623 research outputs found
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
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.
Constraints On The Topology Of The Universe From The WMAP First-Year Sky Maps
We compute the covariance expected between the spherical harmonic
coefficients of the cosmic microwave temperature anisotropy if the
universe had a compact topology. For fundamental cell size smaller than the
distance to the decoupling surface, off-diagonal components carry more
information than the diagonal components (the power spectrum). We use a maximum
likelihood analysis to compare the Wilkinson Microwave Anisotropy Probe
first-year data to models with a cubic topology. The data are compatible with
finite flat topologies with fundamental domain times the distance to
the decoupling surface at 95% confidence. The WMAP data show reduced power at
the quadrupole and octopole, but do not show the correlations expected for a
compact topology and are indistinguishable from infinite models.Comment: 16 pages, 5 figure
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