Does the CMB itself tell us that it started at 3000K?

[clawsoon]

TL;DR Summary

Could the original temperature of the cosmic microwave background be deduced without a theory of a cooling universe? If we didn't know that the universe started hotter than 3000K, would we be able to tell that it was generated at 3000K and not 1000K?

Hi.

Could the original temperature of the cosmic microwave background be deduced without a theory of a cooling universe? I.e. if we didn't know that the universe started hotter than 3000K and cooled to below 3000K, would we be able to look at the CMB and say, "Yep, that was definitely originally generated at 3000K, and not at 2000K or 1000K"?

So far I've learned that blackbody radiation which has been redshifted has an identical distribution to blackbody radiation from a cooler source that hasn't been redshifted, so I'm guessing that the answer to my question is "no".

However, I do know of other examples where we can use the radiation itself to learn more about its origins. E.g., using spectral absorption lines you can figure out how fast something is moving away from you even if you don't have a theory of an expanding universe by comparing those lines to what you get in the lab. Or with Cepheid variable stars you can figure out the absolute brightness of a star even if you don't have a theory of star formation by comparing its period to the period of nearby Cepheids whose distance you've measured via parallax. Is there any equivalent for CMB (or blackbody radiation in general) where you can figure out the original temperature even if you don't have a theory of universe formation?

Apologies if this has been asked before; a few hours of searching around the web hasn't turned up an answer, so I hope it's a fresh-ish question.

 

[hutchphd]

I know of no direct way from the nature of the present blackbody spectrum.
Of course a good metallurgist, for instance, can look at a knife blade and infer its thermal history from a variety of clues but not simply from its present temperature. And, in my experience, a good cosmologist can weave detailed scenarios from mere wisps data: some of them actually may be true!

 

[PeterDonis]

Could the original temperature of the cosmic microwave background be deduced without a theory of a cooling universe?

Not only can it, it is. Basically, it's deduced from the ionization temperature of hydrogen--the temperature at which the average photon in the universe has enough energy to knock the electron out of a hydrogen atom and ionize it. (There are more complications to the detailed calculation than that, but that's the basic idea.). Comparing this with the currently observed temperature of the CMB tells us that the universe has expanded by a huge factor (about 1000) since the CMB was formed.

 

[PeterDonis]

I know of no direct way from the nature of the present blackbody spectrum.

That's true, you can't deduce it from the spectrum alone (since AFAIK we are unable to detect any spectral lines in the CMB spectrum, so we have no direct measurement of its redshift). But you can deduce it from knowing the physics of the ionization of hydrogen, since the CMB was formed when the temperature of the universe dropped below the ionization temperature of hydrogen, so that photons stopped interacting constantly with matter and began propagating freely (with, as I noted in post #3 just now, plenty of complications in the details, but that's the basic idea).

 

[clawsoon]

Basically, it's deduced from the ionization temperature of hydrogen--the temperature at which the average photon in the universe has enough energy to knock the electron out of a hydrogen atom and ionize it.

To put my question a different way: Let's say we had a theory that the CMB was generated by some other process that occurs at, say, 500K instead of 3000K. Would the data from COBE and Planck by itself force us to throw out that theory and develop a 3000K theory? And, if so, what about the COBE and Planck data would force us to throw away our 500K theory?

 

[PeterDonis]

Let's say we had a theory that the CMB was generated by some other process that occurs at, say, 500K instead of 3000K. Would the data from COBE and Planck by itself force us to throw out that theory and develop a 3000K theory?

No. As I noted in post #4, none of our measurements of the CMB can detect spectral lines, which would give us a direct determination of the CMB redshift (and that in turn would tell us by what factor the CMB temperature had decreased from emission to now). So we need some independent knowledge of the process that produced the CMB to determine at what temperature it was emitted.

 

[Ibix]

And, in my experience, a good cosmologist can weave detailed scenarios from mere wisps data: some of them actually may be true!

It's worth noting that the CMB is pretty convincing. It was predicted by cosmologists before its discovery, but actually discovered by a couple of communications engineers who knew nothing of cosmology and just characterised some noise in their microwave receiver. So it's more or less an ideal example of experimentalists being blind to the theories they are testing.

 

[hutchphd]

I think it is wonderful that the Bell Labs guys were at Holmdale NJ almost within shouting distance of the group at Princeton wwho predicted it.

 

[Orodruin]

Let's say we had a theory that the CMB was generated by some other process that occurs at, say, 500K instead of 3000K.

It is hard to imagine such a theory. We know the physics governing the CMB much too well for that to be viable.

