Andrew Zentner (Ohio State University)
"Inflation, cold dark matter, and the central density problem"
ABSTRACT
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.
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 Mpc. Our successful models have values of sigma_8~0.75,
which is within the range of recent
determinations. Finally, models with n>1 (or sigma_8 > 1) are highly
disfavored.
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