354 research outputs found

### Isolating the decay rate of cosmological gravitational potential

The decay rate of cosmological gravitational potential measures the deviation
from Einstein-de Sitter universe and can put strong constraints on the nature
of dark energy and gravity. Usual method to measure this decay rate is through
the integrated Sachs-Wolfe (ISW) effect-large scale structure (LSS) cross
correlation. However, the interpretation of the measured correlation signal is
complicated by the galaxy bias and matter power spectrum. This could bias
and/or degrade its constraints to the nature of dark energy and gravity. But,
combining the lensing-LSS cross correlation measurements, the decay rate of
gravitational potential can be isolated. For any given narrow redshift bin of
LSS, the ratio of the two cross correlations directly measures $[d\ln
D_{\phi}/d\ln a]H(z)/W(\chi,\chi_s)$, where $D_{\phi}$ is the linear growth
factor of the gravitational potential, $H$ is the Hubble constant at redshift
$z$, $W(\chi,\chi_s)$ is the lensing kernel and $\chi$ and $\chi_s$ are the
comoving angular diameter distance to lens and source, respectively. This
method is optimal in the sense that (1) the measured quantity is essentially
free of systematic errors and is only limited by cosmic variance and (2) the
measured quantity only depends on several cosmological parameters and can be
predicted from first principles unambiguously. Though fundamentally limited by
inevitably large cosmic variance associated with the ISW measurements, it can
still put useful independent constraints on the amount of dark energy and its
equation of state. It can also provide a powerful test of modified gravity and
can distinguish the Dvali-Gabadadze-Porrati model from $\Lambda$CDM at
$>2.5\sigma$ confidence level.Comment: 5 pages, 3 figures. Accepted to ApJ. Added more discussions and
presented more detailed explanation of a key formula used in the pape

### Confirmation of the Copernican principle at Gpc radial scale and above from the kinetic Sunyaev Zel'dovich effect power spectrum

The Copernican principle, a cornerstone of modern cosmology, remains largely
unproven at Gpc radial scale and above. Here we will show that, violations of
this type will inevitably cause a first order anisotropic kinetic Sunyaev
Zel'dovich (kSZ) effect. If large scale radial inhomogeneities have amplitude
large enough to explain the "dark energy" phenomena, the induced kSZ power
spectrum will be much larger than the ACT/SPT upper limit. This single test
confirms the Copernican principle and rules out the adiabatic void model as a
viable alternative to dark energy.Comment: 4 pages, 2 figures. v2: updated with ACT result. v3: updated with SPT
result. Expanded discussions. Accepted to PR

### Non-negative matrix factorization for self-calibration of photometric redshift scatter in weak lensing surveys

Photo-z error is one of the major sources of systematics degrading the
accuracy of weak lensing cosmological inferences. Zhang et al. (2010) proposed
a self-calibration method combining galaxy-galaxy correlations and galaxy-shear
correlations between different photo-z bins. Fisher matrix analysis shows that
it can determine the rate of photo-z outliers at a level of 0.01-1% merely
using photometric data and do not rely on any prior knowledge. In this paper,
we develop a new algorithm to implement this method by solving a constrained
nonlinear optimization problem arising in the self-calibration process. Based
on the techniques of fixed-point iteration and non-negative matrix
factorization, the proposed algorithm can efficiently and robustly reconstruct
the scattering probabilities between the true-z and photo-z bins. The algorithm
has been tested extensively by applying it to mock data from simulated stage IV
weak lensing projects. We find that the algorithm provides a successful
recovery of the scatter rates at the level of 0.01-1%, and the true mean
redshifts of photo-z bins at the level of 0.001, which may satisfy the
requirements in future lensing surveys.Comment: 12 pages, 6 figures. Accepted for publication in ApJ. Updated to
match the published versio

### Testing eternal inflation with the kinetic Sunyaev Zel'dovich effect

Perhaps the most controversial idea in modern cosmology is that our
observable universe is contained within one bubble among many, all inhabiting
the eternally inflating multiverse. One of the few way to test this idea is to
look for evidence of the relic inhomogeneities left by the collisions between
other bubbles and our own. Such relic inhomogeneities induces a coherent bulk
flow over gigaparsec scales. Therefore, bubble collisions leave unique imprints
in the cosmic microwave background (CMB) through the kinetic Sunyaev Zel'dovich
(kSZ) effect, temperature anisotropies induced by the scattering of photons
from coherently moving free electrons in the diffuse intergalactic medium. The
kSZ signature produced by bubble collisions has a unique directional dependence
and is tightly correlated with the galaxy distribution; it can therefore be
distinguished from other contributions to the CMB anisotropies. An important
advantage of the kSZ signature is that it peaks on arcminute angular scales,
where the limiting factors in making a detection are instrumental noise and
foreground subtraction. This is in contrast to the collision signature in the
primary CMB, which peaks on angular scales much larger than one degree, and
whose detection is therefore limited by cosmic variance. In this paper, we
examine the prospects for probing the inhomogeneities left by bubble collisions
using the kSZ effect. We provide a forecast for detection using
cross-correlations between CMB and galaxy surveys, finding that the
detectability using the kSZ effect can be competitive with constraints from CMB
temperature and polarization data.Comment: 33 pages, 17 figures. Minor clarifications added in version 2,
conclusions are unchange

### Testing Gravity Against Early Time Integrated Sachs-Wolfe Effect

A generic prediction of general relativity is that the cosmological linear
density growth factor $D$ is scale independent. But in general, modified
gravities do not preserve this signature. A scale dependent $D$ can cause time
variation in gravitational potential at high redshifts and provides a new
cosmological test of gravity, through early time integrated Sachs-Wolfe (ISW)
effect-large scale structure (LSS) cross correlation. We demonstrate the power
of this test for a class of $f(R)$ gravity, with the form $f(R)=-\lambda_1
H_0^2\exp(-R/\lambda_2H_0^2)$. Such $f(R)$ gravity, even with degenerate
expansion history to $\Lambda$CDM, can produce detectable ISW effect at z\ga
3 and l\ga 20. Null-detection of such effect would constrain $\lambda_2$ to
be $\lambda_2>1000$ at $>95%$ confidence level. On the other hand, robust
detection of ISW-LSS cross correlation at high $z$ will severely challenge
general relativity.Comment: 5 pages, 2 figures. Accepted to PRD. v2: Revised to address to more
general audience. v3: added discussion

### Type Ia supernovae as speed sensors at intermediate redshifts

The application of large scale peculiar velocity (LSPV), as a crucial probe
of dark matter, dark energy and gravity, is severely limited by measurement
obstacles. We show that fluctuations in type Ia supernovae (SNe Ia) fluxes
induced by LSPV offer a promising approach to measure LSPV at intermediate
redshifts. In the 3D Fourier space, gravitational lensing, the dominant
systematical error, is well suppressed, localized and can be further corrected
effectively. Advance in SN observations can further significantly reduce shot
noise induced by SN intrinsic fluctuations, which is the dominant statistical
error. Robust mapping on the motion of the dark universe through SNe Ia is thus
feasible to $z\sim 0.5$.Comment: 6 pages, 1 figure. v2: expanded discussions. Accepted to PRD. Also
refer to the news report at Physics world
http://physicsworld.com/cws/article/news/3509

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