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Is the Big Bang instant considered a beginning of all the coordinates of our universe? If we consider the BB to be the Beginning of of our universe, can we call this the Origin of all of the 3 dimensions of space and the 1 dimension of time? Whether it be a point, Planck volume, tiny loop, hole,... it seems we should be able to define it as an Origin that all points of our universe had in common 13.8 BY ago. Why cant we call this a temporal center, and a center of gravity, that happens to be very far away back in time?

Qmechanic
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The Outsider
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  • What for? An alternate terminology? The title in the question asks a different question than the content , btw – anna v Feb 22 '15 at 04:54
  • The Big Bang didn't happen at a point – John Rennie Feb 22 '15 at 06:18
  • @anna v: to define a center. The questions are different ways to express the same thing: Can we define an Origin at t=0? – The Outsider Feb 22 '15 at 06:25
  • @John Rennie: it happened in a small volume did it not? How small is still perhaps not well known, or is it? Were not all the points in the universe in this volume at t=0? Is t=0 the only point on the time domain (at least) the one thing we all have in common in this universe? – The Outsider Feb 22 '15 at 06:31
  • No it did not. Read the article I have linked for the details. – John Rennie Feb 22 '15 at 06:37
  • @John Rennie: I did read it. Here is an essential pat of it: "So to conclude: the Big Bang is the zero time limit of the FLRW metric, and it's a time when the spacing between every point in the universe becomes zero and the density goes to infinity" What I ask is at the place that every point in the universe becomes zero, can we define this to be the Origin in our domains? If not why not? – The Outsider Feb 22 '15 at 06:44
  • And what is the difference between saying that every point is in one place when it began, and it all began in one point? – The Outsider Feb 22 '15 at 06:50
  • And the rest of that paragraph says ... ? – John Rennie Feb 22 '15 at 06:50
  • We can define the zero of time to be at the Big Bang for all comoving observers. But at coordinates of all comoving observers remain distinct apart from at $t = 0$ when the geometry is undefined. – John Rennie Feb 22 '15 at 06:51
  • @John Rennie: Good point! The point is undefined so we cant define it as a point. But can we define it as a point in time? – The Outsider Feb 22 '15 at 06:58
  • Yes. We can defined $t = 0$ everywhere in the universe at the moment of the Big Bang. – John Rennie Feb 22 '15 at 07:04
  • And so my point is this: what are we doing when we look way out in space? Looking back in time. Then why can't we say that the universe origin is very far out there back in time in every direction for every observer and call this our center? I know, it would be thinking of time as analogous to space, but is that not what Master Einstein taught us we must do? – The Outsider Feb 22 '15 at 07:19
  • Right now, the comoving time on a planet a billion light years from Earth is the same as it is on Earth. But if you try to look at a clock on that planet you'll have to wait a billion years for the light leaving the clock to reach you so the clock will appear to be running a billion years slow. But it isn't really running slow - that's just a result of the time it takes us to see what the clock reads. – John Rennie Feb 22 '15 at 10:53
  • Actually it will appear to be running at a slower rate. By a factor $(1+z)$. – ProfRob Feb 22 '15 at 14:30

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The big bang may have two different meanings: First, a phase of very strong inflation of the early universe. In this sense the big bang is widely acknowledged.

But we have very little ideas about what was happening before this inflation phase as it seems that the known laws of physics cannot be applied. There are two main possibilities: Initially the universe may have been very small (condensed) or it may actually have been pointlike (point, planck volume or similar).

Also, you have to distinguish between space, spacetime and time. Spacetime is a relative and observer-dependent notion. It is a mechanism which is conjugating space and time. Time in an objective, Lorentz-invariant sense is the proper time, the duration of a mass particle. Spacetime is telling us if two particles which are travelling through the same point in space are doing this simultaneously (causing an event) or not.

By consequence, time is a fourth dimension only within the limits of spacetime. There is no other similarity between 3D space and time which is just duration of particles. Every observer perceives 4D spacetime, but simultaneity is only a relative concept. There is no objective 4th time dimension.

By consequence, the answer to your title question may be: yes, initially spacetime coordinates of our universe were strongly reduced, perhaps even pointlike. But it would not be meaningful to call it a temporal center: As I pointed out, time is not a fourth dimension, but only duration of particles. And even within spacetime (where time may be considered as a sort of fourth dimension) this does not seem to make any sense because spacetime is relative and observer-dependent, and it is not "analogous" to space. Every particle developed differently since the big bang (example: fast moving electrons aged less since the beginning of the universe than comoving reference frames)

If we consider the BB to be the Beginning of of our universe, can we call this the Origin of all of the 3 dimensions of space and the 1 dimension of time?

The beginning of spacetime: If we suppose that there was a pointlike big bang, we can say that this was the beginning of spacetime. However, there might have existed some causality which produced the big bang. There may have existed (a) spacetime or something else before the big bang.

The beginning of time: The answer is similar to the answer about spacetime. Time in an absolute sense is proper time and duration, as far as we know only mass particles have a proper time.

The beginning of space: It is currently supposed that inflation (even in the sense of inflation of a pointlike big bang) does not happen into something but that space itself expanded. That would mean that the answer would be similar to spacetime and time.

And so my point is this: what are we doing when we look way out in space? Looking back in time.

We see the universe as it was in the past, that is right. But when we are looking out in space, that means that photons reach our eyes which have travelled for billions of years. A photon coming from another galaxy one million light years away has travelled one million years. By consequence, these are photons of our present time, not of our past.

Moonraker
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  • thank you for that answer. I like that you say "Every observer perceives 4D spacetime" as I do not agree with the often stated “spacetime is impossible to visualize.” On the contrary I think space without the passage of time would have been impossible to percieve for the simple fact that our perception devices need time to process. Spacetime is a mathematical concept, but I will always believe that our universe can be visualized as 3D figuration that evolves through time. – The Outsider Feb 23 '15 at 02:36
  • We can understand the overall shape of the universe if we consider two things:
    1. all directions out in space point back in time.
    2. The universe gets smaller farther out back in time.

    The simple shape of the horn torus satisfies these two things, but would require a complete cycle of a symmetric Big Bang-Big Crunch/Bounce Universe in order to make a complete horn torus shape. In such a shape there are two endpoints, two boundaries., the beginning and the end, our Center, the only place we all have in common.

     – The Outsider Feb 23 '15 at 02:42