Indirect Evidence for Dark Matter

I’ve already gone over some of the main pieces of evidence for the existence of dark matter. There are also a lot of more indirect reasons why we think dark matter should exist. I’ve already mentioned these in earlier posts, but here are some of the other measurements and models supporting the dark matter hypothesis.

Big Bang Nucleosynthesis

Using a model of the universe assuming the universe started in a very hot and dense state that expanded adiabatically, we can estimate the relic abundances of the lightest few isotopes, which are mostly produced here rather than in the centers of stars.

The abundances (typically ratios rather than absolute abundances) of objects thought to have compositions close to what’s left after BBN have been measured and have been found to match predictions quite well. There is some disagreement on lithium-7 but the lighter isotopes match predictions from BBN models.

While this doesn’t necessarily tell us that dark matter exists, it does help validate our model of the early universe. It also helps to further challenge models where the “dark matter” is really just some regular matter that we aren’t measuring for some reason.

Structure Formation

Large structures in the universe are believed to have coalesced out of small primordial density fluctuations. Matter is pulled into regions that start with a slight overdensity, leading that overdensity to keep increasing in magnitude. This eventually leads to dense (compared to the average density of the universe) objects like galaxies, clusters, etc.

It turns out that simulations suggest that a universe containing only regular matter will not lead to the structures we see in the universe. The clumps of matter will not form quickly enough. However, if the mass overdensities are made of dark matter, large gravitationally bound objects will form properly. The dark matter provides mass to increase the gravitational forces in these overdense regions but does not clump into compact objects. The compact objects are made of baryonic matter trapped by the dark matter’s gravitational forces.

Cosmic Microwave Background

Skymap of temperature fluctuations in the cosmic microwave background measured by Planck. Credit/copyright: ESA and the Planck Collaboration.

The CMB is the result of the photon-baryon fluid in the early universe (at a redshift around 1100) becoming transparent to photons. When the universe cools sufficiently, the baryonic atomic nuclei start capturing electrons, changing from charged ions to neutral atoms. This means that there are no (or at least few) charged particles left, so photons, which couple to electric charge, have trouble interacting with anything. The photons then are able to continue on without interacting with anything until we measure them billions of years later. The photons follow the standard blackbody spectrum, which is then redshifted until today where the spectrum corresponds to a temperature of 2.7 K (a few degrees above absolute 0) and peaks at microwave wavelengths.

The CMB is not uniform in the sky. It’s temperature is seen to feature tiny fluctuations in space. These can be seen as relics of oscillations in the photon-baryon fluid (to lowest order the different oscillatory modes do not interact with one another). The power spectrum tells us the strengths of these different modes. The properties of the modes (basically the size of the temperature fluctuations on different spatial scales) are related to the properties of the photon baryon fluid. So, we can extrapolate the composition of the early universe from the CMB power spectrum. Fits of a generic early universe model to the measured power spectrum show that a large fraction of the energy content of the early universe (and the current universe) appears to be from non-relativistic, non-baryonic dark matter. Measurements of other things such as Type Ia supernovae (a popular standard candle) help us refine our model, and everything points to the existence of dark matter.