Comments on: Explain to me like I'm first-year university: Age of cosmic background
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background/
Comments on Ask MetaFilter post Explain to me like I'm first-year university: Age of cosmic backgroundSat, 15 Jan 2022 08:08:23 -0800Sat, 15 Jan 2022 08:29:59 -0800en-ushttp://blogs.law.harvard.edu/tech/rss60Question: Explain to me like I'm first-year university: Age of cosmic background
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background
My Google-fu is failing me on this. I've found many articles for laymen saying that the cosmic microwave background radiation can be used to date the universe, and I've found the occasional math-heavy paper or Wikipedia article assuming that I already know how the CMB is used to date the universe. The explanation I can't seem to find is the one in the middle: Explain to a first semester physics or chemistry student how the CMB is used to find the age of the universe. <br /><br /> Here's what I think I've gathered so far:<br>
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<ol><br>
<li>The spectrum of the CMB is mostly shaped like blackbody radiation, but red-shifted. Does this mean that blackbody radiation has such a specific shape at different temperatures that the original temperature can be deduced even after redshifting? I can see how redshifting is detected in the case of spectral absorption lines, since they form unique patterns, but graphs of blackbody radiation at different temperatures always kinda look to me like they could be squished and squashed to look like each other, not giving a unique signature of the original signal.<br>
<li>As a possible alternate way to reconstruct the original signal, I've seen a bunch of references to a specific model of how the CMB originated: Protons and electrons started hooking up to make hydrogen, making the universe transparent, since free-floating electrons no longer gobbled up all the photons. Maybe there are spectral absorption lines in the CMB indicating the presence of hydrogen, which allows us to reconstruct the original signal?<br>
<li>So let's say we can reconstruct the original signal, and thus deduce its redshift. If we had the Hubble constant, we could do some multiplication and figure out how far away from us, and thus how long ago, the signal must have started. Great! ...but from what I've been reading, the CMB can be used to calculate the Hubble constant, and the value we get from the CMB is a bit different from the Hubble constant that we get from supernovae and variable stars. So if everything can be calculated from the CMB, how is the chicken-and-egg resolved, where to find the age from the redshift we need the Hubble constant, but we can get the Hubble constant from the redshift... or... from... something else about the CMB?<br>
<li>But the CMB came from everywhere at once, didn't it? So how does a redshift distance even make sense if we're trying to measure from us to everywhere at once?<br>
</li></li></li></li></ol><br>
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And that's about as far as I've gotten so far. A confused muddle of contradictory guesses based on basic physics knowledge about how the CMB could be used to measure the age of the universe.post:ask.metafilter.com,2022:site.360392Sat, 15 Jan 2022 08:08:23 -0800clawsooncosmicmicrowavebackgroundageofuniverseBy: Nelson
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141653
<a href="https://wmap.gsfc.nasa.gov/universe/uni_age.html">Here's a start</a> (from 2012).<blockquote>Measurements by the WMAP satellite can help determine the age of the universe. The detailed structure of the cosmic microwave background fluctuations depends on the current density of the universe, the composition of the universe and its expansion rate. As of 2013, WMAP determined these parameters with an accuracy of better than than 1.5%. In turn, knowing the composition with this precision, we can estimate the age of the universe to about 0.4%: 13.77 ± 0.059 billion years!<br>
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How does WMAP data enable us to determine the age of the universe is 13.77 billion years, with an uncertainty of only 0.4%? The key to this is that by knowing the composition of matter and energy density in the universe, we can use Einstein's General Relativity to compute how fast the universe has been expanding in the past. With that information, we can turn the clock back and determine when the universe had "zero" size, according to Einstein. The time between then and now is the age of the universe. There is one caveat to keep in mind that affects the certainty of the age determination: we assume that the universe is flat, which is well supported by WMAP and other data. If we relax this assumption within the allowed range, the uncertainty increase a bit. Inflation naturally predicts a very nearly flat universe.</blockquote><br>
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<a href="https://www.scientificamerican.com/article/how-a-dispute-over-a-single-number-became-a-cosmological-crisis/">Scientific American has a recent article</a> that also describes it.<br>
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The point of confusion for me is I have it in my head that there's a discrepancy in the two main methods measuring the age of the universe (the other is the age of the oldest stars, based on models of how stars change as they age). I thought the difference was significant but I'm finding some recent articles now saying the difference is resolved, others including the SciAm article saying it's still a problem.comment:ask.metafilter.com,2022:site.360392-5141653Sat, 15 Jan 2022 08:29:59 -0800NelsonBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141662
(One other thing that I think I've figured out: When the NASA article says, with scare quotes, "zero" size, I believe that they actually mean infinite density, since apparently the math for the <a href="https://en.wikipedia.org/wiki/Lambda-CDM_model">most popular</a> model of the Big Bang <a href="https://physics.stackexchange.com/a/24020/203679">assumes an infinitely large universe</a> to make the math easier, as one does.)comment:ask.metafilter.com,2022:site.360392-5141662Sat, 15 Jan 2022 08:45:18 -0800clawsoonBy: OHenryPacey
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141665
I've recently rewatched Leonard Susskind's <a href="https://www.youtube.com/watch?v=P-medYaqVak&ab_channel=Stanford">Cosmology</a> lectures on youtube. And the gist of his explanation is that there was a time when things were so close together and hot that they were a plasma, like the surface of the sun, and therefore opaque, and that's as far back as we can "see", and what we see is the CMB.<br>
