149 research outputs found
The Real and Redshift Space Density Distribution Function for Large-Scale Structure in the Spherical Collapse Approximation
We use the spherical collapse (SC) approximation to derive expressions for
the smoothed redshift-space probability distribution function (PDF), as well as
the -order hierarchical amplitudes , in both real and redshift space.
We compare our results with numerical simulations, focusing on the
standard CDM model, where redshift distortions are strongest. We find good
agreement between the SC predictions and the numerical PDF in real space even
for \sigma_L \simgt 1, where is the linearly-evolved rms
fluctuation on the smoothing scale. In redshift space, reasonable agreement is
possible only for \sigma_L \simlt 0.4. Numerical simulations also yield a
simple empirical relation between the real-space PDF and redshift-space PDF: we
find that for \sigma \simlt 1, the redshift space PDF, P[\delta_z], is, to a
good approximation, a simple rescaling of the real space PDF, P[\delta], i.e.,
P[\delta/\sigma] d[\delta/\sigma] = P[\delta_z/\sigma_z] d[\delta_z/\sigma_z],
where and \sigma_z are the real-space and redshift-space rms
fluctuations, respectively. This result applies well beyond the validity of
linear perturbation theory, and it is a good fit for both the standard CDM
model and the Lambda-CDM model. It breaks down for SCDM at ,
but provides a good fit to the \Lambda-CDM models for as large as 0.8.Comment: 9 pages, latex, 12 figures added (26 total), minor changes to
conclusions, to appear in MNRA
Stochastic Biasing and Weakly Non-linear Evolution of Power Spectrum
Distribution of galaxies may be a biased tracer of the dark matter
distribution and the relation between the galaxies and the total mass may be
stochastic, non-linear and time-dependent. Since many observations of galaxy
clustering will be done at high redshift, the time evolution of non-linear
stochastic biasing would play a crucial role for the data analysis of the
future sky surveys. In this paper, we develop the weakly non-linear analysis
and attempt to clarify the non-linear feature of the stochastic biasing. We
compute the one-loop correction of the power spectrum for the total mass, the
galaxies and their cross correlation. Assuming the local functional form for
the initial galaxy distribution, we investigate the time evolution of the
biasing parameter and the correlation coefficient. On large scales, we first
find that the time evolution of the biasing parameter could deviate from the
linear prediction in presence of the initial skewness. However, the deviation
can be reduced when the initial stochasticity exists. Next, we focus on the
quasi-linear scales, where the non-linear growth of the total mass becomes
important. It is recognized that the scale-dependence of the biasing
dynamically appears and the initial stochasticity could affect the time
evolution of the scale-dependence. The result is compared with the recent
N-body simulation that the scale-dependence of the halo biasing can appear on
relatively large scales and the biasing parameter takes the lower value on
smaller scales. Qualitatively, our weakly non-linear results can explain this
trend if the halo-mass biasing relation has the large scatter at high redshift.Comment: 29pages, 7 postscript figures, submitted to Ap
Skewness as a probe of non-Gaussian initial conditions
We compute the skewness of the matter distribution arising from non-linear
evolution and from non-Gaussian initial perturbations. We apply our result to a
very generic class of models with non-Gaussian initial conditions and we
estimate analytically the ratio between the skewness due to non-linear
clustering and the part due to the intrinsic non-Gaussianity of the models. We
finally extend our estimates to higher moments.Comment: 5 pages, 2 ps-figs., accepted for publication in PRD, rapid com
Galaxy clustering constraints on deviations from Newtonian gravity at cosmological scales II: Perturbative and numerical analyses of power spectrum and bispectrum
We explore observational constraints on possible deviations from Newtonian
gravity by means of large-scale clustering of galaxies. We measure the power
spectrum and the bispectrum of Sloan Digital Sky Survey galaxies and compare
the result with predictions in an empirical model of modified gravity. Our
model assumes an additional Yukawa-like term with two parameters that
characterize the amplitude and the length scale of the modified gravity. The
model predictions are calculated using two methods; the second-order
perturbation theory and direct N-body simulations. These methods allow us to
study non-linear evolution of large-scale structure. Using the simulation
results, we find that perturbation theory provides reliable estimates for the
power spectrum and the bispectrum in the modified Newtonian model. We also
construct mock galaxy catalogues from the simulations, and derive constraints
on the amplitude and the length scale of deviations from Newtonian gravity. The
resulting constraints from power spectrum are consistent with those obtained in
our earlier work, indicating the validity of the previous empirical modeling of
gravitational nonlinearity in the modified Newtonian model. If linear biasing
is adopted, the bispectrum of the SDSS galaxies yields constraints very similar
