The cosmic abundances tell a consistent story in which the preponderance of the mass in the universe consists of an unknown dark matter component. The cosmic microwave background provides the most powerful measurements of the cosmological parameters; primordial nucleosynthesis restricts the abundance of baryonic matter; Type IA supernovae provided the first evidence for the acceleration of the universe, possibly explained by dark energy as the major constituent of the cosmic energy density.
3.1. The Cosmic Microwave Background
Further evidence for dark matter comes from measurements on cosmological scales of anisotropies in the cosmic microwave background. [1, 2] The CMB is the remnant radiation from the hot early days of the universe. The photons underwent oscillations that froze in just before decoupling from the baryonic matter at a redshift of 1100. The angular scale and height of the peaks (and troughs) of these oscillations are powerful probes of cosmological parameters, including the total energy density, the baryonic fraction, and the dark matter component, as shown in Fig. 5. The sound horizon at last scattering provides a ruler stick for the geometry of the universe: if the light travels in a straight line (as would be the case for a flat geometry), then the angular scale of the first Doppler peak was expected to be found at 1 degree; indeed this is found to be correct. Thus the geometry is flat, corresponding to an energy density of the universe of ∼ 10−29 gm / cm3. The height of the second peak implies that 5% of the total is ordinary atoms, while matching all the peaks implies that 26% of the total is dark matter. Indeed the CMB by itself provides irrefutable evidence for dark matter.
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Figure 5. Planck's power spectrum of temperature fluctuations in the cosmic microwave background. The fluctuations are shown at different angular scales on the sky. Red dots with error bars are the Planck data. The green curve represents the standard model of cosmology, ΛCDM. The peak at 1 degree is consistent with a flat geometry of the universe, the height of the second peak with 5%, and the second and third peaks with 26% dark matter. |
3.2. Primordial nucleosynthesis
When the universe was a few hundred seconds old, at a temperature of ten billion degrees, deuterium became stable: p + n → D + γ. Once deuterium forms, helium and lithium form as well. The formation of heavier elements such as C, N, and O must wait a billion years until stars form, with densities high enough for triple interactions of three helium atoms into a single carbon atom. The predictions from the Big Bang are 25% Helium-4, 10−5 deuterium, and 10−10 Li-7 abundance by mass. These predictions exactly match the data as long as atoms are only 5% of the total constituents of the universe.
The first evidence for the ∼70% dark energy in the universe came from observations of distant supernovae (Perlmutter et al., [10] Riess et al., [11] Riess et al. [12]). The supernovae are dimmer than expected, as is most easily explained by an accelerating universe. There are two different theoretical approaches currently pursued to explain the dark energy: (i) a vacuum energy such as a cosmological constant or time-dependent vacuum [13] may be responsible, or (ii) it is possible that General Relativity is incomplete and that Einstein's equations need to be modified. [14, 15, 17, 16] Note, however, that this dark energy does not resolve or contribute to the question of dark matter in galaxies, which remains as puzzling (if not more) than twenty years ago. We now have a concordance model of the universe, in which roughly a quarter of its content consists of dark matter.