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**The beginning and the end!**

About 13,600 million years ago the universe began in a big bang. Space and time were created in an instant and flowed from a point in space of no dimension called a ‘singularity’. The universe was given one mighty kick, and has been coasting outwards ever since. This is all pretty straightforward stuff, and there is lots of evidence for it. What is a bit trickier is finding out how the universe will end.

Up until very recently there were thought to be three possible endings for the universe.

**The open universe**

1. The universe goes on expanding for ever, getting cooler and thinner all the time until it just fades to become a cold, dark and tenuous place where the energy is so dilute that nothing interesting can ever happen. This is known as an open universe.

**The closed universe**

2. The universe contains so much matter that the collective pull of gravity would slow the expansion down until it stopped and then reversed. The universe would then collapse back in on itself and end in a big crunch. This is known as a closed universe.

**The flat universe**

3. The universe contained just the right amount of matter so that neither of the first two possibilities would happen. This meant that the universe would expand until the gravitational pull of all the matter just balanced the initial push. The universe would therefore grow out to a certain size and then stop. It would then be held a state of perfect balance for eternity and is known as a flat universe.

The one that was favoured by most physicists was, of course, the third option, the flat universe. Largely because this was the ‘prettiest’ solution and mathematically very cute!

In the late 1990s two teams of astronomers set out to prove which one of these theories was correct. They worked independently to see if they both came up with the same answer. They used the universe’s best ‘standard candle’, the type 1a supernovae. This is a type of exploding star that always produces exactly the same amount of energy each time one goes bang. This is really useful because if you can measure how bright they appear and you know how bright they actually are, it is easy to work out their distance. They are also incredibly bright: about five billion times brighter than the sun! So you can see one from a very long way away. More than half way across the universe in fact. If we remember that the further away we look, the further back in time we see, we can do some fairly interesting things.

Finding them is not the easiest job in the world as type 1a supernovae are rare. A whole galaxy only produces two or three type 1as every thousand years and they only last a couple of weeks. The last we had one in our galaxy was Tycho’s star, which appeared in 1572. With a detailed sky survey it is possible to locate enough in different galaxies at a variety of distances.

When you do this, it is possible to work out the size of the universe at different times in the past. This is what allows us to check the ultimate fate of the universe. If the universe has grown to double the size in double the time, then it’s expanding at a steady rate and the universe will be open. If it has less than doubled in size then it is slowing down and we have a closed or flat universe, depending on the speed.

**What did they find out?**

Well actually nothing they were expecting. The universe was not slowing down nor was it expanding at a steady rate it was actually accelerating. The expansion was getting faster. No one expected that. What’s more, both teams got the same answer.

**What does it mean?**

For anything to accelerate, an unbalanced force must act on it. This means an unknown force must be acting on everything in the universe. Where does it come from? And remember because:

Work done = force x distance moved by the force

Energy is needed. This is dark energy!

How much energy is involved? About three times as much energy as we can see in the whole rest of the universe!

A tiny touch of the Einstein’s

This is a colossal amount and yet we don’t have the slightest idea of where it comes from.

This is the most famous equation in science. What it says is that matter (mass) and energy are really just two ways of looking at the same thing. If you change the energy of something its mass will change. So boil a kettle of water, that is, increase its heat energy its mass will go up. If you run, you will increase your Kinetic energy, so again your mass will increase. This is why the speed of light is the speed limit for the universe. As any object approaches the speed of light its mass approaches infinity. If you have an infinite mass you need an infinite force to accelerate it. This is clearly impossible. So the speed of light is as fast as anything can go.

So matter and energy are really just two aspects of the same thing and we can think of matter as frozen energy and energy as potential mass. We can say that 1Kg of matter is equivalent to 9 x1016 J of energy. This is how much energy we would need to make 1 Kg of matter or how much energy we could extract from 1Kg of matter if we could get it all out. So we can look at how much dark energy is out there by imaging how much mass it is equivalent to.

It works out as being the mass equivalent of about 10-26 Kg per cubic metre. This doesn’t seem much but there are an awful lot of cubic metres of space out there. This is enough energy to make all the material we know about in the universe about eighteen times over.

**What use might it be? **

Just imagine if we could work out where this energy comes from and could extract and use it to power our civilisation. Free, clean energy forever! At the moment this prospect is probably as far away as nuclear energy would have seemed to the early pioneers of radioactivity. This took about 60 years, so actually not that long and technological progress is much faster now!

So my young Padawan, the future may just lead us all to the dark side!