Where Is The Center Of The Big Bang Universe?

[Olowkow]

This link pretty well sums up all the basics, at least for me:
http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html

The "horizon problem" is even more interesting:
http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/HorizonProblem.html

When I think I have grasped it, I always have to read about it again to be sure.

 

[Speed of Light]

....Get a balloon and a marker. Place some dots on the outer surface of the balloon. The balloon's outer surface represents the spacetime fabric of our universe, and the dots represent galaxies and galaxy clusters.

Strictly speaking, I read somewhere that one should glue pennies to the balloon to represent galaxies in this analogy, since the galaxies are not themselves expanding as they would if they were dots on the fabric of the balloon.

(I have added 'bold' to the relevant statement)

If the universe is both Isotropic and Homogeneous, shouldn't the galaxies themselves, also be expanding? If space is expanding eveywhere equally, then shouldn't also the distance between the earth and the sun, be increasing? Shouldn't the size of an atom also increase?, and also the space beween the atoms (?)
If eveything is increasing isotropically and homogeneously, then doesn't eveything increase in unison, wouldn't the relative positions and sizes of eveything in the universe, stay exactly the same?
So how could we tell that the universe was expanding?

 

[Olowkow]

So how could we tell that the universe was expanding?

Check the two links in post #21 above. The galaxies and clusters are bound by gravitational force.

 

[~enigma~]

Sol's answer is a good one, but here's a way to illustrate it for yourself.

Get a balloon and a marker. Place some dots on the outer surface of the balloon. The balloon's outer surface represents the spacetime fabric of our universe, and the dots represent galaxies and galaxy clusters. You can even mark one dot with MW for Milky Way (our galaxy).

Now, slowly inflate the balloon and observe what happens. From the point of view of MW, all the other galaxies and clusters appear to move away -- likewise, from the point of view of another galaxy, the MW and all others would appear to move away. So, the idea is that no matter where you are in the universe, you will observe everything to be stretching away from you and could easily conclude that you're at the "center".

This is an illustration of what's known as the Cosmological Principle.

Two important points to note about this example:

1. People will often ask, "What's inside the balloon? Isn't that where the 'center' is?"

The answers are "nothing" and "no." Remember that in this model the entire universe is the stretched fabric of the outer surface of the balloon. The question of what's "inside" isn't relevant, unless you wish to speculate about hypothetical extra dimensions of space, etc.

2. Note that the dots on your balloon do not actually move along the surface of the balloon; rather they move with the balloon's surface as it stretches. In addition, dots further from the MW dot will appear to stretch away faster than nearby ones.

This points out a common misconception about big bang cosmology: that galaxies and galaxy clusters are actually moving through space. They aren't (for the purposes of an expanding universe, that is), instead they are being carried along with the spacetime fabric of the universe as it is undergoing an expansion or stretching.

Lastly, about the further points moving faster as compared to the closer ones: this is an illustration of Hubble's Law, which was one of the first ways we quantified the age of the universe to be many billions of years (roughly 13.7 billion years by current estimates).

I hope this helps. Have fun with your balloons!

IOW, it is relative

 

[~enigma~]

There is no "center" -- see my post above. Btw, great point about the CBR!

No ABSOLUTE center.

 

[Speed of Light]

So how could we tell that the universe was expanding?

Check the two links in post #21 above. The galaxies and clusters are bound by gravitational force.

With two bodies in orbit, eg Sun and Earth, gravity is neutralised - There is no net gravitational force between them, as the gavitational force is used to maintain the orbit.

Besides, gravity ought not to matter, as the 'creation of new space' is an independent process, and should simply increase the distances.

Why should gavitational attraction stop the creation of space in the vicinity?

 

[RecoveringYuppy]

1. People will often ask, "What's inside the balloon? Isn't that where the 'center' is?"

The answers are "nothing" and "no." Remember that in this model the entire universe is the stretched fabric of the outer surface of the balloon. The question of what's "inside" isn't relevant, unless you wish to speculate about hypothetical extra dimensions of space, etc.

Would it also be accurate to say that the history of the universe is inside the balloon? The radius of the balloon would represent time wouldn't it? Then the center of the universe would be a point in time and space at the time of the original singularity.

ETA: I seem to be echoing Fredrik and Gate2501.

 

[Blue Mountain]

AstronomyCast, the weekly astronomy podcast, just covered this topic as well. Their discussion mentions all of the points made here.