Since we can infer the exact temperature of the plasma, we can use red shift and the hubble constant to determine how long ago the universe was this temperature.comment:ask.metafilter.com,2022:site.360392-5141665Sat, 15 Jan 2022 08:56:41 -0800OHenryPaceyBy: teraflop
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141667
<i>graphs of blackbody radiation at different temperatures always kinda look to me like they could be squished and squashed to look like each other, not giving a unique signature of the original signal. </i><br>
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I'm far from having enough knowledge to fully answer your question, but on this point, your intuition is right. <a href="https://www.astro.umd.edu/~miller/teaching/astr422/lecture04.pdf">An ideal blackbody that has been redshifted by a Doppler factor of 2 has an identical spectrum to a blackbody with half the original temperature.</a><br>
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So the overall shape of the spectrum doesn't give you enough information to deduce <i>both</i> the original temperature and how much it has been redshifted. That information has to be deduced from more detailed analysis of the CMB anisotropy.comment:ask.metafilter.com,2022:site.360392-5141667Sat, 15 Jan 2022 09:02:19 -0800teraflopBy: zengargoyle
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141686
<a href="https://www.youtube.com/watch?v=73ZXk_I9h5s">We still don't know the actual age of the Universe | Night Sky News July 2019 - YouTube</a> - at about 13m in.<br>
<a href="https://www.youtube.com/watch?v=R30ratQanWw">The Age of the Universe - Sixty Symbols - YouTube</a> - sorta same thing but a year earlier.<br>
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They're up to like 5 different answers to what is the Hubble Constant.<br>
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<a href="https://www.youtube.com/watch?v=Y5rZ58Q_0zk">Observing the Birth of the Universe - with Lyman Page - YouTube</a> - this might help.<br>
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As far as I can tell by fast-forwarding through some videos.... The plasma state was about 1/2 the temperature of the surface of the Sun. We have a theory that this transition from plasma to atoms happened some 380My after the bang. What the theory goes is that in that time the splotches/fluctuations should be around a certain size. Those fluctuations are only about 1/4000 of a degree C. We then use some trig to use the predicted size of the splotches to their angular cover on the sky to find the distance. The distance is the age of the universe taking into account the speed of light from here to the edge of what we can see (the CMB). So it's not "redshift" or spectrum per-se, it's the size of the splotches of the minute variations of temperature should be some particular size. We calculate the age / Hubble constant from that.comment:ask.metafilter.com,2022:site.360392-5141686Sat, 15 Jan 2022 10:48:25 -0800zengargoyleBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141688
<i>So the overall shape of the spectrum doesn't give you enough information to deduce both the original temperature and how much it has been redshifted. That information has to be deduced from more detailed analysis of the CMB anisotropy.</i><br>
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Any additional information on how CMB anisotropy can be used to reconstruct the original signal would be great. The answer that I'm seeing virtually everywhere (summarized mostly briefly <a href="https://astronomy.stackexchange.com/a/29181">here</a>, but also <a href="https://www.ringnebula.com/science/cmb-notes-calculations.htm">here</a> and <a href="https://webhome.phy.duke.edu/~kolena/cmb.htm">here</a> and <a href="https://thecuriousastronomer.wordpress.com/2016/06/13/the-temperature-of-the-universe-at-recombination-decoupling/">here</a> and <a href="https://astronomy.stackexchange.com/questions/47931/why-was-the-cosmic-microwave-background-cmb-released-at-a-blackbody-temperatur">here</a> and <a href="https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect31/lecture31.html">here</a>) seems to be theory-based rather than observation-based: Assuming the universe started out hot and cooled down, photons would start flying around freely at around 3000K, therefore the original signal must have been 3000K blackbody radiation. So far I haven't found anything saying clearly how the signal itself can be used to reconstruct the original signal.comment:ask.metafilter.com,2022:site.360392-5141688Sat, 15 Jan 2022 10:50:59 -0800clawsoonBy: kms
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141711
The CMB is produced at a very specific moment when the early universe had a temperature that allowed electrons and protons to form atoms for the first time. Before that moment, the universe was hotter and all the gas was ionized and because photons bounce off of electrons very effectively, the universe was opaque, like being in a dense fog, and the photons were "coupled" to the electrons (=they share the same temperature because they come into equilibrium by bouncing off each other all the time). At a temperature of around 3000 K, atoms form and then the photons could travel without bouncing off electrons anymore. Those photons that were "set free" at that time when the universe hit T=3000 K (380,000 years after the Big Bang) are the CMB, and they started off with the spectrum of a blackbody with T=3000K. This spectrum is a smooth curve with a peak at a wavelength of 1000 nm (= 1 micron).<br>
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When we measure the CMB now, it has a spectrum of a blackbody with T=2.73 K, which has a peak wavelength of around 1 mm. The difference in the spectra is because of the expansion of the universe, basically stretching out the wavelength of light in the time since the CMB was emitted and now. From the comparison of the temperatures that characterize the two blackbodies (3000 K vs ~3K, or equivalently the comparison of the two peak wavelengths of 1 µm and 1 mm) we can see that the wavelength of light has grown by a factor of 1000 over that time period. From observations of the expansion rate of the universe ~today from looking at Hubble's Law (how fast distant galaxies move away from us as a function of their distance), you can figure out how long it would take to expand that wavelength by a factor of 1000. And you arrive at the time between CMB photons being released and now. There are some assumptions in there that are more complicated if you dig into cosmology, but that is the basic explanation.<br>
