Please explain the statement "the big bang happened everywhere at once"

[Ken G]

My take on the question "how can something happen everywhere at once" is that it is asking the question "how seriously should we take the cosmological principle." My answer to that is, "no more seriously than any other useful organizational principle in physics, like conservation laws, symmetry principles, etc." Organizational principles are how we make sense of a vastly complicated universe, but we generally run into trouble when we take them too seriously. At what point do we stop asking "how can we think about things to make better sense of it" and start asking "is this how things really are?" I don't think there's any reason to ever go from the first type to the second type. So I would reframe the question, "why do we find success in seeking a time coordinate that allows the universe to behave similarly at large scales at given times"? There could be a lot of answers to that, but if one adopts inflation, the answer is probably "because due to inflation, the spatial extent of the universe that we perceive is too small for us to notice any differences other than those that have developed later due to gravitational instabilities."

 

[mes314159]

It seems to me that a problem with the balloon analogy is that while all points on the surface of the balloon are equivalent, the "center of expansion" of the balloon is unique. But that center is not actually on the balloon of course, and is "outside" the balloon universe. Since this whole analogy is in three dimensions however, people may still think that there is a center of expansion within the universe. So a question, does the big bang have a "mathematical center" in a higher dimension of space-time (as in Sphereland), or is that still a poor analogy?

 

[David Carroll]

One thing I never understood was this: When I was in nuke school in the Navy (which is the extent of my knowledge of physics), the teachers told us that Temperature was the measure of the random molecular kinetic energy in a system. But I never understood how scientists define, in a mathematically rigorous way, how "random" is measured in this context.

When I imagine the "Big Bang" - and I'm probably imagining it in the wrong way - I imagine a large mass of particles expanding uniformly from a single point. But if they are expanding uniformly, how could the mass of these particles have a temperature? "Uniformly", by any definition that I am personally aware of, seems about as opposite from "random" as one could possibly get. Can someone enlighten me on this matter?

 

[russ_watters]

Temperature is a measure of average kinetic energy. The fact that the direction of motion or the distribution of energies is random or has random components is irrelevant to that.

...and I'm not sure what that has to do with the Big Bang...

 

[David Carroll]

Temperature is a measure of average kinetic energy. The fact that the direction of motion or the distribution of energies is random or has random components is irrelevant to that.

...and I'm not sure what that has to do with the Big Bang...

Okay. Thanks, russ_watters.

Well, I asked because physicists will come out and say that the universe had "x" temperature when the universe was "y" minutes old.

 

[DaveC426913]

It seems to me that a problem with the balloon analogy is that while all points on the surface of the balloon are equivalent, the "center of expansion" of the balloon is unique. But that center is not actually on the balloon of course, and is "outside" the balloon universe. Since this whole analogy is in three dimensions however, people may still think that there is a center of expansion within the universe.

Yup, all analogies fall short of the real thing.

So a question, does the big bang have a "mathematical center" in a higher dimension of space-time (as in Sphereland),...?

No, the curvature works in 3 dimensions without there having to be a centre in a 4th dimension.

 

[David Carroll]

Temperature is a measure of average kinetic energy. The fact that the direction of motion or the distribution of energies is random or has random components is irrelevant to that.

...and I'm not sure what that has to do with the Big Bang...

But then if a projectile crashes, do we not say that the kinetic energy in the projectile was partly converted to heat? And wouldn't that imply that, post-crash, the temperature of the system registers a higher temperature than it did pre-crash? But the average kinetic energy of the system pre-crash would simply be the kinetic energy derived from the projectile's motion plus its pre-crash temperature, wouldn't it. So where did the higher temperature come from, post-crash?

I don't mean to take it off-topic. But we ARE on the topic of the big bang happening everywhere at once. And this is precisely relevant to a question I have on the big bang happening everywhere at once.

 

[vela]

Temperature is a measure of average kinetic energy. The fact that the direction of motion or the distribution of energies is random or has random components is irrelevant to that.

I wouldn't say it's irrelevant. When a pitcher throws a baseball, the average kinetic energy of all the particles increases, but you wouldn't say the temperature of the baseball has increased.

 

[vela]

When I imagine the "Big Bang" - and I'm probably imagining it in the wrong way - I imagine a large mass of particles expanding uniformly from a single point.

This thread is precisely about how this is not the correct way to picture the Big Bang.

 

[David Carroll]

This thread is precisely about how this is not the correct way to picture the Big Bang.

Good to know. But, at the very least, no one can accuse me of being off-topic. ;)

 

[russ_watters]

I wouldn't say it's irrelevant. When a pitcher throws a baseball, the average kinetic energy of all the particles increases, but you wouldn't say the temperature of the baseball has increased.

You tried to break my explanation, but swung and missed [/baseball analogy]: I said "the fact that...the direction of motion is random...", so a baseball is not a correct example of my explanation.

