# Big Bang Location ?

1. Dec 3, 2009

### Blakesterc8

I'm sure there is an obvious answer for this, but i don't know it.

The big bang, where is the center thought to be? what direction with respct to the constellations?

2. Dec 3, 2009

### mgb_phys

Everywhere.
The big bang didn't happen in space - it created space

3. Dec 3, 2009

### marcus

Blake, mgb gave you the exact right answer. If you find it hard to comprehend there is a helpful analogy or toy model to think about---but not to take too seriously.

Suppose space were 2D instead of 3D. Suppose we were 2D animals sliding around on the 2D surface of a balloon. The only directions you can point your finger are in the 2D surface. You can't point towards the inside or outside. As far as we know all existence is concentrated on that 2D spherical surface. There IS no inside or outside or surrounding 3D space, as far as we know.

And suppose the balloon is expanding, so distances are gradually increasing between points on the balloon that aren't moving in the sense of latitude and longitude. They stay the same place on the balloon but the distances between them increase.

So somebody else on the balloon asks you "where did this expansion originate?"

If you want, think about how you might answer that other 2D creature, if they asked you that. There is no direction you can point your finger and say "it started over there". So what do you say?

Feel free to propose more than one way of answering. Or ask other people here what they would answer. If you are still around and interested.

4. Dec 3, 2009

### mgb_phys

No there wasn't anything to push outward

Don't think of it as an explosion in black empty space.
Think of it as a white blank page and the black empty space was created from a point.

5. Dec 3, 2009

### Blakesterc8

it is extremely hard to comprehend

we are inside of the black space on the white page though right? so shouldnt there be a direct line from us to the center?

i understand now there's not, but i can't see how

wouldn't that point on the "white page", be the center of the "balloon" thats expanding outward? so its hard to see how theres no exact point

6. Dec 3, 2009

### tipsung

no center, it is the same everywhere.

7. Dec 3, 2009

### Staff: Mentor

A very long time ago, some people believed that the earth was flat and if you sailed in a ship too far in one direction, you'd fall off the edge (that may actually be a myth, but go with it...). But it turns out the earth is curved, and if you sail far enough in one direction, you'll eventually end up back where you started. No edge, no center.

And so it is with space as well, but in 3d instead of 2d. Yes, our minds are not equipped to visualize a 3d space curved in higher dimensions, so instead of trying to picture it (other than by analogy), think more about the description and logical implications of it.

8. Dec 3, 2009

### mgb_phys

Yes

We are in the black space. The black space was created as a single point and then expanded out to form the entire universe

Think of the white page as outside the universe / imaginary / another dimension, or like a cartoon character running off the side of the film onto the sprockets.

It helps to not think of it as an explosion of stars in an empty black space - there wasn't any space.
That's why it's easier to think of it as a blank white page.

9. Dec 4, 2009

### Blakesterc8

so what im getting is that there is no way of knowing where it all started, from inside of our universe. but in another dimension, is there an outside of it?

so from another dimension is there a universe with a measurable center?

kind of? maybe?

because it had to start somewhere

but we just have no way of knowing where "somewhere" is. correct? getting there?

10. Dec 4, 2009

Staff Emeritus
What people are telling you is that this is wrong. The Big Bang was not an explosion into pre-existing space. It was the creation of space.

11. Dec 4, 2009

### Blakesterc8

ok, i totally understand its not an explosion now (thanks all), but what i'm hearing is that space was created at a single point and expanded. that point is not concievable in the 3rd dimension because of the balloon effect. but it is expanding so it should be a conceivable in a 4th dimension for that point to have a location.

if i'm close let me know.

i just wish i could just understand haha

whats a good class for this kind of science?

12. Dec 4, 2009

### Fredrik

Staff Emeritus
You're still suggesting that it started at a point in space. It didn't. The problem isn't that we don't know which point that would be. The best theory we have about space and time (general relativity) tells us very clearly that there is no such point. It's conceivable that the answer would be different if we ever find a better theory than GR, but for now, GR will have to do.

There are mathematical theorems that say that spacetime can be thought of as embedded in a higher-dimensional space, but there's no need to think of it that way. GR certainly doesn't describe it in those terms.

The answer would still be no. Imagine a 3D cartesian coordinate system (three perpendicular axes: x, y and z), with a grid of infinitely long lines in the x-y plane that divides the whole plane into 1 cm x 1 cm squares. Now imagine that the distance between the lines is growing with time. Let's say that the distance at time t>0 is R(t). Suppose also that this function is such that R(t)→0 when t→0+. (The + means that t goes to zero from the right). Note that I haven't defined a t=0 or t<0.

