## Omega Syndrome

Starring: Doug McClure, spatial curvature

Increasing the density of dark matter is seen to
have rather opposite-looking effects on the CMB (top)
and galaxy (bottom) power spectra.
The scale where the galaxy power spectrum
turns over is the so-called
horizon at matter-radiation equality. In plain English,
this is essentially the speed of light times
the age of the Universe when the dark matter density
equaled that of the CMB photons (long ago).
Increasing the dark matter density pushes this epoch
further back in time and moves the turnover to
the right, to smaller scales. Since the normalization is
kept fixed to the far left, this also slides the curve upwards.
If you're astute, you'll notice that h is changing as well,
as in the last movie, also causing a shift
- the other shift is a stronger effect, though.
The CMB acoustic peaks result from a complicated interplay
between dark matter and baryons - see
Wayne's
CMB tutorial> for details.
All you need to know here is that baryons boost the bumps,
so increasing the dark density density reduces the baryon
fraction and lowers the peaks. If you look carefully at
the lower panel, you'll see that the baryon bumps in
the galaxy power spectrum shrink as well.