 

[David Carroll]

Basically, my question boils down to this: in the first few seconds of the universe, was the expansion uniform? If so, how could the universe "have a temperature" in those first few seconds if temperature is a measure of the random kinetic energy of the particles (though russ_watters says that "random" has nothing to do with it)?

 

[russ_watters]

Basically, my question boils down to this: in the first few seconds of the universe, was the expansion uniform? If so, how could the universe "have a temperature" in those first few seconds if temperature is a measure of the random kinetic energy of the particles (though russ_watters says that "random" has nothing to do with it)?

The expansion was essentially uniform, but in addition to the uniform expansion, there was random motion.

Now we can bring the baseball in: if the baseball is moving in a straight line (or even: if it is sitting still on the ground!), can it have a temperature?

 

[dragoneyes001]

one question the term "infinite" is is tossed around as if its a guaranteed fact.

without any ability to prove the universe is in fact infinite doesn't that make most of the theories based on shaky ground?

 

[phinds]

one question the term "infinite" is is tossed around as if its a guaranteed fact.

without any ability to prove the universe is in fact infinite doesn't that make most of the theories based on shaky ground?

I'm not aware of any serious physicists who say that the universe is infinite. We don't know and that has no impact at all on any theories I'm aware of. What theories did you think would be impacted by discovering that the universe is finite but unbounded, as it may well be?

 

[dragoneyes001]

this discussion about the big bang happening everywhere at once because its a mater of time not space. implies the universe is everywhere at its beginning it kind of puts expansion into question as opposed to redistribution of what's in the universe.

 

[PeterDonis]

it kind of puts expansion into question as opposed to redistribution of what's in the universe.

What do you mean by this? "Expansion" of the universe has a precise technical meaning (and that meaning matches what we observe). What does "redistribution" mean?

 

[dragoneyes001]

we observe expansion of the mater within the universe (everything getting further apart from everything) but what makes us sure the universe is not static and the mater is simply expanding within a pre-existing space however big or infinite

 

[PeterDonis]

what makes us sure the universe is not static and the mater is simply expanding within a pre-existing space however big or infinite

We know the universe is not static because "static" also has a precise technical meaning, and it does not match what we observe for the universe as a whole. If the universe were static, there would be some worldline we could follow through spacetime along which physical quantities of interest (such as the average density of matter) would not change. But there isn't; no matter what path we choose through spacetime, all of the key physical quantities that describe the universe as a whole change with time. That means the universe is not static.

 

[dragoneyes001]

there's no way that all the observable matter can be moving within a pre existing space?

 

[PeterDonis]

there's no way that all the observable matter can be moving within a pre existing space?

If we go by models that are consistent with GR, no; the only GR models we have that match observations do not have a "pre-existing space" into which the universe expands. ("Eternal inflation" models complicate this somewhat, but what I've said is still basically true, AFAIK, with them as well.)

Even if we put aside that theoretical fact, how would we tell, physically, that there is a "pre-existing space" into which the universe is expanding? How would we observe it?

 

[dragoneyes001]

if its a void into which everything we can observe is expanding within. there wouldn't be a way of observing it as an it. the only effect it has is being there to allow expansion within its boundaries if it has any

not trying to be contrary just trying to grasp why we're sure the universe is limited to what we can observe of it.

 

[PeterDonis]

the only effect it has is being there to allow expansion within its boundaries if it has any

Why is it necessary for that? Why must there be something extraneous to the universe "to allow expansion"?

just trying to grasp why we're sure the universe is limited to what we can observe of it.

Who said we're sure of that? According to our best current model of the universe, it is spatially infinite, so what we can observe of it is certainly not all there is according to that model. We may end up having to modify the model based on future observations, but even if we do, I don't think any physicists are advocating picking models based on the assumption that the universe is limited to what we can observe of it.

However, saying that there is more to the universe than what we can observe is not, by any means, the same as saying there is some pre-existing "space" into which the universe is expanding. There doesn't have to be any such thing for our current model (including its prediction that the universe is spatially infinite) to be valid.

 

[russ_watters]

there's no way that all the observable matter can be moving within a pre existing space?

No. That would mean that there is an empty space outside of what contains all the matter/galaxies in the universe. That would make the universe have a center of gravity/mass toward which everything would be attracted and, I assume you mean, a center from which the expansion is occurring. This would be observable in the motion and distribution of galaxies. As would the "edge" of this expansion if it existed and was inside our horizon.

...just trying to grasp why we're sure the universe is limited to what we can observe of it.

