5,098 research outputs found
Measuring the cosmological constant with redshift surveys
It has been proposed that the cosmological constant might be
measured from geometric effects on large-scale structure. A positive vacuum
density leads to correlation-function contours which are squashed in the radial
direction when calculated assuming a matter-dominated model. We show that this
effect will be somewhat harder to detect than previous calculations have
suggested: the squashing factor is likely to be , given realistic
constraints on the matter contribution to . Moreover, the geometrical
distortion risks being confused with the redshift-space distortions caused by
the peculiar velocities associated with the growth of galaxy clustering. These
depend on the density and bias parameters via the combination , and we show that the main practical effect of a geometrical
flattening factor is to simulate gravitational instability with . Nevertheless, with datasets of sufficient size it is
possible to distinguish the two effects; we discuss in detail how this should
be done. New-generation redshift surveys of galaxies and quasars are
potentially capable of detecting a non-zero vacuum density, if it exists at a
cosmologically interesting level.Comment: MNRAS in press. 12 pages LaTeX including Postscript figures. Uses
mn.sty and epsf.st
A Model for Structure Formation Seeded by Gravitationally Produced Matter
This model assumes the baryons, radiation, three families of massless
neutrinos, and cold dark matter were mutually thermalized before the baryon
number was fixed, primeval curvature fluctuations were subdominant, and
homogeneity was broken by scale-invariant fluctuations in a new dark matter
component that behaves like a relativistic ideal fluid. The fluid behavior
could follow if this new component were a single scalar field that interacts
only with gravity and with itself by a pure quartic potential. The initial
energy distribution could follow if this component were gravitationally
produced by inflation. The power spectra of the present distributions of mass
and radiation in this model are not inconsistent with the measurements but are
sufficiently different from the adiabatic cold dark matter model to allow a
sharp test in the near future.Comment: 4 pages, 2 figures submitted to ApJ Letter
The Angular Power Spectrum of EDSGC Galaxies
We determine the angular power spectrum, C_l, of the Edinburgh/Durham
Southern Galaxy Catalog (EDSGC) and use this statistic to constrain
cosmological parameters. Our methods for determining C_l, and the parameters
that affect it are based on those developed for the analysis of cosmic
microwave background maps. We expect them to be useful for future surveys.
Assuming flat cold dark matter models with a cosmological constant (constrained
by COBE/DMR and local cluster abundances), and a scale--independent bias, b, we
find good fits to the EDSGC angular power spectrum with 1.11 < b < 2.35 and 0.2
< Omega_m < 0.55 at 95% confidence. These results are not significantly
affected by the ``integral constraint'' or extinction by interstellar dust, but
may be by our assumption of Gaussianity.Comment: 11 pages, 9 figures, version to appear in Ap
The finite size effect of galaxies on the cosmic virial theorem and the pairwise peculiar velocity dispersions
We discuss the effect of the finite size of galaxies on estimating
small-scale relative pairwise peculiar velocity dispersions from the cosmic
virial theorem (CVT). Specifically we evaluate the effect by incorporating the
finite core radius in the two-point correlation function of mass, i.e.
and the effective gravitational force
softening on small scales. We analytically obtain the lowest-order
correction term for which is in quantitative agreement with the
full numerical evaluation. With a nonzero and/or the cosmic virial
theorem is no longer limited to the case of . We present accurate
fitting formulae for the CVT predicted pairwise velocity dispersion for the
case of . Compared with the idealistic point-mass approximation
(), the finite size effect can significantly reduce the small-scale
velocity dispersions of galaxies at scales much larger than and .
Even without considering the finite size of galaxies, nonzero values for
are generally expected, for instance, for cold dark matter (CDM) models with a
scale-invariant primordial spectrum. For these CDM models, a reasonable force
softening r_s\le 100 \hikpc would have rather tiny effect. We present the CVT
predictions for the small-scale pairwise velocity dispersion in the CDM models
normalized by the COBE observation. The implication of our results for
confrontation of observations of galaxy pair-wise velocity dispersions and
theoretical predictions of the CVT is also discussed.Comment: 18 pages. LaTeX text and 8 postcript figures. submitted to Ap
Gravitational lens magnification by Abell 1689: Distortion of the background galaxy luminosity function
Gravitational lensing magnifies the luminosity of galaxies behind the lens.
