B Is the Big Bang only about the observable Universe?

[kuartus4]

I was watching a video where Lawrence Krauss describes the big bang in terms of the observable universe. He says regions outside the observable universe need not have come from the big bang.

Starts At minute 3.

 

[PeterDonis]

He says regions outside the observable universe need not have come from the big bang.

I have no idea what he's talking about. I would want to see a reference to an actual paper.

 

[Ibix]

Just to expand on Peter's answer, the problem with interviews like this is that a scientist has just minutes to express concepts that barely fit into an entire physics career. They leave a lot out. Sometimes they leave so much out that it's difficult to work out what they were actually talking about. With established science this isn't so much of a problem because there's only one thing they could mean, but once you get to the frontiers, where scientists are still arguing about it, it becomes harder to work out which of our many hypotheses he's talking about.

My guess is that he's talking about "bubble" universes that form in inflation theory - have a look at https://en.wikipedia.org/wiki/Multiverse, in particular the bit on "classification schemes" - Tegmark's level I and II.

 

[ohwilleke]

By definition, we don't know what exists beyond the observable universe, which is part of a light-cone from the Big Bang in any theory that accepts a Big Bang of any kind (even a bouncing cosmology, for example).

There is nothing we know about physics that is contrary to the idea that there is another universe created by some other means, perhaps its own Big Bang, whose light cone ends, perhaps 100 billion light years from the outer reaches of our own universe.

Between what is truly unobservable, and what doesn't have its origins in our own Big Bang, is the possibility that there may be matter and energy, originating outside our Big Bang light cone, whose gravitational effects at some point lead to anisotropy in the structure of the observable universe, because the light cone of this other universe may be starting to overlap with our own universe and creating observable effects.

These structures would be outside of the observable universe now, but might be within the observable universe at some point in the future when they start to overlap with ours. These are among the kind of scenarios I take him to be taking pains to avoid ruling out.

In contrast, he doesn't really mean when he says "observable universe" to limit himself to what we actually observe with current instrumentation and as a consequence of the engineering resources available to us. He means the area of space-time that is observable in theory consistent with the laws of physics, and not merely what it was possible, as a matter of practical reality, to observe. Something that could only be seen from Earth with a flawless 500,000 tonne diamond lens, for example, would still be part of the observable universe in the technical sense that he is using the term.

 

[ArcLight]

My guess is that he's talking about "bubble" universes that form in inflation theory - have a look at https://en.wikipedia.org/wiki/Multiverse, in particular the bit on "classification schemes" - Tegmark's level I and II.

Ibix,
May I ask a question about "Bubble Universes" and Guths idea of Inflation, and if I am understanding them correctly?

My understanding (which may be wrong) is that the universe is infinite, mostly empty (not counting vacuum fluctuations), and is expanding rapidly (due to the Inflaton field or other non-0 fields). A Bubble Universe occurs when the Inflaton (or other field) collapses and releases an insane amount of energy/matter into an extremely small area (possibly the Planck length). This concentration undergoes rapid expansion for a brief time (say 10^-35 seconds). This may be how the our Bubble Universe (of which the observable universe is but a tiny part) formed. The collapse of the Inflaton field was localized, rather than occurring everywhere.

My questions:

1 - if one could teleport an infinite distance would the universe look like "islands" of universes (Bubbles) filled with matter and energy surrounded by vast voids?

2 - If so then do these bubble universes have a soft edge? Not a barrier just a lessening of density so that here there is matter and energy, over here this is less matter and energy, and over there there is just empty space?

The problem I see with a soft edge theory is the Schwartzchild radius of the observable universe is around 13.7 Billion Light years. At some point the rate of the expansion of the universe was less than the speed of light so a black hole should have formed along the soft edge and continues inward. Because you would have a large density differential.

I've read of other theories of the origin of the universe, that it may have been due to quantum tunneling.
In that scenario you either have a Bubble Universe in a sea of nothing, or you have a self contained universe which I assume is positively curved, so if you move in one direction you come back to the start point.

 

[PeterDonis]

My guess is that he's talking about "bubble" universes that form in inflation theory

If so, he's not describing that very well, because our own universe is just one such "bubble" in this model, and that "bubble" is not limited to our observable universe.

 

[PeterDonis]

My understanding (which may be wrong) is that the universe is infinite, mostly empty (not counting vacuum fluctuations), and is expanding rapidly (due to the Inflaton field or other non-0 fields).

"Universe" is a bad term here because you are using it to mean the spacetime region which is undergoing "eternal inflation", not the spacetime region in which we live (which is not inflating now). Unfortunately, I'm not sure what the standard term in "eternal inflation" models is for the region that is eternally inflating. But with reference to that region, you are basically correct: in an "eternal inflation" model the eternally inflating region is infinite in extent, the only stress-energy in it is the "inflaton" field (which is in a "false vacuum" state that has a very high energy density; all other fields are in their "true vacuum" states, with zero energy density), and it is expanding rapidly (because of the high energy density in the inflaton field, which acts like a very large positive cosmological constant).

A Bubble Universe occurs when the Inflaton (or other field) collapses and releases an insane amount of energy/matter into an extremely small area (possibly the Planck length).

Sort of. I don't think the notion of the "size" of the bubble is meaningful. But it is true that a "bubble universe" in this model is formed when the inflaton field transitions from its "false vacuum" state to its "true vacuum" state, and the energy density released is transferred to the Standard Model fields we are familiar with (quarks, leptons, and gauge bosons), which end up in a hot, dense, rapidly expanding state that leads to a universe like ours. This transition event is what the term "Big Bang" properly refers to.

AFAIK in this model a "bubble universe" like ours, from the standpoint of observers inside it like us, can actually be spatially infinite. At the very least, AFAIK there is no requirement that the size of our "bubble universe" must be exactly the same as the size of our observable universe (the portion of our universe we have received light signals from since the Big Bang event that started our "bubble universe" in this model.

if one could teleport an infinite distance

One can't, so this is a meaningless question. However, one can ask what a spacelike slice of the full whatever-it-is we-call-the-whole-model looks like. Or, more precisely, we can ask whether the concept of "a spacelike slice" makes sense for the full whatever-it-is-we-call-the-whole model. See below.

would the universe look like "islands" of universes (Bubbles) filled with matter and energy surrounded by vast voids?

I'm not sure this question makes sense, because I'm not sure the concept of a single "spacelike slice" through the full whatever-it-is-we-call-the-whole model makes sense. This is one reason why I think we really need a specific reference to a specific paper that describes the model mathematically, as a basis for discussion. This is emphatically not the sort of model where you can trust pop science presentations or Wikipedia articles or informal videos to properly convey what is going on.

do these bubble universes have a soft edge?

This is one of the questions that I think we need a specific reference to a specific paper in order to answer.

The problem I see with a soft edge theory is the Schwartzchild radius of the observable universe is around 13.7 Billion Light years.

The concept of "Schwarzschild radius" makes no sense for our observable universe because it is not static. That concept only applies to static spacetimes. Our observable universe is expanding.

At some point the rate of the expansion of the universe was less than the speed of light so a black hole should have formed along the soft edge and continues inward.

No. Your reasoning is not valid, both because an expanding universe is not static and the model of black hole formation you are implicitly using requires a static spacetime, and because the universe does not have a single "rate of expansion" that can be compared with the speed of light.

I've read of other theories of the origin of the universe

Please give a specific reference.