Did the universe expand from a point?

[murrdpirate0]

Or could it be infinitely massive? I'm confused since both scenarios seem to be able to predict the same observations.

 

[marcus]

The standard model of the universe that nearly all cosmologists (and astronomers in general as well) use comes in two basic flavors. Each version is consistent (to within statistical error) with the data accumulated so far. In neither version of the standard model does space have a boundary. Overall uniformity is assumed.

One reason to get more data and more precise accurate data is to finally be able to distinguish which version of the standard model we are in.

One version has the universe expanding from infinite initial volume and infinite initial mass.
So what we see is only a small finite portion, but for simplicity we assume matter is roughly uniformly distributed throughout---so the rest of the world is approximately like what we see.
Same density on average. Same expansion rate on average.

The other version has finite volume space. Space has no boundary so if it has finite volume it must have some closed topology like e.g. analogous to a sphere. Like the 2D surface of a balloon except that it is 3D. All existence is in that hypersphere, it has no "outside" or "inside" so in that sense it is unlike a balloon surface. Just think of the pure surface of the balloon, all existence concentrated there with no inside or outside. Then imagine it is 3D instead of 2D. That is an example of finite volume space.
It is almost the only case of finite volume that is considered.

So more data is needed to distinguish. Because if a very slight positive curvature can be deduced from the data then we will think we are in the finite volume hypersphere.

But if the data favor a zero curvature interpretation, that will favor infinite volume.

There is a particular number that you look for, every time new data comes out, to tell you the estimated curvature. The error bar has been shrinking down in recent years and it still contains zero---still consistent with the zero curvature infinite volume case. But it has been slightly lopsided, on the positive curvature side. So it isn't obvious how it is going to go.

A new spacecraft observatory was launched this year, called Planck. It is a European mission, not NASA. It is now taking data. That data will improve the estimate of the curvature and help decide if it is zero or not.

If space is finite volume, it would become more reasonable to say that the "whole universe expanded from a point". But we still won't know. Maybe it expanded from a small, very high density, high temperature state, but not a mathematical point. But still, if it is finite volume then roughly speaking it makes sense to think of it as starting as a "point".

But if it is infinite volume, then the "point-ish" beginning makes no sense at all! It would always have had infinite volume, even at the very start. Talking about the whole universe, not just the part we see.

 

[Chronos]

Perhaps that is true, but unprovable. Marcus has a good point, however, Planck numbers are the only known parameters.

 

[Wallace]

What marcus said is mostly correct, as always, but I'd be a little careful with this statement:

There is a particular number that you look for, every time new data comes out, to tell you the estimated curvature. The error bar has been shrinking down in recent years and it still contains zero---still consistent with the zero curvature infinite volume case. But it has been slightly lopsided, on the positive curvature side. So it isn't obvious how it is going to go.

You can't treat the posterior likelyhood as a measurement and try and do a statistical analysis of the statistics! It would be entirely suspicious if the error bars were not lopsided to some extent even if \Omega_k is truly zero. The statistics tell you that zero spatial curvature is very consistent with current data, end of story! (for now at least).

If space is finite volume, it would become more reasonable to say that the "whole universe expanded from a point". But we still won't know. Maybe it expanded from a small, very high density, high temperature state, but not a mathematical point. But still, if it is finite volume then roughly speaking it makes sense to think of it as starting as a "point".

But if it is infinite volume, then the "point-ish" beginning makes no sense at all! It would always have had infinite volume, even at the very start. Talking about the whole universe, not just the part we see.

The word 'point' implies a special location. As you have well explained, even in a finite universe with positive curvature, as long as the cosmological principle holds, the expansion does not start from any individual point. I'd caution that it still makes no sense to talk about a point of expansion in a postively curved universe. I know what you're saying, but both the everyday and the mathematical meanings on the word 'point' are at odds with BB cosmology, even for a postively curved universe.

There is also the somewhat depressing issue that even Planck (or anything built beyond that) may never actually answer this question. If the curvature is sufficiently far from zero then a detection could be made*, but if not then we will never be able to tell the difference. As far as measurements are concerned flat and very close to flat are the same. On the other hand, at the a theoretical level clearly the two options are very different (as different as knowing if we live on a flat Earth or a globe, but even more significant in terms of fundamental physics), but if the observable universe is a sufficiently small piece of the possibly finite universe we will not be able to tell the difference.

*Note that it gets even tricker when you throw dynamical dark energy into the mix. There is a degeneracy between the dark energy parameters and curvature which smears the error bars out even more if you allow DE to be dynamical. Unless we have a breakthrough at the theoretical level to know what a good model of DE is, even Planck+next gen dark energy missions won't really improve the WMAP LCDM assumed error bars on curvature.

 

[friend]

It's difficult to understand how a universe could exist that has always been and has always been infinite in extent. For me it defies any attempt to explain how it came to be. Wouldn't theories with a causal structure contradict the idea of portions of spacetime that never had any causal relationship to other portions?

 

[Wallace]

It is difficult to understand how a Universe could be infinitely old. It is equally difficult to understand one that has a finite age, since trying to imagine what existed 'before time' is kinda hard I find... It's equally difficult to explain how nothingness, real nothingness without even time, spontaneously became somethingness, as it is to explain why there has always been something. Note that Big Bang theory is mute on this, it's equally compatible with an infinitely old Universe and one that has a finite age, just as it is compatible with both an infinite and finite spatial extent.

These certainly aren't simple questions, but I think both 'sides' are equally implausible and difficult to justify from pure logic. I have my doubts that we will ever get an answer from science (and we certainly won't get any answers from anywhere else) and therefore all we can do is follow the evidence and build a model that can describe as much of time and space as possible.