ELI5: If every mass in the universe has gravitational effect on space time, then why don’t everything just crumbled into each other as everything “attracts” every other things?
ELI5: If every mass in the universe has gravitational effect on space time, then why don’t everything just crumbled into each other as everything “attracts” every other things?
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Everything is expanding outward and distancing it self from everything else. Except black holes and dead stars but even those are expanding away.
Everything does attract everything else through gravity, but space is really big and most objects are super far apart, so those gravitational pulls are usually tiny. On top of that, a lot of things are moving—planets, stars, galaxies so their motion creates a balance. Like how Earth orbits the Sun instead of falling straight into it. The gravitational pull is there, but the sideways motion keeps things in stable orbits instead of everything clumping together.
Because the amount of attraction everything has isn’t enough to counteract the expansion.
Imagine two cars are driving toward each other on a road. They’re each driving 60 mph (or 100 kph if you prefer). They’re 60 miles or 100km apart. They should reach each other in a half hour. But the road is magically getting longer. It’s expanding between them. In fact, every hour, every mile or kilometer is stretching to four miles or kilometers. After an hour, the cars have each driven 100 km toward each other, but the 100 km road is now 400 km, so they’re FURTHER away from each other than they started.
Space is a bit like that. It expands much more subtly, but the distances are so vast that the expansion is more than can ever be overcome by gravity for most things that aren’t already in each other’s region.
One common prediction of the eventual fate of the universe is that all the stars will eventually burn out and all matter will condense into black holes which gradually evaporate over enormous time scales we can’t even comprehend, until all that’s left is a dead, cold universe.
But this whole process takes a long, long, long time. The universe is really big and there’s a lot of stuff and a lot of energy. It’s possible that everything will eventually collapse in, but we’re nowhere near close to that point yet.
It’s like getting on a train in Boston, and asking why you’re not in New York right now. The train may be heading in that direction, but there still a lot of time before it gets there.
Part of it is because of the expansion. And part of it is probably because the universe os so big that gravitational waves (and thus, attraction) from some (most?) matter in the universe has not reached other matter.
That is one of the possibilities of how the universe might “end” (Google: big crunch), though it’s not currently thought to be the most likely. But it would take a long time, because space is huge and things are far apart.
The reason why it’s not considered likely is because the universe is expanding (and the expansion is accelerating). That is, the distance between any two points is getting larger. So space itself is getting larger, making things further apart. And the effect from this is larger than the effect of gravity pulling things together.
The mechanism behind the expansion of the universe is not well understood. We can see that it’s happening, and we have named the cause dark energy – but there’s not a huge amount known about it.
Some recent research (which I believe still needs to be confirmed?) suggests that dark energy is not constant. If this is true, then it is possible that the effect of dark energy eventually gets weak enough that gravity takes over and a big crunch is still on the table. And so your intuition may end up being correct after all. However, there are definitely too many unknowns to say anything about the effect of this research on the universe’s future for certain at this time.
The earliest point in time is the moment the big bang started. At this time, everything in the observable universe was contained to a single point. As so much energy cannot be contained to a single point, it very rapidly speed off in all directions.
The strength of gravity is directly related not just to the amount of mass, but the distance between the objects or points in space you’re considering. The big bang spread things out very, very far. Local things still affect one another, but the rest is so far away that it’s effect is minor.
Gravity is the weakest of the four fundamental interactions, also.
Most objects in the universe are hurdling through space at a very high rate of speed in opposite directions. This has lead scientists to theorize that once all the mass in the Universe was in one place that exploded. (The Big Bang Theory) Gravity caused by the mass of matter is strong, but it is weaker as distance increases. So we end up with clumps of matter, galaxies, hurling through space. Either the Universe will keep expanding or eventually gravity will win out over the force of the Big Bang and the Universe will collapse together again.
That’s actually one of the theories of how the universe will end, The Big Crunch, where the universe stops expanding and starts contracting due to gravity. Another related theory is The Big Bounce, where the universe goes from Bing Bang to Big Crunch and repeats for a possible infinite amount of time.