 

[BenHad]

Thanks to All

Thanks to all who have participated. Now I need to read some of the links provided.

I have a problem with a surface that has 3D At least that is what it seems to be since the telescope scans show galaxies in every direction. If it's like the penny on a ballon, then it appears there are a number of concentric surfaces each with pennies. So how many surfaces??


BenHad

 

[JoeTheJuggler]

Wouldn't the *center* tho of the big bang universe be impossible to define as a location in space (our 3 dimensions)?

I would think there there would be a *center* or *place* somewhere down the axis of time, where the big bang occurred.

The other answers on this thread are good, but what clicked for me was this way of thinking about it:

Imagining that there was space before the Big Bang and that the Big Bang took place at some location in that space is wrong.

There was no space before the Big Bang--there was no universe for it to be located within. The Big Bang is what became the universe. Just as it makes no sense to talk about 3-D space outside the universe, so too is it nonsensical to think of the Big Bang having happened inside 3-D space.

Hope that helps.

 

[bjornart]

Thanks to all who have participated. Now I need to read some of the links provided.

I have a problem with a surface that has 3D At least that is what it seems to be since the telescope scans show galaxies in every direction. If it's like the penny on a ballon, then it appears there are a number of concentric surfaces each with pennies. So how many surfaces??


BenHad

You can't turn the 2D+1 model into 3D+1 by adding surfaces. You need to make a "balloon" with a 3D "surface" in a 4D space. And that's not something we're equipped to visualize.

 

[sol invictus]

I have a problem with a surface that has 3D At least that is what it seems to be since the telescope scans show galaxies in every direction. If it's like the penny on a ballon, then it appears there are a number of concentric surfaces each with pennies. So how many surfaces??

It's not hard to imagine going from a point to line to a plane to a volume, right? And you can imagine a line or plane which is expanding with time; stretching in all directions?

A flat cosmology is simply a volume which, as time goes on, stretches and expands in all directions.

 

[sol invictus]

If the universe is both Isotropic and Homogeneous, shouldn't the galaxies themselves, also be expanding?

When we say isotropic and homogeneous, we're talking about on average, and the average must be taken over very large scales (even bigger than galaxies). It's like toothpaste - observed at by eye it looks smooth, but under a microscope anything but.

If space is expanding eveywhere equally, then shouldn't also the distance between the earth and the sun, be increasing? Shouldn't the size of an atom also increase?, and also the space beween the atoms (?)
If eveything is increasing isotropically and homogeneously, then doesn't eveything increase in unison, wouldn't the relative positions and sizes of eveything in the universe, stay exactly the same?

Nope. Imagine pennies glued to the balloon - they don't expand when the balloon grows.

With two bodies in orbit, eg Sun and Earth, gravity is neutralised - There is no net gravitational force between them, as the gavitational force is used to maintain the orbit.

It's not correct that gravity is "neutralised". For example that system has a negative gravitational energy. To pull it apart would require force and energy. The expansion, if it's not too fast, doesn't pull hard enough to do that, so the system remains bound.

 

[baron]

With two bodies in orbit, eg Sun and Earth, gravity is neutralised - There is no net gravitational force between them, as the gavitational force is used to maintain the orbit.

Besides, gravity ought not to matter, as the 'creation of new space' is an independent process, and should simply increase the distances.

Why should gavitational attraction stop the creation of space in the vicinity?

I think you're describing what I've never been able to understand about this process. I don't doubt the whole theory is correct, of course, just that I can't fully grasp how the entire "fabric" of spacetime expands.

A better way to imagine what I mean is by drawing a line between two sand grains (galaxies) on the balloon surface. That line is 1cm long.

The balloon expands, and the line still joins the galaxies.

So within the bounds of the analogy, the galaxies haven't got any further apart. Within the scope of the surface balloon universe, that line is still 1cm long.

Of course, we can physically measure the line and prove that it's isn't actually 1cm any more, but to do that we have to introduce a comparison with a measurement from "elsewhere" and this is not allowed.

The only way I can reconcile this in my mind is to assume it's simply space that's expanding, but we're told no, it's the fundamental fabric of spacetime. So I don't get it.

 

[sol invictus]

A better way to imagine what I mean is by drawing a line between two sand grains (galaxies) on the balloon surface. That line is 1cm long.

