Will the Universe collapse upon itself?

Dark energy has mass, since it’s energy?

Mass has gravity?

Space is currently expanding, forming more dark energy, right?

And if the universe is mostly dark energy, the mass distribution of the universe is fairly even.

Black holes’ Schwarzchild radius increases proportionately with the mass, not the mass squared, so eventually, any large volume of even distributed mass will exceed its own Schwarzchild radius simply by taking up a given volume and mass.

How big will the universe have to be for this to happen, given the theoretical concentration of dark energy? Is there something inherent in dark energy that will prevent the entire universe from becoming one big black hole?

The person who answers this should immediately begin investigating flights to and lodging in Sweden.

I have no idea. But I’m certain the sun will supernova before any of this. And there’s no chance of us ever getting to another solar system. So we’re fucked in about 5 billion years.

I’ll write a note for the milkman.

Dark energy has negative pressure, which is causing an accelerating expansion of space over and above the expansion that started with inflation.

It’s this accelerating expansion that mere gravity cannot overcome.

No, it won’t.

We found out, about ten years ago, that the universe is not only expanding, but the expansion is accelerating over time. So there clearly isn’t enough mass in the universe to stop the expansion and make the universe contract.

Nitpick: the sun will not go supernova. A normal Main Sequence star has to have a mass of about nine times that of the sun to be able to go supernova. There is another kind of supernova that happens in white dwarf stars, but even those have to have a mass of at least 1.44 times that of the sun.

But dark energy still has mass, no? Gravity doesn’t have to overcome anything if it warps spacetime to make a super supermassive black hole out of the expanding universe. Unless there is some mechanism whereby inflation will negate the warpage of spacetime due to gravity.

On the other hand, at a certain distance inflation appears to happen superliminally, in which case gravity waves will not have a chance to reach anything beyond, so I can see how beyond this distance a supersupermassive black hole would be impossible.

Gravity is attractive. Dark energy is propulsive. If two such forces interact, then the stronger force predominates. This is no different from the fact that a magnet can pull a pin up into the air against the entire gravitational force of the earth. In fact, maybe thinking of the electromagnetic force will help you. That force helps atoms exist despite the tendency of gravity to pull everything together. But like dark energy, it is much more powerful than the extremely weak force of gravity so it prevails.

Many people have calculated the Schwarzchild radius of the mass in the universe. Surprise, it comes out to approximately the radius of the universe. Does that mean we are living inside of a black hole now?

Probably not. It probably says that there is no meaning to a black hole of that size in our current physics. And that a steadily decreasing density of matter cannot form a black hole of the type that exists at the center of galaxies. You have to have a force that will attract matter against all the forces that are pushing it away. Gravity is not strong enough to be that force over the entirety of the universe. That is why the universe is not only expanding, but the expansion is accelerating.

You’re essentially playing with words rather than doing physics. Sometimes the words just aren’t equivalent.

You’re playing with words, as well, since there aren’t any well-understood reasons why short-range, strong phenomena should not reach across the universe or at small quantities. Sure, quantum gravity perhaps alters the reality of long-distance interactions of this sort, but the default assumption is that phenomena scale.

I was asking if any of these phenomena were understood well enough to explain this apparent discrepancy.

Or, we could be living in a black hole, and we can’t get out due to inflation!

So we’re living within the event horizon of a black hole. So what? All that would mean is that we could never get out past it.

But we can’t get out of the universe either.

That’s why this whole issue is meaningless.

The universe will not and can not collapse into a black hole. But it were one now there is nothing we could do to tell the difference.

There is no discrepancy to be resolved. Your misunderstanding is that you think there is one.

I said there was an apparent discrepancy if there is not a collapse and we become inside our own Schwarzchild radius. Surely you did not mistake my last sentence as saying anything other than we are possibly inside a black hole already?

Dark energy does have gravity, and it’s because of its gravity that it causes accelerating expansion. To understand this, you first have to understand that the source of gravity isn’t just mass, but a second-rank tensor, called the stress-energy tensor, with 16 components (10 of which are independant). Energy density is one of those components, and for most of the substances which you encounter every day, it’s the largest by far (in the form of mass), so we can usually get away with ignoring the other parts. But pressure, shear stress, and momentum flux are also components of the tensor, and the vacuum energy (in the simplest models) has a pressure as large as its energy density (but the opposite sign). Since there are three components corresponding to pressure in the stress-energy tensor, but only one for energy density, the pressure of dark energy turns out to be three times as significant as its energy density, so the negative effect of the pressure ends up overwhelming the positive effect of the energy density.

Oh, and to address the Universe-as-black-hole possibility, the condition for the Universe to be a black hole is exactly the same as the condition for it to be closed. The Universe will recollapse iff it is a black hole, and the Big Crunch singularity is the singularity in the center of the hole. Current observations do not support this possibility, however.

Thanks: this seems pretty clear which is a sure sign I don’t understand any of it :slight_smile:

Okay: if, for some reason, there were to be a massive increase in the negative pressure on the inside of a black hole (how? A wizard did it!), would it tear the black hole apart, or would it stay together? If it would be torn apart by this then the laws do indeed seem consistent. But if it can’t then I don’t see how a large would-be black hole could be prevented from forming from negative pressure when an already existing one could not.

At the very least it would imply that if there a black hole were to grow to be the size of the observable universe and inflation was the same as today, the black hole would then be torn apart by the inflation.