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You mentioned the measurement of the Hubble constant from Planck, which is a topic of great interest now. That comes from a different property of the CMB than the spectrum/temperature. That measurement is based on the structure of very tiny deviations of the CMB from a perfect blackbody. Those are called CMB anisotropies and their structure (angular size on the sky) tells us a lot about the state of the early universe. Currently there is some disagreement about the Hubble constant you would measure from galaxies in the present day universe and what is observed from the structure of CMB anisotropies. Astronomers are very interested/concerned/excited about this, it is called the "Hubble tension".comment:ask.metafilter.com,2022:site.360392-5141711Sat, 15 Jan 2022 12:04:08 -0800kmsBy: zengargoyle
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141720
<i>Explain to a first semester physics or chemistry student how the CMB is used to find the age of the universe.</i><br>
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This is <b>so</b> not first semester physics or chemistry by a long shot. Looking at your links, I still don't understand what you mean by "how the signal itself can be used to reconstruct the original signal."comment:ask.metafilter.com,2022:site.360392-5141720Sat, 15 Jan 2022 12:40:16 -0800zengargoyleBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141736
<i>I still don't understand what you mean by "how the signal itself can be used to reconstruct the original signal."</i><br>
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I'm thinking specifically of spectral absorption lines, where you can look at the unique pattern of lines and say, "Oh, that pattern looks exactly like the pattern that would be created by light going through oxygen, but shifted over by 300nm," so you can reconstruct the original signal by shifting all of the signal you're looking at over by 300nm in the other direction. (Or however it is that the math actually works with a doppler shift.)<br>
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Cepheid variable stars would be another example of this: If you measure the star's period, you can use data from parallax measurements to nearby Cepheids to reconstruct the absolute brightness of the light the Cepheid you're looking at is emitting.comment:ask.metafilter.com,2022:site.360392-5141736Sat, 15 Jan 2022 13:56:14 -0800clawsoonBy: zengargoyle
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141737
<a href="https://www.youtube.com/watch?v=PPpUxoeooZk">Sound Waves from the Beginning of Time - YouTube</a> - about the sound like density fluctuations in the opaque age of the universe. Once transparency happened they pretty much froze in place. Now due to deep sky surveys we can still see those density patterns in the visible universe in the location of galaxies. I suspect this is how the CMB was maybe bootstrapped a bit by using our best guesses for the Hubble constant to start, then once it was studied well enough sorta run backwards to try and tweak the Hubble constant.<br>
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So yeah, the CMB stuff took a good guess for the Hubble constant to get started, but once found and sorta confirmed could be used to calculate it's own ideal Hubble constant based on mapping the minor fluctuations in temperature/density onto the surveyed universe of galaxies.comment:ask.metafilter.com,2022:site.360392-5141737Sat, 15 Jan 2022 14:00:20 -0800zengargoyleBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141743
<i>I'm thinking specifically of spectral absorption lines</i>... and what I'm looking for is a similar way to look at blackbody radiation and say "this was definitely emitted by a 3000K source, not a 2000K source." Does such a technique exist?comment:ask.metafilter.com,2022:site.360392-5141743Sat, 15 Jan 2022 14:16:40 -0800clawsoonBy: kms
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141747
I'm not sure I totally understand your question, but the peak of the blackbody spectrum uniquely identifies the temperature. So when you see a blackbody peaking at ~1mm, you know the temperature is ~3 K.comment:ask.metafilter.com,2022:site.360392-5141747Sat, 15 Jan 2022 14:38:28 -0800kmsBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141753
<i>So when you see a blackbody peaking at ~1mm, you know the temperature is ~3 K.</i><br>
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But does that tell you anything about the temperature of the original source of the radiation if there's redshift involved?comment:ask.metafilter.com,2022:site.360392-5141753Sat, 15 Jan 2022 15:02:45 -0800clawsoonBy: kms
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141776
The temperature of the original source (i.e. T=3000 K) is set by the temperature when electrons and protons combine to form atoms.comment:ask.metafilter.com,2022:site.360392-5141776Sat, 15 Jan 2022 16:55:08 -0800kmsBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141777
I will take that as a "no".comment:ask.metafilter.com,2022:site.360392-5141777Sat, 15 Jan 2022 17:01:43 -0800clawsoonBy: zengargoyle
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141786
I'm undecided. Above my level. I can see it multiple ways.<br>
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The opaque soup formed from quantum weirdness in the ionized state. It didn't start with neutral hydrogen being ionized and de-ionized in that way that would create an absorption line.<br>
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Due to the equilibrium state before the time of re-combination (which should just be combination because there's no 're-' in there) if for a moment a neutral hydrogen atom was formed by emitting that hydrogen wavelength photon, that photon was quickly absorbed by some other neutral hydrogen atom thereby ionizing it. We're at equilibrium here.<br>
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The energy/amount of photons in the opaque soup has a bazillion times the energy of the mass having particles (protons/electrons/baryons).<br>
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When things went transparent at the 3000K temperature whether or not there would even be a hydrogen absorption line... Not really sure.<br>
