The Big Bang time paradox

[Zkribbler]

Originally posted by Odin
KH, what's you opinion on the "Ekpyrotic" cyclical model of Paul Steinhardt?

Oh don't get me started on that! I'm still trying to get my head around Hubble's inconstant Constant.

[KrazyHorse]

Don't blame me; blame the ****ing astronomers. They had a hold of cosmology for far too long...

[Diadem]

Originally posted by KrazyHorse
It's very simple: at a point some thirteen and a half billion years ago the Universe emerged from an exponential expansion driven by an unknown physical process. Our knowledge of the history of the expansion of the Universe is cut off by the existence of this process. At the point where somewhat-confident projections give way to pure speculation the Universe already had finite extent. Projecting any further back toward a singularity is ridiculous until we understand the process which drove inflation.

And is there any reason not to call this point the Big Bang, then? Given the inflation factors featuring in most theories, the part of the universe that we can now see was certainly compressed to a very, very tiny point in space at that time. Much tinier than the Planck's Length, for sure.

So we start with the entire (visible) universe in a very tiny point, after which is expands extremely rapidly. Pretty hard to get more Big-Bangy than that.

And why would call this a singularity be ridiculous? It's a singularity just like a Black Hole is. That doesn't mean it's infinitely small, it just means it's so small we can't say anything meaningful about it anymore. General Relavity predicts an infinitely bend spacetime for such a situation. Which doesn't mean spacetime is infinitely bend, but rather that General Relativty breaks down in this situation. Aka: A singularity.

So it's not sloppy language to say that our universe, or at least the visible part, started as a singularity some 13.7 billion years ago, after which the Big Bang occured.

We do know that the Universe was probably significantly smaller at some point prior to inflation than at the moment inflation started, but there is no necessity for it ever to have been infinitesimally small.

Agreed. There is no need for it to have ever been infinitesimally small. Again, I should not have used that word in my other post.

But infinitesimally small or just very very tiny, it's still a singularity in the astrophysical meaning of the word: A concentration of mass so dense that our physics can not describe it anymore.

By the way, you are engaging in sloppy language yourself as well, here. There's a huge difference between the universe and the visible universe. We know the visible universe was once compressed in a singularity (or very small area, if you still dislike this term). We can't say anything about the universe as a whole. Perhaps it always has been and always will be infinitely big.

[Kuciwalker]

And why would call this a singularity be ridiculous? It's a singularity just like a Black Hole is. That doesn't mean it's infinitely small, it just means it's so small we can't say anything meaningful about it anymore. General Relavity predicts an infinitely bend spacetime for such a situation. Which doesn't mean spacetime is infinitely bend, but rather that General Relativty breaks down in this situation. Aka: A singularity.

I'm not a physicist but this is clearly wrong. KH is saying that this condition is not necessary in the pre-inflation period.

[Kuciwalker]

(Understand that I'm working from the premise that any claim KH makes about physics is generally correct )

[Kuciwalker]

But infinitesimally small or just very very tiny, it's still a singularity in the astrophysical meaning of the word: A concentration of mass so dense that our physics can not describe it anymore.

I'm pretty sure there's a more precise definition than that...

[KrazyHorse]

Give up while you're behind, diadem.

This is my house.

[KrazyHorse]

[Kuciwalker]

[KrazyHorse]

The relevant definition of a singularity is the one from general relativity, which is where the observer time for all paths across the singularity is infinite (i.e. what happens when you try to cross the event horizon of a black hole). There is no such boundary to as far back as we can project the state of the Universe. In fact, there's not even a simple coordinate singularity if you use the standard FRW coordinates.

If there was such a singularity then we would have to talk about a "beginning of time". As it is, there's no necessary breakdown in the coordinate system we use. Just because known physics breaks down doesn't mean our coordinate system has to break down. Otherwise we'd have to invent a new coordinate system to deal with the GUT energy scale, and nobody's suggesting that.



edit: spl

[Diadem]

Originally posted by Kuciwalker

And why would call this a singularity be ridiculous? It's a singularity just like a Black Hole is. That doesn't mean it's infinitely small, it just means it's so small we can't say anything meaningful about it anymore. General Relavity predicts an infinitely bend spacetime for such a situation. Which doesn't mean spacetime is infinitely bend, but rather that General Relativty breaks down in this situation. Aka: A singularity.

I'm not a physicist but this is clearly wrong. KH is saying that this condition is not necessary in the pre-inflation period.

I do not see where he said that. And if he did, he's wrong. Sorry

[Diadem]

Originally posted by KrazyHorse
Give up while you're behind, diadem.

This is my house.

Ah. A very good argument.

The argument to end all arguments, in fact. Used by millions upon millions of people before you, when they ran out of rational things to say.

I see that discussing this further is not very useful. So perhaps I should give up. But 'while I'm behind'? Nah, if I had been behind you wouldn't have used that argument. So that can not be the case. I'm sorry.

[KrazyHorse]

Diadem, I suggest you compute the Schwarzschild radius for a Universe of critical density. Compare to the radius of the visible Universe.

You might be surprised at the result.

[KrazyHorse]

Diadem's taking his ball and going home.

It's sort of sad how badly I just schooled you, son.

[KrazyHorse]

Don't leave! Our physics discussion won't be complete without a participant from the Lower Rotterdam School of Technical Arts.