If the universe is constantly expanding, how could the big crunch happen ?
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As I understand it, the expansion could be slowing down due to gravitational attraction.
Eventually the expansion would stop and then reverse (start contracting).
In other words, the current expansion is simply inertia from the big bang, once the momentum stops, the contraction begins.
However, some say that the expansion isn’t slowing but is actually increasing. This is attributed to Dark Energy.
I think Dark Energy is a type of coffee (a stupid joke, sorry).
The idea behind the big crunch was that while the universe was expanding, that rate of expansion would slow over time and eventually reverse. Like throwing a ball attached to an elastic band – it will travel for a while, but eventually slow and return to the point of origin when the forces pulling it back overwhelmed the forces pushing it forward. The theory was that eventually gravitational forces would overcome expansion forces, pulling all the matter in the universe back to a single point – a new singularity.
That said, big crunch is not a popular theory anymore. From what we can tell, the speed of expansion is _accelerating_, not decelerating so the universe is unlikely to come back together. The current theory is that the universe will just keep on expanding until [heat death](https://en.wikipedia.org/wiki/Heat_death_of_the_universe).
We don’t know what’s causing the expansion.
We don’t know what’s causing the expansion to accelerate.
Since we don’t know these things, it could turn out that they are cyclical, as many things in physics are cyclical. If the acceleration stopped and the expansion stopped the Universe might start contracting and that contraction might accelerate and it could all end in a Big Crunch.
We need to develop a testable theory that explains expansion. Once we’ve tested it we might be able to make predictions. If our predictions pan out, then it will be time to start answering Heat Death vs Big Crunch and related questions.
In the Big Crunch hypothesis the idea is that the there are two forces in play – an initial outward momentum of everything caused by the Big Bang and the small and persistent gravitation attraction trying to pull everything back in. Like a model rocket launched upwards at first the initial momentum is so huge everything flies outward, but gradually overtime gravity sucks away this momentum, the mass eventually stops, and then starts to rebound in on itself, eventually recollecting in a Big Crunch.
But that’s not what we are observing in the real world. We are observing the outward expansion *accelerating*. So *something* else must be in play, some sort of anti-gravity outwards repelling force. The issue we can’t *find* the source of this force, but we can estimate the size of it. So we can say there is a force of size “x” out there, being caused by unseeable, undetectable things that we have no physical understand of. We call these unseeable, undectable things “Dark” energy and matter. We currently believe that “Dark” energy is the biggest factor in the mysterious expansion force, but remember, we can’t *find* it.
So we currently don’t accept the Big Crunch theory, because it doesn’t fit with our observations, and we accept that the universe is expanding, increasingly quickly, and will do so for ever. Eventually this expansion with result in everything just being so spread apart and dispersed that nothing meaningful can ever happen again. We call this theory the “Inevitable Heat Death of the Universe”, you can have some fun googling researching that topic.
You’re right, it couldn’t! That’s why we’ve kind of settled on the idea that the big crunch probably *won’t* happen.
Obviously there’s a lot we don’t know, and a lot we *think* we know which might be wrong, but until a handful of years ago the scientific community was split on whether the universe would keep expanding forever, or if it would at some point start to contract instead, ending in a “big crunch”. More recent experiments and measurements have suggested that we’re probably looking at the first scenario. Not only is the universe expanding, but the rate of expansion is *increasing*. That would imply that the universe will probably *keep* expanding, and if that’s the case, there will be no big crunch.
Big crunch is an obsolete hypothesis, it relied on expectation that gravitational attraction would slow and eventually reverse expansion, but when it was measured this was proven to be not true. Expansion of the universe is not slowing down, instead it’s accelerating, why that is the case is unknown, it’s the substance of unsolved problem called dark energy.
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