I Big bang location

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Ozfer

So if we summed up all the gravitational force acting on everything (planets stars etc) would the final end point for everything be the location of the big bang? Or does this make no sense due to space bending?
 

phinds

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So if we summed up all the gravitational force acting on everything (planets stars etc) would the final end point for everything be the location of the big bang? Or does this make no sense due to space bending?
You are implying that there is a center to the expansion of the universe. There is not, so no. I recommend the link in my signature.
 

Ozfer

Well that was informative but it still leaves me wondering. In the analogy the pennies never touch, how does that make sense for the big bang singularity where everything is touching?
 

phinds

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Well that was informative but it still leaves me wondering. In the analogy the pennies never touch, how does that make sense for the big bang singularity where everything is touching?
Everything is NOT touching. If it were that would imply a center. I found this very hard to grasp when I started learning this stuff but "the big bang happened everywhere". It did not happen at a point in space but a point in time.
 

PeroK

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So if we summed up all the gravitational force acting on everything (planets stars etc) would the final end point for everything be the location of the big bang? Or does this make no sense due to space bending?
The universe is possibly infinite, which makes this perhaps a bit easier to explain. So, imagine an infinite box, going off in all directions. There is no centre. Every point is the same. From every point you can look out at a infinite universe in every direction.

If that box contracts in every direction, then it gets smaller and smaller in the sense that things that were previously a certain distance apart are now closer and closer. But, it's still infinite. In that sense, it's not getting any smaller.

This leads to a greater and greater density of matter as things get closer and closer.

There is no way out of this (physically or mathematically) and that it what is called a "singularity".

It actually makes little physical or mathematical sense to imagine the entire universe reduced to a single point. Although that is the popular misconception of a "singularity".

In any case, every point in the box will continue to see the same contraction all around and no point will be special.

If you do imagine that everything ends up at a single point, then that single point will correspond to every point in the original infinite box, so again no point is special.
 
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Ozfer

So what we are saying here is everything existed in a singularity, and everything had something in between it since stuff didn't all touch (not space?), and this stuff in this point could be infinite, and then for some reason it decided to shoot outwards (more space?), but from the stuffs perspective it didn't really shoot outwards? And where was time while all this was happening?
 

PeroK

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So what we are saying here is everything existed in a singularity, and everything had something in between it since stuff didn't all touch (not space?), and this stuff in this point could be infinite, and then for some reason it decided to shoot outwards (more space?), but from the stuffs perspective it didn't really shoot outwards? And where was time while all this was happening?
Given that even a single electron has a finite size, it's clear that you can't even fit an electron in a singularity. So, from that point of view, everything can't have "existed in a singularity".

What actually happened is, of course, unknown.

Also, it is space that is expanding, not stuff that is moving. Instead of thinking of things "flying apart", try to think of things not moving. But, every time they measure the distance between themselves, that distance has got bigger. That's the expansion of space. No "bang", no "explosion", no motion, in fact.
 

russ_watters

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Well that was informative but it still leaves me wondering. In the analogy the pennies never touch, how does that make sense for the big bang singularity where everything is touching?
Touching or not (the actual singularity is problematic), the geometry is a closed curve in a higher dimension than can be visualized. So think of lower dimension analogies: which point on the edge of a circle is the center? Which point on the surface of a sphere is the center?
 

phinds

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So what we are saying here is everything existed in a singularity ...
"Singularity" does not mean what you seem to think it means. It is just a place-holder phrase so that physicists aren't always having to say "the place where the math model breaks down and gives nonphysical results and we don't know WHAT is/was happening".
 

sophiecentaur

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"Singularity" does not mean what you seem to think it means. It is just a place-holder phrase so that physicists aren't always having to say "the place where the math model breaks down and gives nonphysical results and we don't know WHAT is/was happening".
Yes. You cannot use intuition to find a description what happens when you get 'too near' to a singularity. The Laws have to bend when things are compressed into a small enough space. It's analogous to imagining that School Chemistry would still work the same inside a Neutron Star and we would never imagine that - would we?
 

russ_watters

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"Singularity" does not mean what you seem to think it means. It is just a place-holder phrase so that physicists aren't always having to say "the place where the math model breaks down and gives nonphysical results and we don't know WHAT is/was happening".
It depends on the model. In some cases it refers to a divide by zero error, but if the equation doesn't have the dependent variable in the denominator, you can go all the way to zero without it breaking down. This means that some features may still be capable of being evaluated.

Nor does the "singularity" being described as a point require that it is the center of something.
 

russ_watters

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Interesting. I had not thought of it that way.
Do you need an example? Directly applicable is two points on a shrinking circle, a common analogy for the universe. Neither point is the "center". As the circle shrinks, the distance between them shrinks, but a "center" never gets established. The distance can go all the way to zero without any errors in some models of the geometry (d=x in particular). At d=0, the points collapse into one point, but that's still fine: a point doesn't have a "center".

What does become problematic is the density, which goes infinite/singularity at d=0.
 

Ozfer

So how do we know we arn't currently in a singularity?
 

phinds

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Do you need an example? Directly applicable is two points on a shrinking circle, a common analogy for the universe. Neither point is the "center". As the circle shrinks, the distance between them shrinks, but a "center" never gets established. The distance can go all the way to zero without any errors in some models of the geometry (d=x in particular). At d=0, the points collapse into one point, but that's still fine: a point doesn't have a "center".
Helpful. Thanks.

What does become problematic is the density, which goes infinite/singularity at d=0.
Yep, and a good reason to interpret "singularity" as meaning "the place where the math model breaks down and gives nonphysical results and we don't know WHAT is/was happening".
 

jbriggs444

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So how do we know we arn't currently in a singularity?
A singularity is a feature of a model. Something like a boundary where the results do not make sense. It is not a place at all. Certainly not a place where we can be since we build the laws of physics to make sense where we are.

Consider the point where x=0 on the graph of f(x)=1/x. There is no such point on the graph. If you are at a point on the graph at all, you are not at x=0.
 

Ozfer

So what laws don't work at the singularity?
 

jbriggs444

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So what laws don't work at the singularity?
Pretty much none of them. Many of the laws of physics take the form of differential equations that require quantities to be continuous and differentiable to work. Quantities that diverge to infinity as the singularity is approached are not continuous there.
 

Ozfer

Quantities of what? If there is still space, matter, and time it seems like everything should work. If space bends and stuff doesn't touch it seems that solves the density problem.
 

phinds

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Quantities of what? If there is still space, matter, and time it seems like everything should work. If space bends and stuff doesn't touch it seems that solves the density problem.
Density, as one example. One REASON that the big bang singularity is called a singularity is that the math says that the density goes to infinity, which is not physically meaningful.
 

jbriggs444

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Quantities of what? If there is still space, matter, and time it seems like everything should work. If space bends and stuff doesn't touch it seems that solves the density problem.
If you tried to extend the notion of position to the singularity, it would become discontinuous. You could extend time. The singularity could be given a time coordinate. But that does not give you a coordinate system (a "manifold") that extends to include the singularity.
 

russ_watters

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Quantities of what? If there is still space, matter, and time it seems like everything should work. If space bends and stuff doesn't touch it seems that solves the density problem.
If you have two objects traveling around a circle and they collide, you can say they collided at a point and a chosen coordinate system can identify the location of the collision. If instead the circle shrinks to zero diameter, the coordinate system itself disappears when they meet.

The Wikipedia article on the concept of a singularity uses traveling North on Earth as a similar example: when you get to the North Pole, the concept of longitude becomes meaningless.
 

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