Science a GoGo's Home Page Forums General Discussion Physics Forum The Big Bang "site".

It seems strange that scientists can talk about running the expansion of the Universe backwards until it reaches a single point; yet maintain that we should not consider this same expansion as emanating from a single point.

I understand that the BB happened everywhere in the Universe, but wouldn't that "everywhere" have been extremely small in the first instant?

Since my OP I have come across the idea that the Universe could have been infinite at the "time" of the BB.

Anyone have any thoughts about how that could be possible?

Bill, you've hit upon one of the more perplexing aspects of modern cosmology. The thing to keep in mind is that the big band is not an explosion. You do not have a bunch of matter flying away from a central point. Instead that matter is "stationary" (in quotes, as the matter does move due to gravitation interactions and whatnot), and instead of the matter moving, its the space between the matter which is expanding.

It is unclear at this point if our universe is finite or infinite in size, but in either case at the big bang you have the entierty of the universe compacted into a highly dense area - this area may be zero in volume (only possible with a finite universe) or may have a non-zero volume (non-zero but non-infinite for a finite universe, infinite for an infinitely large universe). The BB involves the expansion of the universe from that initial dense state.

If the universe is infinite in size, the BB was infinite in volume. One of the odd things about infinites is that they can be made bigger or smaller, and yet still be infinite.

Bryan

This is the bit that causes me trouble. I understand that mathematical infinities can be of different sizes. E.g. the sum of all whole numbers is infinite, and the sum of all odd numbers is infinite, but the latter must be only half the size of the former.
However, these are only mathematical infinities. No-one can actually produce any of these - "look, here is the sum of all whole numbers." - but a physical infinity must be a real thing, and by its very definition, must be unique.

The more you think about physical infinity, the more you realise that size is in no way relevant to it. It cannot have measurements or dimensions as we understand them, (Jim).

Can you describe to me how something comprising a minuscule, point can be infinite. I can see how it could be unbounded, but not infinite.

I believe that physicists differentiate between mathematical infinites and physical ones by referring to physical infinites as "unbounded" universes. AFAIK, both bounded (finite-sized) and unbounded (infinite-sized) universes "work" for the big bang.

That said, the unbounded universes are exactly what the mathematical infinites would suggest - without definable size.


Not necessarily true. But I do a lot of math in my work, so maybe I visualise things oddly, but I have no issues visualising an unbounded universe.


Because the big bang was not a minuscule point, but rather a state of high density. At the big bang the distances between particles was as small as it can be (supposedly whatever it is in a singularity). Whether the universe in bounded or not, this does not imply zero volume; in the case of a finite universe this singularity could be physically quite large (basically the amount of mass * the volume this mass takes up in a singularity). In an infinite universe you'd have infinite mass, all packed as close together as the mass can be, but since its an infinite amount of mass it takes up an infinite volume. Post big bang, you still have infinite mass, taking up infinite volume, but with a greater separation between particles.

Bryan

Equating unbounded with infinite creates as many problems as does equating mathematical infinities with physical infinity.

I have no problem accepting that out Universe might have started with the BB, and might be unbounded;but accepting that it can have had a beginning, yet be infinite, is a different matter.

Another difficulty I have is accepting that there can be a passage of time in infinity. You say "Post big bang, you still have infinite mass, taking up infinite volume, but with a greater separation between particles. This involves change. Change involves time. Time has no place in physical infinity.

Such as? From a relativistic point-of-view (i.e. the universe as we currently understand it) they are equivalent.


Why?


Once again, why? Infinities do not preclude change; either mathematically or physically.

Bryan

bryan, let's take these points one at a time; make a concession to an oldie!

I understand that if you use a lot of maths, you are probably locked into a mathematical view of infinity. I, on the other hand probably suffer from the opposite problem. However I have put quite a lot of effort into coming to grips with mathematical infinities, and trying to equate these ideas with the concept of physical infinity. I have made sundry notes along the way, so perhaps you wouldn't mind if I run some past you in the interests of clarity.

Is there a difference between “infinite” and “boundless"? Etymologically, infinite is simply endless. It has been argued that something can, therefore, be said to be infinite if it has a beginning, as long as it has no end. However, this line of reasoning is spurious, as the concepts of beginning and end are dependent on the orientation of the observer. For example, if you are journeying across the Atlantic from Europe to America, the Atlantic Ocean begins at the coast of Europe and ends at the American coast. When you turn round to come back, though, the situation is reversed. Similarly, in the case of time, the concepts of beginning and end depend on the perceived direction of that elusive entity, the “arrow of time”. There is another difficulty which, in a way, is an even greater problem; that is the difficulty of terminology. We tend to talk of infinity and eternity as though they were measurable in terms of time and space. They are not. Eternity and infinity are not measurable, so time and space, which are essentially finite measures, have no relevance with reference to the “infinite”, but, because of our finite state of being, it is very difficult for us to take time and space out of our thought processes.

Bryan, looking back through the posts, I find a statement I made: “physical infinity must be a real thing, and by its very definition, must be unique”.
I would be interested to know if you agree with this, or not.

Bryan, you didn't respond. Could that be because you thought I was talking rubbish, and were too polite to say so?

Looking at some of your posts in other threads I find that difficult to believe.

The more you think about physical infinity, the more you realise that size is in no way relevant to it. It cannot have measurements or dimensions as we understand them, (Jim).

Another Star Trek fan, and one who agrees with me about the nature of infinity, it gets better all the time!

I look forward to lots of posts from you, saroy123.