arXiv subjects

Claus Kiefer

Publications and source records attributed to Claus Kiefer.

At least 91 records · Page 5Linked to original sources

arXiv2003

Will LIGO see RIGO's? - Radion-induced graviton oscillations in the two-brane world

One of the most interesting features of braneworld models is the existence of massive gravitational modes in addition to the usual massless one. Mixing of the modes which depends nontrivially on the inter-brane distance can be interpreted as radion-induced gravitational-wave oscillations, a classical analogy to meson and neutrino oscillations. We show that these oscillations arising in M-theory inspired two-brane setups could lead to effects detectable by gravitational-wave interferometers.

astro-ph↗

arXiv2003

Graviton oscillations in the two-brane world

We study the braneworld effective action in the two-brane Randall-Sundrum model. In the framework of this essentially-nonlocal action we reveal the origin of an infinite sequence of gravitational wave modes -- the usual massless one as well as the tower of Kaluza-Klein massive ones. Mixing of the modes, which parametrically depends on the background value of the modulus of the extra dimension, can be interpreted as radion-induced gravitational-wave oscillations, a classical analogue to meson and neutrino oscillations. We show that these oscillations arising in M-theory-inspired braneworld setups could lead to effects detectable by gravitational-wave interferometers.

hep-th↗

arXiv2003

Radion-induced gravitational wave oscillations and their phenomenology

We discuss the theory and phenomenology of the interplay between the massless graviton and its massive Kaluza-Klein modes in the Randall-Sundrum two-brane model. The equations of motion of the transverse traceless degrees of freedom are derived by means of a Green function approach as well as from an effective nonlocal action. The second procedure clarifies the extraction of the particle content from the nonlocal action and the issue of its diagonalization. The situation discussed is generic for the treatment of two-brane models if the on-brane fields are used as the dynamical degrees of freedom. The mixing of the effective graviton modes of the localized action can be interpreted as radion-induced gravitational-wave oscillations, a classical analogy to meson and neutrino oscillations. We show that these oscillations arising in M-theory-motivated braneworld setups could lead to effects detectable by gravitational-wave interferometers. The implications of this effect for models with ultra-light gravitons are discussed.

hep-th↗

arXiv2003

Is there an information-loss problem for black holes?

Black holes emit thermal radiation (Hawking effect). If after black-hole evaporation nothing else were left, an arbitrary initial state would evolve into a thermal state (`information-loss problem'). Here it is argued that the whole evolution is unitary and that the thermal nature of Hawking radiation emerges solely through decoherence -- the irreversible interaction with further degrees of freedom. For this purpose a detailed comparison with an analogous case in cosmology (entropy of primordial fluctuations) is presented. Some remarks on the possible origin of black-hole entropy due to interaction with other degrees of freedom are added. This might concern the interaction with quasi-normal modes or with background fields in string theory.

gr-qc↗

arXiv2002

Nonlocal braneworld action: an alternative to Kaluza-Klein description

We construct the nonlocal braneworld action in the two-brane Randall-Sundrum model in a holographic setup alternative to Kaluza-Klein description: the action is written as a functional of the two metric and radion fields on the branes. This action effectively describes the dynamics of the gravitational field both on the branes and in the bulk in terms of the brane geometries directly accessible for observations. Its nonlocal form factors incorporate the cumulative effect of the bulk Kaluza-Klein modes. We also consider the reduced version of this action obtained by integrating out the fields on the negative-tension brane invisible from the viewpoint of the Planckian brane observer. This effective action features a nontrivial transition (AdS flow) between the local and nonlocal phases of the theory associated with the limits of small and large interbrane separation. Our results confirm a recently proposed braneworld scenario with diverging (repulsive) branes and suggest possible new implications of this phase transition in brane cosmology.

hep-th↗

arXiv2002

Accurate results for primordial black holes from spectra with a distinguished scale

We perform an accurate computation of the production rate for primordial black holes (PBHs). The reason is that the underlying mass variance had been overestimated systematically, as was shown recently. For scale-free powerlaw primordial spectra, and for a Universe with critical density, the mass variance is less than 34% of its value thought earlier for the spectral index in the range $1\leq n\leq 1.3$. We then extend our study to spectra with a characteristic scale and find the accurate shape of the corresponding mass variance. For a pure step in the primordial spectrum, the step in the variance is smoothed around the characteristic scale $k_s$. For a spectrum with large oscillations near $k_s$, we find a pronounced bump in the variance. This could yield a significant part of the cold dark matter in the form of PBHs with mass $M$ in the range $5\times 10^{15} {\rm g}\lesssim M \lesssim 10^{21} {\rm g}$.

astro-ph↗

arXiv2002

Quantum general relativity and Hawking radiation

In a previous paper we have set up the Wheeler-DeWitt equation which describes the quantum general relativistic collapse of a spherical dust cloud. In the present paper we specialize this equation to the case of matter perturbations around a black hole, and show that in the WKB approximation, the wave-functional describes an eternal black hole in equilibrium with a thermal bath at Hawking temperature.

gr-qc↗

arXiv2002

On the interpretation of quantum theory - from Copenhagen to the present day

A central feature in the Copenhagen interpretation is the use of classical concepts from the outset. Modern developments show, however, that the emergence of classical properties can be understood within the framework of quantum theory itself, through the process of decoherence. This fact becomes most crucial for the interpretability of quantum cosmology - the application of quantum theory to the Universe as a whole. I briefly review these developments and emphasize the importance of an unbiased attitude on the interpretational side for future progress in physics.

quant-ph↗

arXiv2002

Can Primordial Black Holes be a Significant Part of Dark Matter ?

