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I agree with you; you can't build up a perfect blackbody spectrum by superposing different near-blackbody spectra, and that led to the cosmological thermalizati

I agree with you; you can't build up a perfect blackbody spectrum by superposing different near-blackbody spectra, and that led to the cosmological thermalization problem. Furthermore, baryonic intergalactic dust isn't like dark matter: we see redshifted spectral features of components of such dusts imprinted on all sorts of backgrounds from quasars to the CMB. The dusts tend to be greybodies. (I'm ignoring the warm-hot intergalactic medium here). The CMB has narrow spectral lines imposed on it by galaxies, dust clouds, and similar objects. Those lines can be used to measure the CMB effective temperature at an object's redshift. A specific example of how this works is linked at the bottom of this comment. If a similar highly dusty object is found in the foreground of a bright quasar (merely nearby quasars were used in the case below), we could get an even tighter measurement of the CMB effective temperature at the dust, and perhaps look in new ways at how the CMB deviated (at the dusty object) very slightly from a blackbody spectrum. This amounts to a new line of evidence in the study of the expansion history. More generally, constraining the evolution of the CMB temperature in the matter-dominated era (T_cmb > 4 K, z > ~ 0.4) will prove fatal to ideas like some in this thread (which is already in trouble because of other spectroscopy e.g. the Lyman-alpha forest and Gunn-Peterson trough). I don't really understand the summary of the "'fringe' theory" at the top of the thread (and I can't identify which specific theory throwawaymaths means). I guess the idea could be that some unknown mechanism might homogenize the emissive components of baryonic intergalactic dust but not circumgalactic medium spectral features. I suppose one could try to investigate a non-adiabatic expansion to explain those features found in the CMB in a nonstandard way. Non-adiabatic components in the expansion have been examined by various theoretical teams in the context of quantum cosmology and/or "fifth force" dark energy, although mostly at much larger lookback times than the the standard surface of last scattering. I don't understand what was meant by '"longer" (aka galactic expansion) timeframe'. Finally, "echo of the big bang" isn't really helpful in understanding the origin of the CMB photons, why they're now filling space with a thermal Planck spectrum, and why we detect small variations in that spectrum in narrow views along different directions, but like "fabric of spacetime" I guess we're kinda stuck with the expression. - -- (Open Access) Riechers, D.A., Weiss, A., Walter, F. et al. Microwave background temperature at a redshift of 6.34 from H2O absorption. Nature 602, 58–62 (2022). https://doi.org/10.1038/s41586-021-04294-5 https://doi.org/10.1038/s41586-021-04294-5

throwawaymaths 3y ago
I am not a physicist, but iirc the idea of the alternative explanation of the CMB is that if you time evolve backwards intergalactic dust over expansion, you reach a point where the dust gets really dense and effectively opaque. So this isn't summing all of the dust you go through, but rather the redshift all the way back to an era when the universe was super dusty (with the expected thermal signature of that era)
- raattgift 3y ago
  Maybe you can dig up a reference, because you're on the verge of telling the conventional cosmology in a time-reversed way. Under time reversal eventually the very first stars disintegrate into mostly atomic hydrogen, which compresses and heats adiabatically. Eventually it becomes hot enough that the hydrogen ionizes. Ignoring processes which alter photon number, the hot nuclei and electrons form a dense fog. As with a fog here on earth, where incoming light can get bounced in a random direction off a fog droplet ("Mie scattering"), photons scattering off these hot charged particles are scattered in a random direction ("Compton scattering" at high energies, "Thomson scattering" at low energies). When we densify the fog, the free-streaming length of light decreases, increasing the fog's opacity. Under the normal direction of time, therefore, the CMB is when de-ionization enormously shrinks the analogue of the fog droplet size ("Thomson cross-section"), and expansion increases the distance between the fog droplet analogues. The free-streaming length can become effectively infinite, like clear night air after a fog dissipates. The spectrum of the light at that point encodes the effective temperature of the scattering medium. In an expanding universe, that spectrum will lose energy (i.e., redshift). The electrically neutral mostly-hydrogen then takes three principal forms: collapsing clouds of gas, which eventually form the first low-metallicity stars; cold dusts of neutral atoms at various sparser densities; and a warm-to-hot sparse intergalactic medium. The latter two is where we should find the so-called "missing baryons", the large fraction of atoms not found in stars and galaxies. Backlighting by active galactic nuclei and UV-hot stars ruins the idea that a microwave-bright diffuse dust of electrically neutral atoms or molecules could generate the CMB with its spectral features. You'd have to keep the stars from heating the dust elements, while keeping the dust dense enough to generate the CMB photon-density. How do you preserve that density during expansion? At the top of the thread you said "the primary proponent of this theory is trying to justify some sort of cyclic model", which doesn't escape this point. (It also didn't lead me to a reference). However, if instead whoever you are struggling to remember is the proponent of an eternal and static cosmology -- with no expansion, ever -- some of these problems with the idea that the CMB can be the product of cold dust could be overcome. The idea might be that we still have a speed of light lookback, with more distant galaxies being older. The older galaxies being redder could be some sort of dust that is very thick in the distant past, thick enough to completely shroud whole galaxies like a lampshade, turning hot thermal Planck spectra into cold thermal Planck spectra. Then you have to (a) get rid of the dust over time without sending more or hotter photons in our direction, and (b) add spectral features to the cold thermal Planck spectra before it gets to us. I don't see how either could be done without very different atomic and/or gravitational physics than we have in our solar system. Adding in supernovae -- whose light-curves we see redshifted as per the article at the top -- makes this idea even harder, because then you need a lampshade-dust that down-converts photon energies in even more ways. I'm not aware of any published or even serious attempt to do this, although I didn't look too deeply into the literature beyond confirming that this wasn't something proposed by Jayant Narlikar.