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Given the balance between the like 5% of universe is baryonic (plain-old-mass) and the rest is other stuff including photons, dark matter, dark energy, neutrinos? I doubt we have the actual technology to even find the hydrogen absorption line in the CMB even if there should be one. Or maybe it's a peak.<br>
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We over the years have just barely hit down the the 1/10,000 degree fluctuation determination that gets us to where we are. First there was just a blob, enough to determine the relative movement across the average universe of our galaxy by gross redshift. Then there was finer resolution to pick out the part of microwave radiation that was due to things in our own galaxy (gas clouds and things). Then it was the Plank mission that finally got us down to the degree-angle sort of resolution. And all we'd be able to resolve is that big 3000K peak that's now 2.7K +- 1/10000K over a large area.<br>
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I don't know if there should be a peak or absorption or nothing at that 3000K pre-redshift temperature. But I doubt we could even resolve it at the moment.<br>
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I sorta think it's just that peak at 2.7K and the distribution of the tiny fluctuations that match the predictions and math of "it should be around 3000K" and that the calculations from that assumption closely match other things like the observed distances between dense clusters of galaxies vs sparse empty voids that we have a handle on measuring is where we're at right now.<br>
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You'd have to catch an early era quantum cosmologist to tell you something like there should be a 1/1000000000000000 blip for the hydrogen in that signal.comment:ask.metafilter.com,2022:site.360392-5141786Sat, 15 Jan 2022 17:48:59 -0800zengargoyleBy: doctord
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141894
<strong>kms</strong> provided an excellent explanation of how the pieces of the puzzle fit together, but based on your follow-up questions it seems like some of your preconceived ideas are getting in the way of your ability to understand, so let's start from the points of your original post:<br>
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<em> 1. The spectrum of the CMB is mostly shaped like blackbody radiation, but red-shifted.</em><br>
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Replace this with "The spectrum of the CMB, as measured now, is the spectrum of a blackbody with T=2.73 K." This is shown clearly in your Duke link COBE graphic. Drop the "but redshifted" for now (we'll get back to it). <br>
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<em> Does this mean that blackbody radiation has such a specific shape at different temperatures that the original temperature can be deduced even after redshifting? </em><br>
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Drop the "even after redshifting" and "original" for now and you have a true statement: "blackbody radiation has such a specific shape at different temperatures that the temperature can be deduced". It may look like it could be "squished and squashed", but it is unique. <br>
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<em> 2. As a possible alternate way to reconstruct the original signal ...</em><br>
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Change this to "The way to construct the original signal ...". As many of your "here" links show, the original signal is constructed from simple models that predict that the original signal is the spectrum of a blackbody with T=3000K. There is no "working backwards" reconstruction from current data and no spectral lines, just the modeling that leads to the T=3000K blackbody (which is unique). <br>
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<em> 3. So let's say we can reconstruct the original signal, and thus deduce its redshift. </em><br>
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I think this may be easier to understand using the historical sequence of events. Construct the original spectrum (blackbody T=3000K) from the model and then use it to predict the current redshifted spectrum from expansion of the universe. This was done in the 40s. The next step is where Nobel prizes get won. The models predicted microwave background radiation, but at levels beyond the reach of then current technology. Then in the 60s Penzias and Wilson used a new instrument to measure an anomalously high intensity in the microwave background, and further measurements confirmed that it had the expected blackbody spectrum (T=2.73K). <br>
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<em> If we had the Hubble constant ...</em><br>
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Why force yourself into this "chicken and egg scenario" if you don't have to? Use the observationally determined value of the Hubble constant. As <strong>kms</strong> described, Hubble constant predictions come from a different property of the CMB than the spectrum/temperature, so this is a distraction at this level of understanding. <br>
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<em> 4. But the CMB came from everywhere at once, didn't it? So how does a redshift distance even make sense if we're trying to measure from us to everywhere at once? </em><br>
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Don't picture this like Doppler redshift from a receding star. The redshift of the blackbody radiation is due to the wavelength of this radiation (which is everywhere) being "stretched" by the expansion of the universe.<br>
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Your later questions seem related to each other and seem to have a common theme of not understanding that the original spectrum comes from the models. FWIW, I thought that your <em> I will take that as a "no".</em> came off as dismissive of the time and effort that <strong>kms</strong> put into responding.comment:ask.metafilter.com,2022:site.360392-5141894Sun, 16 Jan 2022 08:22:47 -0800doctordBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141906
<a href="https://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141894">doctord</a>: <i>Why force yourself into this "chicken and egg scenario" if you don't have to?</i><br>
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It's my way of trying to understand the relative importance and independent power of the various pieces of evidence that have gone into creating our current picture of the universe. Spectral redshift without CMB can (and did) motivate a theory of an expanding universe. Could CMB without spectral redshift have done the same?<br>
