Section 5: From Astronomy to Particle Physics
The abundance of astronomical evidence for dark matter in the early 1970s intrigued physicists working in other fields. Cosmologists and nuclear physicists were developing our current model of cosmology, trying to understand how the universe we live in—dark matter and all—formed. Concurrently, others wondered how the dark matter fit, if at all, into the Standard Model we learned about in Unit 1.
By the late 1970s, the Standard Model of particle interactions had gained a firm experimental footing. At the same time, physicists were refining their standard model of cosmology in which the universe began its existence when a singularity, a point of infinite density and infinite temperature, exploded in the Big Bang and began a process of expansion that continues today. Application of the Standard Model and nuclear theory to the Big Bang model allowed physicists to quantify nucleosynthesis, the process responsible for creating elements out of the protons, neutrons, electrons, and energy that suffused the infant universe.
Figure 12: This series of reactions created the lightest elements in the infant universe.
Source: © Wikimedia Commons, Public Domain, Author: Lokal_Profil, 5 August 2008. More info
This model of Big Bang nucleosynthesis, supported by careful astronomical observations of the abundance of light elements in the universe, makes a particularly significant prediction about the density of baryons in the first few minutes: The Big Bang could not have created enough normal matter at the start of the universe to account for dark matter. Astrophysicists concluded that dark matter must be some new form of matter not yet observed, possibly even a new type of particle.
New dark matter particles
One of the first attempts to explain dark matter with new particles arose in a surprising place: the Homestake Gold Mine in South Dakota that we first encountered in Unit 1. The Homestake neutrino detector was monitoring neutrinos thought to come from the Sun. In 1976, it became apparent that this experiment only counted about half the predicted number. One explanation was that some new form of heavy particles that did not interact much would collect in the center of the Sun, cooling it off very slightly. This new heavy particle would have the same properties required by dark matter: very weak interaction with other particles, copious in our solar system, and left over from the Big Bang.
We now know that the deficit of neutrinos is due to their oscillation; but at the time, it was an intriguing hint that dark matter could be made up of a new type of particle, possibly not included in the Standard Model. Heavy neutrinos were once considered a candidate for particle dark matter, but large-scale structure simulations of neutrino dark matter have ruled them out. The remainder of this unit will focus on particle dark matter in both theory and experiment. In section 8, we will explore the two leading non-Standard Model candidates for particle dark matter and experimental efforts to detect them. We also will examine how the constant theoretical effort to explain dark matter often generates new possibilities for particle dark matter. The table below summarizes all the possibilities for dark matter that appear in this unit.
|Astronomical objects (failed stars, black holes, MACHOs...)||1050-1063 eV||Rather conservative scenario; lensing searches effective||Amount of ordinary matter made in Big Bang falls short of total dark matter we need; not detected via lensing searches|
|Neutrinos||< 2 eV||Known to exist, and have mass so a natural candidate||Tiny neutrino mass inhibits clumping on small scales needed to hold galaxies together|
|Axions||10-6 eV||Postulated to solve a different problem altogether; dark matter aspect comes for free||Tough to detect|
|Weakly Interacting Massive Particles (WIMPs)||1010 eV||Plausible class of new elementary particles that emerge from multiple theories beyond the Standard Model||Have evaded detection in accelerators to date|
|Alternative Gravity Scenarios||N/A||No mysterious new matter needed, but rather a modification of gravity||Hard to reconcile with Bullet Cluster observations; theories seen as "inelegant"|
|Dark Sector Interactions||N/A||Two new pieces of physics: exotic dark matter particles plus new interactions between them; might help reconcile experiments||Added complexity; wider range of phenomenology; tougher to rule out|