87 research outputs found
Inflation, cold dark matter, and the central density problem
A problem with high central densities in dark halos has arisen in the context
of LCDM cosmologies with scale-invariant initial power spectra. Although n=1 is
often justified by appealing to the inflation scenario, inflationary models
with mild deviations from scale-invariance are not uncommon and models with
significant running of the spectral index are plausible. Even mild deviations
from scale-invariance can be important because halo collapse times and
densities depend on the relative amount of small-scale power. We choose several
popular models of inflation and work out the ramifications for galaxy central
densities. For each model, we calculate its COBE-normalized power spectrum and
deduce the implied halo densities using a semi-analytic method calibrated
against N-body simulations. We compare our predictions to a sample of dark
matter-dominated galaxies using a non-parametric measure of the density. While
standard n=1, LCDM halos are overdense by a factor of 6, several of our example
inflation+CDM models predict halo densities well within the range preferred by
observations. We also show how the presence of massive (0.5 eV) neutrinos may
help to alleviate the central density problem even with n=1. We conclude that
galaxy central densities may not be as problematic for the CDM paradigm as is
sometimes assumed: rather than telling us something about the nature of the
dark matter, galaxy rotation curves may be telling us something about inflation
and/or neutrinos. An important test of this idea will be an eventual consensus
on the value of sigma_8, the rms overdensity on the scale 8 h^-1 Mpc. Our
successful models have values of sigma_8 approximately 0.75, which is within
the range of recent determinations. Finally, models with n>1 (or sigma_8 > 1)
are highly disfavored.Comment: 13 pages, 6 figures. Minor changes made to reflect referee's
Comments, error in Eq. (18) corrected, references updated and corrected,
conclusions unchanged. Version accepted for publication in Phys. Rev. D,
scheduled for 15 August 200
Constraints on inflation in closed universe
We investigate inflation in closed Friedmann-Robertson-Walker universe filled
with the scalar field with power-law potential. For a wide range of powers and
parameters of the potential we numerically calculated the slow-roll parameters
and scalar spectral index at the epoch when present Hubble scale leaves the
horizon and at the end of inflation. Also we compare results of our numerical
calculations with recent observation data. This allows us to set a constraint
on the power of the potential: alpha < (3.5 - 4.5).Comment: 5 pages, 5 figures; extended versio
Testing dark energy beyond the cosmological constant barrier
Although well motivated from theoretical arguments, the cosmological constant
\emph{barrier}, i.e., the imposition that the equation-of-state parameter of
dark energy () is , seems to introduce
bias in the parameter determination from statistical analyses of observational
data. In this regard, \emph{phantom} dark energy or \emph{superquintessence}
has been proposed in which the usual imposition is relaxed.
Here, we study possible observational limits to the \emph{phantom} behavior of
the dark energy from recent distance estimates of galaxy clusters obtained from
interferometric measurements of the Sunyaev-Zel'dovich effect/X-ray
observations, Type Ia supernova data and CMB measurements. We find that there
is much \emph{observationally} acceptable parameter space beyond the
\emph{barrier}, thus opening the possibility of existence of more exotic forms
of energy in the Universe.Comment: 5 pages, 5 figures, to appear in Phys. Rev.
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