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<i>I think this may be easier to understand using the historical sequence of events.</i><br>
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I will propose instead considering an alternate history: If we had ended up with no progress in optical telescopes beyond what Galileo had, but fantastic progress in radio telescopes, so that theorists in the 1930s had detailed, COBE-quality CMB data but no cosmological redshift data, would they have proposed the Big Bang? Is there anything about the CMB data itself that would've told them it started out hotter?<br>
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Reading a <a href="https://iopscience.iop.org/article/10.1088/1361-6633/aa94d5/meta">couple</a> of <a href="https://academic.oup.com/ptep/article/2014/6/06B101/1561631">papers</a> that I don't understand about CMB anisotropies and watching the Lyman Page video that zengargoyle linked <a href="https://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141686">above</a>, it seems like there are two plausible answers that I don't know how to decide between:<br>
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- There are other things out there, like spinning dust and spiraling electrons, which generate similar-ish radiation that has to be filtered out of the CMB data. Theorists with CMB-but-not-spectral-redshift data might have been able to prove to their satisfaction that sources like spinning dust and spiraling electrons explained all of what we see in the CMB data, and they might never got a clue from it that the universe is expanding.<br>
- The uniformity of the CMB suggests that it comes from a source that is/was completely connected, which isn't possible in the universe we have now because it's too big, and the polarization differences and tiny ripples in the CMB have a distribution which forces the theorist to come up with physical explanations (which in our current models are acoustical waves in the early universe, gravitational waves, diffusion damping, etc.). It's possible that all proposed physical explanations would end up being eliminated one by one by these features of the CMB data until we ended up with the model we have now.<br>
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I wonder if anyone with the knowledge to decide between those possibilities has ever felt it was worth making the effort.<br>
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<i>Your later questions seem related to each other and seem to have a common theme of not understanding that the original spectrum comes from the models.</i><br>
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I completely understand that now, thanks to detailed explanations here and elsewhere. I'm still trying to find out whether the model could have come from the CMB data rather than (as happened historically) the original spectrum coming from the model. Spectral redshift data basically forced an expanding-universe model on us; is there anything about the CMB that has the same model-generative power, or is it merely confirmatory?<br>
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Or, put another way: Redshift data led to prediction of CMB. Would CMB have led to prediction of redshift?<br>
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<i>FWIW, I thought that your I will take that as a "no". came off as dismissive of the time and effort that kms put into responding.</i><br>
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That's fair, and I apologize. I was feeling frustrated because more effort was being put into answering questions that I didn't have than was being put into understanding the questions that I did have, but that is my fault for not asking very clear questions up front.comment:ask.metafilter.com,2022:site.360392-5141906Sun, 16 Jan 2022 09:19:44 -0800clawsoonBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141922
<i>(which in our current models are acoustical waves in the early universe, gravitational waves, diffusion damping, etc.)</i><br>
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(...and also, if I'm reading correctly, specific percentages of dark matter and dark energy in the early universe. As far as I can tell, our models don't explain the precise CMB data without those.)comment:ask.metafilter.com,2022:site.360392-5141922Sun, 16 Jan 2022 10:07:56 -0800clawsoonBy: zengargoyle
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141949
Probably, we called is RADAR and would be using Doppler Effect instead of redshift. We discovered the speed of light using a toothed wheel and a mirror. Discovered that light and radio were the same electromagnetic waves at different frequencies.<br>
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Do we still see the sun and planets and stars at all? Enough to realize we orbit the sun and not the other way around?<br>
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We figured out the periodic table, prisms and mirrors, polarization, a lot of stuff that didn't necessarily involve looking closely optically at stars and further away galaxies.<br>
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If we were way advanced in the radio end of things before the optical... that doesn't have that much impact on understanding a hydrogen plasma or different elements glow differently when heated. We had alchemy and fireworks and metal forging.<br>
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If we figured out we were rotating and orbiting and found an quasar buzzing away in a particular point out there somewhere we'd notice the change in frequency every six months from when we were going towards it or going away from it.<br>
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We would probably have still discovered fission and fusion and going back to ancient earth/water/fire/wind sort of atomic thinking would have still figured helium was two hydrogens crammed together. We'd still have gravity.<br>
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I think we might still have just thought that everything started as hydrogen.<br>
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That's actually a fun question. :)comment:ask.metafilter.com,2022:site.360392-5141949Sun, 16 Jan 2022 11:53:41 -0800zengargoyleBy: zengargoyle
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5141953