 

[clawsoon]

It is hard to imagine such a theory. We know the physics governing the CMB much too well for that to be viable.

It's worth noting that the CMB is pretty convincing. It was predicted by cosmologists before its discovery

The CMB is great confirmation for the theory. I think what I might ultimately be looking for is whether the CMB could have motivated the theory.

Imagine an alternate history where optical telescopes stayed stuck with 1620s technology but radio telescopes advanced rapidly. Instead of a 100-inch reflector at Mt. Wilson, Hubble has a radio telescope that gives him COBE-quality data. Scientists have no redshift data from receding galaxies, but they do have a detailed map of CMB anisotropies.

Could they have figured out that the universe is expanding from that CMB data alone? Could they have come up with the Big Bang?

If there's something about the CMB data itself which conclusively indicates that it started at 3000K and not 3K, then presumably they could've. But reading a couple of papers that I don't understand about CMB anisotropies, it seems like there are other generators of similar signals (spinning dust, spiraling electrons, etc.) that have to be filtered out from the CMB signal, and it seems reasonable that a theorist of the 1930s who had no redshift data would almost inevitably come up with spinning dust or spiraling electrons or something similar as an explanation for the CMB.

Is there anything about the CMB itself that would force a scientist who had no other evidence for an expanding universe to conclude that the universe is expanding?

 

[PeterDonis]

Imagine an alternate history where optical telescopes stayed stuck with 1620s technology but radio telescopes advanced rapidly. Instead of a 100-inch reflector at Mt. Wilson, Hubble has a radio telescope that gives him COBE-quality data. Scientists have no redshift data from receding galaxies, but they do have a detailed map of CMB anisotropies.

Could they have figured out that the universe is expanding from that CMB data alone?

Maybe not from the CMB data alone, but unless in your alternate history scientists also have zero knowledge of chemistry and atomic spectra, they aren't stuck with just the CMB data alone. And with knowledge of chemistry and atomic spectra, scientists would know that there is no process that would be expected to produce black body radiation at 3 K uniformly all over the sky, with anisotropies only at the 1 part per 100,000 level. So they would know that this uniform 3 K black body radiation could not have originally been produced at that temperature. See further comments below.

reading a couple of papers that I don't understand

If you don't understand them, how can you presume to conclude anything from them?

it seems like there are other generators of similar signals (spinning dust, spiraling electrons, etc.) that have to be filtered out from the CMB signal

Unless you give references to the actual papers, so we who might understand them can read them, there is no way of responding to this. My understanding from what I have read is that there are plenty of processes that can add noise to the CMB signal, but the known processes that could produce a CMB signal--black body radiation all over the sky, uniform to 1 part in 100,000--are few and far between.

Also, if in your alternate history scientists are able to take into account processes like the ones you mention, they obviously do know about chemistry and atomic spectra (and quantum mechanics and electrodynamics, etc., etc.), so the claim that the only data they have relevant to explaining the CMB is the CMB data itself is obviously wrong.

 

[Ibix]

The other thing is that Friedman's Equations were developed before Hubble's discovery of red shifts. The implication of an expanding universe and a singularity in the past actually caused Einstein to develop the cosmological constant in a (mostly) unsuccessful effort to avoid it. So the basics of expanding universe theory were known without experimental support. My feeling is that running an expanding universe backwards would necessarily imply a hot dense state in the past, and a near-uniform glow across the sky is consistent with that theory whether you know about galactic redshifts or not.

 

[clawsoon]

If you don't understand them, how can you presume to conclude anything from them?

Good point. That's why I'm asking questions rather than going with any of my own conclusions. :-)

Unless you give references to the actual papers, so we who might understand them can read them, there is no way of responding to this.

Fair enough. Here they are:

https://iopscience.iop.org/article/10.1088/1361-6633/aa94d5/meta

https://academic.oup.com/ptep/article/2014/6/06B101/1561631

so the claim that the only data they have relevant to explaining the CMB is the CMB data itself is obviously wrong.

Fair point again. So let's say that the only other relevant data they have is ground-based and not cosmological.

My understanding from what I have read is that there are plenty of processes that can add noise to the CMB signal, but the known processes that could produce a CMB signal--black body radiation all over the sky, uniform to 1 part in 100,000--are few and far between.

That is one of the vague impressions I'm forming from these papers that I'm not understanding, though so far I haven't seen anyone say what you're saying explicitly and definitively. Definitely plausible, but could all other possible sources of CMB really be ruled out if we didn't already know the universe was expanding?

