102 research outputs found

### Backgrounds of squeezed relic photons and their spatial correlations

We discuss the production of multi-photons squeezed states induced by the
time variation of the (Abelian) gauge coupling constant in a string
cosmological context. Within a fully quantum mechanical approach we solve the
time evolution of the mean number of produced photons in terms of the squeezing
parameters and in terms of the gauge coupling. We compute the first (amplitude
interference) and second order (intensity interference) correlation functions
of the magnetic part of the photon background. The photons produced thanks to
the variation of the dilaton coupling are strongly bunched for the realistic
case where the growth of the dilaton coupling is required to explain the
presence of large scale magnetic fields and, possibly of a Faraday rotation of
the Cosmic Microwave Background.Comment: 9 pages in LaTex styl

### Relic Gravitational Waves and Their Detection

The range of expected amplitudes and spectral slopes of relic (squeezed)
gravitational waves, predicted by theory and partially supported by
observations, is within the reach of sensitive gravity-wave detectors. In the
most favorable case, the detection of relic gravitational waves can be achieved
by the cross-correlation of outputs of the initial laser interferometers in
LIGO, VIRGO, GEO600. In the more realistic case, the sensitivity of advanced
ground-based and space-based laser interferometers will be needed. The specific
statistical signature of relic gravitational waves, associated with the
phenomenon of squeezing, is a potential reserve for further improvement of the
signal to noise ratio.Comment: 25 pages, 9 figures included, revtex. Based on a talk given at
"Gyros, Clocks, and Interferometers: Testing General Relativity in Space"
(Germany, August 99

### CBR Anisotropy from Primordial Gravitational Waves in Two-Component Inflationary Cosmology

We examine stochastic temperature fluctuations of the cosmic background
radiation (CBR) arising via the Sachs-Wolfe effect from gravitational wave
perturbations produced in the early universe. We consider spatially flat,
perturbed FRW models that begin with an inflationary phase, followed by a mixed
phase containing both radiation and dust. The scale factor during the mixed
phase takes the form $a(\eta)=c_1\eta^2+c_2\eta+c_3$, where $c_i$ are
constants. During the mixed phase the universe smoothly transforms from being
radiation to dust dominated. We find analytic expressions for the graviton mode
function during the mixed phase in terms of spheroidal wave functions. This
mode function is used to find an analytic expression for the multipole moments
$\langle a_l^2\rangle$ of the two-point angular correlation function
$C(\gamma)$ for the CBR anisotropy. The analytic expression for the multipole
moments is written in terms of two integrals, which are evaluated numerically.
The results are compared to multipoles calculated for models that are {\it
completely} dust dominated at last-scattering. We find that the multipoles
$\langle a_l^2\rangle$ of the CBR temperature perturbations for $l>10$ are
significantly larger for a universe that contains both radiation and dust at
last-scattering. We compare our results with recent, similar numerical work and
find good agreement. The spheroidal wave functions may have applications to
other problems of cosmological interest.Comment: 28 pgs + 6 postscript figures, RevTe

### Novel approach to the study of quantum effects in the early universe

We develop a theoretical frame for the study of classical and quantum
gravitational waves based on the properties of a nonlinear ordinary
differential equation for a function $\sigma(\eta)$ of the conformal time
$\eta$, called the auxiliary field equation. At the classical level,
$\sigma(\eta)$ can be expressed by means of two independent solutions of the
''master equation'' to which the perturbed Einstein equations for the
gravitational waves can be reduced. At the quantum level, all the significant
physical quantities can be formulated using Bogolubov transformations and the
operator quadratic Hamiltonian corresponding to the classical version of a
damped parametrically excited oscillator where the varying mass is replaced by
the square cosmological scale factor $a^{2}(\eta)$. A quantum approach to the
generation of gravitational waves is proposed on the grounds of the previous
$\eta-$dependent Hamiltonian. An estimate in terms of $\sigma(\eta)$ and
$a(\eta)$ of the destruction of quantum coherence due to the gravitational
evolution and an exact expression for the phase of a gravitational wave
corresponding to any value of $\eta$ are also obtained. We conclude by
discussing a few applications to quasi-de Sitter and standard de Sitter
scenarios.Comment: 20 pages, to appear on PRD. Already published background material has
been either settled up in a more compact form or eliminate

