2,194 research outputs found
Observational tests for oscillating expansion rate of the Universe
We investigate the observational constraints on the oscillating scalar field
model using data from type Ia supernovae, cosmic microwave background
anisotropies, and baryon acoustic oscillations. According to a Fourier
analysis, the galaxy number count from redshift data indicates that
galaxies have preferred periodic redshift spacings. We fix the mass of the
scalar field as such that the scalar
field model can account for the redshift spacings, and we constrain the other
basic parameters by comparing the model with accurate observational data. We
obtain the following constraints: (95% C.L.),
(95% C.L.) (in the range
). The best fit values of the energy density parameter of the scalar
field and the coupling constant are and ,
respectively. The value of is close to but not equal to .
Hence, in the scalar field model, the amplitude of the galaxy number count
cannot be large. However, because the best fit values of and
are not , the scalar field model has the possibility of accounting for
the periodic structure in the -- relation of galaxies. The variation of
the effective gravitational constant in the scalar field model is not
inconsistent with the bound from observation.Comment: 9 pages, 11 figures, 1 table, Accepted for publication in Physical
Review
Finite-time singularities in f(R, T) gravity and the effect of conformal anomaly
We investigate gravity models ( is the curvature scalar and
is the trace of the stress-energy tensor of ordinary matter) that are able to
reproduce the four known types of future finite-time singularities. We choose a
suitable expression for the Hubble parameter in order to realise the cosmic
acceleration and we introduce two parameters, and , which
characterise each type of singularity. We address conformal anomaly and we
observe that it cannot remove the sudden singularity or the type IV one, but,
for some values of , the big rip and the type III singularity may be
avoided. We also find that, even without taking into account conformal anomaly,
the big rip and the type III singularity may be removed thanks to the presence
of the contribution of the theory.Comment: 18 pages; Accepted for publication in Canadian Journal of Physics
(CJP
Primordial fractal density perturbations and structure formation in the Universe: 1-Dimensional collisionless sheet model
Two-point correlation function of galaxy distribution shows that the
structure in the present Universe is scale-free up to a certain scale (at least
several tens Mpc), which suggests that a fractal structure may exist. If small
primordial density fluctuations have a fractal structure, the present
fractal-like nonlinear structure below the horizon scale could be naturally
explained. We analyze the time evolution of fractal density perturbations in
Einstein-de Sitter universe, and study how the perturbation evolves and what
kind of nonlinear structure will come out. We assume a one-dimensional
collisionless sheet model with initial Cantor-type fractal perturbations. The
nonlinear structure seems to approach some attractor with a unique fractal
dimension, which is independent of the fractal dimensions of initial
perturbations. A discrete self-similarity in the phase space is also found when
the universal nonlinear fractal structure is reached.Comment: 17 pages, 19 jpeg figures. Accepted for publication in ApJ. Figures
are also available from
http://www.phys.waseda.ac.jp/gravity/~tatekawa/0003124/figs.tar.g
Contiguous redshift parameterizations of the growth index
The growth rate of matter perturbations can be used to distinguish between
different gravity theories and to distinguish between dark energy and modified
gravity at cosmological scales as an explanation to the observed cosmic
acceleration. We suggest here parameterizations of the growth index as
functions of the redshift. The first one is given by that
interpolates between a low/intermediate redshift parameterization
and a high
redshift constant value. For example, our interpolated form
can be used when including the CMB to the rest of the data while
the form can be used otherwise. It is found that the
parameterizations proposed achieve a fit that is better than 0.004% for the
growth rate in a CDM model, better than 0.014% for
Quintessence-Cold-Dark-Matter (QCDM) models, and better than 0.04% for the flat
Dvali-Gabadadze-Porrati (DGP) model (with ) for the entire
redshift range up to . We find that the growth index parameters
take distinctive values for dark energy models and
modified gravity models, e.g. for the CDM model
and for the flat DGP model. This provides a means for future
observational data to distinguish between the models.Comment: 7 pages, 6 figures, matches PRD accepted versio
Limits on the integration constant of the dark radiation term in Brane Cosmology
We consider the constraints from primordial Helium abundances on the constant
of integration of the dark radiation term of the brane-world generalized
Friedmann equation derived from the Randall-Sundrum Single brane model. We
found that -- using simple, approximate and semianalytical Method -- that the
constant of integration is limited to be between -8.9 and 2.2 which limits the
possible contribution from dark radiation term to be approximately between -27%
to 7% of the background photon energy density.Comment: 8 page
CMB Constraint on Radion Evolution in the Brane World Scenario
In many versions of brane model, the modulus field of extra dimensions, the