It seems like the history of the role that CMB played made it unnecessary to figure that out: First it was used to decide between theories that already accepted expansion (steady state vs. big bang), then later to decide between theories that already accepted big bang (inflation vs. the others). No-one with the knowledge and smarts to rule out all non-3000K sources of CMB seems to have bothered, since it wasn't a good use of their time.

Unless maybe somebody did it during the '50s and '60s in the steady state-vs-big bang debates? I'm not familiar enough with the field to even know where to start looking, and sci-hub doesn't seem to have many articles that are that old anyway...

 

[clawsoon]

The other thing is that Friedman's Equations were developed before Hubble's discovery of red shifts.

Although... surely his equations would've ended up being a curious footnote if we had discovered that the universe is not, in fact expanding.

 

[Ibix]

Although... surely his equations would've ended up being a curious footnote if we had discovered that the universe is not, in fact expanding.

The thing is, if GR is accurate (and it was already known to be better than any other theory we had, even at the time it was created), and if everything is pretty much the same everywhere, Friedman's equations follow. That was why Einstein had to start adding a cosmological constant without any real basis when he wanted to duck the implications of Friedman - because there was nothing to attack in Friedman's argument. You can't have a non-expanding universe (unless it's closed and already into its contracting phase) without developing a lot of extra theory. An expanding universe is really the very simplest thing you can have in a relativistic universe.

Unless maybe somebody did it during the '50s and '60s in the steady state-vs-big bang debates? I'm not familiar enough with the field to even know where to start looking, and sci-hub doesn't seem to have many articles that are that old...

I think you are coming at this backwards, philosophically speaking. We cannot seriously propose a non-3000K last scattering surface because there's no known way to have such a thing. You can't just say "what if the last scattering surface were a lower temperature" - that's just saying "what if physics didn't work the way it does". It's not an answerable question. To make an answerable question, you have to propose a model that causes light not to propagate freely in the interstellar medium at temperatures below 3000K. Then we can consider the implications of the model, and use it to make predictions which we might test. And since we know that light does propagate through low density non-ionised cool gas, the model would need to explain why it didn't in the past but does now.

So I don't think you'll find papers from the sixties eliminating "low temperature CMB" theories, because I don't think there ever were any. The expanding universe is the simplest hypothesis, and a CMB at the ionisation temperature of hydrogen is a fairly straightforward implication. You need to propose unknown physics in the middle of fairly well-trodden ground to have anything else.

For something that worked out the other way, look at the development of special relativity. Maxwell's equations were problematic, and the simplest way to start thinking about fixing them was to try proposing an ether. People looked for it and failed to find it, eventually leading to the development of new physics at the edges of what we already knew.

 

[PeterDonis]

that's why I'm asking questions rather than going with any of my own conclusions.

Your statements here...

it seems like there are other generators of similar signals (spinning dust, spiraling electrons, etc.) that have to be filtered out from the CMB signal, and it seems reasonable that a theorist of the 1930s who had no redshift data would almost inevitably come up with spinning dust or spiraling electrons or something similar as an explanation for the CMB.

...are conclusions, not questions.

 

[clawsoon]

...are conclusions, not questions.

I tried to say "seems" as many times as possible to indicate that I wasn't coming to any conclusions, though I see how it could be read differently.

 

[PeterDonis]

So let's say that the only other relevant data they have is ground-based and not cosmological.

Data about chemistry and atomic spectra and quantum mechanics and electrodynamics is ground-based. So this proviso doesn't change anything I said.

could all other possible sources of CMB really be ruled out if we didn't already know the universe was expanding?

It's not a question of ruling out "all other possible sources". It's a question of what "possible sources" there are in the first place. What process could produce background radiation that is uniform all over the sky to 1 part in 100,000? That is the difficulty: any process that produced such a thing would have to be uniform all over the universe to 1 part in 100,000. And no process that we currently observe is like that. Spinning dust, spiraling electrons, etc., happen differently in different places--there's a lot of dust in some places and virtually no dust in others, there are strong magnetic fields for electrons to spiral in in some places and no such fields in others. There is no process like that that is occurring uniformly all over the universe to 1 part in 100,000.

In other words, the universe when the CMB was produced had to be very, very different from our universe today. It had to be uniform to 1 part in 100,000 (which our universe today is not, it has non-uniform structures on much larger scales than that). And it had to have some process happen at some point that produced radiation of the same uniformity, that then propagated freely thereafter. The only process that is known to meet that requirement is a transition from plasma (electrons and ions) to gas (neutral atoms), and all such transitions occur at temperatures of a few thousand degrees. So the only known way for the CMB we see today to have been produced is for the universe at some point to have been filled with plasma, uniform to 1 part in 100,000, that transitioned to gas and emitted radiation at some temperature of a few thousand degrees, and for that radiation to then have redshifted by a factor of 1000 or so.

