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Physics LibreTexts

17.15 Dark Matter and Dark Energy

Cosmology is in an extraordinary situation. The big bang model is supported by robust evidence, and cosmological parameters such as the age, shape, size, and expansion rate of the universe have been measured with increasing precision. Yet two of the three major ingredients of the universe are still poorly understood. We know from microwave satellite observations that the universe is made of approximately 5% normal matter, 27% dark matter, and 68% dark energy. These ingredients control the expansion rate and the fate of the universe. Dark matter is like the "brake," acting to slow down the initial expansion, and it dominated the expansion rate for the first 8 billion years. Dark energy is like the "accelerator," acting to speed up the expansion, and as the dark matter has been diluted dark energy has come to dominate the expansion rate for the past 5 billion years, and it will continue to do so in the future.

Dark matter is enigmatic but it is very hard to avoid the conclusion that it exists and outweighs normal particles — protons and neutrons. Invisible material that exerts gravity has been seen in the Milky Way, in every galaxy inspected carefully, in hundreds of clusters of galaxies, and even in the intergalactic voids of space. All dark objects from black holes down to microscopic dust grains have been ruled out, leaving only subatomic particles as an explanation (assuming that we do not want to jettison our very successful theory of gravity). The favored candidate is a kind of weakly interacting massive particle (WIMP) that has not yet been seen in any physics lab or particle accelerator. There are two flavors of WIMP: particles that moved relativistically as galaxies were starting to form (hot dark matter) or particles that were moving more slowly as galaxies were starting to form (cold dark matter). The best hot dark matter candidate is a neutrino, but its mass may not be sufficient to account for dark matter and neutrinos may result in large scale structure that disagrees with galaxy survey. The best cold dark matter candidate is a hypothetical particle called a neutralino. It is predicted from theories that seek to unify fundamental forces by super symmetry, leading to a set of shadow particles of which the neutralino is the most massive, stable one. Physicists are working flat out in a dozen or more experiments around the world to detect WIMPS. If they exist, millions pass through every square centimeter of the Earth's surface (and our bodies!) every second. But by definition they interact very weakly with normal matter so these are difficult experiments. Another route to detecting WIMPs is through collisions in accelerators like the Large Hadron Collider. Physicists are cautiously optimistic that they will succeed in the next 3-5 years.

Dark energy presents an even deeper mystery than dark matter, where at least there are plausible candidate particles that are being sought. The evidence for dark energy hinges on the 1995 discovery that the cosmic expansion is accelerating. This results has been confirmed observationally so it is not in doubt, but the interpretation is heavily debated by both astronomers and physicists. Another indication that something like dark energy is required comes from the microwave background data indicating that space is flat. Since visible and dark matter only account of 32% of the mass-energy density needed to make space flat, the other 68% is attributed to dark energy. In terms of mass-energy equivalence, the density of dark energy is very low, 6.9 × 10-27 kg/m3, but it comes to dominate the universe because it is uniform across space. One interpretation of dark energy is that it's a property of space — this is Einstein's classic cosmological constant, Λ. Because it's a property of space itself, it would not be diluted as space expanded. As more space come into existence, more of the energy-of-space would appear. But no theory in physics predicts why space should have this property, or a value to cause the acceleration we see. A second interpretation is that dark energy is the manifestation of "virtual particles and antiparticles" continually coming into existence and then disappearing. Although this is known to happen, a calculation shows that its effect on space-time is 120 orders of magnitude too large! A third interpretation is a new kind of dynamical energy or field; theorists call this "quintessence" in a nod to ancient Greek philosophers. It's also possible our gravity theory is wrong. Dark energy has no current explanation and understanding it represents a great challenge to both physicists and astronomers.