I'd still recommend <a href="https://www.youtube.com/watch?v=XtMmeAjQTXc&list=PL8_xPU5epJddRABXqJ5h5G0dk-XGtA5cZ">Episode 1: Introduction - The Mechanical Universe - YouTube</a>.<br>
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Caltech, The Annenberg Foundation, on PBS in 1985. 52 lectures on physics including history, highly produced, general audience. You might find a cut-off point on optical development in there and be able to decide if maybe we'd have figured things out the same way with just way better radio technology.<br>
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That's your real first-year university physics course. I'm biased because I watched it at 15 and then at 16 spent the summer at Caltech in the same lecture room doing three semesters worth of physics....<br>
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It's just a little bit more in depth than something like NOVA or COSMOS.comment:ask.metafilter.com,2022:site.360392-5141953Sun, 16 Jan 2022 12:06:05 -0800zengargoyleBy: flug
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5142154
Everyone has covered all these topics very well above, but the references below might give you something more to think about:<br>
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<strong><em>> CMB is mostly shaped like blackbody radiation</em></strong><br>
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You can read about blackbody radiation <a href="https://en.wikipedia.org/wiki/Black-body_radiation">here</a> and <a href="https://opentextbc.ca/universityphysicsv3openstax/chapter/blackbody-radiation/#fs-id1163713090546">here</a>. Also helpful are reading about <a href="https://en.wikipedia.org/wiki/Planck%27s_law">Planck's law</a> and <a href="https://en.wikipedia.org/wiki/Wien%27s_displacement_law">Wien's Displacement Law</a>. The <a href="https://en.wikipedia.org/wiki/Planck%27s_law#Different_forms">different forms of Planck's Law</a> are particularly helpful if you want to try any calculations.<br>
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What might not be immediately apparent even after reading through all that, is this important fact: <em>The shape of the blackbody radiation curve is that same for every temperature (!).</em><br>
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For example, look at <a href="https://en.wikipedia.org/wiki/Planck%27s_law#/media/File:Black_body.svg">the family of blackbody radiation curves for different temperatures here</a>. The 5000K curve and the 3000K curve look quite different. But if you just squeeze in the 3000K curve in the X direction (by a factor of exactly 3/5!) and then stretch it out in the Y direction (by a factor of exactly (5/3)^5=12.86!) then those curves are in fact exactly the same.<br>
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This is the point of <a href="https://en.wikipedia.org/wiki/Wien%27s_displacement_law">Wien's Displacement Law</a> - that the peak frequency of blackbody radiation is exactly proportional to temperature. Since the shape of the blackbody radiation curve is the same for every temperature - only squished or stretched on the X axis and the Y axis by some factors - if you simply know the peak frequency that tells you the blackbody temperature immediately.<br>
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If you mess around with, say, the <a href="https://en.wikipedia.org/wiki/Planck%27s_law#Different_forms">wavelength form of Planck's Law</a>, you can soon convince yourself as to why this is true - for example put (2λ) in place of λ and just see how it looks.<br>
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So that is a long answer to one of your questions: <em>Is there some defining factor of blackbody radiation curves that allows you to identify the original temperature of the blackbody?</em> <br>
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The answer is no, because all blackbody radiation curves look the same, no matter the temperature. They are all a stretched out or squeezed together version of the same characteristic curve.<br>
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This is different from say a spectral curve of a star, where we can recognize characteristic notches as specific frequencies, if those characteristic spectra are shifted in the red direction we can say "Aha! We know what the frequency of this notch <em>should</em> be and the fact that it is shifted red-wards tells us the redshift of this particular object relative to us."<br>
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With blackbody radiation, there is no "notch" or distinctive spectrum that works in this same way.<br>
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You had it exactly correct in your question: "graphs of blackbody radiation at different temperatures always kinda look to me like they could be squished and squashed to look like each other." That is in fact exactly true.<br>
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<em><strong>> how does a redshift distance even make sense if we're trying to measure from us to everywhere at once</strong></em><br>
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It is simply stretched out - that is, space has stretched out by some factor. The waves have been in that space all along so they were stretched along with it. That is the same mechanism behind redshift, just thinking of it in in a somewhat different way.<br>
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As kms said, what was a micrometer is now a millimeter. It has been stretched out - in every dimension - by a factor of almost exactly 1000. So in this case "detecting the redshift" doesn't mean looking for the shift of characteristic spectral lines left or right on the frequency spectrum but rather that the wavelength of a thing that was one micrometer back then but that has now been stretched out by a factor of 1000 to become one millimeter. <br>
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<strong><em>> I will propose instead considering an alternate history: If we had ended up with no progress in optical telescopes beyond what Galileo had, but fantastic progress in radio telescopes, so that theorists in the 1930s had detailed, COBE-quality CMB data but no cosmological redshift data, would they have proposed the Big Bang?</em></strong><br>
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Back in the day I got into a gigantic argument with a guy on <a href="https://www.usenetarchives.com/threads.php?id=sci.astro&y=0&r=0&p=1">sci.astro</a> or some similar group who was trying to argue that Archimedes or the ancient Babylonians should have been able to look up, see that the sky is black rather than all white, and <a href="https://en.wikipedia.org/wiki/Olbers%27_paradox">immediately deduce that the universe is not infinite in extent and age</a>.<br>
