501 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
Cosmological parameters from large scale structure - geometric versus shape information
The matter power spectrum as derived from large scale structure (LSS) surveys
contains two important and distinct pieces of information: an overall smooth
shape and the imprint of baryon acoustic oscillations (BAO). We investigate the
separate impact of these two types of information on cosmological parameter
estimation, and show that for the simplest cosmological models, the broad-band
shape information currently contained in the SDSS DR7 halo power spectrum (HPS)
is by far superseded by geometric information derived from the baryonic
features. An immediate corollary is that contrary to popular beliefs, the upper
limit on the neutrino mass m_\nu presently derived from LSS combined with
cosmic microwave background (CMB) data does not in fact arise from the possible
small-scale power suppression due to neutrino free-streaming, if we limit the
model framework to minimal LambdaCDM+m_\nu. However, in more complicated
models, such as those extended with extra light degrees of freedom and a dark
energy equation of state parameter w differing from -1, shape information
becomes crucial for the resolution of parameter degeneracies. This conclusion
will remain true even when data from the Planck surveyor become available. In
the course of our analysis, we introduce a new dewiggling procedure that allows
us to extend consistently the use of the SDSS HPS to models with an arbitrary
sound horizon at decoupling. All the cases considered here are compatible with
the conservative 95%-bounds \sum m_\nu < 1.16 eV, N_eff = 4.8 \pm 2.0.Comment: 18 pages, 4 figures; v2: references added, matches published versio
Redshifting Rings of Power
The cosmic microwave background (CMB) has provided a precise template for
features in the linear power spectrum: the matter-radiation turnover, sound
horizon drop, and acoustic oscillations. In a two dimensional power spectrum in
redshift and angular space, the features appear as distorted rings, and yield
simultaneous, purely geometric, measures of the Hubble parameter H(z) and
angular diameter distance D_A(z) via an absolute version of the
Alcock-Paczynski test. Employing a simple Fisher matrix tool, we explore how
future surveys can exploit these rings of power for dark energy studies. High-z
CMB determinations of H and D_A are best complemented at moderate to low
redshift (z < 0.5) with a population of objects that are at least as abundant
as clusters of galaxies. We find that a sample similar to that of the ongoing
SDSS Luminous Red Galaxy (LRG) survey can achieve statistical errors at the ~5%
level for D_A(z) and H(z) in several redshift bins. This, in turn, implies
errors of sigma(w)=0.03-0.05 for a constant dark energy equation of state in a
flat universe. Deep galaxy cluster surveys such as the planned South Pole
Telescope (SPT) survey, can extend this test out to z~1 or as far as redshift
followup is available. We find that the expected constraints are at the
sigma(w)=0.04-0.08 level, comparable to galaxies and complementary in redshift
coverage.Comment: 8 pages, 5 figures submitted to PR
Towards an Optimal Reconstruction of Baryon Oscillations
The Baryon Acoustic Oscillations (BAO) in the large-scale structure of the
universe leave a distinct peak in the two-point correlation function of the
matter distribution. That acoustic peak is smeared and shifted by bulk flows
and non-linear evolution. However, it has been shown that it is still possible
to sharpen the peak and remove its shift by undoing the effects of the bulk
flows. We propose an improvement to the standard acoustic peak reconstruction.
Contrary to the standard approach, the new scheme has no free parameters,
treats the large-scale modes consistently, and uses optimal filters to extract
the BAO information. At redshift of zero, the reconstructed linear matter power
spectrum leads to a markedly improved sharpening of the reconstructed acoustic
peak compared to standard reconstruction.Comment: 20 pages, 5 figures; footnote adde
Non-linear matter power spectrum from Time Renormalisation Group: efficient computation and comparison with one-loop
We address the issue of computing the non-linear matter power spectrum on
mildly non-linear scales with efficient semi-analytic methods. We implemented
M. Pietroni's Time Renormalization Group (TRG) method and its Dynamical 1-Loop
(D1L) limit in a numerical module for the new Boltzmann code CLASS. Our
publicly released module is valid for LCDM models, and optimized in such a way
to run in less than a minute for D1L, or in one hour (divided by number of
nodes) for TRG. A careful comparison of the D1L, TRG and Standard 1-Loop
approaches reveals that results depend crucially on the assumed initial
bispectrum at high redshift. When starting from a common assumption, the three
methods give roughly the same results, showing that the partial resumation of
diagrams beyond one loop in the TRG method improves one-loop results by a
negligible amount. A comparison with highly accurate simulations by M. Sato &
T. Matsubara shows that all three methods tend to over-predict non-linear
corrections by the same amount on small wavelengths. Percent precision is
achieved until k~0.2 h/Mpc for z>2, or until k~0.14 h/Mpc at z=1.Comment: 24 pages, 7 figures, revised title and conclusions, version accepted
in JCAP, code available at http://class-code.ne
Isocurvature modes and Baryon Acoustic Oscillations
The measurement of Baryonic Acoustic Oscillations from galaxy surveys is well
known to be a robust and powerful tool to constrain dark energy. This method
relies on the knowledge of the size of the acoustic horizon at radiation drag
derived from Cosmic Microwave Background Anisotropy measurements. In this paper
we quantify the effect of non-standard initial conditions in the form of an
isocurvature component on the determination of dark energy parameters from
future BAO surveys. In particular, if there is an isocurvature component (at a
level still allowed by present data) but it is ignored in the CMB analysis, the
sound horizon and cosmological parameters determination is biased, and, as a
consequence, future surveys may incorrectly suggest deviations from a
cosmological constant. In order to recover an unbiased determination of the
sound horizon and dark energy parameters, a component of isocurvature
perturbations must be included in the model when analyzing CMB data.
