Explain the Big Crunch to me

Matter can neither be created nor destroyed. Therefore, the amount of matter in the universe now is the same amount as was around directly after the big bang.
Matter attracts itself to other matter through gravitational pull. Therefore, the natural tendency of matter is to join together with another mass.

So, given that there was enough force during the Big Bang to break apart the most powerful gravitational attraction ever (i.e. all the mass of the universe), why would there in the future be enough of a gravitational pull to bring them all back together, when galaxies are now so far apart from one another?
In other words, if there is enough mass in the universe to bring about a Big Crunch, how did the Big Bang ever occur in the first place? How did matter escape the pull of all other matter?

But, it can be converted to/from energy. I think this is the basic fact your above post fails to take into account, but I could be wrong.

In other words, if there is enough mass in the universe to bring about a Big Crunch, how did the Big Bang ever occur in the first place? How did matter escape the pull of all other matter?

The same way you can throw a ball into the air and have it fall back to you. The amount of energy that you can give the ball is not enough to have it escape the Earth’s gravity, so it comes back. If you could launch the ball hard enough (that is, if it left your hand with a velocity of 11 km/s, Earth’s escape velocity), it wouldn’t come back down.

In a Big Crunch universe, there was enough energy in the Big Bang to make everything start expanding, but eventually the expansion would slow down and stop and then reverse-- just like the ball slows down and stops and changes direction, and then falls towards you faster and faster.

In a ‘Big Chill’ universe, the energy from the Big Bang was so great that all the matter just keeps flying outwards forever, like the ball would if you could throw it fast enough.

[bit of a hijack]

In a similar vein, I seem to recall reading that time would also flow backwards during any big crunch.

Now, I’m no Hawking, but does that mean (assuming a crunch) we would experience every moment twice, once forward and once backward? Would we even be able to tell the difference between an expanding universe and a contracting universe? And what does that say for free will vs pre-determination?

I think Hawking originally suggested the idea that time would move backwards, but later changed his mind. There isn’t any reason to think that everything that had happened would have to un-happen in order for the universe to collapse. It’s independent of the direction that the matter is moving. We’d just notice that everything seemed to be moving towards us instead of away from us as we observe now.

I thought about this exact thing before posting and while I understand your point, I’m not entirely certain the scenarios are analogous.

The Big Bang made everything expand outward, away from everything else. There no longer is an “Earth” for the “ball” to return to. Certainly some galaxies may collide and merge as it all continues to expand outward, but I fail to see how everything will come back together when there appears to be no focal point that’s drawing us.

If there was an explosion large enough to shatter the Earth and send it into a thousand pieces in every direction, would you expect the Earth to reform again at any point in the future? I wouldn’t.

The Earth’s shattered fragments might re-coalesce (not necessarily re-form) if they existed in a space where there was no other mass exerting a gravitational attraction.

All the matter in the universe is exerting a gravitational force upon all other matter in the universe. The energy from the Big Bang has been forceful enough, so far, that the expanding universe has overcome that gravitational force. So far. But the Big Bang’s energy may someday expend itself, leaving only uncounteracted gravity, in which case the force of gravity will keep all matter attracting all other matter until it all falls together in a Big Crunch. The process does not require a “focal point,” only a set of mutually attracting masses.

How can we tell that it isn’t happening right now?


Miranda, one of the moons of Uranus, appears to have come apart and re-coalesced at some time in the past. It’s more-or-less spheroidal, but its surface is chunky, as if it had been taken apart and thrown back together in a pile.


There are other theories about its appearance, but the breakup-and-reformation is entirely possible, physically speaking. If something happened that was forceful enough to break the planet but not throw the pieces away from each other, they’d keep on going around together in the same orbit, and eventually fall together again (although some of the pieces might be jumbled up, which is what Miranda looks like).

If you hit a hard-boiled egg with a hammer, you can hit it lightly so that only the shell cracks, or you can hit it a little harder so that the white splits into pieces but the pieces don’t go very far, or you can hit it so hard that egg goes all over the room. Not every force or impact has enough energy to blast an object to smithereens. :slight_smile: