What distinguishes the CMB as primordial, rather than just the background temperature of intergalactic gas and dust?
The fact that it’s both everywhere and very uniform, for one. Gas and dust and such varies much more in both temperature and distribution. The CBR only varies very slightly, and is everywhere.
It really isn’t primordial. It is actually the background temperature of ambient gas. But the trick is, it’s the background temperature of gas that was emitting light billions of years ago, and which has since been extremely redshifted.
Way back when, the Universe was hot enough that all of the matter in it was ionized. Ionized matter interacts very strongly with light, which means that it’s very opaque. The gas was constantly producing light, but then constantly absorbing it again, such that any given photon only went for a very short distance before being reabsorbed.
Then, about 380,000 years after the Big Bang, the Universe cooled down enough that neutral atoms could form. And neutral atoms are much more transparent than ions. All of a sudden, all of that light that kept on getting emitted and reabsorbed, wasn’t getting reabsorbed any more, and just kept going on its merry way. The vast majority of it still hasn’t been stopped by anything. But occasionally, some of it hits a planet or a TV antenna or something.
Since then, the Universe has actually re-ionized, mostly due to starlight. But by the time of re-ionization, the density had decreased so much that, even ionized, it was still pretty darned transparent.
Interesting factoid about the CMBR: When first emitted, were there any humans around to see it, it would look white. Actually a bit yellowish, but since it would be the only light around, the human visual system would normalize it to look white.
The other thing about the CMBR is that it has a really high amplitude for what it is. 3 K is extremely cold, and that cold should be overwhelmed by everything warmer out there. And sure there is hotter stuff around. But the CMBR is omnipresent. It is the most perfect black body radiation known. And everywhere you look into empty space, there it is.
That it be so is because it is the remains of the radiation from the entire mass of the universe when it was incandescently hot. Just now seriously stretched out. There was a heck of a lot of energy to start with.
When Penzias and Wilson first found the signal it was 100 times stronger than predicted for general background noise expected at the wavelengths measured in their equipment. They were diligent enough to work through the entire system to eliminate extraneous noise sources and still it persisted. And from everywhere. The confluence of events where they obtained a preprint of the paper predicting the CMB within a big bang cosmology and how it became clear what it was so soon after discovery was remarkable. (It is still strange that they got the Nobel for it, and the theorist Jim Peebles who predicted the CMB didn’t get one for another 4 decades. Theorists never seem to garner the attention.)
Possible nitpick: Black holes should be even more perfect blackbodies. But we’ve never actually detected the Hawking radiation from one, so you can still say that the CMB is the most perfect blackbody ever detected.
It’s seriously impressive, to anyone who knows how to read a graph. You’ll usually see it presented with teeny-tiny error bars… each of which represents 1000 standard deviations, because that’s what it takes to even make the error bars visible at all.
In other words, Science: It works, bitches.
The exception being if the theoreticians are men and experimentalists women. The theoretician who predicted parity violation got the Nobel, but the woman who actually verified it got bupkes.
Actually, it would appear to violate Nobel’s will to ever give it to a theoretician. He wanted to support practical results.
Well, that certainly favors the experimentalists. But a theory can still have practical results.
Practical results should tend to rule out astronomers and research in cosmology in general. Little practical in worrying about black holes or the nature of the universe and the like. It would be fun (or perhaps depressing) to be a fly on the wall at discussions for awarding the Nobel for physics. An almost impossible task really. One with a huge dose of damned if you do, and damned if you don’t.
The modern world of big science experimental physics is something nobody would have envisioned in Nobel’s time. At least there is a lot more to physics than just poking the boundaries of the Standard Model.
Structure of DNA (Rosalind Franklin), discovery of pulsars (Jocelyn Bell) etc. It isn’t a good look. The rule that can only be three does of course make for some bad decisions. None-the-less, some omissions are egregious.
(IMHO (and others) Fred Hoyle should have got one, but he blotted his copybook, and essentially made it untenable. Which is huge shame.)
Rosalind Franklin died before that prize was awarded. Not giving one to Jocelyn Bell was an absolute outrage. You might mention Lise Meitner as well. This, IMHO, is what DEI is–or should be–about.
I once spent several hours in the dark with her and Dame Janet Baker the opera singer. Both fascinating characters.
This just popped up in my feed: