49 research outputs found
The Acoustic Peak in the Lyman Alpha Forest
We present the first simulation of the signature of baryonic acoustic
oscillations (BAO) in Lyman alpha forest data containing 180,000 mock quasar
sight-lines. We use eight large dark-matter only simulations onto which we
paint the Lyman alpha field using the fluctuating Gunn-Peterson approximation.
We argue that this approach should be sufficient for the mean signature on the
scales of interest. Our results indicate that Lyman alpha flux provides a good
tracer of the underlying dark matter field on large scales and that redshift
space distortions are well described by a simple linear theory prescription. We
compare Fourier and configuration space approaches to describing the signal and
argue that configuration space statistics provide useful data compression. We
also investigate the effect of a fluctuating photo-ionizing background using a
simplified model and find that such fluctuations do add smooth power on large
scales. The acoustic peak position is, however, unaffected for small amplitude
fluctuations (<10%). Larger amplitude fluctuations make the recovery of the BAO
signal more difficult and may degrade the achievable significance of the
measurement.Comment: 10 pages, 8 figures; v2: minor revision matching version accepted by
JCAP (new references, better figures, clarifications
The effect of neutrinos on the matter distribution as probed by the Intergalactic Medium
We present a suite of full hydrodynamical cosmological simulations that
quantitatively address the impact of neutrinos on the (mildly non-linear)
spatial distribution of matter and in particular on the neutral hydrogen
distribution in the Intergalactic Medium (IGM), which is responsible for the
intervening Lyman-alpha absorption in quasar spectra. The free-streaming of
neutrinos results in a (non-linear) scale-dependent suppression of power
spectrum of the total matter distribution at scales probed by Lyman-alpha
forest data which is larger than the linear theory prediction by about 25% and
strongly redshift dependent. By extracting a set of realistic mock quasar
spectra, we quantify the effect of neutrinos on the flux probability
distribution function and flux power spectrum. The differences in the matter
power spectra translate into a ~2.5% (5%) difference in the flux power spectrum
for neutrino masses with Sigma m_{\nu} = 0.3 eV (0.6 eV). This rather small
effect is difficult to detect from present Lyman-alpha forest data and nearly
perfectly degenerate with the overall amplitude of the matter power spectrum as
characterised by sigma_8. If the results of the numerical simulations are
normalized to have the same sigma_8 in the initial conditions, then neutrinos
produce a smaller suppression in the flux power of about 3% (5%) for Sigma
m_{\nu} = 0.6 eV (2
sigma C.L.), comparable to constraints obtained from the cosmic microwave
background data or other large scale structure probes.Comment: 38 pages, 21 figures. One section and references added. JCAP in pres
Early Dark Energy at High Redshifts: Status and Perspectives
Early dark energy models, for which the contribution to the dark energy
density at high redshifts is not negligible, influence the growth of cosmic
structures and could leave observable signatures that are different from the
standard cosmological constant cold dark matter (CDM) model. In this
paper, we present updated constraints on early dark energy using geometrical
and dynamical probes. From WMAP five-year data, baryon acoustic oscillations
and type Ia supernovae luminosity distances, we obtain an upper limit of the
dark energy density at the last scattering surface (lss), (95% C.L.). When we include higher redshift
observational probes, such as measurements of the linear growth factors,
Gamma-Ray Bursts (GRBs) and Lyman- forest (\lya), this limit improves
significantly and becomes (95%
C.L.). Furthermore, we find that future measurements, based on the
Alcock-Paczy\'nski test using the 21cm neutral hydrogen line, on GRBs and on
the \lya forest, could constrain the behavior of the dark energy component and
distinguish at a high confidence level between early dark energy models and
pure CDM. In this case, the constraints on the amount of early dark
energy at the last scattering surface improve by a factor ten, when compared to
present constraints. We also discuss the impact on the parameter , the
growth rate index, which describes the growth of structures in standard and in
