Weekly meetings hosted by the "Gravitation Relativiste et
Cosmologie" (GreCO) group at IAP
Every Monday at 1.30pm in room 281 (2nd floor) Salle du Conseil (sous-sol)
To suggest a paper please send a message to bertone.AT.iap.fr
Monday,
14 May 2007 @ 1.30pm,
**Salle du Conseil** (sous-sol)
A combined analysis of Lyman-alpha
forest, 3D Weak Lensing and WMAP year three data (Alain)
(Submitted on 4 May 2007)
Abstract: We present
constraints on the amplitude and shape of the matter power
spectrum and the density of dark matter within the framework of a
standard
LambdaCDM model. We use a Markov Chain Monte Carlo approach to combine
independent measurements of the Ly-alpha forest flux power spectrum,
the three
dimensional weak gravitational lensing shear field and the Cosmic
Microwave
Background temperature and polarization anisotropies. We find good
agreement
between the amplitude of the matter power spectrum on intermediate and
small
scales as inferred from low and high resolution Ly-alpha forest data
and the
lensing data from the COSMOS survey. The Ly-alpha forest data thereby
helps to
break the sigma_8-Omega_m degeneracy characteristic of weak lensing
results.
The combined analysis of the Ly-alpha forest and lensing data gives
sigma_8=0.87+-0.05. This is somewhat larger than the value preferred by
the
WMAP year three CMB data. Combining all three data sets significantly
tightens
the constraints on sigma_8, the spectral index of primordial density
fluctuation n_s, a possible running of the spectral index n_run and the
matter
density Omega_m. The joint constraints are sigma_8=0.80+-0.02,
n_s=0.97+-0.01,
Omega_m=0.25+-0.02 (1-sigma error bars).
PostScript
PDF
Dark matter and the first stars: a new
phase of stellar evolution
(Submitted on 3 May 2007)
Abstract:
A mechanism is identified whereby dark matter (DM) in protostellar
halos
dramatically alters the current theoretical framework for the formation
of the
first stars. Heat from neutralino DM annihilation is shown to overwhelm
any
cooling mechanism, consequently impeding the star formation process and
possibly leading to a new stellar phase. A "dark star'' may result: a
giant
($\gtrsim 1$ AU) hydrogen-helium star powered by DM annihilation
instead of
nuclear fusion, and detectable via annihilation products (gamma-rays,
neutrinos, antimatter) possibly in combination with hydrogen lines.
PostScript
PDF
Primordial nucleosynthesis as a probe of
fundamental physics parameters
(Submitted on 7 May 2007)
Abstract:
We analyze the effect of variation of fundamental couplings and mass
scales
on primordial nucleosynthesis in a systematic way. The first step
establishes
the response of primordial element abundances to the variation of a
large
number of nuclear physics parameters, including nuclear binding
energies. We
find a strong influence of the n-p mass difference (for the 4He
abundance), of
the nucleon mass (for deuterium) and of A=3,4,7 binding energies (for
3He, 6Li
and 7Li). A second step relates the nuclear parameters to the
parameters of the
Standard Model of particle physics. The deuterium, and, above all, 7Li
abundances depend strongly on the average light quark mass hat{m}
\equiv
(m_u+m_d)/2. We calculate the behaviour of abundances when variations
of
fundamental parameters obey relations arising from grand unification.
We also
discuss the possibility of a substantial shift in the lithium abundance
while
the deuterium and 4He abundances are only weakly affected.
PostScript
PDF
Models of f(R) Cosmic Acceleration that
Evade Solar-System Tests
(Submitted on 8 May 2007)
Abstract:
We study a class of metric-variation f(R) models that accelerates the
expansion without a cosmological constant and satisfies both
cosmological and
solar-system tests in the small-field limit of the parameter space.
Solar-system tests alone place only weak bounds on these models, since
the
additional scalar degree of freedom is locked to the high-curvature
general-relativistic prediction across more than 25 orders of magnitude
in
density, out through the solar corona. This agreement requires that the
galactic halo be of sufficient extent to maintain the galaxy at high
curvature
in the presence of the low-curvature cosmological background. If the
galactic
halo and local environment in f(R) models do not have substantially
deeper
potentials than expected in LCDM, then cosmological field amplitudes
|f_R| >
10^{-6} will cause the galactic interior to evolve to low curvature
during the
acceleration epoch. Viability of large-deviation models therefore rests
on the
structure and evolution of the galactic halo, requiring cosmological
simulations of f(R) models, and not directly on solar-system tests.
