36 research outputs found
Magnification-Temperature Correlation: the Dark Side of ISW Measurements
Integrated Sachs-Wolfe (ISW) measurements, which involve cross-correlating
the CMB with the foreground large-scale structure (e.g. galaxies/quasars), have
proven to be an interesting probe of dark energy. We show that magnification
bias, which is the inevitable modulation of the foreground number counts by
gravitational lensing, alters both the shape and amplitude of the observed ISW
signal. This is true especially at high redshifts because (1) the intrinsic
galaxy-temperature signal diminishes greatly back in the matter dominated era,
(2) the lensing efficiency increases with redshift and (3) the number count
slope generally steepens with redshift in a magnitude limited sample. At z >~
2, the magnification-temperature correlation dominates over the intrinsic
galaxy-temperature correlation and causes the observed ISW signal to increase
with z, despite dark energy subdominance -- a result of the fact that
magnification probes structures between the observer and the sources. Ignoring
magnification bias can then lead to erroneous conclusions about dark energy.
While the lensing modulation opens up an interesting high z window for ISW
measurements, high z measurements are not expected to add much new information
to low z ones if dark energy is the cosmological constant. This is because
lensing introduces significant covariance across redshifts. The most compelling
reason to pursue high z ISW measurements is to look for a potential surprise
such as early dark energy domination or the signature of modified gravity. We
conclude with a discussion of existing measurements, the highest z of which is
at the margin of being sensitive to magnification bias. We also develop a
formalism which might be of general interest: to predict biases in estimating
parameters when certain physical effects are ignored in interpreting data.Comment: 14 pages, 12 figures, references added, minor typos corrected,
accepted for publication by PR
A Horizon Ratio Bound for Inflationary Fluctuations
We demonstrate that the gravity wave background amplitude implies a robust
upper bound on the ratio: \lambda / H^{-1} < e^60, where \lambda is the proper
wavelength of fluctuations of interest and H^{-1} is the horizon at the end of
inflation. The bound holds as long as the energy density of the universe does
not drop faster than radiation subsequent to inflation. This limit implies that
the amount of expansion between the time the scales of interest leave the
horizon and the end of inflation, denoted by e^N, is also bounded from above,
by about e^60 times a factor that involves an integral over the first slow-roll
parameter. In other words, the bound on N is model dependent -- we show that
for vast classes of slow-roll models, N < 67. The quantities, \lambda / H^{-1}
or N, play an important role in determining the nature of inflationary scalar
and tensor fluctuations. We suggest ways to incorporate the above bounds when
confronting inflation models with observations. As an example, this bound
solidifies the tension between observations of cosmic microwave background
(CMB) anisotropies and chaotic inflation with a \phi^4 potential by closing the
escape hatch of large N (< 62).Comment: 4 pages, 1 figure; revised to close a loophole in the earlier version
and clarify our assumption
Anisotropic Inflation and the Origin of Four Large Dimensions
In the context of (4+d)-dimensional general relativity, we propose an
inflationary scenario wherein 3 spatial dimensions grow large, while d extra
dimensions remain small. Our model requires that a self-interacting d-form
acquire a vacuum expectation value along the extra dimensions. This causes 3
spatial dimensions to inflate, whilst keeping the size of the extra dimensions
nearly constant. We do not require an additional stabilization mechanism for
the radion, as stable solutions exist for flat, and for negatively curved
compact extra dimensions. From a four-dimensional perspective, the radion does
not couple to the inflaton; and, the small amplitude of the CMB temperature
anisotropies arises from an exponential suppression of fluctuations, due to the
higher-dimensional origin of the inflaton. The mechanism triggering the end of
inflation is responsible, both, for heating the universe, and for avoiding
violations of the equivalence principle due to coupling between the radion and
matter.Comment: 24 pages, 2 figures; uses RevTeX4. v2: Minor changes and added
references. v3: Improved discussion of slow-rol
Probing the dark energy: methods and strategies
The presence of dark energy in the Universe is inferred directly from the
accelerated expansion of the Universe, and indirectly, from measurements of
cosmic microwave background (CMB) anisotropy. Dark energy contributes about 2/3
of the critical density, is very smoothly distributed, and has large negative
pressure. Its nature is very much unknown. Most of its discernible consequences
follow from its effect on evolution of the expansion rate of the Universe,
which in turn affects the growth of density perturbations and the age of the
Universe, and can be probed by the classical kinematic cosmological tests.
