Questions about the Big Bang

[hongi]

Bit of background: I paid no attention to maths at high school, and it was the worst mistake I ever made. If I could go back in time, I'd beat the living daylights out of myself. So I'm pretty ignorant of physics.

1) I often hear that the singularity was a point or the size of a pea. But as I understand it, it's a misconception. The singularity was an infinitely dense state, but it could have been really big as well - and that only the visible part of the universe that was that small. Am I correct?

2) But what about the parts of the Universe that light hasn't reached, what did that look like at the singularity?

2) How do we know there isn't a centre to the Universe? I've read that wherever you are, it seems like the Universe is expanding away from you - is this part of the Cosmological Principle, where the Universe is thought to be homogenous and everything looks the same wherever you are?

[Covenant]

1) You're right, it's a misconception, but yours is as well because of the nature of space. The idea of a universal singularity actually having a size is actually flawed because if it was a singularity, no 'size' that we ascribe to it would make any variety of sense because space-time itself was so wrapped up that there was no 'space' or 'time'. This means that the way we measure size is essentially irrelevant. The size of the universe would have been incomprehensibly small, which is why it's called a singularity. If that sounds like bafflegab then I apologize, but most of our laws begin to break down at these levels, and it really is an incredibly strange thing to think about.

2) Light isn't 'reaching' parts of the universe, it's being emitted from it. The microwave background is light being emitted from the structure of space and all the things in it as it expanded and cooled. Kinda goofy thing to think about, but so was the inflationary period. The parts of the universe we haven't gotten light from (IE, have not been able to observe) are beyond the limits of what we call the observable universe, and part of that we will never see, as inflation seems to be pushing it away from us to such a degree that there will be bits of light that seemingly will never reach us. As such they'll be forever beyond our grasp.

3) Similarly, all points appear to be expanding away from all other points. The 'structure' of the universe is relative to our frame of reference in that way, but there is no preferred frame of reference according to current models. Since the structure of the universe is such that all points are expanding, there's no center. And since we cannot discern the size and shape of the universe, there's no way to even poist a hypothetical one. Furthermore, since the structure of space is determined in part by those things inside of it, and those things are roughly homogeneous, there's no actual center and there would be no meaning to it if we even tried to point to one.

Here's a funny bit by Hawking that describes a smidge of this:

"The early universe could not have been exactly homogeneous and uniform, because that would violate the Uncertainty Principle of quantum mechanics. Instead, there must have been departures from uniform density. The no boundary proposal, implies that these differences in density, would start off in their ground state. That is, they would be as small as possible, consistent with the Uncertainty Principle. However, during the inflationary expansion, they would be amplified. After the period of inflationary expansion was over, one would be left with a universe that was expanding slightly faster in some places, than in others. In regions of slower expansion, the gravitational attraction of the matter, would slow down the expansion still further. Eventually, the region would stop expanding, and would contract to form galaxies and stars. Thus, the no boundary proposal, can account for all the complicated structure that we see around us."

Basically, the concept of both size and center are mostly irrelevant. We assume the universe has finite size, but we aren't entirely sure, and we wouldn't be able to determine where the center would be, even if there was one, since by now no center would be discernable. With no 'wall' at the end of the universe it's like trying to find the 'front' or the 'beginning' of a perfect sphere.

[Surlethe]

Bit of background: I paid no attention to maths at high school, and it was the worst mistake I ever made. If I could go back in time, I'd beat the living daylights out of myself. So I'm pretty ignorant of physics.

1) I often hear that the singularity was a point or the size of a pea. But as I understand it, it's a misconception. The singularity was an infinitely dense state, but it could have been really big as well - and that only the visible part of the universe that was that small. Am I correct?

Sort of. The singularity was everywhere. Everything (we can see) was in the singularity. We're not really sure about the part of the universe beyond visible sight (let alone whether it exists), but one tentative hypothesis is that our part of the universe has a higher vacuum state than the rest of the universe, which is driving expansion. Bear in mind that this is a guess --- the observed vaccum density of the universe is something like 100 orders of magnitude off from the vacuum density predicted by QM.

2) But what about the parts of the Universe that light hasn't reached, what did that look like at the singularity?

Short answer: we don't know. Long answer: we can't know. But we can speculate that it was infinitely hot and dense, just like everything else.

2) How do we know there isn't a centre to the Universe? I've read that wherever you are, it seems like the Universe is expanding away from you - is this part of the Cosmological Principle, where the Universe is thought to be homogenous and everything looks the same wherever you are?

Technically, we don't know that there isn't a center to the universe. For all we know, we could be the center of the universe. However, two very persuasive arguments suggest that the universe has no center. The first is Occam's Razor: if you can describe the universe without assuming it has a center, such a description, all else equal, is superior to one which does assume it has a center. The second is the historical trend of moving the Earth away from the center of the universe. Consider this progression. Pre-Copernicus, the Earth was the center and the sun, moon, planets, and stars all orbited around it. After Copernicus, the Sun was at the center and the Earth, moon, planets, and stars all orbited it. As we began to model the Milky Way, even into the 1800s we thought the Sun was at the center*. Until the 1920s, we thought the Milky Way was the universe. Anything that bucks this trend smacks of anthropomorphic exceptionalism, and astronomers are very leery of it -- that's why they've written the Cosmological Principle into models of the universe.

