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.