### Is the squeezing of relic gravitational waves produced by inflation detectable?

Grishchuk has shown that the stochastic background of gravitational waves
produced by an inflationary phase in the early Universe has an unusual
property: it is not a stationary Gaussian random process. Due to squeezing, the
phases of the different waves are correlated in a deterministic way, arising
from the process of parametric amplification that created them. The resulting
random process is Gaussian but non-stationary. This provides a unique signature
that could in principle distinguish a background created by inflation from
stationary stochastic backgrounds created by other types of processes. We
address the question: could this signature be observed with a gravitational
wave detector? Sadly, the answer appears to be "no": an experiment which could
distinguish the non-stationary behavior would have to last approximately the
age of the Universe at the time of measurement. This rules out direct detection
by ground and space based gravitational wave detectors, but not indirect
detections via the electromagnetic Cosmic Microwave Background Radiation
(CMBR).Comment: 17 pages, 4 Postscript figures, uses revtex, psfig, to be submitted
to PRD, minor revisions - appendix B clarified, corrected typos, added
reference

### Detecting relic gravitational waves in the CMB: A statistical bias

Analyzing the imprint of relic gravitational waves (RGWs) on the cosmic
microwave background (CMB) power spectra provides a way to determine the signal
of RGWs. In this Letter, we discuss a statistical bias, which could exist in
the data analysis and has the tendency to overlook the RGWs. We also explain
why this bias exists, and how to avoid it.Comment: 4 pages, 1 figur

### The Primordial Gravitational Wave Background in String Cosmology

We find the spectrum P(w)dw of the gravitational wave background produced in
the early universe in string theory. We work in the framework of String Driven
Cosmology, whose scale factors are computed with the low-energy effective
string equations as well as selfconsistent solutions of General Relativity with
a gas of strings as source. The scale factor evolution is described by an early
string driven inflationary stage with an instantaneous transition to a
radiation dominated stage and successive matter dominated stage. This is an
expanding string cosmology always running on positive proper cosmic time. A
careful treatment of the scale factor evolution and involved transitions is
made. A full prediction on the power spectrum of gravitational waves without
any free-parameters is given. We study and show explicitly the effect of the
dilaton field, characteristic to this kind of cosmologies. We compute the
spectrum for the same evolution description with three differents approachs.
Some features of gravitational wave spectra, as peaks and asymptotic
behaviours, are found direct consequences of the dilaton involved and not only
of the scale factor evolution. A comparative analysis of different treatments,
solutions and compatibility with observational bounds or detection perspectives
is made.Comment: LaTeX, 50 pages with 2 figures. Uses epsfig and psfra

### Inflationary Perturbations: the Cosmological Schwinger Effect

This pedagogical review aims at presenting the fundamental aspects of the
theory of inflationary cosmological perturbations of quantum-mechanical origin.
The analogy with the well-known Schwinger effect is discussed in detail and a
systematic comparison of the two physical phenomena is carried out. In
particular, it is demonstrated that the two underlying formalisms differ only
up to an irrelevant canonical transformation. Hence, the basic physical
mechanisms at play are similar in both cases and can be reduced to the
quantization of a parametric oscillator leading to particle creation due to the
interaction with a classical source: pair production in vacuum is therefore
equivalent to the appearance of a growing mode for the cosmological
fluctuations. The only difference lies in the nature of the source: an electric
field in the case of the Schwinger effect and the gravitational field in the
case of inflationary perturbations. Although, in the laboratory, it is
notoriously difficult to produce an electric field such that pairs extracted
from the vacuum can be detected, the gravitational field in the early universe
can be strong enough to lead to observable effects that ultimately reveal
themselves as temperature fluctuations in the Cosmic Microwave Background.
Finally, the question of how quantum cosmological perturbations can be
considered as classical is discussed at the end of the article.Comment: 49 pages, 6 figures, to appear in a LNP volume "Inflationary
Cosmology

### Black-hole information puzzle: A generic string-inspired approach

Given the insight steming from string theory, the origin of the black-hole
(BH) information puzzle is traced back to the assumption that it is physically
meaningful to trace out the density matrix over negative-frequency Hawking
particles. Instead, treating them as virtual particles necessarily absorbed by
the BH in a manner consistent with the laws of BH thermodynamics, and tracing
out the density matrix only over physical BH states, the complete evaporation
becomes compatible with unitarity.Comment: 8 pages, revised, title changed, to appear in Eur. Phys. J.

### The Coherent State Representation of Quantum Fluctuations in the Early Universe

Using the squeezed state formalism the coherent state representation of
quantum fluctuations in an expanding universe is derived. It is shown that this
provides a useful alternative to the Wigner function as a phase space
representation of quantum fluctuations. The quantum to classical transition of
fluctuations is naturally implemented by decohering the density matrix in this
representation. The entropy of the decohered vacua is derived. It is shown that
the decoherence process breaks the physical equivalence between vacua that
differ by a coordinate dependent phase generated by a surface term in the
Lagrangian. In particular, scale invariant power spectra are only obtained for
a special choice of surface term.Comment: 25 pages in revtex 3. This version is completely revised with
corrections and significant new calculation

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