory - and in turn also of the de Rham-Gadabaze-Tolley model, to which the mentioned theory is parent. A way to overcome this instability by means of curvature-type deformations is discussed.

Large-Scale Suppression from Stochastic Inflation

We show nonperturbatively that the power spectrum of a self-interacting scalar field in de Sitter space-time is strongly suppressed on large scales. The cutoff scale depends on the strength of the self-coupling, the number of e folds of quasi-de Sitter evolution, and its expansion rate. As a consequence, the two-point correlation function of field fluctuations is free from infrared divergencies.

Long-range correlated random field and random anisotropy O ( N ) models: A functional renormalization group study

We study the long-distance behavior of the O(N) model in the presence of random fields and random anisotropies correlated as � 1/x d−� for large separation x using the functional renormalization group. We compute the fixed points and analyze their regions of stabili ty within a double = d 4 andexpansion. We find that the long-range disorder correlator remains analyt ic but generates short-range disorder whose correlator develops the usual cusp. This allows us to obtain the phase diagrams in (d,�,N) parameter space and compute the critical exponents to first order in and �. We show that the standard renormalization group methods with a finite number of couplings used in previous studies of system s with long-range correlated random fields fail to capture all critical properties. We argue that our results m ay be relevant to the behavior of 3 He-A in aerogel.

Baryon-number conservation in Bose-Einstein condensate black holes

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 systems 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 remnants mass, yielding approximately 3 x 10(23) kg in the same scheme. The consequences for astrophysically formed black holes are also considered.

Effects of Critical Collapse on Primordial Black-Hole Mass Spectra

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-hole masses 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 15xa0years, 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.

Consistent cosmic microwave background spectra from quantum depletion

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.