The discussion centers on the debate regarding the existence of a fifth fundamental force in physics, particularly in relation to dark energy and the universe's accelerated expansion. Participants argue that while dark energy exhibits properties that could suggest a new force, it is more accurately described as a manifestation of gravity influenced by negative pressure. The conversation highlights the complexities of General Relativity and the need for empirical evidence to support claims of additional forces beyond the established four: gravitational, electromagnetic, weak nuclear, and strong nuclear forces.
PREREQUISITESPhysicists, cosmologists, and students of theoretical physics interested in the fundamental forces of nature and the ongoing research into dark energy and cosmic expansion.
Chalnoth said:Yes, if there was enough mass relative to the current rate of expansion and no dark energy, or if the dark energy at some point in the future goes away and it turns out that the remaining mass is large enough compared to the rate of expansion, then it would recollapse. And in General Relativity, it would collapse to a singularity. Though to be fair, General Relativity cannot be trusted that far.
My impression is no, not if the universe didn't recollapse. My understanding of structure formation (admittedly basic understanding) is that in an expanding universe, matter that is within the Jeans' length collapses in on itself, while matter outside that length does not. So basically, structures will gobble up all the local matter they can, then stop growing. Dark energy turns off this growth sooner, but I think it stops in any expanding universe.Tanelorn said:Is it correct that if it wasnt for the recent increase in dark energy expansion, that the matter density would eventually become higher in the filaments between voids and the sizes of galaxies would increase due to filament collapse and galaxy collisions? If so would this constitute another happy "goldilocks" coincidence?
Think of an expanding universe like a fluid. If this fluid is made out of normal matter, then the gravity between the particles of the fluid wants to pull the whole thing together. So if it is expanding, gravity acts to slow the expansion by pulling everything closer to everything else. If this pull is strong enough, by the density being large compared to the expansion rate, then it will cause the universe to recollapse in on itself.Tanelorn said:Chalnoth In this simple model I can't see what causes the collapse since the universe is infinite and the forces are approximately equal in all directions. Unless the small differences in gravity caused instability and accumulaton of matter in certain locations?
The length depends upon the density. So it is relevant for the formation of objects of all scales, whether asteroids, planets, stars, galaxies, or galaxy clusters. It's just that in an expanding universe, the average density is decreasing, and since it takes larger regions longer to collapse, there is a point where regions don't have enough time to collapse before the expansion lowers the density so far that they can longer collapse (because the Jeans' Length is smaller than the size of the overdensity).Tanelorn said:Chalnoth thanks for the Jeans length reference, I think this answers questions above that I have had for quite a while. Although, what is the estimated value, it seems like it isn't that large? Wouldnt this be relevant for star formation?