Dark energy is the name given to the mysterious force that is accelerating the rate at which our universe is expanding. Astrophysicists are busily engaged in investigating the nature of dark energy. These are difficult studies, but they are worthwhile, because dark energy will determine the ultimate fate of our universe. Our current state of knowledge of dark energy is summarised by Adam Riess and Mario Livio in Scientific American (March 2016).
We have known that our universe is expanding – that is, galaxies and clusters of galaxies are moving away from each other – since the observations of Edwin Hubble in the 1920s. It was thought that the rate of expansion would slow down over time as gravity gradually exerted a braking effect. However, about 20 years ago it was surprisingly discovered that not only is expansion not slowing down, it is actually speeding up. Some repulsive force is pulling the universe apart and this force was dubbed "dark energy".
Riess and Livio describe the three main hypotheses to explain the nature of dark energy. The leading hypothesis is that dark energy is a property of empty space itself and this hypothesis fits the astrophysical evidence better than the other two. A second hypothesis is that the universe is permeated by an energy field called quintessence that counteracts the attractive pull of gravity. The third hypothesis is that there is no such thing as dark energy and that the accelerating expansion of the universe is caused by currently unknown properties of gravity that only manifest themselves over vast scales such as over the entire observable universe.
We think of the space between stars and galaxies as “empty space”, but quantum physics tells us that there is no such thing as empty space. The vacuum is actually a seething sea of virtual particle- antiparticle pairs that suddenly pop into existence and instantly disappear again by annihilating each other. Particles carry energy – and energy, just like mass, can produce gravity. However, unlike mass, energy can produce an attractive or repulsive gravity depending on whether the energy pressure is positive or negative. The vacuum energy in theory has negative energy pressure, and this may be the repulsive force that drives the accelerating expansion of the universe. This idea is equivalent to the cosmological constant used by Albert Einstein in his equations of general relativity and represents a constant energy density throughout space. If, on the other hand, dark energy is a field, it would not be a constant and might change, affecting the universe differently at different times.
Dark energy is now the densest constituent in space but this was not always the case. When the universe was small and young, radiation and matter were dominant, packed tightly in space. As the universe expanded, matter and radiation became diluted and dark energy overpowered them. The three different hypotheses predict different futures for our universe. If dark energy is the energy of empty space, the accelerating expansion of the universe will continue forever and about one trillion years from now all the galaxies more distant than our closest neighbours will be separating faster than the speed of light, making them undetectable.
If dark energy is the energy of a field, the future of the universe depends on how the field evolves over time. If field strength remains the same, the universe will expand forever. If the field strength gets weaker, the universe may stop expanding and contract, ending in a final "big crunch" that mimics the original Big Bang. And, if the field strength gets much stronger, it could rip apart all complex structures in the universe. If the alternative-theory-of-gravity hypothesis is correct, various outcomes are possible depending on the detail of the revised theory.
The universe is composed of familiar ordinary matter, dark matter and dark energy. Dark energy constitutes 73 per cent of the fabric of the universe, dark matter 23 per cent and ordinary matter – the only constituent we understand – accounts for only 4 per cent. We are much closer to the start than to the end of discovering the full nature of reality. So, jobs for physicists won’t run out any time soon.
William Reville is an emeritus professor of biochemistry at UCC http://understandingscience.ucc.ie