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With the entire apparatus of modern physics, cosmology, astronomy, etc etc etc etc behind us this is indeed a conclusion one can reach. But when doing so, you have to acknowledge that, besides the reams of data and knowledge carefully accumulated by millions of tireless scientific workers over thousands of years, there are a <em>ton</em> of assumptions baked into that conclusion - some rather carefully examined and others, not so much.<br>
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Point is, there is literally a zero percent chance anyone in the ancient world <em>could have</em> come to that same conclusion on a simple leap of logic, because many thousands of the building blocks upon which that logical conclusion is based simply did not exist in the ancient world.<br>
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(In fact I note with some amusement that Edwin Hubble himself, as late as 1937, was still arguing for <a href="https://en.wikipedia.org/wiki/Expansion_of_the_universe#Hubble's_concerns_over_the_rate_of_expansion">"a universe extended indefinitely both in space and time"</a>. Shouldn't he have just looked up at the black night sky and in a few milliseconds of pure thought, logically concluded that was impossible? If only I had thought to include that tidbit on sci.astro I'm just <em>certain</em> I would have won that argument . . . )<br>
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To your question, the modern apparatus of physics and cosmology is indeed a large and complex, interrelated structure. There are many blind spots and areas of unknowns of course, but things that are known are typically known through multiple lines of evidence that (mostly, in areas that are well understood) reinforce each other.<br>
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So when you ask a question like "What if we removed this whole part of the structure" it becomes very hard to answer in any sensible way. But most likely the answer is, you would eventually come to the same or similar conclusions, using different methods.<br>
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Just for example, <a href="https://en.wikipedia.org/wiki/Hubble%27s_law#Determining_the_Hubble_constant">the Wikipedia article on measuring Hubble's Constant gives a rather long list of different methods that have been used over the past century or so to measure the Hubble Constant</a>. All the methods more or less agree but there are some really interesting and unresolved differences as well. Besides the optical measurements there are XRay and microwave observations and then also the measurement of anistropies in the cosmic background radiation mentioned above. Just in 2018 and 2019 two new methods were introduced - one of them based on the new gravity wave detectors.<br>
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Also, it's worth pointing out that the expansion of space over time is implied by <a href="https://en.wikipedia.org/wiki/Hubble%27s_law#Discovery">Einstein's Field Equations</a>. With Einstein's theories panning out in so many ways, eventually someone would have thought to test out this rather striking feature of the theory - sooner or later.<br>
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So if we didn't get there one way, we would have gotten there some other way. There is no particular reason to insist on the microwave background radiation as the one and only source of information. It's one piece of the puzzle - and a very interesting one, for certain.<br>
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<strong><em>> Explain to a first semester physics or chemistry student how the CMB is used to find the age of the universe</em></strong><br>
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<strong>#1. Thanks to the <a href="https://en.wikipedia.org/wiki/Saha_ionization_equation">Saha Ionization Equation (1920)</a>, we know the temperature at the point the CMB was created was 3000K.</strong><br>
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The logic there is quite simple: <br>
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A. The temperature can't be more than 3000K because the <a href="https://en.wikipedia.org/wiki/Saha_ionization_equation">Saha Ionization Equation</a> says that above that temperature, no radiation will survive to be seen. So the temperature at the time of creation of the CMB is definitely <em>no higher</em> than 3000 degrees kelvin.<br>
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B. If anything at all like the Big Bang happened, it started out at a temperature <em>far higher than 3000K</em>. This doesn't take a real lot of fancy theorizing - it's just that when you take all the energy in the present-day universe and compress it down to an area that is quite small then you can crank through the numbers and figure out the temperature. It is a lot hotter than 3000K.<br>
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A. tells you the temperature of the CMB at the starting point can't be more than 3000K and B. tells you it can't be less than 3000K. Therefore, it was 3000K.<br>
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<strong>#2. We have had a decent idea of how fast the universe is expanding since <a href="https://en.wikipedia.org/wiki/Hubble%27s_law#Discovery">some calculations on the Einstein Field Equations in the 1920s</a>, then <a href="https://en.wikipedia.org/wiki/Hubble%27s_law#Determining_the_Hubble_constant">Hubble's and other data in the first half of the 1900s, then further refinement in the latter half of the 1900s and even more refinement since 2000</a>.</strong><br>
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With the question of "how fast is the expansion" answered (at least to a reasonable estimate) all we need is the size of something <em>before</em> and <em>after</em> to calculate how long it took to make that change in size - and there is your estimate for the age of the universe, or at least, the time from your <em>before</em> to <em>after</em> time points.<br>
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<em>Before</em> is the time the CMB was created and its wavelength at that time when it was at 3000 degrees kelvin - that is known to be 1 micrometer. So to complete this calculation, we just need the <em>after</em> time point and length.<strong><br>
