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Why is the universe different?

mummymonkey

Did you spill my pint?
mummymonkey
Joined
Dec 3, 2002
Messages
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Scotland
I was re-reading Cosmos the other day and was looking at a picture of a spiral galaxy. Right next to it was a picture of a similar galaxy that had a bar right across it.

I wondered how there came to be different kinds of galaxies. Why aren't they all the same? So I reasoned the conditions in the universe must be different from place to place. So it must have always been so? All the way back to the very start?

If it all started from a single point, why is the universe not an even spread of the same kind of fundamental particles expanding away from each other?

Hope this rather daft question makes some kind of sense.
 
mummymonkey said:
I was re-reading Cosmos the other day and was looking at a picture of a spiral galaxy. Right next to it was a picture of a similar galaxy that had a bar right across it.

I wondered how there came to be different kinds of galaxies. Why aren't they all the same? So I reasoned the conditions in the universe must be different from place to place. So it must have always been so? All the way back to the very start?

If it all started from a single point, why is the universe not an even spread of the same kind of fundamental particles expanding away from each other?

Hope this rather daft question makes some kind of sense.
Click to expand...
The universe is homogeneous over large scales. Are you confusing that with locality?
 
It's a good thing atomic structures aren't all the same also.

I'm no expert but elements present might have something to do with different galactic 'looks', not to mention the age or galactic neighborhood one resides.

To expect the same everywhere implies perfection, with not one single photon any different distance or speed as all of the others. That's asking quite a lot.
 
mummymonkey said:
I was re-reading Cosmos the other day and was looking at a picture of a spiral galaxy. Right next to it was a picture of a similar galaxy that had a bar right across it.

I wondered how there came to be different kinds of galaxies. Why aren't they all the same? So I reasoned the conditions in the universe must be different from place to place. So it must have always been so? All the way back to the very start?

If it all started from a single point, why is the universe not an even spread of the same kind of fundamental particles expanding away from each other?

Hope this rather daft question makes some kind of sense.
Click to expand...
There were quantum fluctuations in the mass-energy density, way back then.

Inflation blew these up.

When photons streamed free (the 'surface of last scattering') the tiny density fluctuations started to interact, via gravity^. Those interactions lead to the formation of what we call 'large scale structure' today.

At a much smaller scale, via physical processes that are not yet well understood or constrained, stars formed, and so did clusters, and proto-galaxies.

These interacted, not only via gravity, to form the weird shapes and clumps that you see in deep Hubble images (why not join Hubble Zoo, and not only marvel at them, but also contribute to understanding them?)

These weird objects interacted - collided, merged, etc - to form more stable galaxies, mostly of the spiral (disk) and elliptical kind.

And how do we know all this? Observations of the cosmic microwave background (CMB) are very helpful, for the early stuff; direct observations for the later. However, there is a gap, between z ~10 and z ~1100, which is unconstrained by any direct observations.

Questions?

^ of course they'd been interacting before too, but when radiation dominated the universe the fluctuations were quickly damped down
 
Dark flow, as far as I understand anyway, suggests that other regions of the universe may have slightly different 'rules', or that there are structures not visable to us that affects these areas.

The universe is a strange and interesting place!
 
Illustronic said:
To expect the same everywhere implies perfection, with not one single photon any different distance or speed as all of the others. That's asking quite a lot.
Click to expand...

Well we know the universe is not the same everywhere. Does this imply that the universe started from a condition of imperfection?
 
mummymonkey said:
I understand fluctuations but not "quantum fluctuations". What are they and what caused them?
Click to expand...
Hmm, I'm not sure how best to explain this, as I don't know what aspects of quantum mechanics you already know at least something about.

Have you heard of the Heisenberg uncertainty principle (HUP)? Of the fact that, in the HUP, a particle's position and momentum cannot be determined more accurately (or precisely) than a certain minimum (having to do with something called the Planck constant)? Or that particle-antiparticle pairs (like electrons and positrons) can pop into existence in a vacuum, for a very short time, before self-annihilating? Or of quantum tunnelling? Or that, at absolute zero, the atoms of the substance in question are still moving?

In the early universe, mass-energy was distributed as smoothly as it could be, but if you examined it on finer and finer scales, you'd find that it wasn't perfectly smooth. Why not? Because of the HUP! Or, because of the quantum nature of mass-energy.

The deviation from perfect smoothness is quantum fluctuations.
 
mummymonkey said:
Well we know the universe is not the same everywhere. Does this imply that the universe started from a condition of imperfection?
Click to expand...
As far as we can tell, as you go to larger and larger scales, the universe becomes more and more isotropic and homogeneous.

At the largest scale we can see, so far (the CMB), the universe is (was) only ~1 part per million out of being perfectly uniform.
 
mummymonkey -- bear in mind that "random" does not mean "uniform." If you toss a handful of salt onto a tablecloth, they will not land in an even spread, without clumps or areas with no grains. The Universe is large, getting larger, and has been subject to increasing randomness for a very very very long time. (For all of time, technically.) The stuff of the Universe has been interacting and has randomized; for this very reason, it is not uniformly distributed.

Did that make sense, or did I just make it worse?

Think of it this way: If you see a bunch of plants growing and they are all 4 feet apart, center to center, they were planted! They would not naturally grow that way. Think of stars and galaxies as being that way on a much larger scale. Just as a forest can 'appear' uniform from a distance, but when you get in close enough you see that the trees are not evenly spaced, so when you get far enough "out" in scale, the size and density of galaxies will appear to be homogenous. It's just that we're looking, necessarily, on a much smaller scale because even a single galaxy is so very large and far away compared to us here on the Earth.

Fun topic! MK
 
I'm not expressing myself very well. I'll sleep on this and think again tommorrow. Thanks for all the interesting replies.
 
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