4,977 research outputs found

### 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.Comment: 6 pages, to appear in Class. Quantum Gra

### Can effects of quantum gravity be observed in the cosmic microwave background?

We investigate the question whether small quantum-gravitational effects can
be observed in the anisotropy spectrum of the cosmic microwave background
radiation. An observation of such an effect is needed in order to discriminate
between different approaches to quantum gravity. Using canonical quantum
gravity with the Wheeler-DeWitt equation, we find a suppression of power at
large scales. Current observations only lead to an upper bound on the energy
scale of inflation, but the framework is general enough to study other
situations in which such effects might indeed be seen.Comment: 5 pages, 1 figure, essay awarded first prize in the Gravity Research
Foundation essay competition 201

### 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.Comment: 5 pages, 1 figure, received honorable mention in the 2001 essay
competititon, to appear in Int. J. Mod. Phys.

### Can the Arrow of Time be understood from Quantum Cosmology?

I address the question whether the origin of the observed arrow of time can
be derived from quantum cosmology. After a general discussion of entropy in
cosmology and some numerical estimates, I give a brief introduction into
quantum geometrodynamics and argue that this may provide a sufficient framework
for studying this question. I then show that a natural boundary condition of
low initial entropy can be imposed on the universal wave function. The arrow of
time is then correlated with the size of the Universe and emerges from an
increasing amount of decoherence due to entanglement with unobserved degrees of
freedom. Remarks are also made concerning the arrow of time in multiverse
pictures and scenarios motivated by dark energy.Comment: 14 pages, to appear in "The Arrow of Time", ed. by L.
Mersini-Houghton and R. Vaa

### Remarks on the issue of time and complex numbers in canonical quantum gravity

We develop the idea that, as a result of the arbitrariness of the factor
ordering in Wheeler-DeWitt equation, gauge phases can not, in general, being
completely removed from the wave functional in quantum gravity. The latter may
be conveniently described by means of a remnant complex term in WDW equation
depending of the factor ordering. Taking this equation for granted we can
obtain WKB complex solutions and, therefore, we should be able to derive a
semiclassical time parameter for the Schroedinger equation corresponding to
matter fields in a given classical curved space.Comment: Typewritten using RevTex, to appear in Phys. Rev.

### Canonical Quantization of Spherically Symmetric Dust Collapse

Quantum gravity effects are likely to play a crucial role in determining the
outcome of gravitational collapse during its final stages. In this contribution
we will outline a canonical quantization of the LeMaitre-Tolman-Bondi models,
which describe the collapse of spherical, inhomogeneous, non-rotating dust.
Although there are many models of gravitational collapse, this particular class
of models stands out for its simplicity and the fact that both black holes and
naked singularity end states may be realized on the classical level, depending
on the initial conditions. We will obtain the appropriate Wheeler-DeWitt
equation and then solve it exactly, after regularization on a spatial lattice.
The solutions describe Hawking radiation and provide an elegant microcanonical
description of black hole entropy, but they raise other questions, most
importantly concerning the nature of gravity's fundamental degrees of freedom.Comment: 19 pages no figures. Contribution to a festschrift in honor of Joshua
N. Goldber

### Quantum Gravitational Contributions to the CMB Anisotropy Spectrum

We derive the primordial power spectrum of density fluctuations in the
framework of quantum cosmology. For this purpose we perform a Born-Oppenheimer
approximation to the Wheeler-DeWitt equation for an inflationary universe with
a scalar field. In this way we first recover the scale-invariant power spectrum
that is found as an approximation in the simplest inflationary models. We then
obtain quantum gravitational corrections to this spectrum and discuss whether
they lead to measurable signatures in the CMB anisotropy spectrum. The
non-observation so far of such corrections translates into an upper bound on
the energy scale of inflation.Comment: 4 pages, v3: sign error in Eq. (5) and its consequences correcte

### Age-dependent decay in the landscape

The picture of the "multiverse" arising in diverse cosmological scenarios
involves transitions between metastable vacuum states. It was pointed out by
Krauss and Dent that the transition rates decrease at very late times, leading
to a dependence of the transition probability between vacua on the age of each
vacuum region. I investigate the implications of this non-Markovian,
age-dependent decay on the global structure of the spacetime in landscape
scenarios. I show that the fractal dimension of the eternally inflating domain
is precisely equal to 3, instead of being slightly below 3 in scenarios with
purely Markovian, age-independent decay. I develop a complete description of a
non-Markovian landscape in terms of a nonlocal master equation. Using this
description I demonstrate by an explicit calculation that, under some technical
assumptions about the landscape, the probabilistic predictions of our position
in the landscape are essentially unchanged, regardless of the measure used to
extract these predictions. I briefly discuss the physical plausibility of
realizing non-Markovian vacuum decay in cosmology in view of the possible
decoherence of the metastable quantum state.Comment: 10 pages, RevTeX4, 1 figure included. Clarification of approximation
used, conclusions weakene

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