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#3. With the discovery of the CMB and measurement of its temperature (~3 degrees kelvin) there is our <em>after</em>. 3 degrees kelvin implies a radiation wavelength of 1 millimeter - so we know the <em>before</em> length was 1 micrometer and the <em>after</em> length was 1 millimeter.</strong><br>
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Put those values into the equations from #2. showing how fast the universe was expanding and you get a good estimate of the time from the <em>before</em> to the <em>after</em> points.<br>
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That is obviously simplified but in general when people say "cosmic microwave background radiation can be used to date the universe" they are talking about those three steps.<br>
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Even though CMB isn't a standalone confirmation of the age of the universe, people view it as a nice confirmation of the age of the universe, because #1 and #2 were well known before the discovery that it was possible to precisely measure the CMB, and the age of the universe had been estimated using other means than the CMB by that time, too.<br>
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So when the precise CMB measurements came along you could plug them straight into #1 and #2 and the answer you got dovetailed relatively well with previous estimates of the age of the universe by other means. <br>
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In short, a scientific prediction - and a very definitely falsifiable one - that panned out very nicely. The measurement of the CMB was the piece that pulled it all together.comment:ask.metafilter.com,2022:site.360392-5142154Mon, 17 Jan 2022 03:21:23 -0800flugBy: flug
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5142157
FWIW here is <a href="https://ui.adsabs.harvard.edu/abs/2000Natur.408..931S/abstract">an article</a> (and accompanying <a href="https://www.nature.com/articles/news001221-8">Nature write-up in plainer English</a>) showing yet another way the CMB can be used to determine the age of the universe etc. <br>
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What they did was find a way to measure the temperature of the CMB about 3 billion years after the Big Bang (12 billion years ago). They did that by measuring very distant dust clouds that have a high redshift (thus, very old). <br>
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The CMB was 3000 degrees Kelvin at the start and is now 2.7 degrees Kelvin. Theory says 12 billion years ago the CMB should have been around 9 degrees Kelvin. Their measurements put it between 6.0 and 14. So not a super-precise measurement but enough to show it was well above the current 2.7 degrees Kelvin and in the ballpark of the right number.<br>
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Again this by itself doesn't demonstrate the age of the universe in an iron clad way (and I'm not sure if those initial measurements from 2000 have ever been followed up or verified by other teams) but it is another piece of the puzzle that helps to fill in the picture.comment:ask.metafilter.com,2022:site.360392-5142157Mon, 17 Jan 2022 03:36:49 -0800flugBy: clawsoon
http://ask.metafilter.com/360392/Explain-to-me-like-Im-first-year-university-Age-of-cosmic-background#5142172
I asked a narrower question on physicsforum and got <a href="https://www.physicsforums.com/threads/does-the-cmb-itself-tell-us-that-it-started-at-3000k.1011315/post-6588445">this detailed response</a> along with a link to <a href="https://www.astro.ucla.edu/~wright/stdystat.htm">this page</a> by UCLA professor Ned Wright.<br>
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Putting together all the answers here and over there, what I'm getting is:<br>
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- We can't tell from the CMB itself that it was generated at 3000K. Blackbody radiation just doesn't carry that information.<br>
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However, the CMB itself can tell us a lot about the universe it was generated in, which eliminates a lot of possible sources and strongly points us in the direction of the models that answers above are talking about:<br>
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- The 1-in-100,000 uniformity of the CMB means that it couldn't have been generated in a universe where everything is as spread out as it is today, since things that are spread out inevitably get lumpy and bumpy and wavy (as we see in the universe today). It must have been generated when everything was much, much closer together.<br>
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- Blackbody radiation can't be generated by something transparent. It is only generated by opaque objects. In every direction we look the universe is transparent right now, except for widely spaced clumps of matter. This is another clue from the CMB itself that it was generated when everything was very close together - close enough together, in fact, to be opaque everywhere. But also transparent enough everywhere that the photons didn't just get reabsorbed right away - hence the transition point that folks are talking about in answers above.<br>
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- Things like spinning dust and spiraling electrons can create radiation around the 3K-ish temperature that we see in the CMB, but they don't create a blackbody distribution.<br>
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- When we look at light from quasars passing through distant (and therefore further back in time) dust clouds, we see that the CMB is hotter for them. This suggests that the CMB started out hotter and only happened once; it's not being continuously generated.<br>
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So the CMB itself tells us that it was generated once, everywhere, from an opaque source that was virtually uniform everywhere, in a universe where everything was much closer together than it is now, and it has been cooling down since then. The signal itself can't tell us that it was generated at 3000K, but it paints a picture of the universe that it was generated in which eliminates most of the other options.<br>
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...which I think is what y'all meant when you said that it fits one model of the universe and not others, so thanks for the effort you put into your answers.comment:ask.metafilter.com,2022:site.360392-5142172Mon, 17 Jan 2022 05:04:00 -0800clawsoon