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# 5.4 Inflationary theories and how to study them

To explore the idea of inflation further, watch Video 2 in which a cosmologist is interviewed about the theory of inflation and how it might be tested. Then answer the questions in the exercise that follows.

Video 2 Inflation and the Universe in a grapefruit (24 minutes)

## Exercise 5

Based on Video 2, answer the following questions:

1. Why is reheating needed after inflation, and what happens as a result?
2. Does inflation happen before or after the big bang?
3. What alternatives are there to inflation?
4. What is the best current evidence for inflation?
5. Are dark energy and inflation related, and if so, how?

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### Discussion

1. Inflation involves a very large increase in volume (and therefore decrease in density) in a short time. Particles present prior to this stage will now be very diluted, not consistent with the hot big bang model. Instead, it is the energy of the inflation field that is thought to be converted into ordinary particles (the cosmic soup of radiation, quarks and leptons) in a stage known as reheating, which begins the evolution of ordinary matter.
2. Professor Copeland argues that it is more logical to consider the big bang as the point at which inflation ends (which is the point towards which the current expansion of the Universe can be extrapolated back). However, he then concedes that some form of starting point, and release of energy, must have been the trigger for inflation. Hence, views differ on whether to label the start or end of inflation as the big bang.
3. A possible alternative (discussed from ~17:40 in the video) involves cyclic Universes, which occur in string-theory models. These expand and contract, so that our assumed big bang was not the ultimate starting point for the Universe.
4. The best tests of inflationary theory come from precision measurements of the angular power spectrum of the CMB, which are generally in extremely good agreement with inflation predictions. However, there are some subtle effects that can be explored further to learn more, or to potentially refute inflation theories.
5. The favoured models for both dark energy and inflation involve scalar fields. In many theories the two scalar fields are not related – the energies involved are very different. However, one theory mentioned in the video is quintessential inflation, in which the same field that drives inflation later evolves to be the origin of dark energy.

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