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10 Apr 2006
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6 Mar 2006
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AGN Outflows and the Matter Power Spectrum
Authors:
Robyn
Levine, Nickolay
Y. Gnedin
Comments: submitted to ApJL
We have investigated the effects of AGN outflows on the amplitude of
the
matter power spectrum in a simple model of spherically symmetric
outflows
around realistically clustered AGN population. We find that two
competing
effects influence the matter power spectrum in two opposite directions.
First,
AGN outflows move baryons from high to low density regions, decreasing
the
amplitude of the matter power spectrum by up to 20%. Second, high
clustering of
the AGN transfers the power from small to larger scales. The exact
balance
between these two effects depends on the details of outflows on small
scales,
and quantitative estimates will require much more sophisticated
modeling than
presented here.
Dynamics of the Primordial Hydrogen and Helium (HeI)
Recombination in the Universe
Also see Cosmo Coffee Discussion on this article Cosmo Coffee
Authors:
V.K.Dubrovich,
S.I.Grachev
Comments: 10 pages, 2 figures, accepted for publication in Astronomy
Letters
The dependences on z of fractional number densities of H+ and He+ ions
are
calculated with a proper allowance for two-photon decays of upper
levels of
hydrgen and parahelium and radiative transfer in intercombination line
2 3P_1
--> 1 1S_0 of helium. It is shown that for hydrogen this gives
corrections for
a degree of ionization in no more than a few percents but for helium
this leads
to a significant acceleration of recombination compared to the recent
papers by
Seager et al. (1999; 2000) where these effects were ignored.
An Observational Test for the Anthropic Origin of the
Cosmological Constant
Authors:
Abraham
Loeb (Harvard)
Comments: 5 pages, submitted to JCAP
The existence of multiple regions of space beyond the observable
Universe
(within the so-called "multiverse") where the vacuum energy density
takes
different values, has been postulated as an explanation for the low
non-zero
value observed for it in our Universe. It is often argued that our
existence
pre-selects regions where the cosmological constant is sufficiently
small to
allow galaxies like the Milky Way to form and intelligent life to
emerge. Here
we propose a simple empirical test for this anthropic argument within
the
boundaries of the observable Universe. We make use of the fact that
dwarf
galaxies formed in our Universe at redshifts as high as z~10 when the
mean
matter density was larger by a factor of ~10^3 than today. Existing
technology
enables to check whether planets form in nearby dwarf galaxies and
globular
clusters by searching for microlensing or transit events of background
stars.
The oldest of these nearby systems may have formed at z~10. If planets
are as
common per stellar mass in these descendents as they are in the Milky
Way
galaxy, then the anthropic argument would be weakened considerably
since
planets could have formed in our Universe even if the cosmological
constant was
three orders of magnitude larger than observed. For a flat probability
distribution, this would imply that the probability for us to reside in
a
region where the cosmological constant obtains its observed value is
lower than
\~10^{-3}. A precise version of the anthropic argument could then be
ruled-out
at a confidence level of ~99.9%, which constitutes a satisfactory
measure of a
good experimental test.
Signals of Inflation in a Friendly String Landscape
Authors:
John
March-Russell, Francesco
Riva
Comments: 20 pages, 11 figures
Report-no: OUTP-0510P
Following Freivogel {\it et al} we consider inflation in a predictive
(or
`friendly') region of the landscape of string vacua, as modeled by
Arkani-Hamed, Dimopoulos and Kachru. In such a region the dimensionful
coefficients of super-renormalizable operators unprotected by
symmetries, such
as the vacuum energy and scalar mass-squareds are freely scanned over,
and the
objects of study are anthropically or `environmentally' conditioned
probability
distributions for observables. In this context we study the statistical
predictions of (inverted) hybrid inflation models, where the properties
of the
inflaton are probabilistically distributed. We derive the resulting
distributions of observables, including the deviation from flatness
$|1-\Omega|$, the spectral index of scalar cosmological perturbations
$n_s$
(and its scale dependence $dn_s/d\log k$), and the ratio of tensor to
scalar
perturbations $r$. The environmental bound on the curvature implies a
solution
to the $\eta$-problem of inflation with the predicted distribution of
$(1-n_s)$
indicating values close to current observations. We find a relatively
low
probability ($<3%$) of `just-so' inflation with measurable
deviations from
flatness. Intermediate scales of inflation are preferred in these
models.