Fortunately, doing so does not increase parameter errors significantly.Comment: 23 pages, 3 figure
A dynamical dark energy model with a given luminosity distance
It is assumed that the current cosmic acceleration is driven by a scalar
field, the Lagrangian of which is a function of the kinetic term only, and that
the luminosity distance is a given function of the red-shift. Upon comparison
with Baryon Acoustic Oscillations (BAOs) and Cosmic Microwave Background (CMB)
data the parameters of the models are determined, and then the time evolution
of the scalar field is determined by the dynamics using the cosmological
equations. We find that the solution is very different than the corresponding
solution when the non-relativistic matter is ignored, and that the universe
enters the acceleration era at larger red-shift compared to the standard
model.Comment: 4 pages, 3 figures, accepted for publication in GER
Next-to-leading resummation of cosmological perturbations via the Lagrangian picture: 2-loop correction in real and redshift spaces
We present an improved prediction of the nonlinear perturbation theory (PT)
via the Lagrangian picture, which was originally proposed by Matsubara (2008).
Based on the relations between the power spectrum in standard PT and that in
Lagrangian PT, we derive analytic expressions for the power spectrum in
Lagrangian PT up to 2-loop order in both real and redshift spaces. Comparing
the improved prediction of Lagrangian PT with -body simulations in real
space, we find that the 2-loop corrections can extend the valid range of wave
numbers where we can predict the power spectrum within 1% accuracy by a factor
of 1.0 (), 1.3 (1), 1.6 (2) and 1.8 (3) vied with 1-loop Lagrangian PT
results. On the other hand, in all redshift ranges, the higher-order
corrections are shown to be less significant on the two-point correlation
functions around the baryon acoustic peak, because the 1-loop Lagrangian PT is
already accurate enough to explain the nonlinearity on those scales in -body
simulations.Comment: 18pages, 4 figure
Cosmological constraints on unparticle dark matter
In unparticle dark matter (unmatter) models the equation of state of the
unmatter is given by , where is the scaling factor.
Unmatter with such equations of state would have a significant impact on the
expansion history of the universe. Using type Ia supernovae (SNIa), the baryon
acoustic oscillation (BAO) measurements and the shift parameter of the cosmic
microwave background (CMB) to place constraints on such unmatter models we find
that if only the SNIa data is used the constraints are weak. However, with the
BAO and CMB shift parameter data added strong constraints can be obtained. For
the UDM model, in which unmatter is the sole dark matter, we find that
at 95% C.L. For comparison, in most unparticle physics models it is
assumed . For the CUDM model, in which unmatter co-exists with
cold dark matter, we found that the unmatter can at most make up a few percent
of the total cosmic density if , thus it can not be the major component
of dark matter.Comment: Replaced with revised version. BAO data is added to make a tighter
constraint. Version accepted for publication on Euro.Phys.J.
Large-scale periodicity in the distribution of QSO absorption-line systems
The spatial-temporal distribution of absorption-line systems (ALSs) observed
in QSO spectra within the cosmological redshift interval z = 0.0--4.3 is
investigated on the base of our updated catalog of absorption systems. We
consider so called metallic systems including basically lines of heavy
elements. The sample of the data displays regular variations (with amplitudes ~
15 -- 20%) in the z-distribution of ALSs as well as in the eta-distribution,
where eta is a dimensionless line-of-sight comoving distance, relatively to
smoother dependences. The eta-distribution reveals the periodicity with period
Delta eta = 0.036 +/- 0.002, which corresponds to a spatial characteristic
scale (108 +/- 6) h(-1) Mpc or (alternatively) a temporal interval (350 +/- 20)
h(-1) Myr for the LambdaCDM cosmological model. We discuss a possibility of a
spatial interpretation of the results treating the pattern obtained as a trace
of an order imprinted on the galaxy clustering in the early Universe.Comment: AASTeX, 13 pages, with 9 figures, Accepted for publication in
Astrophysics & Space Scienc
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