Quickie: CP and power spectrum of the CMB

honestrosewater

Would the power spectrum of the CMB tell you whether the cosmological principle is correct?

Ack, I was going to try to explain my reasoning, but it doesn't really count as reasoning. I guess I was wondering what the CMB power spectrum tells you. If it's complicated, nevermind; I'm just curious.

marcus

what was really on your mind? what were you wondering about Rosewater?

marcus

why dont you tell us your reasoning? even if you think it might not count as reasoning.


I expect you have already read the Wiki article. have you?
I mean whatever Wiki says about "cosmological principle" (which isnt very much but is a start)

http://en.wikipedia.org/wiki/Main_Page

http://en.wikipedia.org/wiki/Cosmological_principle

This Wiki on the CMB looks pretty good. Have you glanced at it? It has a picture of the power spectrum, in terms of variation of temperature over various amounts of angle in the sky----how MOTTLED the temperature map is at various angular scales. It is a nice picture. Lot of nice pictures on that page including the beautiful oval temperature map itself.

http://en.wikipedia.org/wiki/CMB

huge amoung of info about early universe in that map, and the finer they look and more detail they can see in the map the more exciting it gets

honestrosewater

Eh, I'm not actually studying any of this yet; I'm just doing a little reading for fun. (I was reading this when I thought of this question.)

My basic thinking was that if you know how something, call it a 'system', changes over time and you know the 'state of the system' at one point in time, then you know the state of the system at any other point in time. So if the CMB tells you the state of the universe at one point in time (which it may not) and if you know how the universe changes over time (which you may not), then you know the state of the universe at every other point in time.
I don't doubt the argument's validity; I doubt its soundness. I don't know what the CMB or its power spectrum tell you; I don't know what I mean by 'state of the universe', only that it would include the distribution of 'stuff' ; And so on.

I'll take a look at those links. Thanks.

marcus

a secret: people are often disconcerted (or even alarmed) when they ring the cosmology doorbell expecting to meet a grownup, and the door opens and it's a child

or even a baby.

[EDIT: ROSEWATER YOU SHOULD READ WHAT RATZINGER REPORTED HERE:
http://www.physicsforums.com/showpos...298#post785298
you see it is true, what Lee Smolin said, about "what is at stake"]

marcus

the big question in cosmology is what is the right quantum model of gravity (i.e. spacetime)
and (therefore) what preceeded the onset of expansion

the more detailed map of the CMB we can get the more able we will be to distinguish the right model of gravity (that is, spacetime geometry), and to understand the beginning of expansion.

these questions are just now being asked, and we only recently got good maps of the CMB-----and are only recently studying the short intense Gammaray Bursts that also have information for us about quantum gravity.

so cosmology is still an infant

All during 20th century starting with that Belgian priest George Lemaître and the Russian Alexander Friedmann, cosmology meant applying a simplified version of Einstein 1915 Gen Rel to fit the data. But vintage 1915 Gen Rel BREAKS DOWN and gives silly meaningless answers at the beginning of expansion----which is where it needs to be replaced by a more realistic quantized theory.

The replacement models are now being proposed and they will have to be matched to the data, and then we will have possibly another century of people doing cosmology, but with an improved model. And puzzles like dark energy and accelerating expansion and what determines the fundamental laws (or constants) of physics will have a better chance of being resolved.

So if you had a great sense of urgency to get authoritative answers I would feel sorry for you, but judging from your tone of voice I suspect you arent bothered by any great sense of urgency----so it is a good time to be watching, a lot of new stuff is in the works.

wolram

Heck Marcus, what a stand, good for you, now let us wait and see.

marcus

I am glad it met with your approval, Wolram.

SpaceTiger

The fact that the CMB is almost completely uniform is certainly evidence for the cosmological principle. Your reasoning:

is basically correct. Although we don't have a complete model for the universe at this point in time, the homogeneity and isotropy of the laws of physics suggests that any universe that is completely uniform at some point in the past ought to remain so in the future (at least when averaged over large enough scales).

As I mentioned in a previous post, the correctness of the cosmological principle will likely never be proven for the universe as a whole (particularly for regions beyond our horizon), but it does seem to be valid for those parts we can see.

honestrosewater

Are there any models that predict for the universe what you observed of the beach: at which scales you will find 'peaks' and how the size of these peaks relate to each other? I'm specifically wondering if there's a scale at which the peaks are expected to start decreasing in size as the scales increase.

SpaceTiger

Yes, in fact, this is the reason that the CMB is such a good probe of cosmological parameters. Lets extend this analogy a little further because I think it's very instructive. If I were a geologist trying to construct a model for the formation of beaches, one of the things my model might predict is the distribution of sand grains. A combination of external variables (like sedimentation rate, local climate, etc.), along with the known laws of physics, will contribute to determining this distribution. A particular model (including the modeller's guass as to the external variables) might predict that the power spectrum of sand grains should be distributed with a peak at around a millimeter. If we then go and observe a particular beach and find that the peak is actually at two millimeters, then we can rule out the model (or, at least, that version of it).

The case of the CMB anisotropies is even simpler because the system is thought to be entirely closed (there should be no unexpected sunbathers to come by and change the result). The unknowns will be cosmological parameters -- that is, the basic numbers that describe our universe. Once we have these numbers, we need only run the physics (which is, for the most part, already known) through our computer.


You mean scales for which the peaks decrease in size as the universe expands? This can certainly occur for perturbations that are growing "non-linearly". Although the specific scale at which this is occurring changes with time, the current non-linear regime is on the order of a few megaparsecs (a few times the distance to the Andromeda galaxy).

honestrosewater

No, but I think I was looking at things incorrectly. I should try to figure it out for myself anyway. Thanks - your explanations are inspiring.