Section 3: Dark Matter in the Early Universe
By the end of the 1970s, two compelling lines of evidence for dark matter had appeared. The motion of galaxies within clusters and the motion of gas clouds around individual galaxies strongly suggested that either our understanding of gravity is fundamentally wrong, or that there is far more matter in the galaxies and clusters than meets the eye. Further, simulations of galaxy formation showed that the spiral and elliptical galaxies we observe in the night sky cannot form without large amounts of dark matter in addition to the luminous stars. A third line of evidence developed in the 1990s, as radio telescopes above the atmosphere mapped the cosmic microwave background (CMB).
Figure 7: The history of the universe according to our standard model of cosmology.
Source: © NASA, WMAP Science Team. More info
This new evidence for dark matter has its origin in the early universe. About one second after the Big Bang, astrophysicists believe, a very dense mixture of protons, neutrons, photons, electrons, and other subatomic particles filled the universe. The temperature was so high that the electrons could not bind with the protons to form atoms. Instead, all the particles scattered off of each other at high rates, keeping all the different species at the same temperature—that is, in thermal equilibrium—with each other. The photons also scattered off of the electrically charged protons and electrons so much that they could not travel very far.
As the universe expanded, the temperature dropped to about one billion degrees Kelvin (K). At that point, the protons and neutrons began to bind together to form atomic nuclei. At roughly 390,000 years after the Big Bang, continued expansion and cooling had dropped the temperature of the universe to about 3000 K. By that point, all the electrons and protons had bound to form electrically neutral hydrogen atoms, and all the other charged particles had decayed. After the primordial hydrogen formed, the universe became so transparent to photons that they have been traveling throughout it for the entire 13.7 billion years since then. These relic photons from the early universe have a microwave wavelength, and are known as the cosmic microwave background, or CMB.
Density fluctuations and dark matter
Before the neutral hydrogen formed, the matter was distributed almost uniformly in space—although small variations occurred in the density of both normal and dark matter owing to quantum mechanical fluctuations. Gravity pulled the normal and dark matter in toward the center of each fluctuation. While the dark matter continued to move inward, the normal matter fell in only until the pressure of photons pushed it back, causing it to flow outward until the gravitational pressure overcame the photon pressure and the matter began to fall in once more. Each fluctuation "rang" in this way with a frequency that depended on its size. The yo-yoing influenced the temperature of the normal matter. It heated up when it fell in and cooled off when it flowed out. The dark matter, which does not interact with photons, remained unaffected by this ringing effect.
When the neutral hydrogen formed, areas into which the matter had fallen were hotter than the surroundings. Areas from which matter had streamed out, by contrast, were cooler. The temperature of the matter in different regions of the sky—and the photons in thermal equilibrium with it—reflected the distribution of dark matter in the initial density fluctuations and the ringing normal matter. This pattern of temperature variations was frozen into the cosmic microwave background when the electrons and protons formed neutral hydrogen. So a map of the temperature variations in the CMB traces out the location and amount of different types of matter 390,000 years after the Big Bang.
Figure 8: Map of the temperature variations in the cosmic microwave background measured by the WMAP satellite.
Source: © NASA, WMAP Science Team. More info
American physicists Ralph Alpher, Robert Herman, and George Gamow predicted the existence of the CMB in 1948. Seventeen years later, Bell Labs scientists Arno Penzias and Robert Wilson detected them. Initial measurements showed the intensity of the relic photons to be constant across the sky to a fraction of 1 percent. In the early 1990s, however, NASA's Cosmic Background Explorer (COBE) spacecraft used a pair of radio telescopes to measure differences among relic photons to one part per million between two points in the sky. A subsequent spacecraft, the Wilkinson Microwave Anisotropy Probe (WMAP), made an even more precise map. This revealed hot and cold spots about 1.8 degrees in size across the sky that vary in intensity by a few parts per million.
The angular size and the extent of variation indicate that the universe contained about five times as much dark matter as normal matter when the neutral hydrogen formed. Combined with measurements of supernovae and the clustering of galaxies, this indicates that dark energy comprises 73 percent of the universe, dark matter 23 percent, and normal matter just 4 percent.