Expansion of the Universe and Time

[agreen]

My standard disclaimer (this is my second post now): "I'm not a physicist... I'm not a mathematician... I'm not even in school, I'm just a computer guy who watches the Science channel a lot :) So don't yell at me... be gentle :)"

From the zillions of TV shows I've watched about the subject in an attempt to gain an increasing understand of the concepts, as I understand it - space and time are interwoven, such that if the Universe is expanding, that means that both space and time are expanding. Is that why we seemingly experience events in a linear "forward" time - because both space AND time are expanding? (forgive my limited vocabulary for trying to explain what I'm thinking :)

 

[Nabeshin]

No. What is expanding is the spatial universe, not the temporal component. I know you're not a physicist or mathematician, but hopefully you know enough algebra to note be scared by a simple equation:
$$ds^2=-dt^2+a(t)^2(dx^2+dy^2+dz^2)$$

This is the equation which describes the expansion of the universe (a flat universe, mind you). You'll recognize the x^2+y^2+z^2, the distance between two points in space (think pythagorean theorem). Now, the a(t)^2 factor out in front is what's called the scale factor. When that factor increases, we say the universe is expanding (the distance between the points is increasing). Similarly, if it were decreasing, the universe is contracting. Now it is the simplest thing to note that such a factor does not appear in front of the time portion of the equation -- i.e. time is unaffected.

 

[agreen]

Hmmm... I thought that at the instant of the Big Bang, there was no time and space, just the "singularity", and that when the Big Bang occurred and progressed, it wasn't just matter expanding into empty space, but the Universe itself was expanding (time and space included)? Or am I incorrect in understanding this part?

 

[Nabeshin]

Hmmm... I thought that at the instant of the Big Bang, there was no time and space, just the "singularity"

I hope you realize that this sentence is not even self-consistent. There was no time and space implies you cannot even talk about the "instant" of the big bang, since there is no concept of time.

It is true though that in standard interpretation there is no empty space to expand into, but time, space, and the universe itself came into being all simultaneously. I really don't want to get into this since it's somewhat silly, given that we know quantum corrections are important here so some quantum gravity theory of cosmology is necessary to speak meaningfully about the universe before a Planck time.

Although, I don't see how this relates to your original question or how I answered it...

 

[agreen]

Well, remember I'm just a lay person hoping that your imparted wisdom will make me smarter :) My second question was just an attempt to help me better understand your answer; I was trying to get a better grip on the concepts that made me ask the question in the first place.

Technically, I CAN talk about the "instant of the Big Bang" because, once it started, there would then be time and space to speak of, right? :)

Regardless, thanks for your help :)

 

[phinds]

I think that although it's hypothetical, at best, to talk about space-time at t=0 (the moment of the singularity) right after that, time just started rolling along. Space was doing some weird stuff, though, then and since then, inflating and then expanding and now expanding at an accelerated rate.

Time (or maybe I should say "perception of time") gets weird under relativistic effects, but "absolute time" (defined as relative to an observer) has, I believe, always just gone along at the rate of 1 second per second (if you see what I mean). We consider the U to be ~14B years old in absolute time.