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Sean Carroll (Caltech) wrote up a quick reaction here http://www.preposterousuniverse.com/blog/2018/02/28/dark-matter-and-the-earliest-stars/ http://www.prepos

Sean Carroll (Caltech) wrote up a quick reaction here http://www.preposterousuniverse.com/blog/2018/02/28/dark-matter-and-the-earliest-stars/ http://www.preposterousuniverse.com/blog/2018/02/28/dark-mat...

chopin 9y ago
Nice write-up, thanks for sharing. I am wondering, can't there be something like gravitational cooling? When playing around with gravitation simulators one observes that sometimes high velocity parts leave the bulk, which in my understanding leads to a cool down of the pack (the escaping part carries away kinetic energy, after all). If I let some "cool" matter interact with "hot" matter only gravitationally, I'd expect that this leads to an equal "temperature" eventually. Is it, that due to the very weak interaction, time scales are too long for this?
- raattgift 9y ago
  From studies of the cosmic microwave background we can be pretty confident that temperatures were very uniform at the time of recombination, and that the cooling of the CMB (which was fixed at that time [1]) closely resembles the adiabatic expansion of an ideal gas. So we're already seeing cooling-by-separation. If we have a decent idea of the history of the expansion, then we also have a decent idea of the temperature of the CMB at the surface of last scattering. Since we look so intently for tiny overdensities or underdensities to seed structure formation (collapse of matter into early galaxies), it's hard to imagine where one could hide an enormous underdensity or overdensity that could cool the CMB across an enormous region (we see the same temperature along widely angular-separated lines of sight; this is the horizon problem [2]) without leaving fingerprints. The period between recombination and "first light" from the first stars (triggering reionization) is not especially well understood, but the expectation is that the expansion of the universe would be pretty smooth in that epoch. So the neutral hydrogen around the time of first light being cold enough to produce the unexpectedly strong 21cm absorption line is an oddity. If evidence holds up suggesting that this gas is uniformly colder than expected then we could take the position that our idea of the history of the expansion is somehow wrong, or alternatively that some other process was at work at the end of the dark ages. We have constraints on both; and in particular "some other process" is tricky because the almost-exactly-black-body spectrum of the CMB puts constraints on early cooling-by-clumping (where cooling is up to "chemistry", where that's a(n exothermic, in this case) reaction between particles of different species). There could be "chemistry" in the dark matter sector that allows dark matter to cool by extremely local clumping (for example, if in the "dark ages" before recombination there are two oppositely-charged dark matter particles and at recombination they combine into one neutral dark matter composite particle, analogously to how electrons and protons recombined into neutral hydrogen), but then you have to figure out how to have the dark matter be colder than the hydrogen and how to thermalize the hydrogen to the dark matter. You also have to avoid smearing out small matter density fluctuations. If structure formation works the way we think, everything at recombination seems too uniform and diffuse for local gravitational interactions to do the job of transferring much momentum from the matter sector to the dark matter sector in time for the 21 cm absorption line seen by EDGES. There are also constraints on extra long-range interactions, so Carroll's raising the idea of a zero-mass boson mediating the interaction is hard to imagine (a massive mediator would be shorter-range, in general, and could carry more momentum between matter and dark matter, but there are constraints on such interactions from the much later universe -- for instance, we would expect missing momentum in particle physics experiments, along the lines of the missing momentum that led to the discovery of the neutrino). The problem is that there are lots of strongly correlated observations and pushing at one tends to cause problems with one or more of the others. But that's what theorists in this area (model building) seem to like to do. :-) - -- [1] Peter Coles (cosmology, Cardiff) has a good, accessible write-up from some years ago, adapted here: https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html [2] https://en.wikipedia.org/wiki/Horizon_problem?oldformat=true https://en.wikipedia.org/wiki/Horizon_problem?oldformat=true
  - chopin 9y ago
    Thanks for the elaborate answer. As far as I understand your fifth paragraph refers directly to my question. How would uniformity rule out purely gravitational interaction? As far as I see it gravitational forces should be enough to get two reservoirs of different temperature onto equal temperature. It's just the time scale which may be a problem. And if I start already with two (almost) uniformly distributed reservoirs, long range interactions shouldn't make a difference. Inflation should have flattened out everything already anyways.
    - raattgift 9y ago
      I don't understand your idea. Gravitation squashes collisional matter together, heating it. The hot matter radiates, allowing it to fall deeper into dense structures. I don't see how this cools non-infallen hydrogen, rather than lighting it up.
      - chopin 9y ago
        I'll try to elaborate: Let's say we have to reservoirs of baryonic matter, one cool and one hot. If these interact through radiation (electromagnetic forces) both reservoirs eventually get the same Boltzmann distribution and thus temperature. Now we have the same situation but we switch one reservoir to dark matter (the cooler one) and the force between the reservoirs is purely gravitational. Beside the time scale, would that be any different? In a large collection of masses, particles can exchange momentum and energy through gravitation, afair. Therefore one could expect that the two reservoirs equilibrate. I think what differs may be that gravitation is only attractive and contraction may be faster than equilibration. But is this the case if the initial temperature of the constituents is high enough, especially of the baryonic stuff?
        raattgift 9y ago
        What would source the gravitational radiation that would thermalize the two reservoirs? The metric sourced by a uniform cloud of hot gas doesn't really lend itself to periodic change (under a slicing) unless it's collapsing far from spherically symmetrically, and even then the momentum-energy carried out by electromagnetic radiation would dwarf the gravitational radiation. Also, if you let the hot and cold clouds fall into each other, then electromagnetic scattering will do the bulk of the work of thermalization (or more likely the heating of both clouds of inelastically colliding matter; cf. collisonless dark matter clouds sliding through one another).

