# HEP/Astro Journal Club - Friday, 10 May 2002

## Is there a cosmological small-scale structure problem?

## Abstract

In the standard, LCDM model of cosmology, the Universe is dominated by
collisionless, cold dark matter (CDM) and a cosmological constant
($\Lambda$). According to this model, structure formed hierarchically
via the growth of Gaussian density
fluctuations generated during an inflationary epoch in the very early
Universe. The standard assumption is that the primordial power spectrum
of density fluctuations took on the scale-invariant, Harrison-Zel'dovich
form (\ie, $P(k) \propto k^n$ with $n=1.0$). While this standard
model can account for an impressive range of large-scale observations,
it has difficulties explaining observations on galactic and sub-galactic
scales.

I will begin with an overview of the standard model of
structure formation and a brief discussion of a semi-analytic model
that can be used to predict the central densities of CDM halos. I will
then describe the {\em central density problem}: LCDM with $n=1.0$ predicts
dark halos that are overdense by a factor of $\sim 8$ compared to the
densities inferred from observed galaxy rotation curves. I will propose
two possible resolutions to the central density problem that do not require
a modification of the CDM properties. First, generic models
of inflation do not predict primordial power spectra that are {\em exactly}
scale-invariant. I will show that by accounting for the small, but important,
deviations from $n=1.0$ and the scale-dependence of the effective spectral
index
predicted by broad classes of inflationary models, the central density problem
can be alleviated or even eliminated. Second, massive neutrinos tend to
damp power on small scales due to ``free-streaming.'' I will show that
a neutrino mass of $m_{\nu} \sim 0.5$ eV can also eliminate the central
density problem. My conclusion is that galaxy rotation curves may be
teaching us about inflation and/or neutrinos as well as the properties of
CDM.

## 12:30, Smith Lab 4079

Matthew Wingate
(
wingate.9@osu.edu
),
last updated 30 April 2002.