Was the entire universe created by the Big Bang, or is the space/time generated by the Big Bang part of a larger universe?

June 23, 2009 10:55 AM

Was the entire universe created by the Big Bang, or is the space/time generated by the Big Bang part of a larger universe?

I was astounded recently to discover that the known universe has been measured from end to end, and it is 165 billion light years wide.

So, if you look at it that way, all of space/time is now a big egg-like ovoid of matter and energy.

If all the matter and energy generated by the Big Bang is now a gigantic ovoid:

1. Where is the Milky Way located in that ovoid?

2. If we are closer to one end than the other, do the photos we have of the most distant reaches of the known universe depict the far end or the near end?

3. Is this giant egg the entire universe, or is there space/time beyond the ovoid in which other Big Bangs could have occurred.

4. If there are other ovoids, will we ever be able to detect them?

posted by Lownotes to science & nature (23 answers total)

Okay, let's settle one misconception, which is that there's this ovoid shape. The Universe is not like that. It's "finite but unbounded." This is a somewhat mathy way of describing cosmology and it means that, like an old game of Asteroids, if you go up through the "top" of the screen, you end up at the bottom. Go left, end up right. The screen still has a finite size but it has no boundaries against which you would bounce.

So, the Milky Way has no place in an absolute sense, only a place relative to other things in the Universe.
posted by adipocere at 11:00 AM on June 23, 2009 [3 favorites]

2. If we are closer to one end than the other, do the photos we have of the most distant reaches of the known universe depict the far end or the near end?

I'll take this one. Our observations about the size of the known universe are equally limited in all directions. In our current model, we will only be able to see as far back as photons were allowed to propagate. Shortly after the big bang, space was filled with a high energy soup of quarks and gluons, then for a much longer time, a plasma of electrons and protons. Light cannot pass through this charged gas, so it sets a distinct limit in how far back we can probe by measuring light from distant sources, or, more practically, light in the form of the cosmic microwave background radiation.
posted by fatllama at 11:06 AM on June 23, 2009 [1 favorite]

Big Bang theory and current cosmology takes the view that space/time did not exist before the big bang. Questions 3 and 4 are unanswerable as we cannot look outside our own universe.
posted by damn dirty ape at 11:15 AM on June 23, 2009

"Finite but unbounded". Think of the surface of the moon. There is a finite amount of it, but there isn't any edge. If you're standing on the moon and start walking, you can walk forever without running into an edge or a wall. The surface of the moon isn't an infinite flat Euclidean plane, it's the surface of a sphere (approximately) and applying Euclidean concepts to it leads to confusion. The right conceptual model is Spherical geometry.

The universe is the same way, except that it's 3 dimensional instead of 2 dimensional. Part of the source of your confusion is that you're trying to think in terms of Euclidean geometry, and the universe is non-Euclidean. That's what the General Theory of Relativity showed. Applying Euclidean concepts to the universe is just as fallacious as trying to apply them to the surface of the moon.

There isn't any "end". There isn't any "edge". There isn't any "center".
posted by Chocolate Pickle at 11:16 AM on June 23, 2009 [3 favorites]

3. Is this giant egg the entire universe, or is there space/time beyond the ovoid in which other Big Bangs could have occurred.

4. If there are other ovoids, will we ever be able to detect them?

The short answer to these questions is - nobody knows. String theory says that there may be as many as 10⁵⁰⁰ universes, each with different laws of physics, all existing in a multiverse. Some of these universes may not have been able to have Big Bangs, others may have collapsed instantaneously. Others might have had a Big Bang but could not produce stars and planets.

However, string theory (an attempt to reconcile general relativity with quantum mechanics) is so far unproven and a sizeable contingent of physicists believe it to be a dead-end theory. Even if the other universes did exist, it would be impossible to observe them directly because the edge of the observable Universe is expanding away from us at the speed of light. Our only hope would be observe immensely subtle indirect effects.
posted by Electric Dragon at 11:17 AM on June 23, 2009

This is the known universe, not the UNIVERSE AS IT EXISTS.

Which we don't know all of yet.
posted by kldickson at 11:26 AM on June 23, 2009

Turtles all the way down.
posted by aquafortis at 11:27 AM on June 23, 2009 [3 favorites]

I think the ovoid you usually see comes from the fact that we usually project the celestial sphere, i.e. our essentially 2D view of the universe from earth (or the sun), on such an ovoid. This has nothing to do with the "shape" of the universe, which is a concept that doesn't translate too well to cosmology.

