We have an idea of how the universe began, essentially, as a point, and we have an idea of how old the universe is. Then, from our observation of the way light red-shifts - as described by another respondent - we have an idea of the size of our universe, and we can see how fast the universe in now expanding. Then,we are able to do some calculations that allow us to put these pieces together so that we can build a picture of a sort of 3 dimensional trajectory - like you might plot the trajectory of a ball thrown into the air. Even when it was only part-way through its flight, you could calculate the whole of its flight path.
Assuming that we are correct so far, we come to a sort of snag in being able to predict the future 'trajectory'. We think we have an idea of how much mass is in the universe, and we think we have an idea of how much mass it would take, spread out over the universe, to create enough gravitational pull to eventually pull it all back together again. And the amount of mass we 'see' is far too little to do that.
But we are convinced that there is more mass than we can observe, even much more. We can observe the effects of this mass in the way certain galaxies wobble - as if there is a lot of mass where it does not look like there is any mass. We call this mass 'dark matter'. Since we do not know how much dark matter is in the universe - or even what it is - we are in no position to say if the amount of mass is sufficient to pull the universe back together eventually.
However, even this whole way of thinking may be flawed since we have quite recently observed that contrary to our expectations, the universe's expansion seems to be accelerating - exactly the opposite of what we would expect. And so you could say that 'all bets are off".
· 9 years ago