Neutrino Coannihilation on Dark-Matter Relics?
Authors:
Gabriela
Barenboim, Olga
Mena Requejo, Chris
Quigg
Comments: 5 pages, 3 figures, uses RevTeX 4
Report-no: FERMILAB-Pub-06/050-T
High-energy neutrinos may resonate with relic background neutralinos to
form
short-lived sneutrinos. In some circumstances, the decay chain that
leads back
to the lightest supersymmetric particle would yield few-GeV gamma rays
or
charged-particle signals. Although resonant coannihilation would occur
at an
appreciable rate in our galaxy, the signal in any foreseeable detector
is
unobservably small.
On the status of superheavy dark matter
Authors:
R.
Aloisio, V.
Berezinsky, M.
Kachelriess
Comments: 7 pages, 4 eps figures
Superheavy particles are a natural candidate for the dark matter in the
universe and our galaxy, because they are produced generically during
inflation
in cosmologically interesting amounts. The most attractive model for
the origin
of superheavy dark matter (SHDM) is gravitational production at the end
of
inflation. The observed cosmological density of dark matter determines
the mass
of the SHDM particle as $m_X=$(a few)$\:\times 10^{13}$ GeV, promoting
it to a
natural candidate for the source of the observed ultra-high energy
cosmic rays
(UHECR). After a review of the theoretical aspects of SHDM, we up-date
its
predictions for UHECR observations: no GZK cutoff, flat energy spectrum
with
$dN/dE\approx 1/E^{1.9}$, photon dominance and galactic anisotropy. We
analyze
the existing data and conclude that SDHM as explanation for the
observed UHECRs
is at present disfavored but not yet excluded. We calculate the
anisotropy
relevant for future Auger observations that should be the conclusive
test for
this model. Finally, we emphasize that negative results of searches for
SHDM in
UHECR do not disfavor SHDM as a dark matter candidate. Therefore,
UHECRs
produced by SHDM decays and with the signatures as described should be
searched
for in the future as subdominant effect.
Implications of a Running Spectral Index for Slow Roll
Inflation
Authors:
Richard
Easther (Yale), Hiranya
Peiris (KICP/EFI, U. Chicago)
Comments: 4 pages
We analyze the weak (2 sigma) evidence for a running spectral index
seen in
the three-year WMAP dataset and its implications for single field, slow
roll
inflation. We assume that the running is comparable to the central
value found
from the WMAP data analysis, and use the Hubble Slow Roll formalism to
follow
the evolution of the slow roll parameters. For all parameter choices
consistent
with a large, negative running, single field, slow roll inflation lasts
less
than 30 efolds after CMB scales leave the horizon. Thus, a definitive
observation of a large negative running would imply that any
inflationary phase
requires multiple fields or the breakdown of slow roll. Alternatively,
if
single field, slow roll inflation is sources the primordial
fluctuations, we
can expect the observed running to move much closer to zero as the CMB
is
measured more accurately at small angular scales.
