229 research outputs found
Termination of the Phase of Quintessence by Gravitational Back-Reaction
We study the effects of gravitational back-reaction in models of
Quintessence. The effective energy-momentum tensor with which cosmological
fluctuations back-react on the background metric will in some cases lead to a
termination of the phase of acceleration. The fluctuations we make use of are
the perturbations in our present Universe. Their amplitude is normalized by
recent measurements of anisotropies in the cosmic microwave background, their
slope is taken to be either scale-invariant, or characterized by a slightly
blue tilt. In the latter case, we find that the back-reaction effect of
fluctuations whose present wavelength is smaller than the Hubble radius but
which are stretched beyond the Hubble radius by the accelerated expansion
during the era of Quintessence domination can become large. Since the
back-reaction effects of these modes oppose the acceleration, back-reaction
will lead to a truncation of the period of Quintessence domination. This result
impacts on the recent discussions of the potential incompatibility between
string theory and Quintessence.Comment: 7 pages a few clarifying comments adde
Investigation of g_f0-rho-gamma and g_a0-rho-gamma coupling constants in light cone sum rules
We present a calculation of the coupling constant f0-->rho-gamma of and
a0-->rho-gamma decays from the point of the light cone QCD sum rules. We
estimate the coupling constants g_f0-rho-gamma and g_a0-rho-gamma which are an
essential ingredient in the analysis of physical processes involving isoscalar
f0(980) and isovector a0(980) mesons.Comment: 15 pages- 5 figure
Analysis of CMB polarization on an incomplete sky
The full sky cosmic microwave background polarization field can be decomposed
into 'electric' and 'magnetic' components. Working in harmonic space we
construct window functions that allow clean separation of the electric and
magnetic modes from observations over only a portion of the sky. Our
construction is exact for azimuthally symmetric patches, but should continue to
perform well for arbitrary patches. From the window functions we obtain
variables that allow for robust estimation of the magnetic component without
risk of contamination from the probably much larger electric signal. For
isotropic, uncorrelated noise the variables have a very simple diagonal noise
correlation, and further analysis using them should be no harder than analysing
the temperature field. For an azimuthally-symmetric patch, such as that
obtained from survey missions when the galactic region is removed, the
exactly-separated variables are fast to compute allowing us to estimate the
magnetic signal that could be detected by the Planck satellite in the absence
of non-galactic foregrounds. We also discuss the sensitivity of future
experiments to tensor modes in the presence of a magnetic signal generated by
weak lensing, and give lossless methods for analysing the electric polarization
field in the case that the magnetic component is negligible.Comment: 27 pages, 8 figures. New appendix on weak signal detection and
revised plots using a better statistic. Other changes to match version
accepted by PRD. Sample source code now available at
http://cosmologist.info/pola
Inflation with a constant ratio of scalar and tensor perturbation amplitudes
The single scalar field inflationary models that lead to scalar and tensor
perturbation spectra with amplitudes varying in direct proportion to one
another are reconstructed by solving the Stewart-Lyth inverse problem to
next-to-leading order in the slow-roll approximation.
The potentials asymptote at high energies to an exponential form,
corresponding to power law inflation, but diverge from this model at low
energies, indicating that power law inflation is a repellor in this case. This
feature implies that a fine-tuning of initial conditions is required if such
models are to reproduce the observations. The required initial conditions might
be set through the eternal inflation mechanism.
If this is the case, it will imply that the spectral indices must be nearly
constant, making the underlying model observationally indistinguishable from
power law inflation.Comment: 20 pages, 7 figures. Major changes to the Introduction following
referee's comments. One figure added. Some other minor changes. No conclusion
was modifie
Dark Energy and the quietness of the Local Hubble Flow
The linearity and quietness of the Local () Hubble Flow (LHF) in
view of the very clumpy local universe is a long standing puzzle in standard
and in open CDM cosmogony. The question addressed in this paper is whether the
antigravity component of the recently discovered dark energy can cool the
velocity flow enough to provide a solution to this puzzle. We calculate the
growth of matter fluctuations in a flat universe containing a fraction
of dark energy obeying the time independent equation of state
. We find that dark energy can indeed cool the LHF. However the
dark energy parameter values required to make the predicted velocity dispersion
consistent with the observed value have been ruled out
by other observational tests constraining the dark energy parameters and
. Therefore despite the claims of recent qualitative studies dark
energy with time independent equation of state can not by itself explain the
quietness and linearity of the Local Hubble Flow.Comment: 4 pages, 3 figures, accepted in Phys. Rev. D. Minor corrections, one
figure adde
High-redshift objects and the generalized Chaplygin gas
Motivated by recent developments in particle physics and cosmology, there has
been growing interest in an unified description of dark matter and dark energy
scenarios. In this paper we explore observational constraints from age
estimates of high- objects on cosmological models dominated by an exotic
fluid with equation of state (the so-called generalized
Chaplygin gas) which has the interesting feature of interpolating between
non-relativistic matter and negative-pressure dark energy regimes. As a general
result we find that, if the age estimates of these objects are correct, they
impose very restrictive limits on some of these scenarios.Comment: 5 pages, 3 figures, to appear in Phys. Rev.
