The previous 12 posts listed the main discoveries of modern cosmology in chronological order: putting all this information together leads to the Standard Model of cosmology (not to be confused with the Standard Model of particle physics). We conclude our short course with a simple overview of the standard model (also known as the Concordance Model). You will notice that it is also a brief history of time. Keep in mind that what follows is a model: the strength of the evidence for each phenomenon varies (see specific posts on each topic starting here).
1. The Big Bang
- The universe began approximately 13.7 billion years ago when it began expanding from an almost inconceivably hot, dense state. Ever since, the cosmos has been expanding and cooling, eventually reaching the cold, sparse state we see today.
- In the first 10-34 seconds, the universe experiences a brief period of extremely fast expansion known as inflation. This period smooths out initial inhomogeneities, leaving the universe with the homogeneity and isotropy we see today. Quantum mechanical fluctuations during this process are imprinted on the universe as density fluctuations that later seed the formation of structure.
- The infant universe is a soup of matter and energy in which particle/antiparticle pairs are constantly born and annihilated. As the universe cools, it becomes too cold to produce heavier particles, while the creation of lighter particles continues until temperatures cool to a few billion Kelvin. At this point, most of the remaining particle/antiparticle pairs are annihilated. A small amount of matter survives due to a slight asymmetry in the decay of between matter and antimatter.
- After a few minutes, nuclei of the light elements (hydrogen, helium and lithium) are formed by the combination of free protons and neutrons, a process known as nucleosynthesis.
- After about 100,000 years, the universe is cold enough for free nuclei and electrons to to combine into atoms (recombination). At this point, the universe becomes transparent due to reduced scattering by free electrons. Radiation now permeates the universe – seen today as the cosmic microwave background. By this time, dark matter (unaffected by the behavior of the baryonic matter) has already begun to collapse into halos.
- After a few hundred million years, galaxies and stars form, as baryonic gas and dust collapse to the center of the pre-existing dark matter halos.
A brief history of time
2. The Composition of the Universe
- Baryonic Matter: ~3% of the mass in the universe
This is ordinary matter composed of protons, neutrons, and electrons. It comprises gas, dust, stars, planets, people, etc.
- Cold Dark Matter: ~23%
This is the “missing mass” of the universe. It comprises the dark matter halos that surround galaxies and galaxy clusters, and aids in the formation of structure in the universe. Dark matter is believed to be composed of weakly interacting massive particles or WIMPs.
- Dark Energy: ~73%
- Observations of distant supernovae suggest that the expansion of the universe is currently accelerating. This observation is backed up by the flatness of the universe as measured from the cosmic microwave background. Cosmologists believe that the acceleration may be caused by some kind of energy of the vacuum, possibly left over from inflation.
Matter/energy composition of the universe
That concludes our short course in cosmology. You can find details on any of the topics above by scrolling through the last 12 posts of this blog. Alternately, you can find slides from a lecture I gave on the subject (The Big Bang – Theory or Established Fact?) by clicking here
There is a really nice one-page web summary of all of the above on the Talkorigins Archive here, and for readers requiring a slightly more advanced treatment, there is a good review of the current state of play in cosmology on the ARXIV here