If the universe expanded from the Big Bang, why is there no centre?

[Bandersnatch]

It is also misleading to describe the beginning as being a "Singularity." Using that word implies that everything was once within the sphere of a tiny dot and that everything exploded out of that dot.

Does it really? I think it only has that connotation for most lay people, because the first and only time they've heard it before is when used to describe black holes, so they then imagine it being synonymous with something very small and dense.
Unlike with "explosion", it's a misunderstanding of the term by the listener, not a misrepresentation of facts. As such, I don't think it's wrong to use it, any more than it is wrong to call evolution a "theory", just because some people may think it means "unsupported speculation".
It would be great to properly explain an unfamiliar term when first introducing it, though.

 

[phinds]

But all of this did not have a beginning in the form of an "explosion" or as a tiny dot (hence the use of the word "singularity"). The words "explosion" and "singularity" are entirely too misleading in describing what cosmologists think actually occurred and which we are witnessing today via the technology that is now available. Explosion implies a "center" where the so-called explosion had its origin. The Universe has no center. Too the contrary, every observer, no matter where located within the Universe, views himself as being at the center. That word also wrongly implies that there was once a time when objects were ejected from a center much like the bricks of a building became "ejected" as a result of a stick of dynamite exploding within the center of the building. It is also misleading to describe the beginning as being a "Singularity." Using that word implies that everything was once within the sphere of a tiny dot and that everything exploded out of that dot. What 'singularity" really means is that point in time beyond which our physical laws do not apply. Because our physical laws do not apply prior to that point in time, we are not able to say anything regarding what happened prior thereto. A witch doctor in Haiti apparently is just as qualified as Stephen Hawking in guessing what really happened.

It is regrettable that such misleading words have to be used in explaining things to a layman. We are not first graders. It is like a math teacher telling students in the 8th grade that the basics of math can be understood if you know the word 'addition."

You completely misunderstand the meaning of the word "singularity". It does NOT mean explosion and it does NOT imply "point" it just means "the place where our math model breaks down" and it is exactly the right description for what was going on before one Plank time.

 

[julcab12]

You completely misunderstand the meaning of the word "singularity". It does NOT mean explosion and it does NOT imply "point" it just means "the place where our math model breaks down" and it is exactly the right description for what was going on before one Plank time.

--To extend the subject. How did they came up with singularity and what really is "breaking"?^^

--Singularity is what they've labelled to that limit in the actual Einstein Field Equation(spacetime theory). In a sense that the equation shows discontinuity(value of infinity is showing up in the equation). Example. the function f=1/x is singular when x = 0; resulting to undefined value. In the BB model as expressed by the metric tensor. The values of this tensor are given by the Einstein field equations, and solutions to these equations can be singular(infinitesimal density). The equation is entering to where things are undefined which some suggest that it is incomplete. QG is our best bet as far as i know.


http://arxiv.org/pdf/1308.1722.pdf

"I discuss singular spacetimes in the context of the geometrized formulation of Newtonian
gravitation. I argue first that geodesic incompleteness is a natural criterion for when a model
of geometrized Newtonian gravitation is singular, and then I show that singularities in this
sense arise naturally in classical physics by stating and proving a classical version of the
Raychaudhuri-Komar singularity theorem."




--Since GR represents spacetime in terms of a metric (on a manifold of points). We can assume that a feature of certain spacetime i.e singular in a geodesically is in an incomplete manner.

--The question remains whether we should take the singularities of general relativity to be features of our actual universe, or whether we should instead take these singularities as indicating limits in which the classical theory of gravity and spacetime breaks down. The latter option would then suggest that one would need a theory of quantum gravity to describe circumstances that classically lead to singularities. While singularities might be unavoidable in classical context, there are some reasons to suspect that quantum processes might prevent true singularities from developing.

 

[Mordred]

I posted this reply in another thread, as it is so appropriate to the OP of this thread I feel it deserves posting the reply here as well.

There is a simple proof you can perform yourself to see why the universe has no center.

draw on a piece of paper several circles, one within the other. Add as many contained circles as you like 2 will suffice. Draw lines at every 45 degrees. Where the circles cross the lines apply a coordinate letter assignment. Yo will quickly notice or should notice that two coordinates near the center of the circle will not increase in distance at the same rate as two coordinates near the outer circle.

In fact two coordinates at the inner circle will increase in distance at a slower rate than two coordinates at the outer circle.

In cosmology all coordinates separate from all other coordinates at the same rate. Much like coordinates drawn on the ( surface only) of a balloon.

in terms of density, the inner regions will be more dense than the outer regions in the first example.

in the Balloon analogy all points on the surface has the same density, only time varies the diameter of the balloon so as you go back in time the density increases.
the first example has no easy means to describe the time factor so it would be confusing to describe lol. At least I can't think of a way in the first example lol.

Thankfully that isn't important to understand how easy it would be to determine a center if there was one.

 

[CCWilson]

Personally I think "explosion" is an appropriate term. True, the explosions of our acquaintance are three dimensional explosions, but it's not much of a stretch to apply the term to four dimensions. After all, we use the balloon analogy and all sorts of other tricks to try and wrap our minds around spacetime, which - let's face it - is impossible to really visualize for most (maybe all) of us homo sapiens and our three dimensionally-limited brains. And of course the image of a stretched rubber sheet deformed by the mass of heavenly bodies is also not the real thing, just a trick to help us understand GR. If we're going to play with analogies like that, which I think are great, it seems odd to insist that "explosion" - defined as a violent expansion - can't be extended to include four dimensional spacetime.

By the way, the definitions I've seen of "explosion" don't mention a center, or how many dimensions can be involved. It's all just semantics, anyway.

 

[bapowell]

Semantics, yes -- words mean something. And that matters.

Explosions bring to mind material expanding out from a central point. If you polled the general public, I have a good feeling most would agree with this conception. This picture is utterly unlike the early expansion of the universe. If the big bang is considered analogous to an explosion, people will have in mind literally an explosion of matter occurring within pre-existing space. This is not a guess -- on these forums alone we address this misconception more frequently than I think we'd all like. If refraining from a bad analogy prevents people from going down that road, then I'm all for paying attention to semantics.

 

[phinds]

Personally I think "explosion" is an appropriate term. True, the explosions of our acquaintance are three dimensional explosions, but it's not much of a stretch to apply the term to four dimensions.

But it's NOT four dimensions. In the sense that you are using it, a normal fire-cracker is a four-dimensional explosion ... 3 space and 1 time ... from a point. It is too misleading to describe the early universe as an "explosion".

 

[ryan albery]

Let's not knock people who simplify things a bit but get at least some of the idea across rather than make it totally incomprehensible by insisting that they have a full understanding or bust.

good words.