A mysterious dark energy fills the universe...

The universe is flat

Cosmologists can measure the shape of our universe by attempting to add up all of the mass and energy which we can observe. Recent measurements indicate the universe is composed of 74% dark energy, 22% dark matter, and a measly 4% ordinary matter.

Dark matter is material which we can't observe through telescopes or any other method, but we can infer it's existence by observing the rotation of galaxies outside our own. Basically some of these galaxies are rotating at speeds far greater than they should be - and the reason is that material which we can't see is present within the body of the galaxy.

As we have read, dark energy provides a neat explanation for the expansion of the universe but we also need it to solve another dilemma. The shape of our universe is explained by the existence of dark energy. So how do you find the shape of our universe?

What cosmologists mean by 'flat' is that if you were to draw an enormous equilateral triangle in space, all of the angles would add up to 180 degrees. If we imagine a space which is curved, such as on a globe, or our earth, then an equilateral triangle drawn along lines of latitude and longitude will add up to more than 180 degrees! For our universe to be flat, space cannot be curved, and so the total amount of energy and mass in the universe must add up to a particular amount. The total amount of mass and energy tied up in ordinary matter and dark matter only adds up to 26% of what we would need to keep our universe the flat shape it is. Somehow, 74% of our universe is in a mysterious form which we can not account for, dark energy.

We can see it's effects - but what IS dark energy?

We can speculate about the properties of dark energy by studying the universe around us. It has to be very evenly spread out throughout the entire universe, unlike dark matter which clumps and forms structures. It doesn't interact with anything we can measure, other then by causing the universe to expand. This makes it difficult to measure it and advance theories or disprove ideas.

The cosmological constant

There are lots of different theories about dark energy. The most popular harks back to an equation Einstein wrote in 1917 to describe the state of the universe. At the time everybody thought the universe was static - it didn't expand, or contract, it just stayed the same, so Einstein included a constant in his equation to balance the effect of gravity. This 'fudge factor' was named the cosmological constant. Soon after this, Hubble discovered that the universe was actually expanding. Einstein immediately abandoned the extra constant reportedly calling it the 'biggest blunder of his life'. What he didn't realise was that the expansion of the universe was accelerating - so an extra term was needed after all. Quantum mechanics theorises that an empty vacuum has a small amount of fundamental energy. Einstein's extra cosmological constant can be thought of as accounting for this extra energy. This energy pushes on the surrounding space-time, causing the universe to expand. The constant Einstein disregarded might hold the key to dark energy.

Quintessence

The other most mainstream theory is called 'quintessence'. The name comes from the ancient Greek for 'fifth element'. The ancient Greeks thought that a pure fifth element called the aether filled the whole universe. Quintessence differs from the cosmological constant because it changes over time and space. A special form known as 'phantom energy' increases infinitely over time. The universe would accelerate at a faster and faster rate until it tears the whole universe apart in a 'Big Rip'. The ultimate doomsday scenario would depend on the exact nature of quintessence.

Other theories

String theory is also used to explain dark energy, but has not managed to nudge the other two theories off the top spot. It is not as popular amongst the scientific community. Some scientists dismiss the whole concept of an accelerating universe as merely a failure of our current theories. Some argue that the laws of physics which apply to our small part of the universe might need amending when applied to the universe as a whole. Is it high time for some new physical information about dark energy to boost our theoretical ideas.

Future space missions

To determine which of our theories are correct we need to find out more about the properties of dark energy. Nasa are holding a competition for possible space missions to explore dark energy. At the moment there are three possible contenders which all chart the rate of acceleration of the universe using standard candles. Each spacecraft also offers it's own unique instruments which measure redshifts of light from distant stars.

The new observations from these missions will help identify whether we are dealing with a cosmological constant, quintessence or something even weirder. These missions could bring back information which shakes up modern physics and puts us back on track to formulating a fundamental theory of the universe. Embarrassingly, until then we won't know what 74% of our universe is. We live in the dark ages.

For more information

International Journal of High Energy Physics - CERN Courier
http://cerncourier.com

NASA - The Nature of the Universe - Great Debate 1998
http://antwrp.gsfc.nasa.gov/