We use this effect to constrain the total mass in the cluster Abell 1689 by
comparing the lensed luminosities of background galaxies with the luminosity
function of an undistorted field. Since galaxies are assumed to be a random
sampling of luminosity space, this method is not limited by clustering noise.
We use photometric redshift information to estimate galaxy distance and
intrinsic luminosity. Knowing the redshift distribution of the background
population allows us to lift the mass/background degeneracy common to lensing
analysis. In this paper we use 9 filters observed over 12 hours with the Calar
Alto 3.5m telescope to determine the redshifts of 1000 galaxies in the field of
Abell 1689. Using a complete sample of 151 background galaxies we measure the
cluster mass profile. We find that the total projected mass interior to
0.25h^(-1)Mpc is (0.48 +/- 0.16) * 10^(15)h^(-1) solar masses, where our error
budget includes uncertainties from the photometric redshift determination, the
uncertainty in the off-set calibration and finite sampling. This result is in
good agreement with that found by number count and shear-based methods and
provides a new and independent method to determine cluster masses.Comment: 13 pages, 10 figures. Submitted to MNRAS (10/99); Replacement with 1
page extra text inc. new section, accepted by MNRA
Cosmological Lower Bound on Dark Matter Masses from the Soft Gamma-ray Background
Motivated by a recent detection of 511 keV photons from the center of our
Galaxy, we calculate the spectrum of the soft gamma-ray background of the
redshifted 511 keV photons from cosmological halos. Annihilation of dark matter
particles into electron-positron pairs makes a substantial contribution to the
gamma-ray background. Mass of such dark matter particles must be <~ 100 MeV so
that resulting electron-positron pairs are on-relativistic. On the other hand,
we show that in order for the annihilation not to exceed the observed
background, the dark matter mass needs to be >~ 20 MeV. We include the
contribution from the active galactic nuclei and supernovae. The halo
substructures may increase the lower bound to >~ 60 MeV.Comment: 5 pages, 5 figures; accepted for publication in PRD, Rapid
Communicatio
Dynamics of inflationary cosmology in TVSD model
Within the framework of a model Universe with time variable space dimensions
(TVSD), known as decrumpling or TVSD model, we study TVSD chaotic inflation and
obtain dynamics of the inflaton, scale factor and spatial dimension. We also
study the quantum fluctuations of the inflaton field and obtain the spectral
index and its running in this model. Two classes of examples have been studied
and comparisons made with the standard slow-roll formulae. We compare our
results with the recent Wilkinson Microwave Anisotropy Probe (WMAP) data.Comment: 18 pages, 3 figures, accepted in Mod. Phys. Lett.
The Axiverse Extended: Vacuum Destabilisation, Early Dark Energy and Cosmological Collapse
A model is presented in the philosophy of the "String Axiverse" of Arvanitaki
et al (arXiv:0905.4720v2 [hep-th]) that incorporates a coupling of ultralight
axions to their corresponding moduli through the mass term. The light fields
roll in their potentials at late times and contribute to the dark sector energy
densities in the cosmological expansion. The addition of a coupling and extra
field greatly enrich the possible phenomenology of the axiverse. There are a
number of interesting phases where the axion and modulus components behave as
Dark Matter or Dark Energy and can have considerable and distinct effects on
the expansion history of the universe by modifying the equation of state in the
past or causing possible future collapse of the universe. In future such a
coupling may help to alleviate fine tuning problems for cosmological axions. We
motivate and present the model, and briefly explore its cosmological
consequences numerically.Comment: 13 pages, 17 figures, published in PRD. v3: corrected SUSY
interpretation of axion potential scal
- …