I am not a cosmologist but love to read books on the subject and at least in the books I’ve read, the consensus seems to be that there is not enough matter in the universe to cause a BIG CRUNCH and so things must eventually (10 trillion years or so) expand to de facto nothingness. Now there are other debates here about whether this expansion has to happen but my question is more basic - WHY do we say there is not enough matter in the universe? As I understand the Big Bang, in a few seconds, the universe itself expanded from the size of an atom to billions of light years. Though I cannot conceive of this, I have to accept it. But what I do NOT understand is why most scientists DENY the possibility that that expansion went on a few seconds longer. By this I mean, couldn’t it have expanded beyond observable range (about 30 billion light years I think,) so there could be a trillion times more matter in the universe that it is impossible for us to ever detect since it is beyond the TIME at which the Big Bang occured?
The amount of matter in the universe isn’t as important as the overall density of the universe. The farther apart massive objects are, the less gravity they exert on each other. We can observe the local section of the universe, estimate its density, and assume the rest of the universe has about the same density.
Also, more and more dark matter is discovered all the time, so maybe there is enough matter but we just can’t see it and include it in our calculations.
I think you might not be getting the density part. As snailboy said, it’s the density of the universe that matters. We know roughly how big the universe is. We know how much matter we can presently see in the universe. We know how fast the universe is expanding. If you do the math it works out that the amount of matter we have observed is not enough to halt the expansion through gravity. (Note, some of the numbers they use are better known than others)
There is some speculation on ‘dark matter’, matter that is out there but we just can’t see. I haven’t followed the dark matter debate recently so I have no idea what people are thinking about that now. I seem to recall that some were speculting on black holes while other were suggesting exotic particles. IIRC, the amount of dark matter could swing the universe from an ever expanding universe to a colapsing universe.
::wandering over to Borders.com to pick up a newer book on the subject::
One could, in principle, directly measure the size of the observable Universe and the total amount of matter in it, and thereby determine the average density. But this is impractical, largely because of the dark matter, which is (by definition) difficult or impossible to detect directly, and which seems to considerably outweigh the normal matter. What one does instead is to indirectly calculate the average density of the Universe by observing the Universe’s dynamics. That is to say, if the Universe were going to ultimately end up in a Big Crunch, it would already have started slowing down in a particular way. From observations, it seems that the Universe is not slowing down in that particular way (in fact, it’s not slowing down at all, but rather speeding up), so we conclude that, whatever the properties of the matter in the Universe, dark or light, there isn’t enough of it.
[QUOTE=From observations, it seems that the Universe is not slowing down in that particular way (in fact, it’s not slowing down at all, but rather speeding up), so we conclude that, whatever the properties of the matter in the Universe, dark or light, there isn’t enough of it.[/QUOTE]
I few points, again remembering I am not a scientist so could be talking nonsense.
I have read that matter SEEMS to be speeding up. Is it possible that space is slowing down and so the matter in it APPEARS to be speeding up?
If we accept that the “laws of nature” did not apply at the creation of the universe, then certainly matter need not have been evenly distributed at the moment of creation. Past the observable limits of space, why couldn’t the density of the universe be thousands of times greater, so that when space itself stops expanding, the universe will begin to collapse almost immediately?