80 research outputs found
Removable Matter-Power-Spectrum Covariance from Bias Fluctuations
We find a simple, accurate model for the covariance matrix of the real-space
cosmological matter power spectrum on slightly nonlinear scales (k~0.1-0.8
h/Mpc at z=0), where off-diagonal matrix elements become substantial. The model
includes a multiplicative, scale-independent modulation of the power spectrum.
It has only one parameter, the variance (among realizations) of the variance of
the nonlinear density field in cells, with little dependence on the cell size
between 2-8 Mpc/h. Furthermore, we find that this extra covariance can be
modeled out by instead measuring the power spectrum of (delta/sigma_cell), i.e.
the ratio of the overdensity to its dispersion in cells a few Mpc in size.
Dividing delta by sigma_cell essentially removes the non-Gaussian part of the
covariance matrix, nearly diagonalizing it.Comment: Accepted to ApJ. 5 pages, 5 figures; slight clarifications to match
accepted versio
Ringing the initial Universe: the response of overdensity and transformed-density power spectra to initial spikes
We present an experiment in which we 'ring' a set of cosmological
N-body-simulation initial conditions, placing spikes in the initial power
spectrum at different wavenumber bins. We then measure where these spikes end
up in the final conditions. In the usual overdensity power spectrum, most
sensitive to contracting and collapsing dense regions, initial power on
slightly non-linear scales (k ~ 0.3 h/Mpc) smears to smaller scales, coming to
dominate the initial power once there. Log-density and Gaussianized-density
power spectra, sensitive to low-density (expanding) and high-density regions,
respond differently: initial spikes spread symmetrically in scale, both upward
and downward. In fact, in the power spectrum of 1/(1 + {\delta}), spikes
migrate to larger scales, showing the magnifying effect of voids on small-scale
modes. These power spectra show much greater sensitivity to small-scale initial
features. We also test the difference between an approximation of the
Ly-{\alpha} flux field, and its Gaussianized form, and give a toy model that
qualitatively explains the symmetric power spreading in Gaussianized-density
power spectra. Also, we discuss how to use this framework to estimate
power-spectrum covariance matrices. This can be used to track the fate of
information in the Universe, that takes the form of initial degrees of freedom,
one random spike per initial mode.Comment: Accepted to MNRAS Letters. 6 pages, 5 figure
A Map of the Integrated Sachs-Wolfe Signal from Luminous Red Galaxies
We construct a map of the time derivative of the gravitational potential
traced by SDSS Luminous Red Galaxies. The potential decays on large scales due
to cosmic acceleration, leaving an imprint on cosmic microwave background (CMB)
radiation through the integrated Sachs-Wolfe (ISW) effect. With a template fit,
we directly measure this signature on the CMB at a 2-sigma confidence level.
The measurement is consistent with the cross-correlation statistic,
strengthening the claim that dark energy is indeed the cause of the
correlation. This new approach potentially simplifies the cosmological
interpretation. Our constructed linear ISW map shows no evidence for
degree-scale cold and hot spots associated with supervoid and supercluster
structures. This suggests that the linear ISW effect in a concordance
Lambda-CDM cosmology is insufficient to explain the strong CMB imprints from
these structures that we previously reported.Comment: 9 pages, 12 figures, accepted to ApJ. Updated discussion about
redshift cut
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