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How does the CMB support the Big Bang?

Cankerskull

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I was recently watching The Universe on the History Channel, and they mentioned the Cosmic Microwave Background radiation. Sepcifically, they said how it was the largest piece of evidence in supporting the Big Bang theory.

My question is: How? The show briefly explained that it was the left over radiation from the Big Bang. But how do we know that it's a remnant from the Big Bang?

I've read the Wiki entry for CMB, but wasn't able to make much sense of it. Can anyone provide an explanation for "dummies"?
 
Cankerskull said:
My question is: How? The show briefly explained that it was the left over radiation from the Big Bang. But how do we know that it's a remnant from the Big Bang?
Click to expand...

Well, what else would it be?

The CMB was predicted to exist, well before radioastronomy existed, as a theoretical consequence of the model. When Penzias and Wilson actually found it, it matched the theoretical predictions almost perfectly.

And all the other theories of the formation of the universe made predictions that it didn't match. (In particular, the steady-state theory predicted that it wouldn't have the spectrum that it does.)
 
Cankerskull said:
Can anyone provide an explanation for "dummies"?
Click to expand...

First off, the photons in the CMB have a black-body spectrum. In fact they have what's perhaps the most perfect black body spectrum ever observed in nature. What means that they last scattered off some substance that was in nearly perfect thermal equilibrium. That substance had to exist both very far away (because CMB photons come from behind everything else in the sky) and very long ago (because it takes billions of years for light to travel so far). And since the temperature we can infer from the spectrum is almost equal across the sky, whatever stuff those photons scattered from is the same in all directions.

There is no such substance in the universe anywhere near us now - so either it exists in a perfect sphere centered on the earth with a radius of billions of lightyears (which would be extremely bizarre and make the earth the center of the universe), or it existed uniformly everywhere until about ten billion years ago, when the evolution of the universe changed its characteristics so it no longer scatters photons (which happens in the big bang model when the baryon-photon plasma cools enough to form neutral atoms; that process is called recombination).

That's only the beginning of the evidence for it. The next stage is to look at the (small) variations in the temperature, which turn out to have a very distinctive pattern. That pattern is perfectly consistent with the acoustic oscillations that the BB predicts took place in that baryon-photon plasma before recombination. It's very difficult to imagine that's a coincidence, and I don't know of any other model that would predict anything remotely like it.

Polarization of CMB photons is another powerful test. Another is the consistency of the data from the CMB with a host of other cosmological observables - the distribution of galaxies and galaxy clusters on large scales tests much of the same physics in the BB model, and the results are consistent with the CMB. So does the distribution of distant supernovae. In some other model, there's no reason why those would have anything to do with the CMB, let alone produce the same results quantitatively.

All those things are interlocking, quantitative tests of the BB model, any of which could have failed to work (but none did). On top of that, there is no other model which can explain more than one or two of those things, let alone all together. So it's very good evidence.
 
sol invictus said:
First off, the photons in the CMB have a black-body spectrum.
Click to expand...

Sol has a good explanation of the significance of this but let me emphasize its importance anyways. Many opponents of the big bang try to focus on the temperature of the CMB, but the temperature is unimportant compared to the lineshape (blackbody spectra have a characteristic shape).

A blackbody spectrum has an associated temperature, and when the CMB stopped scattering off matter the universe was quite hot and so this temperature was large. As the universe expanded, photons got stretched out over time and this temperature dropped. What exactly this temperature is therefore depends on various parameters like the current age of the universe and its expansion rate, and these parameters have to be measured, they can't come from the model itself. So if you get any of them wrong (and early measurements weren't that accurate), then you'll get the temperature wrong. So some opponents point to early incorrect estimates of the CMB temperature as evidence against the theory, and ignore the fact that the lineshape matches. They may also point to non-big bang predictions of an interstellar or intergalactic temperature which is close to the CMB temperature, but similarly ignore the fact that these predictions do not predict a blackbody spectrum.

So to repeat, the lineshape (a perfect blackbody lineshape) is far more important than the temperature, though the temperature fits with the model as well.
 
Ziggurat said:
So to repeat, the lineshape (a perfect blackbody lineshape) is far more important than the temperature, though the temperature fits with the model as well.
Click to expand...

Yes, and just to amplify on that even more - a line shape is in principle an infinite number of constraints. The temperature is one number.

In other words, the BB predicts that the CMB will have a black body lineshape, but it doesn't predict the temperature very well at all. So you have to determine the temperature from data: you make two measurements of the spectrum at two different frequencies (you can use that to determine the temperature by assuming the spectrum is black body).

