# Big Bang…or…Everywhere Stretch?

Recently I watched a minute-physics video that suggested that a better name for the beginning of time would be "Everywhere stretch" because there wasn't a space-time singularity that formed where the universe began to expand but the expansion happened everywhere.

1. Well, if the expansion did happen everywhere that means that space and stuff had already existed. So what would happen before that expansion and why cant we see it (why don't we receive any signals?

2. In the end, as you can see, he brings up a theory that cannot be excluded, which suggests that the beginning of our universe was the end of another because of the Big Crunch, the collapse of space on itself. If that's true how the does the expansion of the universe decelerates and reverses its direction (becoming contraction)? I mean, the gravitational force becomes weaker while the galaxies move away while dark energy continues to "push".

The only think that comes to mind is that sometime in the future the impact of dark energy will become negligible and the gravitational force will overcome it, thus making the universe contracting. But that would lead to an oscillation of the universe.(pretty stupid ha?)

3. So every now and then I hear about different interpretations of the Big Bang theory. Sometime ago I asked in the forum if the Big Bang really happened, thinking that I knew what I was talking about,but now I realize I am actually confused. So what is really the Big Bang theory to a physicist, because I can understand that media distort the picture that we have (if we have) about it?

So, sorry for having a lot of questions and expressing them so confusingly; my only knowledge on the matter comes from Steven Hawking books and some articles. I have numbered the questions so that I avoid causing any more confusion.

• Seems you put dark matter instead of dark energy in 2), besides that you covered rather nicely one of the theories - we don't know if it's true. – Mithoron Mar 29 '15 at 15:54
• – Mithoron Mar 29 '15 at 15:58
• – John Rennie Mar 31 '15 at 10:10

The Big Bang is a mathematical model of how the observable universe evolved , based on fitting astrophysical observations. Like all models it has its region of validity. When I read cosmology fifty years ago, the model included a singularity at the origin, because that is what the mathematical functions of the General Relativity solutions showed. The model at the time fitted the observed expansion of the universe, which showed clusters of galaxies receding from each other at a certain rate. In that model, all (x,y,z) points at t=0 were at (0,0,0) and it makes no sense to talk outside of this point within this model, a singularity. It was as if an explosion started from the origin, in this model.

It is well known though that when singularities appear in a physics model, it is a sign for the need of an extension of the model or a different model, as happened with the introduction of quantum mechanics for the microcosm of particles, which avoids singularities with the Heisenberg uncertainty principle.

The need for a quantum mechanical framework came from the new astrophysical observations of the last sixty years. The cosmic microwave background observations could not be reconciled with classical thermodynamics within General Relativity. The CMB showed great homogenization, with differences in the temperature map of order of 2*10^-5. This homogenization could not happen at that early time of photon separation, 380.000 years in the history, and quantum mechanics was introduced at the beginning of the universe to homogenize the system.

At the same time observations showed that the expansion of the universe is not constant, a bang and free tracks, but is accelerating and new physics is introduced, dark energy , to explain the phenomenon and still keep a shell of the original Big Bang General Relativity solution.

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe.

1). As I said, everything was at one point at t=0 in the theoretical model. In the present history there is a quantum mechanical uncertainty as to the region which projects to a point from the classical BB.

2). speculations there are many. The standard physics status is in the picture above

3).is answered by my exposition above

The universe is still expanding as if space were the surface of a balloon. An interesting consequence of it being a stretch everywhere is that points that are further away should expand away more quickly. This is indeed so. The speed that two points expand away from each other is given by the equation $v = H_0D$, where $v$ is velocity, $D$ is distance, and $H_0$ is something called Hubble's constant. We can say that as $D$ gets bigger, $v$ gets faster.
1. One of the ways that we try to see what happened at the Big Bang is by working backwards. Imagine space, dotting with planets and other collections of matter. Instead of expanding space, contract it. We approach an infinitely dense yet somehow infinite space. Remember however that $0 \cdot \infty$ is undefined, so we cannot do this reversal all the way.