The place of the milky way on these maps depends on the chosen projection (in galactic coordinates, it's around the equator, since we're sitting at the brink of it looking in - see bright band on the Wikipedia picture).
posted by themel at 11:44 AM on June 23, 2009

4. If there are other ovoids, will we ever be able to detect them?

No other ovoids. But maybe other branes.
posted by Cool Papa Bell at 11:51 AM on June 23, 2009 [1 favorite]

Thanks to all.

My confusion here was that I had conceptualized the universe as a bubble floating in a void.

From the answers above, I think I get it. It is more like an "Asteroids"-ish boundless but finite place, which may or may not include everything in existence, where one would have to travel about 165 billion light years before returning to the spot from which they began.

Correct?
posted by Lownotes at 11:59 AM on June 23, 2009

finite but unbounded.

Cite? The last time I did research on this my impression was that there was no definiteness at all on this, just speculation. I.e. the volume of the universe might be finite and bounded, finite and unbounded, infinite and unbounded, or various other more exotic possibilities, and cosmologists don't even have any really great reasoning for preferring one over the other, much less for being certain. I think that people here are speaking with much more certainty than in general exists in cosmology.

I think what Lownotes is referring to might be more of something like the "knowable universe", the Hubble volume, for one of the lower theoretical values of the Hubble constant maybe? (Because 165 billion ly is pretty big an estimate for the Hubble volume I think.) If that's the case this would be saying that outside of this area, due to various laws of physics, they believe it is not possible to experimentally know anything about what's there.

And in that case the Milky Way is at the exact center. (Not because of any special importance of the Milky Way but simply because the Hubble volume is defined by essentially how far we can see from where we are.)
posted by XMLicious at 12:01 PM on June 23, 2009 [4 favorites]

The analogy that makes most sense to me is that of an inflating balloon. The entire universe exists on the surface of the balloon, but the point where it started, where the big bang occurred, if you will, isn't anywhere on that surface, and it's equally "distant" (through a dimension nonexistent on that surface) from every point in the surface-bound universe.
posted by rocket88 at 12:09 PM on June 23, 2009

Cite?

The mass of the universe is not infinite, and the average mass density is not zero. Therefore the volume of the universe cannot be infinite.
posted by Chocolate Pickle at 12:39 PM on June 23, 2009

Cite?

The mass of the universe is not infinite, and the average mass density is not zero. Therefore the volume of the universe cannot be infinite.

So, the cite is Metafilter's Chocolate Pickle?
posted by Jaltcoh at 12:46 PM on June 23, 2009 [4 favorites]

If you're into this stuff, check out any of the Radio Lab podcasts that feature physicist Brian Green (especially the Space episode).
posted by diogenes at 1:01 PM on June 23, 2009

According to Brane cosmology, the universe is contained inside (if inside is the right term, and it's really not) a 4-dimensional construct called a Brane, which may or may not hold more than one universe. The brane is inside a higher-dimensional construct called a bulk, which can hold any number of branes... and a higher dimensional construct called a super-bulk contains any number of bulks... turtles all the way down. One theory believes that the big bang was caused by two branes banging together, and where they interfered with each other, the violence of it caused a universe to come into being... and there's nothing that says they can't bang together again, at any time.
posted by Slap*Happy at 1:13 PM on June 23, 2009

The contingent of anti-string theorists is much more noisy than it is sizeable.
(Caveat: I am a string theorist.)

String theory also does not predict that there are 10^500 universes; rather, it says there are (at least) 10^500 ways to arrange the physical constants, and we don't have a mechanism to pick between these ways of arranging. That is, we don't know of a physical principle within string theory that says *this* is the particular arrangement of constants you have to end up with. It's possible (although if you're a theorist, depressing) that there is no such principle; maybe all universes happen, and we just happen to be in this one. (I personally prefer to think that there is a principle and we just don't know it yet).

How do you claim to know the mass of the universe is not infinite?

"Finite, bounded" usually goes with positive curvature; but spatially (not space-timewise) our universe is very very close to flat. Include the time dependence and we seem to live in something like a DeSitter universe.