* One of the hitches to realizing the Milky Way is so damned big was interstellar dust. Astronomers didn't realize it existed in such major amounts until one clever fellow showed that if you didn't assume interstellar dust existed, nebulae were huge fingers all pointing toward the solar system. This alone was taken as disproof of the existing model -- as I said, astronomers are leery of anything that indicates the Earth is at the center of the universe.

[Kuroneko]

The universe tends to zero size as one approaches the Big Bang. The Big Bang singularity has no size, but is nevertheless three-dimensional rather than point-like.

Taken together, these three claims may seem rather perverse and contradictory. The first two are pretty straightforward: things get closer together, while the singularity itself is an idealization that's not really part of spacetime, so physical measurement don't quite make sense. But why, if things really get closer together in such a way as to make any finite distance go to zero, is the singularity not point-like?

Suppose there are two particles, A and B, separated by some distance d, and B emits a photon toward A. Before the photon actually reaches A, however, the universe expands a bit, so the distance between A and B increases, as well as the distances both behind and in front of the photon. As a result, A may need to wait a lot longer than d/c for the photon to arrive. For example, the age of the universe is 13.7 billion years, but the most distant light that we see was originally emitted when it was something on the order of 400Kly away. If there was no expansion, it should have reached us in 400K years instead; universal expansion forced it to travel for billions of years. Of course, by the same token, now the stuff that emitted it is even further away.

But the point is simply this: there is a particle horizon that describes the size of the observable universe around any point, which is the largest distance which could send a signal to that point. This horizon is not constant in time--wait a while longer, and more things come into view, but this is still a very important relationship: two things outside their particle horizons could not have communicated at any time prior to now. That the singularity is not point-like is a reflection of the fact that despite the distance between any two particles going to zero as one approaches the Big Bang, there is still a very real, physical sense in which they stay separated from one another.

(This is a lot more obvious on a Penrose diagram, but the significance of such wouldn't make sense without prior familiarity with such.)

[hongi]

Thank you, reading about science is like crack to me. Any suggestions for a book or website to read up on?

Do we know what 'stuff', for lack of a better term, the singularity was comprised of?

The universe tends to zero size as one approaches the Big Bang. The Big Bang singularity has no size, but is nevertheless three-dimensional rather than point-like.

As I understand it, a singularity doesn't even have to look like a point anyway - it can be a ring shape, which is what black holes have (?).

2) Light isn't 'reaching' parts of the universe, it's being emitted from it. The microwave background is light being emitted from the structure of space and all the things in it as it expanded and cooled. Kinda goofy thing to think about, but so was the inflationary period. The parts of the universe we haven't gotten light from (IE, have not been able to observe) are beyond the limits of what we call the observable universe, and part of that we will never see, as inflation seems to be pushing it away from us to such a degree that there will be bits of light that seemingly will never reach us. As such they'll be forever beyond our grasp.

My bad.

It sounds like unobservable Universe is far larger than the observable universe. It's pretty awesome to think there could be a hundred billion other galaxies somewhere out there.

Basically, the concept of both size and center are mostly irrelevant. We assume the universe has finite size, but we aren't entirely sure, and we wouldn't be able to determine where the center would be, even if there was one, since by now no center would be discernable. With no 'wall' at the end of the universe it's like trying to find the 'front' or the 'beginning' of a perfect sphere.

Kinda like how to find a centre on a circle, you need to know the edges or some reference point?

[Kuroneko]

The universe tends to zero size as one approaches the Big Bang. The Big Bang singularity has no size, but is nevertheless three-dimensional rather than point-like.

As I understand it, a singularity doesn't even have to look like a point anyway - it can be a ring shape, which is what black holes have (?).

Some of them do, anyway. But the non-intuitive difference here is a black hole has positive size, whereas the universe shrinks to zero size but the singularity is not point-like.

It sounds like unobservable Universe is far larger than the observable universe. It's pretty awesome to think there could be a hundred billion other galaxies somewhere out there.

It is much larger. Although we can't know whether it is truly infinite, everything we observe is compatible with it being so.

Basically, the concept of both size and center are mostly irrelevant. We assume the universe has finite size, ...

We really don't assume that. In fact, the usual ΛCDM model fitted to observational data has a spatially flat universe, which is therefore infinite in an extent.

Kinda like how to find a centre on a circle, you need to know the edges or some reference point?

Yes, although there's no center on a circle. On a disk, yes, but not on a circle. That's probably a good exercise in itself; just remember when you draw one that the circle is the curved line itself, rather than the region on the paper it encloses (that's a disk).

[Surlethe]

Thank you, reading about science is like crack to me. Any suggestions for a book or website to read up on?

Carroll & Ostlie, Introduction to Modern Astrophysics (also termed BOB). The last several chapters are very helpful.

Do we know what 'stuff', for lack of a better term, the singularity was comprised of?

No. The Standard Model can be extended back almost to the very beginning, but at the beginning everything breaks down. We have difficulty modeling situations where gravity is very strong, particle density is very high, and scale is very small. (Similar things happen in neutron stars and the interiors of black holes.)

[Covenant]

We really don't assume that. In fact, the usual ΛCDM model fitted to observational data has a spatially flat universe, which is therefore infinite in an extent.

Huh, I never thought about it that way. Your point about the circle was also a really good one. Threads like this are illuminating for everyone!