Dark Matter and how WIMPs may save the universe

“Albert, you said earlier that the Belgian priest came up with the Big Bang idea by thinking backwards.”

Father Lemaître. That’s right.

“Well if I start thinking forwards about an expanding universe all I can imagine is a universe that gets bigger and bigger until all the stars would be so far apart you couldn’t see any.”

Excellent. That brain of yours is now starting to imagine things. Now what you say might happen but it isn’t the only way the universe could end. It might just keep on expanding, getting thinner and thinner, older and older. But then there is my great friend gravity. If there is enough stuff in the universe to generate enough gravitational pull it will start to slow down and then start contracting. Eventually everything will end up in same place. The Big Crunch.

"Then what happens?"

Well it might start all over again with Big Bang II: The Sequel. Perhaps it already has. There is one thing that science can’t fathom - what happened before our Big Bang. Gravity is trying to pull everything in the universe back into the centre. The more mass, the more gravity and the more likely it is that gravity will be able to put the brakes on the present expansion and start pulling things back together again. Looking around the universe there isn't enough matter to overcome the present expansion. But there is a lot more to the universe than meets the eye. The fate of the universe rests on how much dark matter there is.

"Dark matter? That sounds like something from Star Wars."

Dark matter is very real and very important. It's all the bits of the universe that can't be seen. Left over hydrogen, dust, interstellar debris, pollution from advanced but ecologically unsound civilisations - that sort of thing. What sounds even more like a Star Wars invention is the idea of the 'Dark Energy.' In the last few years scientists have started to see signs that the expansion of the universe is getting faster, as if there is something pushing galaxies away.

"Something like the opposite of gravity."

That sort of thing. But no-one knows what sort of mysterious force could do this so they've called it 'Dark Energy'. Remember I put something extra into my theory to stop the Universe expanding when I didn’t want to believe it could, the Cosomological Constant.

“Your biggest mistake?”

That one. Well it turns out that today that same extra part of the equations is exactly how this Dark Energy could work.

“So you were right?”

Only in part and for all the wrong reasons. But whatever this dark force is, if it exists, the only thing that will stop this force is gravity. So it comes back to seeing how much dark matter there is. If there is enough dark matter then one day the universe will stop expanding. If not the universe will just keep going and eventually fade away.

"So the fate of the universe depends on how much dust there is lying around?"

Well, basically yes. The problem is that as it’s dark we don’t how much is out there because you can’t see it with a normal telescope.

“So we’ll never know until we start travelling around the galaxy like in Star Trek?”

No, astronomers have already found a way to find invisible things but strangely it wasn’t invented by astronomers but by an American telephone company, Bell Labs, in 1931. Karl Jansky was looking for all the different types of interference or noise that could affect radio telephone signals and he noticed a certain type of interference that appeared at round about the same time each day. This interference was found to come from certain positions in the sky, from outer space. This was the birth of radio astronomy, mapping and understanding radio signals that come from stars and galaxies.

"So are these like alien radio stations?"

No, radio waves can be created deliberately to send music and news, but the radio waves picked up radio telescopes from space seem to be naturally occurring coming from stars, galaxies and other strange objects that live out there in space. And guess what you can detect with a radio telescope?

"Stars?"

Well some stars but the most important things are the things that you can't see with a regular telescope like dust and interstellar gas.

"So astronomers finally have a way of looking for the missing dark matter."

Exactly. Unfortunately, despite all this, they still don't think they have found all of it. Another thing that might make up dark matter are WIMPs that some scientists think will determine the fate of the universe.

“The fate of the universe lies in the hands of wimps? Albert you really have lost it now.”

WIMPs are weakly interacting massive particles. They are called sub-atomic particles because they are smaller than atoms but they don’t really interact with normal matter. A WIMP could fly straight through the whole earth, which makes them even harder to detect than dust. Wimps may make up 80% of the dark matter but no-one is quite sure.

"So most of this dark matter is still missing?"

Either they can't find it or it isn't there.

"Are there clouds of it lying around in this galaxy?"

Oh, yes lots of it. There are also things called MACHO’s which stands for Massive Compact Halo Objects, these are large lumps of dark matter like old dead stars or dense clouds.

"So which are more important for the universe wimps or machos?"

Oh, the wimps I'd say. They could be responsible for the most of the dark matter in the universe.

"And we couldn't tell if we were about to fly into a cloud of this dark matter?"

No. That's why it is called dark matter.

"And we could be absorbed, just like that, before we had reached anywhere?"

True, but we have another two thousand years of this journey to go, so I think we can reasonably assume we are safe for a few years. Anyway I never think of the future. I always find it comes soon enough. But don’t forget this is a thought experiment. So don’t imagine it happening or it just might.