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# 6 Conclusion

The focus of this course has been on what we do, and don’t, understand about how the Universe works. These were some of the key learning points:

- Although dark matter, dark energy and inflation require new physics that is not yet fully understood, they each remain the best explanation for a range of observational data at the present time.
- The most popular particle dark matter candidates are weakly interacting massive particles (WIMPs) and axions, both of which are preferred because they naturally solve other problems in particle physics. Primordial black holes of very low mass may also be a possibility.
- Direct detection experiments have ruled out some mass ranges for both types of particle, but continue to explore parameter space. In the case of WIMPs, experiments involve underground detectors and large volumes of noble gases, while it is hoped axions may be detected via their interaction with magnetic fields.
- The observed acceleration of the Universe may be explained by a cosmological constant , evolving dark energy (quintessence), or modified theories of gravity.
- Observations aim to measure the equation of state parameter for dark energy, , and its evolution. To date, observational constraints are consistent with a cosmological constant and do not require quintessence models.
- Inflation can explain the
  - horizon problem, by enabling now-widely separated regions to have been previously in causal contact
  - flatness problem, by enabling the total density parameter to evolve to be extremely close to 1 at early times
  - monopole problem, by diluting the density of any magnetic monopoles so that they would now be extremely rare.
- Inflationary models are in good agreement with the observed properties of the CMB, but more precise experiments are needed to test inflation models further.

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