The Lyman-alpha forest and WMAP year three
Authors:
Matteo
Viel, Martin
G. Haehnelt, Antony
Lewis
Comments: 5 pages, 4 figs, 2 tables
A combined analysis of Cosmic Microwave Background (CMB) and Lyman-a
forest
data allows to constrain the matter power spectrum from small scales of
about 1
Mpc/h all the way to the horizon scale. The long lever arm and
complementarity
provided by such an analysis has previously led to a significant
tightening of
the constraints on the shape and the amplitude of the power spectrum of
primordial density fluctuations. We present here a combined analysis of
the
WMAP three year results with Lyman-a forest data. The amplitude of the
matter
power spectrum sigma_8 and the spectral index ns inferred from the
joint
analysis with high resolution Lyman-a forest data and low resolution
Lyman-a
forest data as analyzed by Viel & Haehnelt (2006) are consistent
with the new
WMAP results to within 1 sigma. The joint analysis with the low
resolution data
as analysed by McDonald et al. (2005) suggest a value of sigma_8 which
is ~ 2
sigma higher than that inferred from the WMAP three year data alone.
The joint
analysis of the three year WMAP and the Lyman-a forest data also does
not
favour a running of the spectral index. The best fit values for a
combined
analysis of the three year WMAP data, other CMB data, 2dF and the \lya
forest
data are (sigma_8, ns) = (0.78\pm 0.03,0.96 \pm 0.01).
Breaking the scale invariance of the primordial spectrum or
not: the new WMAP data
Authors:
Davor
Palle
Comments: 7 pages, 2 figures, 16 references
It seems that new WMAP data requires a fit with a primordial spectrum
containig small negative tilt index in addition to the featureless
Harrison-Zeldovich- Peebles spectrum thus implying a broken scale
invariance.
We show that the data could be otherwise interpreted by a scale
invariant
primordial spectrum with the scale non-invariant evolution of density
contrast
using the Press-Schechter formalism. The estimate of the acceleration
parameter, as a source of the inhomogeneity of spacetime, is made by
searching
for the minima of the deviation measure defined by the Press-Schechter
mass
functions for this interpretation compared to the assumptions implicit
in the
WMAP fit.
Improved Calculation of the Primordial Gravitational Wave
Spectrum in the Standard Model
Authors:
Yuki
Watanabe, Eiichiro
Komatsu (Univ. of Texas at Austin)
Comments: 25 papes, 11 figures, submitted to PRD
We show that the energy density spectrum of the primordial
gravitational
waves has characteristic features due to the successive changes in the
relativistic degrees of freedom during the radiation era. These changes
make
the evolution of radiation energy density deviate from the conventional
adiabatic evolution, \rho_r~ a^{-4}, and thus cause the expansion rate
of the
universe to change suddenly at each transition which, in turn, modifies
the
spectrum of primordial gravitational waves. We take into account all
the
particles in the Standard Model of elementary particles. In addition,
free-streaming of neutrinos damps the amplitude of gravitational waves,
leaving
characteristic features in the energy density spectrum. Our
calculations are
solely based on the standard model of cosmology and particle physics,
and
therefore these features must exist. Our calculations significantly
improve the
previous ones which ignored these effects and predicted a smooth,
featureless
spectrum.
Weak Lensing of the Cosmic Microwave Background by Foreground
Gravitational Waves
Authors:
Chao
Li (Caltech), Asantha
Cooray (UC Irvine)
Comments: 11 pages, 4 figures, comments are welcome
Weak lensing distortion of the background cosmic microwave background
(CMB)
temperature and polarization patterns by the foreground density
fluctuations is
well studied in the literature. We discuss the gravitational lensing
modification to CMB anisotropies and polarization by a stochastic
background of
primordial gravitational waves between us and the last scattering
surface.
While density fluctuations perturb CMB photons via gradient-type
deflections
only, foreground gravitational waves distort CMB anisotropies via both
gradient- and curl-type displacements. The latter is a rotation of
background
images, while the former is related to the lensing convergence. For a
primordial background of inflationary gravitational waves, with an
amplitude
corresponding to a tensor-to-scalar ratio below the current upper limit
of
$\sim$ 0.3, the resulting modifications to the angular power spectra of
CMB
temperature anisotropy and polarization are below the cosmic variance
limit.
This suggests that lensing by foreground gravitational waves can be
ignored
when interpreting high sensitivity CMB observations.
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