(March 3, 2018 at 12:00 pm)Brian37 Wrote:(March 3, 2018 at 11:42 am)polymath257 Wrote: That's a very good question. What happens is that dark matter affects the rate of universal expansion, and thereby of the cooling that leads to the formation of hydrogen atoms (as opposed to ions, which were there before). So, the amount of dark matter determines *when* those hydrogen atoms form. And *that* leaves a signature on the background radiation, which is what was detected here. There also seems to be a higher level than expected of interaction between the dark matter and ordinary matter at that point--which is very interesting. This also affected the cooling rate.
So the point is that the rate of expansion (and hence, the cooling) is affected by dark matter and *that* is what we are detecting.
Unfortunately, such indirect measurements are required for dark matter because dark matter doesn't interact strongly with light or ordinary matter (hence, why it is termed 'dark'). So we can only really detect it through its gravitational effects (or, if a certain type of particle, by its decay--not done yet, but possible). In this case, it is the gravitational effects changing the rate of expansion at the stage where hydrogen atoms are formed.
It seems to be a long universal tool of discovery in science, that when we can't detect something directly we study indirectly the objects we can detect, just like we cant see into a black hole, but can see the effects of the matter around the even horizon.
So if I am reading you correctly the behavior of hydrogen atoms and ions are how we are indirectly studying dark matter based on gravity?
That is my understanding, yes. I haven't read the detailed paper, though. When I do, I'll report any mistakes I have made.
