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# 4 The nature of dark energy

The origin of the acceleration of the expansion of the Universe is one of the biggest questions in modern astrophysics, and it underpins many ongoing theoretical and observational research programmes. To explore these ideas further, watch Video 1 in which a cosmologist is interviewed about the nature of dark energy and the prospects of its detection, and then answer the questions that follow.

Video 1 Dark energy and the ‘big rip’ (34 minutes)

## Exercise 4

Based on Video 1, answer the following questions:

1. In what way does dark energy behave like ‘anti-gravity’?
2. What is Professor Copeland’s reason for rejecting an explanation for the accelerated expansion in which the cosmological principle is incorrect (i.e. the Universe is not homogeneous)?
3. What is a ‘scalar field’?
4. What is special about ‘chameleon fields’?
5. Will the Universe end in a ‘big rip’?

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### Discussion

1. Ordinary matter and radiation are both sources of gravitational attraction, and in the context of the Universe’s expansion they act to slow it down. Dark energy has a negative pressure, and so its gravitational effect is to push spacetime apart, rather than pull it together, and so it effectively acts in the opposite direction to gravity.
2. At around 09:50 in the video, Professor Copeland argues that the distribution of structure in the CMB (i.e. the uniformity and lack of large scale inhomogeneity) strongly supports the cosmological principle. The CMB structure suggests that the probability of us being located in a region with atypical acceleration (e.g. caused by local effects) must be very small.
3. A scalar field describes a function that takes a single (scalar) value at every point in space. The field has a potential energy and kinetic energy associated with each location, and these may evolve with time.
4. Chameleon fields are one possible model for dark energy, in which the energy associated with the field depends on environmental conditions. This enables it to behave differently on different size scales, which makes it a potentially useful dark energy candidate.
5. Continuing acceleration of the Universe’s expansion would be expected to lead to a big rip. However, Professor Copeland argues that plausible theories for dark energy are likely to involve decay of the energy in the scalar field at some point in the future, which could result in a return to a matter-dominated Universe with different long-term evolution.

Video 1 discussed three main explanations for the acceleration of the Universe. Further information about each of these explanations is provided in the following sections.

◄Contents►Next: 4.1 A cosmological constant

OpenLearn - Unsolved problems in cosmology
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