Galaxies

Why are our galaxies distributed in clumps together and these clumps are maintained even with expansion (to a degree)?

Bernadette

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I’m assuming this is something to do with gravity?

Heather B.

— To my knowledge, the answer is “nobody knows”. Kind of.

Consider a universe entirely filled with hot gas. Gravity will cause the gas to be attracted together, but if the gas was exactly uniform, the molecules would be equally attracted in all directions and nothing would happen. If there’s even the slightest inhomogeneity though, the attractions will be greater in some directions than others, causing the gas to clump together into different regions.

It then becomes a question of where the inhomogeneities came from. One explanation could be to point out that a system described as above is unstable, and hence even the tiniest infinitesimal inhomogeneity would cause clumping. There’s still the question of which way it will actually clump, though.

I suspect there are lots of theories but that no-one really knows. Does anyone know any more about this? Astronomy students, maybe?

Angus

That’s right, Angus. If it was completely homogeneous, it wouldn’t clump (or it would form one big clump). We don’t know why it had the degree of homogeneity it had.

Once the galaxies are roughly the way they’ve turned out, then gravity binds the clusters and superclusters together more strongly than expansion pushes them apart. (E.g. our galaxy and the Andromeda galaxy, M31, are going to fall into each other.) That’s WITHIN the superclusters. BETWEEN the superclusters, expansion seems to be winning.

Jason

I’m a big Brian Greene fan, so I thought i’d add in some of his cool explanatory analogies:

Why clumps are maintained with expansion:

if you imagine many pennies upon a balloon , (the pennies representing superclusters, the expanding ballon representing expansion of space):

Each penny stays fixed in size because the forces holding its zinc and copper atoms together are far stronger than the outward pull of the expanding balloon to which it is glued. Similarly, the nuclear force holding individual atoms together, and the electromagnetic force holding our bones and skin together, and the gravitational force holding planets and stars intact and bound together in galaxies, are stronger than the outward swelling of space, and so none of these objects expands

also why things clump in the first place:

he explains inflationary cosmologies take on the formation of clumpy structures such as galaxies.

my understanding (correct me if wrong) is that inflationary cosmologists claim the initial overall homogeneity (as well as the important tiny nonuniformities) result from quantum mechanics.. that:

  1. the burst of inflationary expansion stretched space by such a huge factor that what initially inhabited the microscopic was drawn out to the macroscopic.
  2. On a microscopic level, there is no perfectly uniformity because of the jitteriness inherent to the uncertainty principle
  3. Therefore prior to inflationary expansion; random differences between quantum jitters between different spatial locations generated slight microscopic inhomogeneities— resulting in differing amounts of energy between locations
  4. thus: the resulting inflationary swelling of space, stretches these tiny variations to macro scales, yielding the lumpiness which over the next few billion years culminated in galaxies and clusters, through gravitational clumping.

Conc: the 100 billion galaxies plus, are nothing but ‘quantum mechanics writ large across the sky’

(though I know this position is contested, am I right in thinking it is the forerunner theory?)

Julien

Very good explanation about pennies on a balloon.

I’m a big Brian Greene fan too, but we’ve had two lectures on why the Uncertainty Principle doesn’t say the things you’d like it to say, and a total of six lectures on varous divergent theories of quantum mechanics, only one of which is considered in that chapter. So I DON’T recommend that people read that chapter for this course, although I do recommend that they read the rest of the book — you’re talking about Fabric of the Cosmos, right?

You may be right that your description describes the front-runner theory, but (a) i wouldn’t be too sure about that, and (b) it wouldn’t be saying much: there are many many theories about this, with none of them having a clear lead.

Jason

yes Fabric of the Cosmos was the one, yes i also found the QM section fuzzy/confusing particularly Bell theorem.. But i did love the idea of the celestial bodies being Quantum mechanics writ large across the sky..

Julien

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I was reading the other day about there being supermassive black holes at the centre of very (or at least most) galaxies.

There is pretty good evidence to suggest that there is one at the center of ours.

With that amount of gravitational force it seems logical that they form clumps? - Maybe?

Vic, yes.

Jason

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