The computation of PBH (primordial black hole) production from primordial perturbations has recently been improved by considering a more accurate relation between the primordial power spectrum and the PBH mass variance. We present here exact expressions which are valid for primordial spectra of arbitrary shape and which allow accurate numerical calculations. We then consider the possibility to have a significant part of dark matter in the form of PBHs produced by a primordial spectrum of inflationary origin possessing a characteristic scale. We show that in this model the relevant PBH mass is constrained to lie in the range $5\times 10^{15} {\rm g}\lesssim M \lesssim 10^{21} {\rm g}$. This is much less than the mass range coming from the QCD phase transition, allowing the two mechanisms to be easily distinguished.

astro-ph↗

arXiv2002

Exact positivity of the Wigner and P-functions of a Markovian open system

We discuss the case of a Markovian master equation for an open system, as it is frequently found from environmental decoherence. We prove two theorems for the evolution of the quantum state. The first one states that for a generic initial state the corresponding Wigner function becomes strictly positive after a finite time has elapsed. The second one states that also the P-function becomes exactly positive after a decoherence time of the same order. Therefore the density matrix becomes exactly decomposable into a mixture of Gaussian pointer states.

quant-ph↗

arXiv2002

Quantum aspects of black holes

This is a brief introduction to quantum aspects of black holes, addressed at an astrophysics-oriented audience. The topics are: The laws of black-hole mechanics, Hawking radiation, interpretation of entropy, and primordial black holes.

astro-ph↗

arXiv2001

Primordial Black Holes from inflationary models with and without Broken Scale Invariance

We review the formalism of primordial black holes (PBHs) production and show that the mass variance at horizon crossing has been systematically overestimated in previous studies. We derive the correct expression. The difference is maximal at the earliest formation times and still very significant for PBH masses $\sim 10^{15}$g, an accurate estimate requiring numerical calculations. In particular, this would lead to weaker constraints on the spectral index $n$. We then derive constraints on inflationary models from the fact that primordial black holes must not overclose the Universe. This is done both for the scale-free case of the power spectrum studied earlier and for the case where a step in the mass variance is superimposed. In the former case we find various constraints on $n$, depending on the parameters. In the latter case these limits can be much more strengthened, so that one could find from an observational limit on $n$ a constraint on the allowed height of the step.

astro-ph↗

arXiv2001

Hawking radiation from decoherence

It is argued that the thermal nature of Hawking radiation arises solely due to decoherence. Thereby any information-loss paradox is avoided because for closed systems pure states remain pure. The discussion is performed for a massless scalar field in the background of a Schwarzschild black hole, but the arguments should hold in general. The result is also compared to and contrasted with the situation in inflationary cosmology.

gr-qc↗

arXiv2001

Singularity avoidance by collapsing shells in quantum gravity

We discuss a model describing exactly a thin spherically symmetric shell of matter with zero rest mass. We derive the reduced formulation of this system in which the variables are embeddings, their conjugate momenta, and Dirac observables. A non-perturbative quantum theory of this model is then constructed, leading to a unitary dynamics. As a consequence of unitarity, the classical singularity is fully avoided in the quantum theory.

gr-qc↗

arXiv2001

Robustness and diffusion of pointer states

Classical properties of an open quantum system emerge through its interaction with other degrees of freedom (decoherence). We treat the case where this interaction produces a Markovian master equation for the system. We derive the corresponding distinguished local basis (pointer basis) by three methods. The first demands that the pointer states mimic as close as possible the local non-unitary evolution. The second demands that the local entropy production be minimal. The third imposes robustness on the inherent quantum and emerging classical uncertainties. All three methods lead to localized Gaussian pointer states, their formation and diffusion being governed by well-defined quantum Langevin equations.

quant-ph↗

arXiv2000

Origin of classical structure from inflation

According to the inflationary scenario, all structure in the Universe can be traced back to quantum fluctuations of the metric and scalar field(s) during inflation. The seeds of this structure can be observed as classical anisotropies in the cosmic microwave background. I briefly review how the transition from the inherent quantum nature of these fluctuations to classical behaviour comes about. Two features play a crucial role: Firstly, the quantum state of the fluctuations becomes highly squeezed for wavelengths that exceed the Hubble radius. Secondly, decoherence due to other fields distinguishes the field-amplitude basis as the classical pointer basis. I also discuss the entropy of the fluctuations and make a brief comparison with chaotic systems.

astro-ph↗

arXiv1999

Conceptual issues in quantum cosmology

I give a review of the conceptual issues that arise in theories of quantum cosmology. I start by emphasising some features of ordinary quantum theory that also play a crucial role in understanding quantum cosmology. I then give motivations why spacetime cannot be treated classically at the most fundamental level. Two important issues in quantum cosmology -- the problem of time and the role of boundary conditions -- are discussed at some length. Finally, I discuss how classical spacetime can be recovered as an approximate notion. This involves the application of a semiclassical approximation and the process of decoherence. The latter is applied to both global degrees of freedom and primordial fluctuations in an inflationary Universe.

gr-qc↗

arXiv1998

Hamiltonian evolution and quantization for extremal black holes

We present and contrast two distinct ways of including extremal black holes in a Lorentzian Hamiltonian quantization of spherically symmetric Einstein-Maxwell theory. First, we formulate the classical Hamiltonian dynamics with boundary conditions appropriate for extremal black holes only. The Hamiltonian contains no surface term at the internal infinity, for reasons related to the vanishing of the extremal hole surface gravity, and quantization yields a vanishing black hole entropy. Second, we give a Hamiltonian quantization that incorporates extremal black holes as a limiting case of nonextremal ones, and examine the classical limit in terms of wave packets. The spreading of the packets, even the ones centered about extremal black holes, is consistent with continuity of the entropy in the extremal limit, and thus with the Bekenstein-Hawking entropy even for the extremal holes. The discussion takes place throughout within Lorentz-signature spacetimes.

gr-qc↗