Drakkith said:
Marcus I never thought of it until you mentioned it, but will high velocity particles moving towards far away galaxies will arrive at some at significantly reduced speeds thanks to expansion? I think that's what you were saying but I want to make sure.
Yes Drakkith, I believe so. To make the discussion more precise it seems to be about the comoving frames of reference or the idea of CMB rest. If it weren't for that the discussion might not be as interesting as it is, for us.
Because of the apparent uniformity of ancient matter before it began to fall together into clumps, and because of the near uniformity of the CMB light emitted by the ancient matter, we have in cosmology a nice concept of the stationary observer. And by extension, the idea of galaxies being nearly at rest (save for a bit of individual random motion).
And it is distances between stationary observers that the Hubble law of expansion is really about.
Well this expansion can be seen as responsible not only for the redshifting of light but also the SLOWING of moving objects if you measure their speed relative to CMB rest wherever they happen to be. Moving objects do lose momentum and kinetic energy in that sense.
And it is completely commonsensical because where they were going was backing away from them. It can all be analysed in Doppler or relative motion terms, if one wishes. The redshifting of light can be resolved into a huge number of little infinitesimal Doppler shifts along the way as the expansion rate changes while the light is in transit. It is completely unmysterious if you want to think of it that way. And objects of course slow down by the same mechanism, due to distance expansion.
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I like to think of Dark Matter particles starting out with a lot of random kinetic energy and being cooled by expansion so they are now moving slowly and can collect in clouds (which formed the basis for structure formation in ordinary matter). And when DM particles fall together and selfgravitate they interact gravitationally and some get slowed down and stay in the cloud while others get speeded up and thrown out of the cloud. THEY CARRY AWAY THE EXCESS KINETIC ENERGY and allow the remainder to condense.
DM has no other way to get rid of excess energy than by sacrificing some of itself, to carry the energy away.
Now because of EXPANSION the sacrificial DM can lose energy in the sense that when it arrives somewhere else it is going slower. So it can be RECYCLED and again participate in cloud condensation. So it is the expansion cooling of DM which provided the seeds of structure.
Expansion was the PARENT of all the structure which we see and which we are. Because stars and galaxies of ordinary matter condensed around an ARMATURE of DM cobwebs and filaments. And those DM filaments and clouds would not be there without the cooling effect expansion.
This is a familiar story, in a sense. We know the parallel story concerning light, expansion cooled the light and thus the ordinary matter in equilibrium with the light. But now I am saying the same thing but talking about DM particles that cannot emit and absorb light. So the only way they have to slow down is by way of expansion.
I wanted to add that, tangentially related to what you asked about particles slowing, even though you didnt ask directly about DM, because it just seems nice. Another role expansion has played in our lives.
because you measure the wavelength of the photons in comoving coordinate "length units"? So the wavelengths cannot have changed!
