Will a Universal Ocean Prolong the Age of Stars for Eternity?

Question

One moment, there are as much as two trillion different universes, all of which have their own collections of galaxies. The next, 4,543,000 universes still have their galaxy collections. What happened to the others?

The unfortunate answer is this--a hyperadvanced civilization had parasitized on those universes by sucking up all of the universes' most common element, hydrogen. First, they merged all the galaxies in their universe into one, revitalizing the hydrogen reserves needed to create stars. But the race is paranoid. The idea of the universe being dead--cold, dark, empty, quiet--is flat-out unacceptable to them. So they had some kind of technology to suck up every atom of hydrogen from other universes, turning their universe into a hyperdense ocean of hydrogen.

The lifespan of a star isn't necessarily defined by its size so much as how quickly it burns off its hydrogen. Blue giants heat up awfully fast, so they can live only like insects. Red dwarves, on the other hand, burn their hydrogen so slowly that they can stand for trillions of years. By feeding all the stars an excessive glut of hydrogen, will this prolong the Age of Stars indefinitely?

Answer 1

This is an interesting question, but the answer probably isn't the one you're looking for. As with normal stars, these constantly-accreting objects will fuse hydrogen into heavier elements, then fuse some of those heavier elements into even heavier elements, and so and so forth, until you end up with an inert iron core that grows progressively bigger - and just like normal stars, they'll run into the issue not of having no fuel to burn but of not being able to support that core against its own gravity. It will collapse, presumably into a black hole.

I would argue, in fact, that this would bring about the end of the Stelliferous Era even quicker, though for slightly different reasons than Monty Wild argues. The dense "ocean" would be a ripe place for star formation to occur; as the initial generation of these massive, accreting stars dies, the resulting supernovae would send out shock waves, as normal supernovae do - but since they would be surrounded by enormous reservoirs of gas, it should be extremely easy to compress the surrounding gas and trigger a massive starburst. We see this in our universe (e.g. in Sco OB2, Preibisch & Zinnecker 2001), but not to the extent we would see in this ocean of hydrogen.

This would lead to something of a positive feedback loop - a starburst galaxy on a scale never-before-seen. It's interesting to think about what the ultimate fate of this galaxy would be. The massive stars should presumably form strong winds, eventually combining into a galactic superwind. This has the potential to bring balance to the system, ejecting some of the gas and putting an end to the starburst.

Once all the gas has either been consumed, ejected, or is simply too diffuse to collapse into new stars, however, you'd be left with a collection of stellar remnants - occasionally doing interesting things like interacting gravitationally or even merging (in exotic three-body encounters), but fairly inactive. The Stelliferous Era would be over - although it would have ended with quite the bang.

Answer 2

No.

By adding all this hydrogen, existing stars will be made heavier, plus new stars will form, sweeping up the excess hydrogen into huge, dense stars, the stellifierous age will burn much more brightly, but also much more briefly, and all of these super-massive stars will eventually burn through their fuel, erupt into supernovas, and ultimately collapse into black holes and neutron stars.

Adding even more hydrogen to a universe so full of black holes and degenerate matter will only hasten the collapse of the degenerate matter into black holes.

If we continue to add hydrogen, this will only add to the mass of the black holes, hastening their mergers into more massive black holes.

If hydrogen is still added, I don't know what would happen when the entire universe becomes a single ultra-massive black hole.

TL;DR

Continuing to add hydrogen in an attempt to prolong the stelliferous era will serve only to hasten the end of the stelliferous era and ensure the collapse of the universe into degenerate matter and black holes, and ultimately produce a single universal black hole.

Answer 3

Nope. Their universe is now more than a trillion times as massive as it was. Gravity wins, they'll get a very big crunch.

Clarifying: Our universe has approximately the amount of mass needed to eventually halt the expansion. They've brought a trillion times the mass into the universe, it's way, way more massive than needed to close it. Gravity will bring the expansion to a halt and everything will fall back in. What happens beyond that we do not know.