modified gravity models.Comment: 11 pages, 9 figures and 4 table
Numerical simulations of the Warm-Hot Intergalactic Medium
In this paper we review the current predictions of numerical simulations for
the origin and observability of the warm hot intergalactic medium (WHIM), the
diffuse gas that contains up to 50 per cent of the baryons at z~0. During
structure formation, gravitational accretion shocks emerging from collapsing
regions gradually heat the intergalactic medium (IGM) to temperatures in the
range T~10^5-10^7 K. The WHIM is predicted to radiate most of its energy in the
ultraviolet (UV) and X-ray bands and to contribute a significant fraction of
the soft X-ray background emission. While O VI and C IV absorption systems
arising in the cooler fraction of the WHIM with T~10^5-10^5.5 K are seen in
FUSE and HST observations, models agree that current X-ray telescopes such as
Chandra and XMM-Newton do not have enough sensitivity to detect the hotter
WHIM. However, future missions such as Constellation-X and XEUS might be able
to detect both emission lines and absorption systems from highly ionised atoms
such as O VII, O VIII and Fe XVII.Comment: 18 pages, 5 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 14; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
WMAP constraints on inflationary models with global defects
We use the cosmic microwave background angular power spectra to place upper
limits on the degree to which global defects may have aided cosmic structure
formation. We explore this under the inflationary paradigm, but with the
addition of textures resulting from the breaking of a global O(4) symmetry
during the early stages of the Universe. As a measure of their contribution, we
use the fraction of the temperature power spectrum that is attributed to the
defects at a multipole of 10. However, we find a parameter degeneracy enabling
a fit to the first-year WMAP data to be made even with a significant defect
fraction. This degeneracy involves the baryon fraction and the Hubble constant,
plus the normalization and tilt of the primordial power spectrum. Hence,
constraints on these cosmological parameters are weakened. Combining the WMAP
data with a constraint on the physical baryon fraction from big bang
nucleosynthesis calculations and high-redshift deuterium abundance, limits the
extent of the degeneracy and gives an upper bound on the defect fraction of
0.13 (95% confidence).Comment: 10pp LaTeX/RevTeX, 6 eps figs; matches accepted versio
Unified dark energy models : a phenomenological approach
A phenomenological approach is proposed to the problem of universe
accelerated expansion and of the dark energy nature. A general class of models
is introduced whose energy density depends on the redshift in such a way
that a smooth transition among the three main phases of the universe evolution
(radiation era, matter domination, asymptotical de Sitter state) is naturally
achieved. We use the estimated age of the universe, the Hubble diagram of Type
Ia Supernovae and the angular size - redshift relation for compact and
ultracompact radio structures to test whether the model is in agreement with
astrophysical observation and to constrain its main parameters. Although
phenomenologically motivated, the model may be straightforwardly interpreted as
a two fluids scenario in which the quintessence is generated by a suitably
chosen scalar field potential. On the other hand, the same model may also be
read in the context of unified dark energy models or in the framework of
modified Friedmann equation theories.Comment: 12 pages, 10 figures, accepted for publication on Physical Review
Is cosmology consistent?
We perform a detailed analysis of the latest CMB measurements (including
BOOMERaNG, DASI, Maxima and CBI), both alone and jointly with other
cosmological data sets involving, e.g., galaxy clustering and the Lyman Alpha
Forest. We first address the question of whether the CMB data are internally
consistent once calibration and beam uncertainties are taken into account,
performing a series of statistical tests. With a few minor caveats, our answer
is yes, and we compress all data into a single set of 24 bandpowers with
associated covariance matrix and window functions. We then compute joint
constraints on the 11 parameters of the ``standard'' adiabatic inflationary
cosmological model. Out best fit model passes a series of physical consistency
checks and agrees with essentially all currently available cosmological data.
