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

http://www.nytimes.com/2008/06/03/science/03dark.html?ref=science

This article describes how some scientists "despair" of ever understanding the current conundrum facing astronomy: namely, the accelerating expansion of the universe. This phenomena is currently attributed to "dark energy" - but it may require a re-write of major scientific theories (e.g. General Relativity).

Can science ever be said to surrender? Or are we just waiting for the next Einstein/Newton/Galileo/Archimedes ?

Bearing in mind the explanation that all scientists resent, that science is really just our 'best explanation' for what we observe going on, it would appear that we are waiting eagerly for the Grand Unified Theory.

Let's face it - Physics is in a bit of a mess. Astronomers can't account for 99.9% of the Universe and particle physicists have broken matter into so many different fundamental particles that they've lost count. Acknowledged constants, such as the speed of light and the cosmological constant, are being rumoured to not be quite as constant as what was once thought.

Rather than original thought a la Einstein or Newton, it really would appear to be time for a theory that will tie up the loose ends, as Mendeleev did for Chemistry and Crick & Watson did for Biology. I don't keep up to date with the latest physics on account of it making my head hurt (23-dimension strings? Ow!), but wasn't there talk recently of applying some of the precepts of quantum mechanics to the movement and interaction of galaxies? Or some such?

B

DId you really mean "resent", or did you mean "present"?

I'd go with resent. Possible slightly less accurate, but much more amusing!

I did actually mean 'resent', but I didn't phrase it very well - what I meant is that they accept it, but secretly resent it.

B

I don't think there are sooo many fundamental particles - I think the problem with the Standard Model are coupling constants (18 parameters derived from experiment rather than first principles).

I don't think Mendeleev "tied" up the loose ends of chemistry - chemistry has advanced huge amounts since him (e.g. the concept of chemical bonds, lewis dot structures, MO theory).

Same with Watson & Crick and bio.

As far as physics being a mess, the same could be said regarding physics prior around 1900 due to the Ultraviolet catastrophe and the photoelectric effect. Only in that case, the theories were overly simple and couldn't account for current observations. It's hard to say whether our current theories are overly complicated or overly simple until we discover the new theories.

Last but not least, let's not forget that during the revolution of modern physics, the concepts changed dramatically, but the basic results under the previously known systems/conditions still were valid. So in the future, if I want to study sub-atomic - nuclear structure at low collision energies, the Standard Model will still be fine to use.

That whole Dark Matter thing is getting too far fetched I think. I´d imagine in 15 years the big bang theory will have been replaced by something else.

if we have all these extra dimensions we cant see, if they were full of matter would this account for all that "missing" universe????

Given that matter exists in three (or possibly more) dimensions, that isn't really feasible, sadly.

B

I've said it before, but if you give something a science fiction name, then no-one will belive it exists.

I was under the impression that dark matter had been observed indirectly by measuring gravitation lensing though? So its likely that there's something there.

As for dark energy, pffft, who knows?

There are alternatives to the dark matter explanation, involving gravity acting differently over long distances. That said I don't find the idea that there's a lot of stuff that we haven't managed to detect that unlikely - after all it's only recently that we've discovered quarks and neutrinos.

By the way, I just dug out a New Scientist from March and it gives 96% of the universe as dark matter, not 99.9%.

Correction: 22% of the universe is dark matter, but only 4% is the stuff we know about, leaving 74% as dark energy. Probably.

The main evidence for dark matter, as far as I know, is the rotation of galaxies. If you look at our solar system, you'll see that the planets which are close in, near the sun, go around the sun quickly, and the ones that are far out go around the sun slowly. This is because the sun makes up almost all the matter of the solar system, and it is concentrated at the centre, and the further from the centre the weaker the gravitation.

Galaxies, on the other hand, spin at a much more constant rate, so that the middle turns only slightly faster than the outside. Some of this is explained by the mass of the galaxy being spread across the whole galaxy rather than concentrated at the centre, but even this is not enough to explain the phenomenon. Dark matter was invented to try and explain it. If the galaxy is actually 10 times as massive as it looks, then it would rotate at the observed rate.

It's not a very good explanation, but it's the best they have so far.

That was certainly the initial evidence, but gravitational lensing also suggests that there is more mass than there should be just in what we can see.

Thanks for the great discussion everyone.

I don't know much about this, but one thing that jumps out at me are the properties of super-massive black holes that are at the center of most? all? galaxies. Apparently their average density can be less than the density of air, and a hypothetical astronaut might not experience tidal forces while crossing the event horizon.

Taking a step back, this seems to be not that well understood. Alternatively, it seems like ripe grounds for an alternative explanation. Or is it well understood?

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couldn't the galaxies be full of normal matter, dust and rock etc. and not some exoctic "dark matter"

Well, one theory of the make-up of dark matter is what they call MACHOs - MAssive Compact Halo Objects. They would be small, high-density and at least fairly dark objects, such as brown dwarfs and black holes, distributed in a halo around a galaxy.

The problem with the idea of it being rocks and dust is that that would obstruct and reflect the light from stars. Dark matter in the form of WIMPs (Weakly Interacting Massive Particles) wouldn't do that so much because it wouldn't have any charged particles and so would barely interact with electromagnetic radiation.

Yeah, it's exotic, Taff, my feeling too, but it seems that that's just the problem - it isn't full enough.

I've been accepting the dark side, more or less, but without trying too hard.
Now it occurs to me to ask:

The percentages quoted are...substantial. With only normal matter, just how full would we need to be?

It strikes me that we'd be continually battered into a homogenous thin soup. Either that or the universe'd be full of 'pangea' type solid 'galaxies'.
Or, perish the thought, the Big Bang would have sprayed us too far, too quickly...

My sanity is at stake here.

Bagpuss: that helps a bit. I missed yours before posting mine.

Xanatic or someone else who knows may have to correct me on this, but I believe there is still a lot of debate about the early expansion of the universe, and a number of models which work. So, it would be quite hard to predict what would happen in a universe without dark matter.