The Universe’s Darkest Hour

"These stars we're travelling past, were they all made in the Big Bang?"

Not at all, the stars you can see now were born well after that. Even though all the matter in the universe was made in the Big Bang, it's been rearranged a few times since them. New stars are being created all the time in space from clouds of gas and space dust. Some of this dust has been here since the Big Bang and the rest is the leftovers from stars that have long since exploded.

"How can a star form out of just gas and dust in the middle of space?"

It all comes back to gravity. After the Big Bang, all the matter started to spread out in all directions. If all the matter spread too evenly then the universe would have just got thinner and thinner and fizzled into nothing; no stars, no galaxies. On Earth they talk about the dark ages lasting from the end of the Roman empire to the time when Europe became civilised again. That was only about 500 years. After the Big Bang, the Universe was dark for 150 million years. Not a single star and no light. Total darkness.

“No stars for 150 million years? Are you serious?”

Absolutely. But remember those microwave ripples formed by the Big Bang? Well, luckily for us, they showed that the universe just after the Big Bang was lumpy. From these microwave ripples scientists at NASA have worked out the lumpiness of the universe 300,000 years after the Big Bang. Look here is a picture of the lumpiness of the universe in terms of temperature back then. The yellow and red bits are hot and the blue bits are cold.


"Why are lumps lucky for us?"

Because without the lumps, there wouldn't be any stars and without our star we wouldn't be here. For gravity to be able to collect matter together into stars there needs to be a few irregularities or lumps. The bigger you are the more gravitational pull you have. So it takes one lump of matter in the centre, or anywhere, to start slowly pulling in other smaller pieces of matter. The central lump then starts getting bigger and bigger and so its gravity gets stronger and stronger. The whole process speeds up with more and more atoms being pulled into a big ball.

“So why did it take 150 million years to make a star?”

Because the ripples were very small so the first lumps didn’t contain much and it took ages for gravity to pull in enough gas to make a star.

"And what exactly is in these balls?"

Three quarters of it would have been hydrogen, remember that's the simplest and smallest type of atom. Nearly all the rest was Helium which is the next smallest. Between them, hydrogen and helium make up most of the universe. As these balls of gas got pulled together by gravity, they got hotter and hotter like I explained before. Now hot things glow, but gravity alone will not make hydrogen glow as brightly as a star. When they get really hot, the hydrogen inside starts combining with itself by nuclear fusion reactions and ....carrummmph..... there you have it.

"Have what exactly?"

A fully fledged star using nuclear fusion energy to shine for millions or billions of years.

“How can we know how stars were made billions of years ago?”

Because we can see it happening today.

“Where”

The easiest place to see where stars are being born is in the constellation Orion, the hunter. That’s one of the easiest constellations to see. It has a belt of three stars, just below the belt is what looks like a hazy area. That is a massive cloud of dust and gas where stars are being made as we speak. It looks like tiny fuzzy blob when you look up at Orion at night but it is huge, 30 light years across. With the Hubble telescope, scientists have even seen new stars with discs of gas around them. Look this picture taken about ten years ago. Those fuzzy beige blobs are stars being born surrounded by swirling discs of dust that one day may become planets.


“Albert, how can you know about all this stuff that happened after you died, like the Hubble telescope?”

Well, when I was rebuilt in this computer system I found I could roam about cyberspace catching up on all the science I missed out on by dying. What do you think I do when I’m not talking to you?

“Be careful there is a lot of rubbish out there on the Internet too.”

Sparks of brilliance and generosity in a sea of dross and unpleasantness.

“Is that one of your famous quotes?”

No, I just made that one up. But it sums up the Internet pretty well don’t you think?

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.