Preheating in Supersymmetric Theories
We examine the particle production via preheating at the end of inflation in
supersymmetric theories. The inflaton and matter scalars are now necessarily
complex fields, and their relevant interactions are restricted by holomorphy.
In general this leads to major changes both in the inflaton dynamics and in the
efficiency of the preheating process. In addition, supersymmetric models
generically contain multiple isolated vacua, raising the possibility of
non-thermal production of dangerous topological defects. Because of these
effects, the success of leptogenesis or WIMPZILLA production via preheating
depends much more sensitively on the detailed parameters in the inflaton sector
than previously thought.Comment: 24 pages, 3 figures; references adde
The Detectability of Departures from the Inflationary Consistency Equation
We study the detectability, given CMB polarization maps, of departures from
the inflationary consistency equation, r \equiv T/S \simeq -5 n_T, where T and
S are the tensor and scalar contributions to the quadrupole variance,
respectively. The consistency equation holds if inflation is driven by a
slowly-rolling scalar field. Departures can be caused by: 1) higher-order terms
in the expansion in slow-roll parameters, 2) quantum loop corrections or 3)
multiple fields. Higher-order corrections in the first two slow-roll parameters
are undetectably small. Loop corrections are detectable if they are nearly
maximal and r \ga 0.1. Large departures (|\Delta n_T| \ga 0.1) can be seen if r
\ga 0.001. High angular resolution can be important for detecting non-zero
r+5n_T, even when not important for detecting non-zero r.Comment: 7 pages, 4 figures, submitted to PR
Large Nc and Chiral Dynamics
We study the dependence on the number of colors of the leading pi pi
scattering amplitude in chiral dynamics. We demonstrate the existence of a
critical number of colors for and above which the low energy pi pi scattering
amplitude computed from the simple sum of the current algebra and vector meson
terms is crossing symmetric and unitary at leading order in a truncated and
regularized 1/Nc expansion. The critical number of colors turns out to be Nc=6
and is insensitive to the explicit breaking of chiral symmetry.
Below this critical value, an additional state is needed to enforce the
unitarity bound; it is a broad one, most likely of "four quark" nature.Comment: RevTeX4, 6 fig., 5 page
Benchmark Parameters for CMB Polarization Experiments
The recently detected polarization of the cosmic microwave background (CMB)
holds the potential for revealing the physics of inflation and gravitationally
mapping the large-scale structure of the universe, if so called B-mode signals
below 10^{-7}, or tenths of a uK, can be reliably detected. We provide a
language for describing systematic effects which distort the observed CMB
temperature and polarization fields and so contaminate the B-modes. We identify
7 types of effects, described by 11 distortion fields, and show their
association with known instrumental systematics such as common mode and
differential gain fluctuations, line cross-coupling, pointing errors, and
differential polarized beam effects. Because of aliasing from the small-scale
structure in the CMB, even uncorrelated fluctuations in these effects can
affect the large-scale B modes relevant to gravitational waves. Many of these
problems are greatly reduced by having an instrumental beam that resolves the
primary anisotropies (FWHM << 10'). To reach the ultimate goal of an
inflationary energy scale of 3 \times 10^{15} GeV, polarization distortion
fluctuations must be controlled at the 10^{-2}-10^{-3} level and temperature
leakage to the 10^{-4}-10^{-3} level depending on effect. For example pointing
errors must be controlled to 1.5'' rms for arcminute scale beams or a percent
of the Gaussian beam width for larger beams; low spatial frequency differential
gain fluctuations or line cross-coupling must be eliminated at the level of
10^{-4} rms.Comment: 11 pages, 5 figures, submitted to PR
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