OK.

The balloon expands, and the line still joins the galaxies.

Right.

So within the bounds of the analogy, the galaxies haven't got any further apart. Within the scope of the surface balloon universe, that line is still 1cm long.

Huh??

The balloon expanded! The line is now, say, 2 cm long. And we can measure it perfectly well with fixed ruler on the surface of the balloon, or in terms of the number of galaxies you could fit along it.

Of course, we can physically measure the line and prove that it's isn't actually 1cm any more, but to do that we have to introduce a comparison with a measurement from "elsewhere" and this is not allowed.

What "elsewhere"? Again, just ask how long the line is in units of the size of the galaxies.

If everything in the universe - from protons to people to galaxies - got bigger by exactly the same amount that might not change anything. But that's not at all what happens here. Protons and people and galaxies are all pretty much fixed in size - they get "tugged" on slightly by the expansion, just like the pennies on the balloon, but they are much too tightly bound internally to fall apart (hopefully!). Two very distant galaxies, on the other hand, will get pulled apart.

 

[imagineNoReligion]

Strictly speaking, I read somewhere that one should glue pennies to the balloon to represent galaxies in this analogy, since the galaxies are not themselves expanding as they would if they were dots on the fabric of the balloon.

How does the balloon model explain that the Andromeda galaxy will collide with Milky way in a few billion years?

 

[sol invictus]

How does the balloon model explain that the Andromeda galaxy will collide with Milky way in a few billion years?

The same force (gravity!) that holds the galaxies themselves together also pulls them towards each other. If they're (relatively) close together gravity wins and they fall towards each other. If they're far enough apart, gravity loses and the expansion forces them apart.

The pennies are supposed to stand for individual gravitationally bound structures. To make the model a little more accurate they should actually represent regions much larger than galaxies. Each penny should be a galaxy cluster - a region containing many galaxies, roughly 100 times bigger in linear scale than the radius of the Milky Way disk.

 

[thatguywhojuggles]

This point, right here, is the center of the Universe --------> .



Oh, and here too: -------------> .

 

[baron]

Huh??

The balloon expanded! The line is now, say, 2 cm long. And we can measure it perfectly well with fixed ruler on the surface of the balloon, or in terms of the number of galaxies you could fit along it.
...
If everything in the universe - from protons to people to galaxies - got bigger by exactly the same amount that might not change anything. But that's not at all what happens here. Protons and people and galaxies are all pretty much fixed in size - they get "tugged" on slightly by the expansion, just like the pennies on the balloon, but they are much too tightly bound internally to fall apart (hopefully!). Two very distant galaxies, on the other hand, will get pulled apart.

But that's my point. If the whole of fabric of spacetime is expanding, not just space, then how can anything be fixed?

I thought that the balloon analogy demanded that the entirity of what we understand as spacetime (i.e. everything) is represented by the surface. That doesn't allow us to have fixed rulers, which must logically be independent of the surface else they wouldn't be fixed.

So is it just space that expands, not spacetime? I'm not denying it, I'm just asking the question, because the explanations I've read have been specific in stating that spacetime is expanding, which to my mind would leave us without a fixed reference against which expansion could be measured.

 

[sol invictus]

So is it just space that expands, not spacetime? I'm not denying it, I'm just asking the question, because the explanations I've read have been specific in stating that spacetime is expanding, which to my mind would leave us without a fixed reference against which expansion could be measured.

The balloon is an analogue of the real universe in one less dimension. Its surface has two spatial dimensions. It expands with time, but at some particular fixed time its surface is just space. A penny or spot on the balloon might or might not stretch, depending on what it's made out of and whether it can hold against the expansion. That's the best analogue anyone has come up with - to be more precise, we would need math.

If it helps, going down again in dimension would mean that the universe is the circumference of a circle, which expands in time. (One less than that would be two points moving apart.) Our universe has three spatial dimensions, and it expands in time, so it's just one dimension up from the balloon.

Incidentally, even if time too were stretching at the same rate as space, we'd still have physical effects. In fact if you want you can re-write these cosmologies that way (although it's not standard). The point is that the rate of stretching is a physical and observable effect, even when both space and time do so together. For example the universe will heat up slightly - particles will be produced by the stretching. What isn't physical is simply changing coordinates - for example re-scaling (once, not with time) all quantities with dimensions of length by the same fixed amount.