throwawaymaths 3y ago

I am not a physicist, but iirc the idea of the alternative explanation of the CMB is that if you time evolve backwards intergalactic dust over expansion, you reach a point where the dust gets really dense and effectively opaque. So this isn't summing all of the dust you go through, but rather the redshift all the way back to an era when the universe was super dusty (with the expected thermal signature of that era)

raattgift 3y ago
Maybe you can dig up a reference, because you're on the verge of telling the conventional cosmology in a time-reversed way. Under time reversal eventually the very first stars disintegrate into mostly atomic hydrogen, which compresses and heats adiabatically. Eventually it becomes hot enough that the hydrogen ionizes. Ignoring processes which alter photon number, the hot nuclei and electrons form a dense fog. As with a fog here on earth, where incoming light can get bounced in a random direction off a fog droplet ("Mie scattering"), photons scattering off these hot charged particles are scattered in a random direction ("Compton scattering" at high energies, "Thomson scattering" at low energies). When we densify the fog, the free-streaming length of light decreases, increasing the fog's opacity. Under the normal direction of time, therefore, the CMB is when de-ionization enormously shrinks the analogue of the fog droplet size ("Thomson cross-section"), and expansion increases the distance between the fog droplet analogues. The free-streaming length can become effectively infinite, like clear night air after a fog dissipates. The spectrum of the light at that point encodes the effective temperature of the scattering medium. In an expanding universe, that spectrum will lose energy (i.e., redshift). The electrically neutral mostly-hydrogen then takes three principal forms: collapsing clouds of gas, which eventually form the first low-metallicity stars; cold dusts of neutral atoms at various sparser densities; and a warm-to-hot sparse intergalactic medium. The latter two is where we should find the so-called "missing baryons", the large fraction of atoms not found in stars and galaxies. Backlighting by active galactic nuclei and UV-hot stars ruins the idea that a microwave-bright diffuse dust of electrically neutral atoms or molecules could generate the CMB with its spectral features. You'd have to keep the stars from heating the dust elements, while keeping the dust dense enough to generate the CMB photon-density. How do you preserve that density during expansion? At the top of the thread you said "the primary proponent of this theory is trying to justify some sort of cyclic model", which doesn't escape this point. (It also didn't lead me to a reference). However, if instead whoever you are struggling to remember is the proponent of an eternal and static cosmology -- with no expansion, ever -- some of these problems with the idea that the CMB can be the product of cold dust could be overcome. The idea might be that we still have a speed of light lookback, with more distant galaxies being older. The older galaxies being redder could be some sort of dust that is very thick in the distant past, thick enough to completely shroud whole galaxies like a lampshade, turning hot thermal Planck spectra into cold thermal Planck spectra. Then you have to (a) get rid of the dust over time without sending more or hotter photons in our direction, and (b) add spectral features to the cold thermal Planck spectra before it gets to us. I don't see how either could be done without very different atomic and/or gravitational physics than we have in our solar system. Adding in supernovae -- whose light-curves we see redshifted as per the article at the top -- makes this idea even harder, because then you need a lampshade-dust that down-converts photon energies in even more ways. I'm not aware of any published or even serious attempt to do this, although I didn't look too deeply into the literature beyond confirming that this wasn't something proposed by Jayant Narlikar.