In this analogy, the big bang corresponds to the limit t→0+. Note that the x-y plane is infinitely large at all times. Even when "viewed from a higher dimension", the limit t→0 doesn't assign any special meaning to any point in the x-y plane. The lines are approaching each other at all points in the plane.

Each solution of Einstein's equation (the main equation of GR) describes a spacetime. The solutions describing homogeneous and isotropic universes fall into three classes. My analogy is an appropriate description of one of them. Marcus's analogy about the balloon is an appropriate description of one of the others. Note that in his analogy, the universe is the surface of the sphere, not the volume inside it. The universe at time t is a sphere with radius R(t), and we have R(t)→0 when t→0+. Note that this limit doesn't assign any special meaning to any point on the surface, and that t=0 and t<0 are still undefined.

You may still be thinking "OK, so GR doesn't describe t=0 or t<0, but something must have happened before that" but that view would be very naive. It would be saying that our intuitive ideas about space and time (which have been proven wrong by experiments and observations that favor GR over Newton's theory) are better than the description provided by the most accurate (or second most accurate) theory in science.

So what you should try to understand is that the best theory of space, time and gravity only defines times t>0, and that the t→0+ limit of the relevant solutions doesn't assign any special meaning to any point in space.

By the way, I disagree with the claim that space was created at t=0. The theory doesn't mention t=0 at all, so claiming that to be the moment of creation is precisly the kind of error I was talking about. There is no t=0 in the theory. The big bang is a limit, not an event. And the big bang theory is a sometimes funny TV show, or the claim that spacetime is represented by a homogeneous and isotropic solution of Einstein's equation.

Last edited: Dec 4, 2009
13. Dec 4, 2009

### Blakesterc8

Ok

i'm sorry for the frustration, theres really no easy explanation on the subject. i learned alot from this though.

i have no background in general relativity...i want to take a course on it.

thank you for your replies

-Blake

14. Dec 4, 2009

### marcus

It sounds to me like you are close. Close to grasping (or simply getting used to) the basic idea that in our presentday 3D space there is no place you can point to and say the expansion originated there.

You wanted to be told if someone thought you were close. I think so. Of course in any group of people there's always some disagreement about the exactly right way to understand something. More important, in this business you can get used to idea One, and feel comfortable for a while and then a little while later be astonished and confused again by the next unintuitive idea Two. The point is not to let this discourage you or make you nervous.

You referred to the balloon model in your post. I suggest you google "wright balloon model" and watch it for a few minutes, if you haven't already done so. Keep in mind that that is a toy 2D model of the case where the universe is spatially finite. We don't know that it is. I would recommend thinking a lot about the finite case for starters and getting used to that. Don't puzzle yourself unnecessarily by trying to grasp several things at the same time, go at it case by case. At least if space has finite but increasing volume, which is one way it could be then I think you are right to say it should be conceivable in a 4th dimension for that point to have a location. It is conceivable--one can imagine setting up an artificial 4th spatial direction, and setting up artificial 4D coordinates, so that in terms of those coordinates the initial state at the start of expansion would have a "location".

That is not too different from just introducing a time coordinate.

I'll get a link for "wright balloon model"
http://www.astro.ucla.edu/~wright/Balloon2.html

The white whirlers are galaxies---they stay in the same place (latitude-longitude-wise) on the balloon while the distances between them grow. The colored wigglers are photons of light---they travel across the expanding space between the galaxies. Each time you click you get a slightly different history, not an exact replay.

At UC Berkeley it is called Astronomy 10. General astro for non-science majors. It is so well taught that even physics majors take it (even tho it is non-calculus). The point is, at whatever level, you should pick up the abstract ideas at the same time as you are learning the practical observational ideas of how we know these things. Not just to understand the model but also understand why it is plausible, why it is a good fit to the data, and get a notion of how the data is gathered. The guy who teaches it has great slides and knows how to convey intuition.

But if you want an abstract pure math class, then take calculus and then take upper division undergrad Differential Geometry. Learn about manifolds, and how curvature can be measured from within the manifold, without there necessarily having to be some surrounding higher dimensional space.

For most people I would recommend approaching the universe via a non-majors General Astro course because its beautiful and soooo easy, compared with the other route. But if you want to do the full Albert, get calculus and differential geometry and then general relativity---there is usually an advanced undergrad course in it.

Last edited: Dec 4, 2009
15. Dec 4, 2009

### nucular1

Very nice discussion of spacetime, Marcus et al. It is difficult to get folks to wrap their minds around the notion that space and time are inseparable and that there is no "center" or "edge" of the universe. Questions about what is "outside" or what came "before" the universe are nonsensical it terms of Einstein's theory. And that is not meant to condescending. You also did well in mentioning that the analogies are not perfect. Well done!