That's not part of the theory either. In fact, it is near certain that we cannot see all of the universe. Since a lot of your questions are based on mistaken beliefs about what the Big Bang Theory says, it would be helpful if you read-up on it some. It may correct many of these misunderstandings more easily. The wiki on the Big Bang and its children have a lot of this information. For example, there is an entire wiki on "the observable universe", which discusses why what we observe isn't all that there is and why we don't know if the universe is finite or infinite:
http://en.wikipedia.org/wiki/Observable_universe#The_universe_versus_the_observable_universe

 

[Quds Akbar]

Take a tiny ball for an example, then inflate it, where would the tiny ball geographically be in the new,large ball? It would be the large ball so the geographical location would be defined as everywhere. The Universe began as an infinitely dense and infinetly small point in space time (if it existed). Any events before the Big Bang would not affect it and would therefore be undefined.

 

[phinds]

The Universe began as an infinitely dense and infinetly small point ...

No, it did NOT and that in fact is the point of this thread. You should read threads before you reply.

 

[Monsterboy]

If space did not exist before the Big Bang but it exists now does that mean it is made/composed of something? what is it ? Is space a "field" just like electric, magnetic , gravitational and Higgs field ?

 

[phinds]

If space did not exist before the Big Bang but it exists now does that mean it is made/composed of something? what is it ? Is space a "field" just like electric, magnetic , gravitational and Higgs field ?

This is a somewhat contentious subject, but the consensus is that space is not "something" tangible, it is just a framework in which things happen and descriptions such as "stretches", "bend", "curves", etc. are a description of the geometry of space-time, not a literal description of space.

Whether anything existed before the Big Bang Singularity is another subject of debate but the "Big Bang Theory" has no comment on it, being just a description of how the universe has evolved since one Plank Time after the singularity.

 

[Doug Huffman]

Do you know what a field is, what it means, and what it is not? An issue here on PF is naivete of space, time, dimensionality, proof, falisifiability. Many of our correspondents are un-read in the topic upon which they are commenting.

 

[Monsterboy]

Many of our correspondents are un-read in the topic upon which they are commenting.

So ? Is there any rule that only those people who have a background in the subject should comment on it ? I ask questions out of curiosity and anybody who knows better can answer or correct me.

 

[jack476]

Edit: It seems that there were some incorrect statements in my post, so I've removed it.

 

[phinds]

My understanding:

Start with the concept that locations in space are separated not just by a measure of distance but also a length of time, the time needed for a phenomenon to propagate from one location to another. So if an object one light year away changes color from blue to red, then I can't know about it for one year, because that signal takes a year to reach me, and it is impossible for me to know about it before that time. Thus, I am not just one light-year away from it in space, but also one year away from the object in time.

So let's say I have a hypothetical infinitely long measuring tape, and I were to try and measure the distance between where I'm sitting now and the edge of the universe. What I'd see from my own position is that, as the other end of the tape gets closer and closer to the edge, it begins to compress so that the marks on the tape become closer and closer together, until eventually the distance between two marks is infinitesimal, that is, infinitely small. This represents space becoming smaller and smaller as I go further away from myself in distance, and therefore am measuring space as it was further back in time. The term "singularity" in this sense refers to this asymptotic decay: it couldn't have been 0, but it was infinitely small, but infinitely small is not 0. In theory, we can say that there was a "big bang" that happened about 13.7 billion years ago because that's when there is an asymptote in those calculations, ie, a "singularity" at -13.7 billion years. And so far, experiment agrees with theory.

And the reason we can say it happened everywhere at once is that the above is true no matter where I try to measure from.

There are a couple of flaws to this post, the post serious being that it is predicated on their being an "edge" to the universe. There isn't.
Second, I'm not clear just what you're saying about the tape measure, but if I DO understand what you are saying, it is incorrect as well.

 

[jack476]

There are a couple of flaws to this post, the post serious being that it is predicated on their being an "edge" to the universe. There isn't.
Second, I'm not clear just what you're saying about the tape measure, but if I DO understand what you are saying, it is incorrect as well.

Okay. Could you explain please?

 

[phinds]

Okay. Could you explain please?

No edge means just that. No edge. An edge implies a center and a center implies a preferred frame of reference and we know empirically that such does not exist.

Your statement about the ruler seems to imply that a MEASURE of distance changes over distance and/or time but that is false. The distance between things in the early universe was less than it is now, but the MEASURE of distance has not changed.

 

[jack476]

No edge means just that. No edge. An edge implies a center and a center implies a preferred frame of reference and we know empirically that such does not exist.

Yes, but I meant that if you look out far enough, don't you see things as they were in the past? So from the perspective of some observer, if you looked out far enough, wouldn't you eventually see space becoming infinitely compressed to a point where you can't see any further?

Your statement about the ruler seems to imply that a MEASURE of distance changes over distance and/or time but that is false. The distance between things in the early universe was less than it is now, but the MEASURE of distance has not changed.

Because space was smaller, and the marks on the ruler that are further away from the observer from the perspective of the observer would start to appear closer and closer together, right? I'm not saying that the measurements are changing, but if there is a mile-long segment of the ruler far away from the observer with a length locally of one mile, to an observer far away won't it look smaller because space itself was smaller?