tag:blogger.com,1999:blog-7315236707728759521.post1896290229465770998..comments2024-03-28T21:52:52.100-06:00Comments on Dispatches From Turtle Island: Notable Views Regarding PhysicsAndrew Oh-Willekehttp://www.blogger.com/profile/02537151821869153861noreply@blogger.comBlogger3125tag:blogger.com,1999:blog-7315236707728759521.post-3928755111869375432017-02-28T19:14:56.080-07:002017-02-28T19:14:56.080-07:00Put it this way:
Suppose we had a perfect underst...Put it this way:<br /><br />Suppose we had a perfect understanding of gravity and were trying to model the behaviour of baryonic matter. How simple of a model could we get away with to capture the rough mass distribution?<br /><br />IIRC most matter is tied up in the intergalactic gas at the galaxy cluster/higher scale, and at the scale of a galaxy a large percentage is in molecular gas clouds. How much would we need to know to model their behaviour in a given environment? Presumably we would need to know about photons and that there is some sort of baryon. Would we need to know about ionization? Looking at some of the models for evolution of the structure of the universe, they have to include the effects of supernova and active galactic nuclei on these media. Do you think we would be able to figure all of that out if the only thing we could observe was the gravitational effects?Ryanhttps://www.blogger.com/profile/07906194112935320590noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-41620764427401778132017-02-27T19:38:10.263-07:002017-02-27T19:38:10.263-07:00Strictly speaking, I say it can't have more th...Strictly speaking, I say it can't have more than two particles that make a dominant contribution.<br /><br />In the early days of dark matter models, there were a lot of "mixed dark matter" models that would have several different kinds of particles. But, these models pretty much across the board failed to match observation in simulations as well as more parsimonious models. There were probably a dozen or more papers that came to that conclusion.<br /><br />Also, the fact that you can make a modified gravity model closely approximate reality with just a couple of free parameters or less, implies that any dark matter particle theory that also approximates reality may have no more degrees of freedom than the modified gravity model does.<br /><br />You actually could have a dark matter particle model in which just like reality, almost all matter was wrapped up in just a couple of kinds of particles (protons and neutrons in the matter world) and almost all other forms of dark matter were extremely rare and unstable (much like heavier baryons in the matter dominated world), because those very rare and short lived particles don't have a meaningful gravitational dynamics impact. And the dominant dark matter particles, for similar reasons, need not be fundamental - they could be stable composite particles instead.<br /><br />But, any dark matter particle theory has to boil down to one or two dominant kinds of dark matter with just one dominant fermion and possibly one dominant boson in addition (in self-interacting dark matter models), or one dominant boson without a dominant fermion. But, since the parameter space of SIDM is pretty much non-existent at this point, you really, at this point, need either one dominant fermion or one dominant boson as the sole dominant source of dark matter. So, the parameter space is really even more constrained (although I'm not sure that two nearly degenerate forms of dark matter particles in analogy to the proton+electron v. the neutron, would be detectable with current experimental methods).andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-6154137133653631412017-02-27T14:00:09.250-07:002017-02-27T14:00:09.250-07:00Why do you say that Dark Matter (if it exists) cou...Why do you say that Dark Matter (if it exists) couldn't have more than 2 particles?Ryanhttps://www.blogger.com/profile/07906194112935320590noreply@blogger.com