In addition to sharp constraints on the cosmic matter budget in good agreement
with those of the BOOMERaNG, DASI and Maxima teams, we obtain a heaviest
neutrino mass range 0.04-4.2 eV and the sharpest constraints to date on gravity
waves which (together with preference for a slight red-tilt) favors
``small-field'' inflation models.Comment: Replaced to match accepted PRD version. 14 pages, 12 figs. Tiny
changes due to smaller DASI & Maxima calibration errors. Expanded neutrino
and tensor discussion, added refs, typos fixed. Combined CMB data, window and
covariance matrix at http://www.hep.upenn.edu/~max/consistent.html or from
[email protected]
Testing the tidal alignment model of galaxy intrinsic alignment
Weak gravitational lensing has become a powerful probe of large-scale
structure and cosmological parameters. Precision weak lensing measurements
require an understanding of the intrinsic alignment of galaxy ellipticities,
which can in turn inform models of galaxy formation. It is hypothesized that
elliptical galaxies align with the background tidal field and that this
alignment mechanism dominates the correlation between ellipticities on
cosmological scales (in the absence of lensing). We use recent large-scale
structure measurements from the Sloan Digital Sky Survey to test this picture
with several statistics: (1) the correlation between ellipticity and galaxy
overdensity, w_{g+}; (2) the intrinsic alignment auto-correlation functions;
(3) the correlation functions of curl-free, E, and divergence-free, B, modes
(the latter of which is zero in the linear tidal alignment theory); (4) the
alignment correlation function, w_g(r_p,theta), a recently developed statistic
that generalizes the galaxy correlation function to account for the angle
between the galaxy separation vector and the principle axis of ellipticity. We
show that recent measurements are largely consistent with the tidal alignment
model and discuss dependence on galaxy luminosity. In addition, we show that at
linear order the tidal alignment model predicts that the angular dependence of
w_g(r_p,theta) is simply w_{g+}*cos(2*theta) and that this dependence is
consistent with recent measurements. We also study how stochastic nonlinear
contributions to galaxy ellipticity impact these statistics. We find that a
significant fraction of the observed LRG ellipticity can be explained by
alignment with the tidal field on scales >~10 h^-1 Mpc. These considerations
are relevant to galaxy formation and evolution.Comment: 23 pages, 5 figures, minor changes to reflect published version,
including updated figures and a minor correction to the measured error
Limits on the gravity wave contribution to microwave anisotropies
We present limits on the fraction of large angle microwave anisotropies which
could come from tensor perturbations. We use the COBE results as well as
smaller scale CMB observations, measurements of galaxy correlations, abundances
of galaxy clusters, and Lyman alpha absorption cloud statistics. Our aim is to
provide conservative limits on the tensor-to-scalar ratio for standard
inflationary models. For power-law inflation, for example, we find T/S<0.52 at
95% confidence, with a similar constraint for phi^p potentials. However, for
models with tensor amplitude unrelated to the scalar spectral index it is still
currently possible to have T/S>1.Comment: 23 pages, 7 figures, accepted for publication in Phys. Rev. D.
Calculations extended to blue spectral index, Fig. 6 added, discussion of
results expande
Next-generation test of cosmic inflation
The increasing precision of cosmological datasets is opening up new
opportunities to test predictions from cosmic inflation. Here we study the
impact of high precision constraints on the primordial power spectrum and show
how a new generation of observations can provide impressive new tests of the
slow-roll inflation paradigm, as well as produce significant discriminating
power among different slow-roll models. In particular, we consider
next-generation measurements of the Cosmic Microwave Background (CMB)
temperature anisotropies and (especially) polarization, as well as new
Lyman- measurements that could become practical in the near future. We
emphasize relationships between the slope of the power spectrum and its first
derivative that are nearly universal among existing slow-roll inflationary
models, and show how these relationships can be tested on several scales with
new observations. Among other things, our results give additional motivation
for an all-out effort to measure CMB polarization.Comment: 10 pages, 8 figures, to appear in PRD; major changes are a reanalysis
in terms of better cosmological parameters and clarifications on the
contributions of polarization and Lyman-alpha dat