I'm pretty sure XMLicious has it; Lownotes is referring to the *knowable* universe, from which we can actually receive photons. Wait a while, it'll get bigger. The "edge" is because looking far away also means looking back in time; you can't look too far back in time because photons didn't propagate that soon after the big bang.
posted by nat at 1:23 PM on June 23, 2009 [2 favorites]

I was astounded recently to discover that the known universe has been measured from end to end, and it is 165 billion light years wide.

If you think about, that is not so astounding - it is the known universe that is known, or measured. So we have measured that part of the universe we have measured.

All the rest is - carefully and intelligently done - speculation. There is a lot of interesting theory, but in the end we can't be sure of much. At the moment the big bang is generally agree on, but whether there was something before it is not agreed on (some physicists theorize a series of bangs, e.g.), and the shape / finitude etc of the universe is certainly not agreed on.

To your #4, though, I would think anything we could detect or contact would by definition be considered part of our universe, so, just logically, no.
posted by mdn at 1:35 PM on June 23, 2009

(disclaimer; I am only an undergrad so feel free to overrule me if you know more)

3rding XMLicious/nat.

The basic answer is infinite or at least much larger than the hubble volume.

Double checking in "An introduction to general relativity, spacetime and geometry" by Carroll and looking at the wiki page on the lambda-CDM model, the spatial curvature of the universe is zero or very nearly so.

This means that locally the universe has euclidean spatial geometry. However there are structures which are locally euclidean but also finite (like the asteroids case, but not like a sphere). If the universe were shaped like this (or a torus or other locally euclidean shape) and the asteroids screen were 'small' one or more directions in the universe would look the same (i.e. light that goes off the screen and comes back at us).

We don't see such a thing, as far as I'm aware (I might be wrong on this), thus the universe must at least be much larger (spatially) than the hubble volume.

The 165b refers to the observable universe. I.e. the co-moving distance to objects for which light could have reached us since the big bang. This is not simply related to the age of the universe since the universe is continually expanding, so at earlier times things were closer together.

In response to 1); the question is meaningless in general relativity since all coordinates are purely relative. You can't say "I am here," but you can say "such and such is so far away." Not that there aren't preferred choices of coordinate systems which make our lives easier, which is why the "age of the universe" makes any sense.

IMO the easiest way to understand a lot of this is to read the wikipedia page on the FRW metric.
posted by Erberus at 2:44 PM on June 23, 2009

The mass of the universe is not infinite, and the average mass density is not zero. Therefore the volume of the universe cannot be infinite.

This kind of logic sounds familiar.
posted by Electric Dragon at 4:13 PM on June 23, 2009 [1 favorite]

This means that locally the universe has euclidean spatial geometry.

Unless you're near anything that has mass. Then space curves in all sorts of strange and wonderful ways. (That's what General Relativity was about.)
posted by Chocolate Pickle at 5:25 PM on June 23, 2009

"The last time I did research on this my impression was that there was no definiteness at all on this, just speculation. I.e. the volume of the universe might be finite and bounded, finite and unbounded, infinite and unbounded, or various other more exotic possibilities, and cosmologists don't even have any really great reasoning for preferring one over the other, much less for being certain. I think that people here are speaking with much more certainty than in general exists in cosmology."
You're right, except that 'finite and bounded' would be considered peculiar if it turned out to be true, and I'd describe it as quite disfavoured as it's not something that comes up in general relativity, and that seems to be mostly correct. Infinite and unbounded is something I'd probably describe as philosophically distasteful, but I do not think there is a strong reason to favour the finite and unbounded solution over it.

"The mass of the universe is not infinite, and the average mass density is not zero. Therefore the volume of the universe cannot be infinite."

This is just wrong. We don't know the mass of the universe, as we can't see all of it, and in GR (at least the usual FRW solutions) the average density (of energy, not just mass) is what determines whether the universe is infinite or not.

The universe is in fact freakishly close to the dividing line between the two cases, so I'd be surprised if we figure the answer out in our lifetimes.
posted by edd at 8:27 AM on June 24, 2009 [1 favorite]

"I was astounded recently to discover that the known universe has been measured from end to end, and it is 165 billion light years wide."
It isn't.
posted by edd at 8:30 AM on June 24, 2009