Why are there so many stars and galaxies?
I wonder how long before we can
The problem is that at such timescales, the CMB will have shifted sooooooooo much that there’s nothing they will be able to deduce, all light has redshifted to absurd scales, and worst of all, everything will be so far away that light will never reach the galaxy.
If their physics is correct, they will figure it out. We did because we verified that expansion is accelerating even though our physics are incomplete. Until the late 90s, physicists accepted a cosmological constant of 0, which meant no acceleration. Turns out that that was wrong.
It may be possible that they figure it out via Quantum Mechanics because it seems that acceleration of the universe is related to the energy density of a vacuum / empty space. The problem is that they won't be able to empirically verify that I think.
For star formation, however, the tension is between the outward flow of matter and simple photonic pressure on interstellar gas, competing against the inward pull of gravity.
That is: stars form as large "dust" clouds (mostly hydrogen atoms, obviously), collapse. Once those reach a critical mass, fusion initiates within the stellar core. The rate of fusion is moderated by the pull of gravity, such that larger stars, fighting more inward mass, burn far hotter (and faster, and live comparatively short lives: a few tens of millions of years for the very largest, as compared with ~10 billion for the Sun, and potentially trillions for brown dwarfs which eke out their fuel slowly).
In a curious bit of physical synchronicity, the energy density of the Sun's core (the part which is actively generating energy), in terms of watts/kg, is roughly equivalent to that of mammalian metabolism. It's just that there's a whole lot more mass to the Sun's core than there are mammals, and of course, we're comparing nuclear reactions with chemical ones (the Krebs Cycle).
Back to stars: as they ignite, they start blowing the dust that's forming them away, and so limit their maximum growth potential. The least massive stars (brown dwarfs) are roughly 10x the mass of Jupiter, or about 1/100th the mass of our Sun, the most massive stars are roughly 100x the mass of the Sun, giving a total effective range of roughly 1:10,000. The upper bound, called the Eddington Mass Limit, is about 120 solar masses, and is based on total luminosity which translates to outward pressure exerted by the star against the stellar medium surrounding it:
<https://en.wikipedia.org/wiki/Eddington_luminosity>
I don't know what the dynamics governing galaxy growth are, though a few ideas come to mind. Apparently the most massive galaxies are about 100x the size of the Milky Way (so we're at roughly similar ends of the scale for both stars and galaxies in that regard) in the Physics StackExchange post below. The discussion there does mention luminosity and pressure, though there's also some disagreement over just what constitutes a galaxy, and where to draw borders.
<https://physics.stackexchange.com/questions/172953/what-is-t...>