Thursday, January 11, 2007

Cosmology and the Age of the Universe

Back to creation/evolution posts...

What is Cosmology?

Cosmology - the study of the physical universe, its structure, dynamics, origin and evolution, and fate
Wiktionary

Basically, cosmology is the study of the universe as a whole. It's the bit of physics that gave us the Big Bang theory, for example. It's also one of the least discussed areas in the debate about the age of the universe, probably because it tends to be trickier to visualise than evolution.

My background in cosmology is that I studied it as part of my taught masters' at university first time round. So I've got the foggiest what I'm talking about, but I'm not an expert. The bits here that are unconventional I've checked out with people who are experts. I'm going to try to explain this at the level of an intelligent layperson. Some of that means that I'm going to simplify to the point where the truth might be slightly obscured - I'm trying to do it to keep the main points clear.

Virtual Photons

Oddly enough, one bit of background that turns out to be really useful comes from Quantum Electrodynamics (QED), which is a completely different area of physics.

Atoms are made up of electrons, which are really small and which kind of go round a nucleus made of protons (and usually neutrons too). The electrons are held into the atom because they have a negative charge and the protons have a positive charge. The force works by the electrons and the protons constantly swapping particles called virtual photons, which are just like the particles that light is made up of, except that they last a very very short period of time while they are between the proton and the electron (coz they go very fast and it's only a small distance).

Evidence for the Big Bang

The Big Bang theory, or some variety of it, is held by I'd guess at least 99% of cosmologists. It basically says that the universe is expanding and all the matter and energy in the universe started off in one place about 14 billion years ago. That must have been much hotter than today, and the idea is a bit like an explosion that threw out all the matter, whch gradually condensed into atoms and galaxies and stars and planets and stuff.

There are three main pieces of evidence which are cited as evidence for the Big Bang. I'm going to discuss them and look at how the evidence is interpreted.

  • Redshift
  • The Cosmic Microwave Background Radiation
  • Abundance of Elements
Hubble Redshift

The idea that originally started the Big Bang theory is called Hubble Redshift. It's not actually quite what is believed today, but it's a good way in to talking about it.

When an ambulance or a racing car is moving away from you and giving out sound waves, the sound waves get stretched out and it sounds lower-pitched than it would do if the ambulance or racing car wasn't moving. (And if it's moving towards us the waves get squashed together and it sounds higher)

There's a great example that shows that here.

The same happens with light, which is made of particles called photons, but they act like waves with this. If an object is moving away from us and giving out photons, the photons get stretched out, and the light appears redder than it normally does - we call this a redshift. We don't usually notice it because light goes very very fast compared to cars and stuff. (And yes, there is a blueshift if it's coming towards us).

When Hubble looked at very distant galaxies, he saw that their light was redder than he was expecting, and the further away the galaxies were, the redderer the light was. This led him to conclude that they were all moving away from us, and the idea of the Big Bang was born.

Cosmological Redshift

The current idea isn't that the galaxies are moving away from us through space, but that space itself is being stretched, kind of like the skin of a balloon when you blow it up. So the galaxies are getting further apart, but it's not because they're moving. And as the photons are going through space, they get stretched too as space stretches, so we get the redshift because light has been travelling through space that is stretching. That's called the cosmological redshift.

So the evidence is that the distant galaxies look more redshifted than nearer ones, which we interpret by using the cosmological redshift idea and saying that that's because when the light set out from the distant galaxies, it was a very long time ago, so the universe has stretched a lot since then and redshifted the light. The further away it is, the longer ago the light set out, the more it's been redshifted.

The Cosmic Microwave Background Radiation

Another thing we observe if we look carefully at the sky is that everywhere we look, there's a load of microwave photons whizzing round the universe. And the cosmological redshift idea helps us to explain this too.

After the Big Bang, the argument goes, everything started off very hot, so there weren't atoms or anything. But as the universe cooled down, it eventually got cold enough for atoms to form. And when they formed, there were a load of photons left behind that didn't quite have enough energy any more to break those atoms up. And as the universe expanded, they got redshifted and ended up as the Microwave Background.

Actually, some scientists had realised that if the Big Bang theory was true, we should see something like this, but they apparently realised it only just before Penzias and Wilson discovered the Microwave Background Radiation, so they didn't get their prediction published in time.

The Problem with the Cosmological Redshift

So far, so good. It all kind of makes sense, that's all on the A-level physics syllabus so far and it's all pretty much what I was taught. The problem is that the cosmological redshift idea doesn't work (and I've checked this out with some experts, who agree that it's a valid criticism).

Redshifted photons have less energy than normal photons (this is because the energy is given by hf, and as they are redshifted, f drops).

