In 1998, an international team of astronomers, known as the High-Z Supernova Search Team, used their survey of type 1a supernovae to determine the distances to several highly-redshifted galaxies. This, combined with the speeds at which these galaxies are receding away from us (determined from the galaxies’ red shifts), provided the best ever picture of how the Universe has evolved since the Big Bang. To their surprise, this new data seemed to show that the rate at which the Universe is expanding is actually increasing over time. Prior to this, it had been assumed that, due to gravity, the expansion of the Universe would be slowing down.

It is still not known what is causing this acceleration in the expansion of the Universe, but in order to account for this, cosmologists have introduced the concept of ‘dark energy’, a mysterious energy, permeating all of space, which is driving the expansion of the Universe at an ever faster rate.

It has been determined from observations of the Cosmic Background Radiation that dark energy accounts for around 68 per cent of the mass-energy of the Universe, based on the standard Big Bang cosmological model.

To read the High-Z Supernova Search Team’s original paper, Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, visit:
arxiv.org/abs/astro-ph/9805201

Vacuum Energy or Inflationary Dark Energy

Einstein’s field equations of General Relativity allow for a ‘cosmological constant’ that can be viewed as the equivalent to the mass-energy of empty space or ‘vacuum energy’. This could have the effect of introducing a repulsive gravitational force over long distances, which could potentially be driving the expansion of the Universe. This leads to the Standard Model of Cosmology, which is the simplest model that agrees well with recent observations.

An alternative suggestion is that the same repulsive force that drove inflation in the early universe is responsible for dark energy.

The Fate of the Universe

If the theory of dark energy, and the prediction that the Universe will expand exponentially with time from now on, is correct, this suggests that everything from galaxies and stars, down to atoms and subatomic particles, and even spacetime itself, will eventually be torn apart by the expansion of the Universe. This possible scenario for the very distant future of the Universe is often known as the ‘Big Rip’.

However, since the nature of the dark energy is currently unknown, it is still possible that the expansion might eventually reverse and cause the Universe to collapse back in on itself in a ‘Big Crunch’. It has been suggested that this might even cause the Universe to ‘rebound’, initiating another Big Bang. It is even possible that this scenario has already occurred and that the Universe has undergone many such rebound events prior to the most recent Big Bang.

A third possibility is that neither dark energy nor gravity will dominate, leading to a universe that neither expands exponentially nor collapses back on itself. This ‘Big Freeze’ scenario will eventually lead to a state where the majority of matter is confined in black holes that will slowly disintegrate via hawking radiation. The Universe will then have reached a state of maximum entropy, also known as the ‘heat death’ of the Universe, where it is no longer possible to exploit temperature differences or other processes to extract energy and perform useful work.

Many alternative theories been proposed, however, such as those leading to a state change in the Universe or the birth of new universes from with our own. However, these theories are considered to be highly speculative and not currently within the realms of scientific testability.