But then you then have a prediction for every single other frequency, and you can make hundreds or more such measurements (you're limited by the frequency resolution of your detectors), any one of which could disprove your prediction.
 
Excellent replies....:thumbsup:

some info here showing many of the interlocking aaspects

http://www.aspera-eu.org/index.php?option=com_content&task=view&id=347&Itemid=97

science at its very best...fortune favours the alert minds ...they weren't looking for CMB but they followed what the evidence produced...

snip

The whole Universe is filled with an abundant density of microwave photons. This cosmic microwave background (CMB) was discovered in 1964 by Arno Penzias and Robert Wilson. They had devoted a lot of effort to develop a very low noise microwave receiver, and a reference source to stabilize and calibrate it. For the first time such a clean receiver was looking to the sky and received a weak constant microwave signal from all directions, corresponding to the emission of a thermal source at a temperature of about 3 K. For this discovery, they were awarded the Nobel Prize in 1978.


This evidence was confirmed with increasing accuracy, using ground-based and balloon-borne experiments covering the microwave and millimetre bands, until, in 1992, the COBE satellite of NASA obtained an extremely accurate measurement of the wavelength distribution (the spectrum) of the CMB. This turned out to have exactly the shape and amplitude of a thermal spectrum at 2.725 K, perfectly fitting the blackbody formula of Planck. Deviations from this shape, if any, must be smaller than 1 part in 10 000. This is an astonishing result: we are comparing the prediction of a physical law - which is at the base of quantum mechanics - to the measurement of radiation coming from everywhere in the Universe. And we find perfect agreement, probably better than in laboratory experiments with hot and cold thermal sources. His discovery was the result of a long sequence of experimental activities, carried out from high-altitude sites (to avoid atmospheric emission and noise) and from stratospheric balloons and rockets. It fits perfectly in the cosmological scenario of the Hot Big Bang, developed in the 1950s by George Gamow.
Click to expand...

somewhat more basic stuff from NASA here including the inadvertent discovery that led the Nobel.

http://map.gsfc.nasa.gov/universe/bb_tests_cmb.html
 
Last edited:
macdoc said:
Excellent replies....:thumbsup:

some info here showing many of the interlocking aaspects

http://www.aspera-eu.org/index.php?option=com_content&task=view&id=347&Itemid=97

science at its very best...fortune favours the alert minds ...they weren't looking for CMB but they followed what the evidence produced...

snip



somewhat more basic stuff from NASA here including the inadvertent discovery that led the Nobel.

http://map.gsfc.nasa.gov/universe/bb_tests_cmb.html
Click to expand...

Your mention of George Gamow reminded me of this paper;

http://prola.aps.org/abstract/PR/v73/i7/p803_1

The Origins Of Chemical Elements

By Alpher, Bethe, and Gamow.

Now physics lore has it that the paper was really by Alpher and Gamow, and Bethe was invited in to make it a pun.
 
sol invictus said:
Yes, and just to amplify on that even more - a line shape is in principle an infinite number of constraints. The temperature is one number.

In other words, the BB predicts that the CMB will have a black body lineshape, but it doesn't predict the temperature very well at all. So you have to determine the temperature from data: you make two measurements of the spectrum at two different frequencies (you can use that to determine the temperature by assuming the spectrum is black body).

But then you then have a prediction for every single other frequency, and you can make hundreds or more such measurements (you're limited by the frequency resolution of your detectors), any one of which could disprove your prediction.
Click to expand...

One might make similar arguments for its isotropy, I guess, limited by the angular resolution of your detectors (and subject to foreground removal and other complications).
 
Cankerskull said:
I was recently watching The Universe on the History Channel, and they mentioned the Cosmic Microwave Background radiation. Sepcifically, they said how it was the largest piece of evidence in supporting the Big Bang theory.

My question is: How? The show briefly explained that it was the left over radiation from the Big Bang. But how do we know that it's a remnant from the Big Bang?

I've read the Wiki entry for CMB, but wasn't able to make much sense of it. Can anyone provide an explanation for "dummies"?
Click to expand...


Hi Cankerskull, WElcome

You may not can an explanation for dummies but keep asking!
 
MattusMaximus said:
Just as a quick aside, it is also worth noting that the CMB is just one of many lines of independent evidence which support the big bang cosmology.
Click to expand...

Yes, I did at least know that much, but thanks for pointing it out :) And thanks to everyone for the replies! It makes much more sense now.
 
Or another way to summarize it: The CMB was predicted by the Big Bang Theory.

No previous theory predicted its existence.

It might be possible (and, that is an awfully big might) that the CMB is not really a remanant of the Big Bang. But, no one else has a better hypothesis for it, yet.
 
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