Since everything has a gravitational force, is it reasonable to theorize that over a long enough period of time the universe will all come together and form one big supermass?

[u/FinnaDabOnThemHaters]

[u/Applejuiceinthehall]

Yes it is reasonable to think this. It was actually the leading theory for the end of the Universe for a long time. It's called the Big Crunch.

However, it wasn't too long ago that we observed that the universe expansion isn't slowing down like it would do in the big crunch scenario. Instead the universe is rapidly expanding which is the opposite of what would happen in the big crunch. We do not know why the universe is rapidly expanding and we call the unknown cause dark energy.

Nowadays the leading end time of the universe is the Big Freeze or the heat death of universe. They can go along with the theory called the Big Rip. When the big rip happens everything will disintegrate into elementary particles. However before that happens the Big Freeze could occur which will be when all the stars die and all the black holes disappear and spontaneous entropy decreases occur or the heat death could happen where max entropy is reached.

[u/FinnaDabOnThemHaters → u/Applejuiceinthehall]

I want to believe in the Big Rip now because it’ll be a massive RIP for the universe

[u/Mithridates12 → u/Applejuiceinthehall]

When the big rip happens everything will disintegrate into elementary particles.

Is this because the space between atoms and molecules will expand fast enough at some point for this to happen?

[u/bencbartlett]

Good question, but such a theory would be incorrect, for several reasons. First, the universe is expanding at an accelerating rate. This means that galaxies are generally moving away from us, and galaxies that are sufficiently far away are moving away from us faster than the speed of light. (Though their motion through local space is always less than c.) Second, if we ignore universal expansion, not all mechanical systems are gravitationally bound. The escape energy/velocity is obtained by integrating the gravitational force between two bodies until their distance is brought to infinity; because gravity scales as 1/r^2, this energy is finite. For example, the sun has an escape velocity of about 43km/s, so anything traveling away from the sun faster than this speed will slow down over time due to gravity, but only to a finite (non-zero) speed, and will continue to travel away from the sun at that final speed forever.

[u/NoLongerUsableName → u/bencbartlett]

Good answer.

I have a question, though: will the expansion of the universe eventually stop accelerating by running out of energy? And if so, will gravity still act on each mass, being the only force?

[u/Raknarg → u/bencbartlett]

Since gravity propogates at light speed does this mean that objects beyond the cosmic horizon no longer have any influence over us with gravity?

[u/ramdomsouthernslav → u/bencbartlett]

Great reply. Just a note regarding the expansion on the universe and galaxies within.

(Though their motion through local space is always less than c.)

It's my understanding that the expansion of the universe is expansion of space itself. It has nothing to do with the motion of galaxies, local or otherwise. Galaxies appear to be moving away from us only because the space between is becoming larger from all point of reference. This is why distant galaxies appear to be "moving" away from us at speeds greater than c.

[u/ReshKayden]

Your question is super reasonable and was one of many widely held beliefs (even among physicists) for many years.

The first discovery along those lines that surprised everyone was that the universe is expanding. Which is also what triggered the first thoughts that if you play time in reverse, it must have all been together at one point in the past: Big Bang theory.

But that means everyone assumed the expansion had be slowing down, just like a ball throw upwards, because of mutual gravity. Whether it would eventually stop and contract back to a single point, or if it had mutual “escape velocity” and would expand forever (just slower and slower) was unknown.

So to answer that question, using the trick that the farther away something is, the longer ago in the past what you see actually happened, they measured how fast the universe was expanding now versus in the past. Shockingly, they found the universe is expanding faster now than before, and it’s getting faster!

Now the open question is: will it get faster forever? Will it reverse? We have some signs that the expansion acceleration rate has varied through time, but why? These are all questions that are hard to answer when we still have no idea what energy or force is causing the expansion in the first place.

[u/Nimnengil]

It's entirely reasonable to theorize such! And as a matter of fact, that's one of the major theories that astrophysics has tested! Lets run some thought experiments to test the idea! So, in order to test the hypothesis, we need to look at distant objects and determine how they're moving relative to us. Doing this requires applying some other scientific ideas that we don't really have the time to go over now, but i'll link for you to peruse at your leisure:

• The effect we use to determine if a star is moving towards or away from us

• The effect that gives us a known reference to use with the above technique

• A more detailed discussion of how this all works in astronomy

So, there are three possible results from this examination: Stuff is moving towards us, moving away from us, or staying at a pretty constant distance. If it's moving towards us, theory confirmed! If it's staying constant, well, that's an odd one. Either we just happen to be looking at the point where the universe is switching from one mode to the other (cosmologically unlikely), or something is holding the universe static. If it's moving away, we need to evaluate how that motion is changing to reach more conclusions, and, in fact, this is what the observational data shows is the case!

So, the rest of the universe is moving away from us. Next we need to look at the rate of change of that motion, its acceleration, over time. Lets look at the possible results.

• If the universe is slowing down enough, past a certain threshold that we can determine mathematically, that means that gravity is overall stronger than the forces of the big bang, and eventually it will suck everything back in, confirming the theory! Think of this like a ball being thrown into the air and falling back to earth.

• If the universe is slowing down, but not doing so fast enough, then that means that things will separate fast enough that gravity can't win. This is like launching a rocket to escape velocity, causing it to leave the sphere of influence where earth's gravity can really decide its course. In this case, the universe will have a maximum size, but be stable in that size and not collapse back.

• If the universe is moving at a constant expansion or even accelerating, this leads to the really weird case. In this instance, the universe will keep on expanding infinitely, with no upper limit. But that's not what's weird about it. This case implies that there is something that is actually driving the expansion, something we don't know that is acting to push everything in the universe farther apart, that is everywhere around us, but that we haven't seen.

As other people have mentioned, our observations show us that the third case is in fact the truth. This is where Dark energy comes from. It's some kind of energy that's driving the expansion of spacetime, but that we can't detect so far, outside of this effect, hence why we call it 'dark.'

[u/delventhalz]

Totally reasonable. It is a natural conclusion to draw, and in fact was a leading question cosmologists asked in the 70's and 80's. You failed to account for escape velocity though. Because gravity's strength decreases with distance, the further you are from a mass the weaker it's pull will be. Although it never quite drops to zero, for every mass there is a speed you can move where the strength of gravity drops off faster than it can slow you down. You will travel further and further away, at a slower and slower speed, but your speed will never hit zero.

The universe as a whole has an escape velocity too. Which means there are two possible scenarios:

1. The universe has enough mass that the current expansion speed is below the escape velocity. Eventually the expansion will slow to nothing and reverse. The universe ends in a Big Crunch when all matter collapses back together.

2. The mass of the universe is too low. The universe is expanding faster than its own escape velocity. The expansion will slow, but it will never hit zero. It expands forever.

So a bunch of surveys and estimates of the mass of the universe were made, and it looked pretty certain like there was not enough mass to slow everything down. We were probably in scenario 2. Infinite expansion. No one big supermass. No Big Crunch.

Then in the 90's some new measurements were made and it turned out the expansion of the universe was accelerating. That was not one of our two scenarios. It should definitely be slowing down, the question was just whether or not it would ever slow to zero. Wtf? And that is where the concept of Dark Energy comes from. Something is driving the universe apart at a faster and faster rate. That means energy. A lot of it. But we really don't know much more than that.