The Big Bang model is typically broken down into a few key eras and events. Standard cosmology, the set of ideas that are most reliable in helping decipher the universe’s history, applies from the present time back to about a hundredth of second after the Big Bang. But before then, particle physics and quantum cosmology ruled the universe.
When the Big Bang occurred, matter, energy, space, and time were all formed, and the universe was infinitely dense and incredibly hot. The often-asked question “What came before the Big Bang?” is outside the realm of science because it can’t be answered by scientific means. In fact, science says little about the way the universe behaved until some 10–43 second after the Big Bang, when the Grand Unification Epoch began (and lasted only until about 10–35 second). Matter and energy were interchangeable and in equilibrium during this period, and the weak and strong nuclear forces and electromagnetism were all equivalent.
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As time went on and particles’ rest-mass energy was greater than the thermal energy of the universe, many were annihilated with their partners, producing gamma rays in the process. As more time crept by, these annihilations left an excess of ordinary matter over antimatter.
The relic radiation of the Big Bang decoupled (picture heavy traffic suddenly clearing) nearly 400,000 years later, creating the resonant echo of radiation observed by Penzias and Wilson with their radio telescope. This decoupling moment witnessed the universe changing from opaque to transparent. Matter and radiation were finally separate.
Observational astronomers consider much of the history of the early universe the province of particle physicists, describing what happened up to the formation of galaxies, stars, and black holes as “a lot of messy physics.” They are more interested in how the first astronomical objects, the large-scale inhabitants of the universe, came to be about 1 billion years after the Big Bang. But before these astronomers can gain a clear picture of that process, they need to consider the role of the wild card — dark matter.