Has the ratio of normal to dark matter remained the same since the beginning?
Define what you mean by "the beginning".
The beginning is usually considered to be the start. How else would you define it?
Present theory - dark (non-baryonic) matter and baryonic matter are believed to be unchanged since the big bang. Dark energy is another story.
This would suggest that there are no decay paths between non- and baryonic matter?!
Sort of. Depends upon how far back you go.
For thermal dark matter models (the most common, but not the only ones by far), the dark matter does interact strongly with normal matter at extremely high temperatures. Once the temperature of our universe dropped below those temperatures, interactions between dark matter and normal matter would have slowed to a standstill, and however much dark matter was around at that time would still be around today.
Note that dark matter is expected to be comprised of equal numbers of matter and anti-matter, so it will slowly annihilate (producing normal matter particles). But this process has to be quite slow to allow for the dark matter in galaxies to still be there today. Observing these annihilations through their high-energy signatures is expected to be one potential way to determine the properties of dark matter.
You are running in circles. There is no such thing in present theory, you can only go back so far and still be certain. Then there are many different possibilities.
If you have thermally produced WIMP dark matter then fine (although if it is a Majorana fermion it is probably quite moot to talk about matter/antimatter distinction), but there are many dark matter models where this is not the case and I would say that they are becoming increasingly popular as WIMP searches continue to turn out negative. Just take axion or asymmetric dark matter models as examples.
Oh my. Thread prefix changed from "A" (Advanced PhD level) to "I" (Intermediate undergraduate level). Please take care to set your thread prefixes at the level of discussion that you want (and can understand). Thank you.
This is new to me. Do you have a reference? My understanding is that we know almost nothing about dark matter, except that it is there. I have never heard of a concept of dark anti-matter.
For thermally produced Dirac fermion dark matter this follows directly from the Boltzmann equations in the early Universe (important caveat: if there is a matter-antimatter asymmetry in the dark sector this is not necessarily true). Of course, Dirac fermions intrinsically come with matter and antimatter components.
For Majorana fermion dark matter the concept of matter vs antimatter makes less sense as the Makorana mass would break dark matter number. However, this would still generally allow dark matter to pair-annihilate. Supersymmetric DM candidates such as neutralinos would be of this type.
On top of this you have other candidates, such as axion DM etc.
It's hard for me to provide a reference, though maybe Orodruin's post will help with some pointers on looking into it in more depth. I thought I'd explain the thermal case a little bit more.
At high enough temperatures, all matter behaves like radiation. One aspect of this is that the number of particles is not a fixed number: when the typical kinetic energy of particles is above the mass, then colliding particles can produce more particles of the same mass.
When normal matter goes non-relativistic, it tends to annihilate rapidly because of the strength of the electromagnetic force: simply put, electrons and positrons attract one another very strongly.
For dark matter, this doesn't happen. Once dark matter goes non-relativistic, it can only interact very weakly with itself and with the surrounding normal matter. So dark matter doesn't get a chance to annihilate. You just get a bunch of dark matter particles which were banging around in the very early universe and started to stream freely throughout the universe once their temperature got low enough. This process is very, very similar to the CMB, except that it happened far earlier and unlike photons, dark matter particles have mass, allowing it to cluster once it slows down enough.
In thermal dark matter models, the dark matter has to decouple early enough that you have enough dark matter at low enough temperatures to make sense of observations.
. This is true of baryonic matter, but we know essentially nothing about dark matter, so why should we assume it holds. Also much of the discussion seems to revolve around various quantum theory equations and we just don't know if they have anything to do with dark matter.
Matter behaves like radiation at sufficiently-high energies simply because it has more kinetic energy than mass energy. Also, there's nothing particularly mysterious about the dark matter particles in thermal dark matter models. They're basically heavy neutrinos (note: not actually heavy neutrinos, but they behave similarly).
There are, as Orodruin noted, non-thermal dark matter models as well. Axions, for instance, have very small masses but are produced in extremely large quantities at extremely low temperatures during inflation.
If dark matter is primordial black holes (PBHs), created less than one second following the "Big Bang", is it not then likely that over time ordinary matter would be captured by the PBHs to become larger BHs. If this were the case, then would it still be reasonable to say that DM is still PBHs since these larger BHs would contain much ordinary matter that was added after one second? Also, if so, would it then be appropriate to say that the ratio of DM to ordinary matter has changed?
The amount of normal matter which would have accreted onto primordial black holes since they very early universe is likely to be effectively negligible, so for all intents and purposes, the ratio between DM and ordinary matter would have been close to constant. There may have been some significant change in the ratio very early-on, but not since then.
Note: PBH's could, in principle, also have pretty small masses. Such small-mass black holes would evaporate, decreasing in density over time. However, current observational limits indicate that such small-mass black holes can't make up more than 1% of dark matter, so don't really contribute here (see here).
I am not knowledgeable enough to doubt that you are correct. I would very much appreciate your helping me learn more about the mathematical analysis that reaches this conclusion, so I can perhaps explain it to some friends who have an interest, and even less knowledge than I do. Can you cite a reference that discusses this calculation?
This is my argument:
1) Observationally, dark matter has to be close to collisionless. Dark matter haloes extend much further than the visible matter in galaxies, indicating that the friction normal matter experiences causes them to collapse inward. Dark matter can't experience this friction and remain in this configuration. Accretion of normal matter would constitute friction.
2) The black holes that we have observed are a tiny fraction of the observed normal matter mass. Even the supermassive black holes at the centers of galaxies are only a couple of percent (at most) of the galaxy mass.
3) My understanding is that current observations have ruled out primordial black holes as being a significant fraction of the dark matter component except in a pretty narrow range: roughly a few tens of solar masses. Black holes of this mass would rarely collide with normal matter for the same reason that stars rarely collide: space is really big, and the separations between stars are much greater than the sizes of stars. Black holes are even more extreme, as they are far more dense objects than stars are. You'd occasionally get a black hole plowing through a gas cloud and collect some of it, but as most of the dark matter isn't even within the confines of the visible galaxy, this will be pretty rare.
4) You could get primordial black holes absorbing matter in the very early universe, before the CMB was emitted. But they can't do very much of that without it showing up as a signature in the CMB.
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