Absent a compelling theoretical model (or even a class of models), we describe
the dark energy by an effective equation-of-state w=p_X/\rho_X which is allowed
to vary with time. We describe and compare different approaches for determining
w(t), including magnitude-redshift (Hubble) diagram, number counts of galaxies
and clusters, and CMB anisotropy, focusing particular attention on the use of a
sample of several thousand type Ia supernova with redshifts z\lesssim 1.7, as
might be gathered by the proposed SNAP satellite. Among other things, we derive
optimal strategies for constraining cosmological parameters using type Ia
supernovae. While in the near term CMB anisotropy will provide the first
measurements of w, supernovae and number counts appear to have the most
potential to probe dark energy.Comment: 20 pages and 23 figures, revtex, submitted to Phys. Rev.
The Angular Correlation Function of Galaxies from Early SDSS Data
The Sloan Digital Sky Survey is one of the first multicolor photometric and
spectroscopic surveys designed to measure the statistical properties of
galaxies within the local Universe. In this Letter we present some of the
initial results on the angular 2-point correlation function measured from the
early SDSS galaxy data. The form of the correlation function, over the
magnitude interval 18<r*<22, is shown to be consistent with results from
existing wide-field, photographic-based surveys and narrower CCD galaxy
surveys. On scales between 1 arcminute and 1 degree the correlation function is
well described by a power-law with an exponent of ~ -0.7. The amplitude of the
correlation function, within this angular interval, decreases with fainter
magnitudes in good agreement with analyses from existing galaxy surveys. There
is a characteristic break in the correlation function on scales of
approximately 1-2 degrees. On small scales, < 1', the SDSS correlation function
does not appear to be consistent with the power-law form fitted to the 1'<
theta <0.5 deg data. With a data set that is less than 2% of the full SDSS
survey area, we have obtained high precision measurements of the power-law
angular correlation function on angular scales 1' < theta < 1 deg, which are
robust to systematic uncertainties. Because of the limited area and the highly
correlated nature of the error covariance matrix, these initial results do not
yet provide a definitive characterization of departures from the power-law form
at smaller and larger angles. In the near future, however, the area of the SDSS
imaging survey will be sufficient to allow detailed analysis of the small and
large scale regimes, measurements of higher-order correlations, and studies of
angular clustering as a function of redshift and galaxy type
KL Estimation of the Power Spectrum Parameters from the Angular Distribution of Galaxies in Early SDSS Data
We present measurements of parameters of the 3-dimensional power spectrum of
galaxy clustering from 222 square degrees of early imaging data in the Sloan
Digital Sky Survey. The projected galaxy distribution on the sky is expanded
over a set of Karhunen-Loeve eigenfunctions, which optimize the signal-to-noise
ratio in our analysis. A maximum likelihood analysis is used to estimate
parameters that set the shape and amplitude of the 3-dimensional power
spectrum. Our best estimates are Gamma=0.188 +/- 0.04 and sigma_8L = 0.915 +/-
0.06 (statistical errors only), for a flat Universe with a cosmological
constant. We demonstrate that our measurements contain signal from scales at or
beyond the peak of the 3D power spectrum. We discuss how the results scale with
systematic uncertainties, like the radial selection function. We find that the
central values satisfy the analytically estimated scaling relation. We have
also explored the effects of evolutionary corrections, various truncations of
the KL basis, seeing, sample size and limiting magnitude. We find that the
impact of most of these uncertainties stay within the 2-sigma uncertainties of
our fiducial result.Comment: Fig 1 postscript problem correcte
Partially ionizing the universe by decaying particles
We show that UV photons produced by decaying particles can partially reionize
the universe and explain the large optical depth observed by Wilkinson
Microwave Anisotropy Probe. Together with UV fluxes from early formed stars and
quasars, it is possible that the universe is fully ionized at z \lesssim 6 and
partially ionized at z \gtrsim 6 as observed by Sloan Digital Sky Survey for
large parameter space of the decaying particle. This scenario will be
discriminated by future observations, especially by the EE polarization power
spectrum of cosmic microwave background radiation.Comment: 5 pages, 6 postscript figures include
Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of
dark matter and dark energy in the universe. In this brief review we outline
how weak lensing helps determine the structure of dark matter halos, measure
the expansion rate of the universe, and distinguish between modified gravity
and dark energy explanations for the acceleration of the universe. We also
discuss requirements on the control of systematic errors so that the
systematics do not appreciably degrade the power of weak lensing as a
cosmological probe.Comment: Invited review article for the GRG special issue on gravitational
lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). V3: subsection on
three-point function and some references added. Matches the published versio