That means that in a universe with the cosmological redshift, the universe is actually losing energy. It's not going anywhere; the amount of energy in the universe is decreasing. Some people might think that's a big problem because it contradicts the laws of thermodynamics, but cosmologists get round it by saying that we can say that the mean energy density of the universe is constant rather than the actual total energy of the universe. I'd say that energy itself not being constant really messes up Quantum Mechanics, but there's a much bigger problem that's easier to see, so I'll focus on that one.

Remember those virtual photons? Well, they need to get redshifted as well. Only a little tiny bit each time, but over 13 billion or so years, it'll add up to quite a lot. That means that atoms have to lose energy too as the universe expands. But atoms can't lose energy - we can work out how much energy an atom has and it doesn't have anything to do with the age of the universe in it. So we have a big problem. We understand atoms pretty well, and we know they can't lose energy. But we know that if the cosmological redshift argument is true, then they must. So I'd conclude from that that the cosmological redshift argument might be nice, but it doesn't actually fit what we already know about the universe.

That kind of undermines the first two pieces of evidence for the Big Bang. The problem then is that the cosmological redshift argument is actually the only argument that explains the CMB. What most cosmologists do is they say that there are "problems with the relationship between General Relativity and Quantum Mechanics" then carry on using the theory regardless and hope someone very very clever sorts the problems out in the end.

Maybe it's true, and the theory just needs some very clever tweaking. Or maybe it isn't and there's some other explanation for the Cosmic Microwave Background and the Redshift other than the Big Bang with cosmological redshift. I honestly don't know.

Abundance of Elements

Some people much better at maths than me have worked out how much of each element there should be in the universe to start with if the Big Bang happened. It's something like 75% hydrogen, 23% helium, 2% everything else. That's just trying to remember the numbers from 6 years ago, but the exact numbers don't matter for this.

Of course, nuclear reactions in stars and stuff have changed that by now, but they won't have made any more hydrogen. So no star or anything should have more than 75% hydrogen (or whatever number it is).

The problem with this piece of evidence is that all kinds of weird stuff can happen to stars. People find stars which really don't fit with pretty much any pattern quite often, then manage to come up with an explanation for why they are the way they are. Here's an example of the kind of thing they come up with.

There used to be two stars here, one heavy and one less heavy. The heavy one burnt its fuel more quickly, but then it got so large that the gravity of the less heavy one started stealing the outside bits of the heavier one. Eventually, the heavier one exploded into nothingness, leaving this star which looks like it's a mixture of two other stars.

Theories like that are fine, but if you've got that kind of potential level of complexity in a star's lifecycle, plus 14 billion years for stuff to happen, I'm sure that current cosmologists and astrophysicists could explain just about any imaginable star in terms of some process or other. So I don't think this piece of evidence is actually falsifiable.

In other words, even if it was false, we wouldn't know because astrophysicists could (and would) always find another explanation for why the star was like that.

Virtual Photons and the Appearance of Age

As I discussed in the comments to this post, virtual photons also undermine the argument from distant starlight.

The basic idea is this - we see things that are a very long way away. That means that the light must have left them a very long time ago to get to us now. So the universe must be at least that old.

It's a nice argument. The problem is that if the universe was created suddenly out of nothing, then for atoms to work for the first tiny bit of a second, virtual photons need to have been created in mid-flight along with the protons and electrons and stuff. Those photons would have look like they'd been emitted by the proton or electron just before, but they actually hadn't - they'd just been created out of nothing suddenly, along with the proton and electron.

If you scale that up, then it makes sense that if the universe was created suddenly then photons are created mid-flight between two objects, even giving the appearance of a history which never actually happened. It's pretty much like the idea of rocks in the garden of Eden, which I discussed here.

Problem - the Fine Structure Constant

One problem which I haven't seen anyone tackle properly, on either side, is the problem with a number called the fine structure constant. It's a number - about 1/137 (if I remember correctly), which is worked out by combining a whole load of important quantities in the universe like the speed of light and the charge of an electron. The problem with it is that in most attempts to unify all the physics theories into one big theory, it needs to have changed as the universe gets older.

Where the problem comes is here - if the fine structure constant is changing, what's that doing to the redshift? What's that doing to the photon energies? What's that doing to the CMB? What's that doing to the stability of atoms?

Maybe one day all the problems with the theory will be solved and cancel each other out. Or maybe they won't and in 1000 years people will be looking back and laughing that people in 2007 believed in the Big Bang. I really don't know.

Conclusion

My personal opinion is that the Big Bang theory, while it has huge holes in, is probably the best current explanation for the evidence, but I think there's plenty of room to think that the universe is young.

The fact is that no-one on either side has yet come up with a comprehesive enough theory to explain all the evidence and fit in with the rest of what we know about how the universe works. Or if they have, they haven't told me...

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