to those from the power spectrum. If we allow for the nonlinear biasing
instead, we find that the ratio of the quadratic to linear biasing
coefficients, b_2/b_1, should satisfy -0.4 < b_2/b_1<0.3 in the modified
Newtonian model.Comment: 12 pages, 7 figure
Dark matter clustering: a simple renormalization group approach
I compute a renormalization group (RG) improvement to the standard
beyond-linear-order Eulerian perturbation theory (PT) calculation of the power
spectrum of large-scale density fluctuations in the Universe. At z=0, for a
power spectrum matching current observations, lowest order RGPT appears to be
as accurate as one can test using existing numerical simulation-calibrated
fitting formulas out to at least k~=0.3 h/Mpc; although inaccuracy is
guaranteed at some level by approximations in the calculation (which can be
improved in the future). In contrast, standard PT breaks down virtually as soon
as beyond-linear corrections become non-negligible, on scales even larger than
k=0.1 h/Mpc. This extension in range of validity could substantially enhance
the usefulness of PT for interpreting baryonic acoustic oscillation surveys
aimed at probing dark energy, for example. I show that the predicted power
spectrum converges at high k to a power law with index given by the fixed-point
solution of the RG equation. I discuss many possible future directions for this
line of work. The basic calculation of this paper should be easily
understandable without any prior knowledge of RG methods, while a rich
background of mathematical physics literature exists for the interested reader.Comment: much expanded explanation of basic calculatio
On the Viability of Bianchi Type VIIh Models with Dark Energy
We generalize the predictions for the CMB anisotropy patterns arising in
Bianchi type VIIh universes to include a dark energy component. We consider
these models in light of the result of Jaffe et al. (2005a,b) in which a
correlation was found on large angular scales between the WMAP data and the
anisotropy structure in a low density Bianchi universe. We find that by
including a term Omega_L > 0, the same best-fit anisotropy pattern is
reproduced by several combinations of cosmological parameters. This sub-set of
models can then be further constrained by current observations that limit the
values of various cosmological parameters. In particular, we consider the
so-called geometric degeneracy in these parameters imposed by the peak
structure of the WMAP data itself. Apparently, despite the additional freedom
allowed by the dark energy component, the modified Bianchi models are ruled out
at high significance.Comment: submitted to Ap
Constraints on Galaxy Bias, Matter Density, and Primordial Non--Gausianity from the PSCz Galaxy Redshift Survey
We compute the bispectrum for the \IRAS PSCz catalog and find that the galaxy
distribution displays the characteristic signature of gravity. Assuming
Gaussian initial conditions, we obtain galaxy biasing parameters
and , with no sign of
scale-dependent bias for h/Mpc. These results impose stringent
constraints on non-Gaussian initial conditions. For dimensional scaling models
with statistics, we find N>49, which implies a constraint on
primordial skewness .Comment: 4 pages, 3 embedded figures, uses revtex style file, minor changes to
reflect published versio
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The architecture of EMC reveals a path for membrane protein insertion.
Funder: Boehringer Ingelheim Fonds; FundRef: http://dx.doi.org/10.13039/501100001645Funder: Naito Foundation; FundRef: http://dx.doi.org/10.13039/100007428Funder: Japanese Biochemical SocietyApproximately 25% of eukaryotic genes code for integral membrane proteins that are assembled at the endoplasmic reticulum. An abundant and widely conserved multi-protein complex termed EMC has been implicated in membrane protein biogenesis, but its mechanism of action is poorly understood. Here, we define the composition and architecture of human EMC using biochemical assays, crystallography of individual subunits, site-specific photocrosslinking, and cryo-EM reconstruction. Our results suggest that EMC's cytosolic domain contains a large, moderately hydrophobic vestibule that can bind a substrate's transmembrane domain (TMD). The cytosolic vestibule leads into a lumenally-sealed, lipid-exposed intramembrane groove large enough to accommodate a single substrate TMD. A gap between the cytosolic vestibule and intramembrane groove provides a potential path for substrate egress from EMC. These findings suggest how EMC facilitates energy-independent membrane insertion of TMDs, explain why only short lumenal domains are translocated by EMC, and constrain models of EMC's proposed chaperone function
Cosmic Strings and the String Dilaton
The existence of a dilaton (or moduli) with gravitational-strength coupling
to matter imposes stringent constraints on the allowed energy scale of cosmic
strings, . In particular, superheavy gauge strings with are ruled out unless the dilaton mass m_{\phi} \gsim 100 TeV,
while the currently popular value imposes the bound \eta
\lsim 3 \times 10^{11} GeV. Similar constraints are obtained for global
topological defects. Some non-standard cosmological scenarios which can avoid
these constraints are pointed out.Comment: 16 page
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