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Dark energy rules the universe

Darth Vader got it wrong -- it's not 'The Force' that rules the universe; it's dark energy. And it's pushing us towards either a very lonely or violent end, says Dr Karl.

Darth Vader

Don't underestimate the power of the dark side ... (Source: Dylan Martinez/Reuters)

Last time, I talked about dark energy and how it makes up the missing 69 per cent of the universe. I also talked about how it's been increasing the rate of expansion of the universe.

Why did the effect of dark energy show itself only five billion years ago? Why not earlier, or later?

Well, it was because there has always been a battle between gravity, which is an 'attractive' force and dark energy, which is a 'repulsive' force. About five billion years ago was when dark energy finally got bigger than gravity.

So first, gravity – gravity is caused by the regular matter and the dark matter that's present in the universe. Gravity has always been trying to slow down the expansion of the universe, but the effect of gravity has been decreasing as the matter in the universe has thinned out.

Okay, second – dark energy. Dark energy seems to be an inherent property of space-time. But not everybody agrees – it's in no way fully proven yet. There seems to be a certain amount of dark energy in each cubic metre of the universe and so, if dark energy is an inherent property of space-time, well then, as the universe expands, it 'creates' more space-time and therefore, it also creates more dark energy, according to some theoretical physicists.

It turns out that about five billion years ago was when the repulsive force of dark energy began to outweigh the attractive force of gravity.

However, the amount of this dark energy, in any given cubic metre of the universe, seems to be always the same and seems to be very, very small. As a comparison, it's roughly a millionth of a millionth of a millionth of a millionth of a millionth of the matter in our planet's crust. But space is very, very big and there are relatively few planets and stars, so, over the whole universe, dark energy adds up to be about 69 per cent of everything.

One big question is: "What is the Universe expanding into?" and, of course, one slightly facetious answer is: "The future".

But another big question is: "What happens in the future?"

Well, in the short term of the next billion years or so, the Sun will heat up so much that life on Earth will be hard-pressed to survive. In five billion years, our Sun will expand to swallow Mercury, probably Venus and possibly the Earth.

But in the long term, dark energy will dominate.

Now, it all depends on what dark energy turns out to be. Now, if the amount of dark energy in each cubic metre is constant, then the universe will thin out and space will be cold and empty. The rest of the universe will disappear from view. This is called the 'Big Chill'.

In 100 billion years, the clusters of galaxies that fill the sky would stretch to the breaking point, leaving us with isolated clumps of galaxies, separated from each other by huge emptiness. The relative speeds of separating galaxies will be faster than the speed of light, so we won't be able to see them. Our home galaxy could be the only galaxy visible in an otherwise empty universe.

Inside our galaxy, the frugal, low-mass stars will burn their fuel slowly, but only for 10 to 20 trillion years. New stars will occasionally form, but most of these will burn out within 100 trillion years. In a cosmic sense, it will truly be the Big Chill.

But let's make a different assumption. Let's assume that dark energy is something different and that the amount of dark energy in each cubic metre increases with time. In that case, we are heading for a different scenario – the 'Big Rip'.

Under the so-called Big Rip scenario, everything gets torn apart by the increasing dark energy, about 50 to 100 billion years in the future. This will be the end of everything.

About 200 million years before the end, the stars in our galaxy get ripped apart from each other, and fly off into empty space. One year before the end, the planets in our solar system get flung into empty space. One hour before the end, the atoms that make up our planet explode away from each other. One tenth of a billionth of a billionth of a second before the end, the electrons get ripped away from the central nucleus in their atom. And at one ten thousandth of a billionth of a billionth of a second before the end, the actual protons and neutrons in the nucleus of atoms get torn apart from each other.

Darth Vader in the Star Wars saga got it wrong. It's not 'The Force' that controls the universe – it's dark energy.

Tags: the-universe, physics

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Published 20 August 2013

© 2014 Karl S. Kruszelnicki Pty Ltd