The only model of the universe we know of in which such a process is expected to occur is a universe that expands from an initial hot, dense state, as in the Friedmann models. And in fact the existence of a CMB was predicted from such models before the CMB was discovered. No other model predicted any such thing. Attempts were made after the CMB was discovered to try to explain it in other models (such as the steady state model), but such attempts were obviously ad hoc and did not work out well.

It is an interesting historical question to ask what would have happened if the CMB had been discovered before GR and the expanding Friedmann models were discovered, or before any evidence of redshifts of distant galaxies was known, so that no model of the universe was known that could produce a CMB. Would the arguments above have led anyone to propose an expanding universe as an explanation for the CMB? But such questions, while interesting, are unanswerable.

 

[clawsoon]

So I don't think you'll find papers from the sixties eliminating "low temperature CMB" theories, because I don't think there ever were any. The expanding universe is the simplest hypothesis, and a CMB at the ionisation temperature of hydrogen is a fairly straightforward implication.

Lots of respectable physicists came up with serious alternatives to those simplest hypotheses, though, didn't they? As I understand it, a feature of steady-state theories is that they proposed expansion without a high-temperature past. Maybe one of the ways to put a stake into the hearts of those theories would be to eliminate all possible low-temperature sources of CMB based on features of CMB itself? I don't picture steady-state theorists who had put their lives into their theory simply rolling over and saying, "Gee, I can't think of any way to save my theory from the CMB, better just give up." Surely they'd come up with some alternate low-temperature CMB sources to save their steady state theories, which the big bang theorists would then have to demonstrate didn't work?

I have the impression that there were some cyclic theories that were taken seriously for a while, too. Did any of them propose a cycle peaking at below 3000K, or did they all propose a very hot and dense universe at the peak of the cycle?

 

[clawsoon]

What process could produce background radiation that is uniform all over the sky to 1 part in 100,000?

I won't quote all of it, but that was a great, detailed explanation, exactly what I was looking for. Thanks!

Would the arguments above have led anyone to propose an expanding universe as an explanation for the CMB? But such questions, while interesting, are unanswerable.

In theory we could answer the "could have" question with answers like the one you've given, even if we can't answer the "would have" question.

 

[PeterDonis]

Lots of respectable physicists came up with serious alternatives to those simplest hypotheses, though, didn't they?

Depends on what you consider a "serious alternative".

As I understand it, a feature of steady-state theories is that they proposed expansion without a high-temperature past.

They proposed apparent expansion. The actual "size" of the universe was constant in steady state models.

Plus, steady state models had to propose new physics, continuous creation of matter, that violates local energy conservation.

I don't picture steady-state theorists who had put their lives into their theory simply rolling over and saying, "Gee, I can't think of any way to save my theory from the CMB, better just give up." Surely they'd come up with some alternate low-temperature CMB sources to save their steady state theories, which the big bang theorists would then have to demonstrate didn't work?

Yes, and that's exactly what happened. Steady state proponents proposed explanations for the CMB after it was discovered, all of which were shot down.

I have the impression that there were some cyclic theories that were taken seriously for a while, too.

Some such proposals still are, at least by some subset of cosmologists.

Did any of them propose a cycle peaking at below 3000K

No.

or did they all propose a very hot and dense universe at the peak of the cycle?

Yes. The main point of cyclic models was to account for a hot, dense universe in the past without having to have an initial singularity.

 

[PeterDonis]

Steady state proponents proposed explanations for the CMB after it was discovered, all of which were shot down.

Ned Wright gives a decent brief discussion here:

https://www.astro.ucla.edu/~wright/stdystat.htm

The steady state proposals for explaining the CMB were what Wright calls "Quasi-Steady-State Cosmologies" or "QSSC".

 

[clawsoon]

Ned Wright gives a decent brief discussion here:

That was really informative and taught me a lot more about how CMB observations have been used to rule out various theories. Thanks again!

 

[PeterDonis]

That was really informative and taught me a lot more about how CMB observations have been used to rule out various theories. Thanks again!

You're welcome! If you found that article helpful, Wright's cosmology tutorial and FAQ, and his articles on the age of the universe and on distance measures (links are at the bottom of the article I linked to) are also worth reading.