radion, could have cosmological evolution, which induces variation of the Higgs
vacuum expectation value, , resulting in cosmological variation of the
electron mass $m_e$. The formation of Cosmic Microwave Background (CMB)
anisotropies is thus affected, causing changes both in the peaks positions and
amplitudes in the CMB power spectra. Using the three-year Wilkinson Microwave
Anisotropies Probe (WMAP) CMB data, with the Hubble parameter $H_0$ fixed to be
the Hubble Space Telescope (HST) result 72 km s$^{-1}$ Mpc$^{-1}$, we obtain a
constraint on $\rho$, the ratio of the value of at CMB recombination to
its present value, to be [0.97, 1.02].Comment: 7 pages, 6 figures, minor changes of format to conform with PRD
forma
Reconstructing the shape of the correlation function
We develop an estimator for the correlation function which, in the ensemble
average, returns the shape of the correlation function, even for signals that
have significant correlations on the scale of the survey region. Our estimator
is general and works in any number of dimensions. We develop versions of the
estimator for both diffuse and discrete signals. As an application, we examine
Monte Carlo simulations of X-ray background measurements. These include a
realistic, spatially-inhomogeneous population of spurious detector events. We
discuss applying the estimator to the averaging of correlation functions
evaluated on several small fields, and to other cosmological applications.Comment: 10 pages, 5 figures, submitted to ApJS. Methods and results unchanged
but text is expanded and significantly reordered in response to refere
The ergodicity bias in the observed galaxy distribution
The spatial distribution of galaxies we observed is subject to the given
condition that we, human beings are sitting right in a galaxy -- the Milky Way.
Thus the ergodicity assumption is questionable in interpretation of the
observed galaxy distribution. The resultant difference between observed
statistics (volume average) and the true cosmic value (ensemble average) is
termed as the ergodicity bias. We perform explicit numerical investigation of
the effect for a set of galaxy survey depths and near-end distance cuts. It is
found that the ergodicity bias in observed two- and three-point correlation
functions in most cases is insignificant for modern analysis of samples from
galaxy surveys and thus close a loophole in precision cosmology. However, it
may become non-negligible in certain circumstances, such as those applications
involving three-point correlation function at large scales of local galaxy
samples. Thus one is reminded to take extra care in galaxy sample construction
and interpretation of the statistics of the sample, especially when the
characteristic redshift is low.Comment: Revised version published as JCAP08(2010)01
Cosmological Three-Point Function: Testing The Halo Model Against Simulations
We perform detailed comparison of the semi-analytic halo model predictions
with measurements in numerical simulations of the two and three point
correlation functions (3PCF), as well as power spectrum and bispectrum. We
discuss the accuracy and self-consistency of the halo model description of
gravitational clustering in the non-linear regime and constrain halo model
parameters. We exploit the recently proposed multipole expansion of three point
statistics that expresses rotation invariance in the most natural way. This not
only offers technical advantages by reducing the integrals required for the
halo model predictions, but amounts to a convenient way of compressing the
information contained in the 3PCF. We find that, with an appropriate choice of
the halo boundary and mass function cut-off, halo model predictions are in good
agreement with the bispectrum measured in numerical simulations. However, the
halo model predicts less than the observed configuration dependence of the 3PCF
on ~ Mpc scales. This effect is mainly due to quadrupole moment deficit,
possibly related to the assumption of spherical halo geometry. Our analysis
shows that using its harmonic decomposition, the full configuration dependence
of the 3PCF in the non-linear regime can be compressed into just a few numbers,
the lowest multipoles. Moreover, these multipoles are closely related to the
highest signal to noise eigenmodes of the 3PCF. Therefore this estimator may
simplify future analyses aimed at constraining cosmological and halo model
parameters from observational data.Comment: Minor corrections. Accepted for publication by Ap
Nonlinear Velocity-Density Coupling: Analysis by Second-Order Perturbation Theory
Cosmological linear perturbation theory predicts that the peculiar velocity
and the matter overdensity at a same point are
statistically independent quantities, as log as the initial density
fluctuations are random Gaussian distributed. However nonlinear gravitational
effects might change the situation. Using framework of second-order
perturbation theory and the Edgeworth expansion method, we study local density
dependence of bulk velocity dispersion that is coarse-grained at a weakly
nonlinear scale. For a typical CDM model, the first nonlinear correction of
this constrained bulk velocity dispersion amounts to (Gaussian
smoothing) at a weakly nonlinear scale with a very weak dependence on
cosmological parameters. We also compare our analytical prediction with
published numerical results given at nonlinear regimes.Comment: 16 pages including 2 figures, ApJ 537 in press (July 1
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