h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hello again

I read your notes, and the TV web site. This is what I know about the subject.


WebSherpa's recent entry on red shift/cosmic expansion (http://www.bbc.co.uk/h2g2/guide/A636897) provides a good introduction to this.

After inventing his general theory of relativity, Albert Einstein applied his theory to the universe. He could not find a solution to his equations which described a static universe. As gravity was an attractive force, a static universe would collapse under the gravitational attraction of all the galaxies. In order to prevent this collapse the universe would have to be continuously expanding with galaxies flying away from each other.

As Albert felt sure the universe must be static, he 'fudged' his equations by introducing the Cosmological Constant. This acts as a sort of repulsive force on large scales, to balance gravity producing a static universe.

Measuring the red shift of a galaxy tells us how fast it is moving away from the earth. Due to the Doppler effect the wavelength of light is shifted due to the relative speed of the light source (the galaxy) and the observer.

Edwin Hubble measured the red shift of distant galaxies in 1927 and showed that the further a galaxy was from the Earth, the faster it was moving away from us. He concluded that the universe is expanding. When Albert heard this, he thought he had been stupid not to realise that the universe would be expanding, and called the Cosmological Constant his 'biggest blunder'

There are three possible futures for an expanding universe: it could expand to a maximum size and then fall back, collapsing in a 'big crunch/Gnab Gib', this scenario is called a closed universe; alternatively the universe could continue expanding forever, this is an open universe; a third possibility is that it will expand to an infinite size, at an ever slowing rate, this is called a flat universe.

There is a lot of evidence from other theories that we live in a flat universe. This can be confirmed by experiments, by measuring the precise rate of expansion of the universe. This is done by repeating Edwin Hubble's work by measuring the red shift of galaxies a long way from the earth. As these galaxies are often billions of light years away, we see them as they were billions of years ago, so we also measure the rate at which the expansion of the universe is changing. In the three cases outlined above, it should be decelerating.

The problem with this experiment is that it is very difficult to determine how far away a galaxy is. A dim galaxy could be a bright but distant galaxy, or a not-so-bright and not-so-distant galaxy. This is where type 1a supernovae are useful.

A 1A supernova is an example of what astronomers call a 'standard candle' – an object whose absolute brightness is known. By measuring the rate at which a supernova brightens and then dims, we can calculate its intrinsic brightness. By comparing this to its apparent brightness, we can calculate how far away it is. Supernovae are the only type of standard candle which are bright enough to be visible from a very distant galaxy. However they are still very dim, so precise measurement of their apparent brightness requires large, high-resolutions telescopes, such as the appropriately named Hubble Space Telescope (http://www.bbc.co.uk/h2g2/guide/A601129)

Two independent teams of astronomers carried out these measurements in 1999. They both reached the same conclusion:
1. The universe is flat
2. The expansion of the universe is accelerating!
While the first result was expected, the second was a real surprise. All established theories said the universe should be decelerating. It shocked everybody, except a small group of theoretical cosmologists who had just completed a theory predicting an accelerating universe. Their theories were based on the idea of dark energy.

As explained above, there is a lot of theoretical and observational evidence that the universe is flat, the expansion of the universe should be balanced by gravity. However the amount of visible matter in all the luminous stars and galaxies we can see, cannot produce nearly enough gravity to do this. Astronomers have believed for a long time that there must be a lot of 'dark matter' out there to provide this (http://www.bbc.co.uk/h2g2/guide/A588224). However in recent years, some astronomers have questioned whether there is enough dark matter out there. Some have proposed that there must be even more 'dark energy' to make up the balance. Current estimates suggest 70% of the density of the universe is dark energy, 30% dark matter, and only a tiny fraction made up of visible matter.

Dark energy is an intrinsic density of space itself which contributes to the gravitational attraction of the universe. However some cosmologists realised that if dark energy existed billions of years ago, galaxies could not have formed. They proposed that it must have been insignificant in the past, and only plays a major role today. If their theories are correct, dark energy may be much more complex than previously thought. In addition to providing space with an intrinsic density (which contributed to gravitational attraction), it would also give space an intrinsic negative pressure. This negative pressure would act as a repulsive force causing the expansion of the universe to accelerate.

These theories did not receive much attention until the supernova results showed that the universe was indeed accelerating.

This is very similar to Einstein's cosmological constant. It worked by redefining the density and pressure of the universe to act as a repulsive force to balance gravity. Reintroducing the constant into modern models of the universe can produce models of an accelerating universe.

At this point you may have a good question: if dark energy was introduced to explain gravitational attraction, how can it also be a repulsive force causing the universe to accelerate? The full answer is rather mathematical. However basically there are two main factors affecting the expansion of the universe: its density, and its pressure. After the big bang, the pressure was very high causing the universe to accelerate and fly apart; today the pressure is negligible, and the dominant effect is from the density which causes gravitational attraction to slow down the expansion of the universe, or so we thought.

However the cosmological constant is just a fudge factor, a way of tailoring the pressure and density in models of the universe to produce a desired results. It is still unclear exactly what dark energy could be. Other theories refer to vacuum energy, and quintessence. This are all variations on the theory that something is causing space to accelerate.

Understanding the true nature of the universe will require more experiments, more precision measurements to determine what factors are at work. Further measurements will also show if the universe really is accelerating, and that the results so far are not just erroneous.


hope you're not asleep by now
MP

Thanks MP, not asleep - not yet, at least...

...but I was thinking of a rather simple and easy-to-read article based on the fact that the chance of having visitors from distant galaxies is diminishing all the time, because the other galaxies is rushing away from us!

...are rushing, even...

Just siging in here, hello.

Did anyone happen to see Horizon on Thurday BBC2?

The whole flat universe interests me as an astronomer and Chrisitan, it would make it a special universe, right on the line, is there a reasong for this? or is it that we just wouldn't be able to evolve in a open/closed one?

A flat universe is indeed rather special, the reason is inflation theory: shortly after the big bang the universe expanded exponentially fast for a short time; this had the effect of stretching any curvature out flat.

But it does appear that many other parameters in cosmology, apparently determined at random, have just the right values to allow galaxies, planets and life to evolve. Some people have said that this suggests the existence of some sort of god; others say it is simply so, because otherwise philosophers would not have evolved to notice it. There are also a few radical theories involving many universes and some sort of evolution process.

These programmes do sound interesting, maybe I should buy a TV set.
If you found my first posting a bit too much, just write the entry in the style you like and I'll post any suggestions. I was trying to give some details around your script and got a bit carried away.
I think the chance of having any visitors from outside the galaxy is pretty remote anyway, given the distances involved. But it's an interesting thought.

Well, I like the way Clarke put it:

"Sometimes I think we're alone in the universe, and sometimes I think we're not. In either case the idea is quite staggering."

And even though the chance might be remote, it's just getting even more remote, with the galaxies speeding away from each other, right?

Yes. But while we may have a visitor from within the galaxy one day, the nearest other galaxy is over two million light years away. The nearest galaxy which is moving away from us is even further away.
If there's anyone else in our galaxy, we can hope to find them one day. However it's extremely unlikely we will ever know about distant galaxies. At these distances we can only just see a supernova.

Ahh but of course Andromeda is ging to collide with us some time so any civilisations there are comming!

"Wow that's big, like a mass of little lights, it needs a bid wide sounding word, axle, galaxle, galaxy, yes that's it, galaxy! Oh I wpnder if it'll be friends with me"