731 research outputs found
Tensor Microwave Background Fluctuations for Large Multipole Order
We present approximate formulas for the tensor BB, EE, TT, and TE multipole
coefficients for large multipole order l. The error in using the approximate
formula for the BB multipole coefficients is less than cosmic variance for
l>10. These approximate formulas make various qualitative properties of the
calculated multipole coefficients transparent: specifically, they show that,
whatever values are chosen for cosmological parameters, the tensor EE multipole
coefficients will always be larger than the BB coefficients for all l>15, and
that these coefficients will approach each other for l<<100. These
approximations also make clear how these multipole coefficients depend on
cosmological parameters.Comment: 19 pages, 9 figures, accepted for publication in Phys. Rev. D,
references and comments on earlier work on the subject added, cosmetic
modification of figure
Signatures of a Graviton Mass in the Cosmic Microwave Background
There exist consistent low energy effective field theories describing gravity
in the Higgs phase that allow the coexistence of massive gravitons and the
conventional 1/r potential of gravity. In an effort to constrain the value of
the graviton mass in these theories, we study the tensor contribution to the
CMB temperature anisotropy and polarization spectra in the presence of a
non-vanishing graviton mass. We find that the observation of a B-mode signal
consistent with the spectrum predicted by inflationary models would provide the
strongest limit yet on the mass of an elementary particle -- a graviton -- at a
level of m\lesssim 10^(-30) eV\approx(10 Mpc)^(-1). We also find that a
graviton mass in the range between (10 Mpc)^(-1) and (10 kpc)^(-1) leads to
interesting modifications of the polarization spectrum. The characteristic
signature of a graviton mass in this range would be a plateau in the B-mode
spectrum up to angular multipoles of l\sim 100. For even larger values of the
graviton mass the tensor contribution to the CMB spectra becomes strongly
suppressed.Comment: 22 pages, 5 figures, v2: references added, accepted for publication
in PR
Comment about quasi-isotropic solution of Einstein equations near cosmological singularity
We generalize for the case of arbitrary hydrodynamical matter the
quasi-isotropic solution of Einstein equations near cosmological singularity,
found by Lifshitz and Khalatnikov in 1960 for the case of radiation-dominated
universe. It is shown that this solution always exists, but dependence of terms
in the quasi-isotropic expansion acquires a more complicated form.Comment: 7 pages, The collective of authors is enlarged and some comments and
references are adde
Semiclassicality and Decoherence of Cosmological Perturbations
Transition to the semiclassical behaviour and the decoherence process for
inhomogeneous perturbations generated from the vacuum state during an
inflationary stage in the early Universe are considered both in the Heisenberg
and the Schr\"odinger representations to show explicitly that both approaches
lead to the same prediction: the equivalence of these quantum perturbations to
classical perturbations having stochastic Gaussian amplitudes and belonging to
the quasi-isotropic mode. This equivalence and the decoherence are achieved
once the exponentially small (in terms of the squeezing parameter )
decaying mode is neglected. In the quasi-classical limit , the
perturbation mode functions can be made real by a time-independent phase
rotation, this is shown to be equivalent to a fixed relation between squeezing
angle and phase for all modes in the squeezed-state formalism. Though the
present state of the gravitational wave background is not a squeezed quantum
state in the rigid sense and the squeezing parameters loose their direct
meaning due to interaction with the environment and other processes, the
standard predictions for the rms values of the perturbations generated during
inflation are not affected by these mechanisms (at least, for scales of
interest in cosmological applications). This stochastic background still
occupies a small part of phase space.Comment: Revised version to appear in Class. Quantum Grav. All prior
conclusions hold. This version contains in particular a Wigner function
calculatio
Origin of Correlations between Central Black Holes Masses and Galactic Bulge Velocity Dispersions
We argue that the observed correlations between central black holes masses
M_{BH} and galactic bulge velocity dispersions \sigma_e in the form
M_{BH}\propto\sigma_e^4 may witness on the pregalactic origin of massive black
holes. Primordial black holes would be the centers for growing protogalaxies
which experienced multiple mergers with ordinary galaxies. This process is
accompanied by the merging of black holes in the galactic nuclei.Comment: 6 pages, 1 figure, submitted to Astron. and Astrophys. Transaction
Trans-Planckian Particle Creation in Cosmology and Ultra-High Energy Cosmic Rays
We consider observational constraints on creation of particles induced by
hypothetical trans-Planckian effects during the current stage of the Universe
expansion. We show that compatibility with the diffuse gamma-ray background
measured by the EGRET experiment strongly restricts this creation. In
particular, it rules out the possibility to detect signatures of such short
distance effects in anisotropies of the cosmic microwave background radiation.
On the other hand, a possibility that some part of the ultra-high energy cosmic
rays originates from new trans-Planckian physics remains open.Comment: Typos are correcte
A Gravitational Wave Background from Reheating after Hybrid Inflation
The reheating of the universe after hybrid inflation proceeds through the
nucleation and subsequent collision of large concentrations of energy density
in the form of bubble-like structures moving at relativistic speeds. This
generates a significant fraction of energy in the form of a stochastic
background of gravitational waves, whose time evolution is determined by the
successive stages of reheating: First, tachyonic preheating makes the amplitude
of gravity waves grow exponentially fast. Second, bubble collisions add a new
burst of gravitational radiation. Third, turbulent motions finally sets the end
of gravitational waves production. From then on, these waves propagate
unimpeded to us. We find that the fraction of energy density today in these
primordial gravitational waves could be significant for GUT-scale models of
inflation, although well beyond the frequency range sensitivity of
gravitational wave observatories like LIGO, LISA or BBO. However, low-scale
models could still produce a detectable signal at frequencies accessible to BBO
or DECIGO. For comparison, we have also computed the analogous gravitational
wave background from some chaotic inflation models and obtained results similar
to those found by other groups. The discovery of such a background would open a
new observational window into the very early universe, where the details of the
process of reheating, i.e. the Big Bang, could be explored. Moreover, it could
also serve in the future as a new experimental tool for testing the
Inflationary Paradigm.Comment: 22 pages, 18 figures, uses revtex
Reheating-volume measure for random-walk inflation
The recently proposed "reheating-volume" (RV) measure promises to solve the
long-standing problem of extracting probabilistic predictions from cosmological
"multiverse" scenarios involving eternal inflation. I give a detailed
description of the new measure and its applications to generic models of
eternal inflation of random-walk type. For those models I derive a general
formula for RV-regulated probability distributions that is suitable for
numerical computations. I show that the results of the RV cutoff in random-walk
type models are always gauge-invariant and independent of the initial
conditions at the beginning of inflation. In a toy model where equal-time
cutoffs lead to the "youngness paradox," the RV cutoff yields unbiased results
that are distinct from previously proposed measures.Comment: Figure 1 updated, version accepted for publication in Phys.Rev.
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