ACCELERATING UNIVERSE AND DARK ENERGY

Like dark matter, cosmic inflation (even if it is not actually proven beyond all doubt) is now usually seen as part of the standard Big Bang theory, and to some extent the two additional concepts rescue the Big Bang theory from being completely untenable. However, other potential problems still remain.

The universe has continued to expand since the Big Bang, albeit at a slower rate since the period of inflation, while at the same time the gravity of all the matter in the universe is working to slow down and eventually reverse the expansion. Two main possibilities therefore present themselves: either the universe contains sufficient matter (known as the "critical mass") for its gravity to reverse the expansion, causing the universe to collapse back to what has become known as the “Big Crunch”, a kind of mirror image of the initial Big Bang; or it contains insufficient matter and it will go on expanding forever.

According to General Relativity, the density parameter, Omega, which is defined as the average density of the universe divided by the critical density (i.e. that required for the universe to have zero curvature) is related to the curvature of space. If Omega equals 1, then the curvature is zero and the universe is flat; if Omega is greater than 1, then there is positive curvature, indicating a closed or spherical universe; if Omega is less than 1, then there is negative curvature, suggesting an open or saddle-shaped universe.

The cosmic inflation model hypothesizes an Omega of exactly 1, so that the universe is in fact balanced on a knife’s edge between the two extreme possibilities. In that case, it will continue expanding, but gradually slowing down all the time, finally running out of steam only in the infinite future. For this to occur, though, the universe must contain exactly the critical mass of matter, which current calculations suggest should be about five atoms per cubic metre (equivalent to about 5 x 10^-30 g/cm³).

(Click for a larger version) Graph of how critical density affect the expansion of the universe (Source: Northern Arizona University Lectures: http://www4.nau.edu/meteorite/ Meteorite/Book-GlossaryD.html)

This perhaps sounds like a tiny amount (indeed it is much closer to a perfect vacuum than has even been achieved by scientists on Earth), but the actual universe is, on average, much emptier still, with around 0.2 atoms per cubic metre, taking into account visible stars and diffuse gas between galaxies. Even including dark matter in the calculations, all the matter in the universe, both visible and dark, only amounts to about a third of the required critical mass, suggesting a continuously expanding universe.

However, in 1998, two separate teams of astronomers observing distant type 1a supernovas (one led by the American Saul Perlmutter and the other by the Australians Nick Suntzeff and Brian Schmidt) made parallel discoveries which threw the scientific community into disarray, and which also has important implications for the expanding universe and its critical mass. The faintness of the supernova explosions seemed to indicate that they were actually further away from the Earth than had been expected, suggesting that the universe’s expansion had actually speeded up (not slowed) since the stars exploded. Contrary to all expectations, therefore, the expansion of the universe actually seems to be speeding up - an accelerating universe!

The only thing that could be accelerating the expansion (i.e. more than countering the braking force of the mutual gravitational pull of the galaxies) is space itself, suggesting that perhaps it is not empty after all but contains some strange “dark energy” or “antigravity” currently unknown to science. Thus, even what appears to be a complete vacuum actually contains energy in some currently unknown way.

(Click for a larger version) Estimated distribution of dark matter and dark energy in the universe (Source: Wikipedia: http://en.wikipedia.org/wiki/Dark_energy)

Like dark matter, we still do not know exactly what this dark energy is, how it is generated or how it operates. It appears to produce some kind of a negative pressure which is distributed relatively homogeneously in space, and thereby exerts a kind of cosmic repulsion on the universe, driving the galaxies ever further apart. As the space between the galaxies inexorably widens, the effects of dark energy appears to increase, although it seems to have little or no influence within the galaxies and clusters of galaxies themselves, where gravity is the dominant force. Interestingly, the effective mass of all this dark energy (calculated according to Einstein’s General Theory of Relativity) comes extremely close to accounting for the missing two-thirds of the critical mass of matter in the universe required for the balancing act predicted by the theory of cosmic inflation.

Although no-one has any idea of what dark energy may actually be, it appears to be unsettlingly similar to the force of cosmic repulsion or “cosmological constant” discarded by Einstein back in 1929 (as mentioned in the section on The Expanding Universe and Hubble’s Law), and this remains the most likely contender, even if its specific properties and effects are still under intense discussion. The size of the cosmological constant needed to describe the accelerating expansion of the universe is very small indeed, of the order of about 1 divided by 10⁶⁰. Indeed, the very closeness of this to zero (without it actually being zero) has worried many scientists.

Other candidates for dark energy are so-called “quintessence” and so-called “phantom energy”, although all of these ideas are essentially still at the hypothesis stage.