Even small
deviations that conservatively satisfy both galactic and solar-system
constraints can still be tested by future, percent-level measurements
of the
linear power spectrum, while they remain undetectable to
cosmological-distance
measures. Although we illustrate these effects in a specific class of
models,
the requirements on f(R) are phrased in a nearly model-independent
manner.
Inflationary Cosmology Connecting Dark
Energy and Dark Matter
(Submitted on 24 Apr 2007)
Abstract:
Kination dominated
quintessence models of dark energy have the intriguing
feature that the relic abundance of thermal cold dark matter can be
significantly enhanced compared to the predictions from standard
cosmology.
Previous treatments of such models do not include a realistic embedding
of
inflationary initial conditions. We remedy this situation by
constructing a
viable inflationary model in which the inflaton and quintessence field
are the
same scalar degree of freedom. Kination domination is achieved after
inflation
through a strong push or "kick" of the inflaton, and sufficient
reheating can
be achieved depending on model parameters. This allows us to explore
both
model-dependent and model-independent cosmological predictions of this
scenario. We find that measurements of the B-mode CMB polarization can
rule out
this class of scenarios almost model independently. We also discuss
other
experimentally accessible signatures for this class of models.
PostScript
PDF
What Can the Cosmic Microwave Background
Tell Us About the Outer Solar System?
(Submitted on 7 May 2007)
Abstract:
We discuss two new observational techniques that use observations of
the
Cosmic Microwave Background (CMB) to place constraints upon the mass,
distance,
and size distribution of small objects in the Kuiper Belt and inner
Oort Cloud,
collectively known as Trans-Neptunian Objects (TNOs). The first new
technique
considers the spectral distortion of the isotropic, or monopole, CMB by
TNOs
that have been heated by solar radiation to temperatures above that of
the CMB.
We apply this technique to the spectral measurements of the CMB by the
Far
Infrared Absolute Spectrophotometer (FIRAS) on the Cosmic Background
Explorer
(COBE). The second technique utilizes the change in amplitude of the
TNO signal
due to the orbital motion of the observer to separate the TNO signal
from the
invariant extra-galactic CMB and construct a map of the mass
distribution in
the outer Solar System. We estimate the ability of future CMB
experiments to
create such a map.
PostScript
PDF
Imprint of Distortions in the
Oort Cloud on the CMB Anisotropies
(Submitted on 7 May 2007)
Abstract:
We study the effect of a close encounter of a passing star on the shape
of
the inner Oort Cloud, using the impulse approximation. The deviation of
the
perturbed Oort Cloud from sphericity adds angular fluctuations to the
brightness of the Cosmic Microwave Background (CMB) due to thermal
emission by
the comets. An encounter with a solar-mass star at an impact parameter
of $1750
\au$, as expected based on the abundance and velocity dispersion of
stars in
the solar neighborhood, leads to a quadrupole moment in the square of
the
fractional CMB intensity fluctuation of $C_2 = 4.5 \times 10^{-15}, 6.7
\times
10^{-12}, 1.1 \times 10^{-9}$ at $\nu = 30, 353, 545 \GHz$ (these being
the
frequency bands of the upcoming Planck satellite). We also quantify the
quadrupole spectral distortions produced by the Scattered Disc, which
will
exist regardless of any perturbation and the subsequent shape of the
Oort
Cloud. For comparison, the square fractional temperature fluctuation
quadrupole
moment predicted by the current cosmological model is $C_2 = 1.76
\times
10^{-10}$, which corresponds to fluctuations in the CMB intensity of
$C_2 = 2.9
\times 10^{-10}, 6.8 \times 10^{-9}, 1.6 \times 10^{-8}$ at $\nu = 30,
353, 545
\GHz$. Finally, we discuss how a measurement of the anisotropic
spectral
distortions could be used to constrain the trajectory of the closest
stellar
fly-by.
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