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webb202
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Has the ratio of normal to dark matter remained the same since the beginning?
Sort of. Depends upon how far back you go.webb202 said:Has the ratio of normal to dark matter remained the same since the beginning?
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.webb202 said:The beginning is usually considered to be the start. How else would you define it?
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.kimbyd said: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).
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.webb202 said:The beginning is usually considered to be the start. How else would you define it?
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.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).
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.mathman said: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.
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.mathman said: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.
. 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.At high enough temperatures, all matter behaves like radiation.
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).mathman said:. 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.
Hi mathman:mathman said:Present theory - dark (non-baryonic) matter and baryonic matter are believed to be unchanged since the big bang. Dark energy is another story.
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.Buzz Bloom said:Hi mathman:
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?
Regards,
Buzz
Hi kimbyd:kimbyd said: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.
This is my argument:Buzz Bloom said:Hi kimbyd:
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?
Regards,
Buzz
The ratio of normal matter to dark matter is approximately 5 to 1. This means that for every 5 units of normal matter, there is 1 unit of dark matter.
No, the ratio of normal matter to dark matter has remained unchanged since the beginning of the universe. This ratio has been consistent throughout the evolution of the universe.
Scientists use a variety of methods to determine the ratio of normal matter to dark matter. These include studying the rotation of galaxies, gravitational lensing, and observations of the cosmic microwave background radiation.
The ratio of normal matter to dark matter is significant because it helps us understand the composition and evolution of the universe. It also plays a crucial role in shaping the large-scale structure of the universe.
While the ratio of normal matter to dark matter is consistent on a large scale, there may be variations in different regions of the universe. This is an area of ongoing research and is currently not fully understood.