chopin 9y ago

Nice write-up, thanks for sharing. I am wondering, can't there be something like gravitational cooling? When playing around with gravitation simulators one observes that sometimes high velocity parts leave the bulk, which in my understanding leads to a cool down of the pack (the escaping part carries away kinetic energy, after all). If I let some "cool" matter interact with "hot" matter only gravitationally, I'd expect that this leads to an equal "temperature" eventually. Is it, that due to the very weak interaction, time scales are too long for this?

raattgift 9y ago
From studies of the cosmic microwave background we can be pretty confident that temperatures were very uniform at the time of recombination, and that the cooling of the CMB (which was fixed at that time [1]) closely resembles the adiabatic expansion of an ideal gas. So we're already seeing cooling-by-separation. If we have a decent idea of the history of the expansion, then we also have a decent idea of the temperature of the CMB at the surface of last scattering. Since we look so intently for tiny overdensities or underdensities to seed structure formation (collapse of matter into early galaxies), it's hard to imagine where one could hide an enormous underdensity or overdensity that could cool the CMB across an enormous region (we see the same temperature along widely angular-separated lines of sight; this is the horizon problem [2]) without leaving fingerprints. The period between recombination and "first light" from the first stars (triggering reionization) is not especially well understood, but the expectation is that the expansion of the universe would be pretty smooth in that epoch. So the neutral hydrogen around the time of first light being cold enough to produce the unexpectedly strong 21cm absorption line is an oddity. If evidence holds up suggesting that this gas is uniformly colder than expected then we could take the position that our idea of the history of the expansion is somehow wrong, or alternatively that some other process was at work at the end of the dark ages. We have constraints on both; and in particular "some other process" is tricky because the almost-exactly-black-body spectrum of the CMB puts constraints on early cooling-by-clumping (where cooling is up to "chemistry", where that's a(n exothermic, in this case) reaction between particles of different species). There could be "chemistry" in the dark matter sector that allows dark matter to cool by extremely local clumping (for example, if in the "dark ages" before recombination there are two oppositely-charged dark matter particles and at recombination they combine into one neutral dark matter composite particle, analogously to how electrons and protons recombined into neutral hydrogen), but then you have to figure out how to have the dark matter be colder than the hydrogen and how to thermalize the hydrogen to the dark matter. You also have to avoid smearing out small matter density fluctuations. If structure formation works the way we think, everything at recombination seems too uniform and diffuse for local gravitational interactions to do the job of transferring much momentum from the matter sector to the dark matter sector in time for the 21 cm absorption line seen by EDGES. There are also constraints on extra long-range interactions, so Carroll's raising the idea of a zero-mass boson mediating the interaction is hard to imagine (a massive mediator would be shorter-range, in general, and could carry more momentum between matter and dark matter, but there are constraints on such interactions from the much later universe -- for instance, we would expect missing momentum in particle physics experiments, along the lines of the missing momentum that led to the discovery of the neutrino). The problem is that there are lots of strongly correlated observations and pushing at one tends to cause problems with one or more of the others. But that's what theorists in this area (model building) seem to like to do. :-) - -- [1] Peter Coles (cosmology, Cardiff) has a good, accessible write-up from some years ago, adapted here: https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html [2] https://en.wikipedia.org/wiki/Horizon_problem?oldformat=true https://en.wikipedia.org/wiki/Horizon_problem?oldformat=true
- chopin 9y ago
  Thanks for the elaborate answer. As far as I understand your fifth paragraph refers directly to my question. How would uniformity rule out purely gravitational interaction? As far as I see it gravitational forces should be enough to get two reservoirs of different temperature onto equal temperature. It's just the time scale which may be a problem. And if I start already with two (almost) uniformly distributed reservoirs, long range interactions shouldn't make a difference. Inflation should have flattened out everything already anyways.
  - raattgift 9y ago
    I don't understand your idea. Gravitation squashes collisional matter together, heating it. The hot matter radiates, allowing it to fall deeper into dense structures. I don't see how this cools non-infallen hydrogen, rather than lighting it up.
    - chopin 9y ago
      I'll try to elaborate: Let's say we have to reservoirs of baryonic matter, one cool and one hot. If these interact through radiation (electromagnetic forces) both reservoirs eventually get the same Boltzmann distribution and thus temperature. Now we have the same situation but we switch one reservoir to dark matter (the cooler one) and the force between the reservoirs is purely gravitational. Beside the time scale, would that be any different? In a large collection of masses, particles can exchange momentum and energy through gravitation, afair. Therefore one could expect that the two reservoirs equilibrate. I think what differs may be that gravitation is only attractive and contraction may be faster than equilibration. But is this the case if the initial temperature of the constituents is high enough, especially of the baryonic stuff?
      - raattgift 9y ago
        What would source the gravitational radiation that would thermalize the two reservoirs? The metric sourced by a uniform cloud of hot gas doesn't really lend itself to periodic change (under a slicing) unless it's collapsing far from spherically symmetrically, and even then the momentum-energy carried out by electromagnetic radiation would dwarf the gravitational radiation. Also, if you let the hot and cold clouds fall into each other, then electromagnetic scattering will do the bulk of the work of thermalization (or more likely the heating of both clouds of inelastically colliding matter; cf. collisonless dark matter clouds sliding through one another).