So I'm not a physicist, so I hoping there are others who can clear up misconceptions people might have about the Big Bang.
So what do I think I know...
There was no "bang". IOW there was no explosion. Just a rapid expansion of what has been termed the singularity.
Why is this accepted, this expansion of the universe?
Because we can see the rest of the universe moving away from us. Two things, we can tell by the color shift when objects ion space are moving away from us and at what speed. The farther away from us the faster they are moving away from us.
Second, what is beyond the edge of the universe? Well, we don't know. The edge of the universe is not the edge of the universe. It is just as far as we can see before, relatively, objects are traveling away from us faster than the speed of light. The light of these objects will never reach the earth so we will never be able to see anything past this point.
I presume the basis of the model of the singularity is that we see all of these objects is space moving away from us. Conceptually, reversing time every object in space gets closer and closer together until everything is compacted into this singularity. Mathematically, not that I'm a mathematician either, this has been validated to just a fraction of a second after the occurrence of the "Big Bang". Prior to this, no one can make any claims about what happened. Any theories about what happened before this point are unsupported speculation.
That kind of exhausts what I think I know.
So I invite others to post what they think they know about the "Big Bang" and maybe we can clear up some of our misconceptions.
OK, there is a LOT to unpack here and I'm not sure one post will do what needs to be done.
First, the physics.
In 1915, Einstein proposed his new theory of gravity. It is called general relativity and is a vast improvement on Newton's theory of gravity.
When general relativity is applied to a 'universe' of uniform density and the same in all directions at all points, the mathematics gives solutions that have very specific characteristics, but among them is that they are either expanding over time or contracting over time. Which happens depends on the density of the mass/energy. This is the basic Big Bang theory.
Hubble discovered the existence of galaxies other than our own (the Milky Way), by figuring out a way to measure distances to those other galaxies. Among the things he discovered is that the farther galaxies have a larger red-shift in their spectra than the nearby ones. The specific relation between distance and red-shift is what was predicted by general relativity.
Later, thermodynamics was added to the model. This allowed the prediction and characteristics of the cosmic background radiation. It also, using the known rules of nuclear reactions, allowed the prediction of the abundances of the lighter elements (hydrogen, helium, lithium, beryllium, boron). These abundances can be detected through spectra and were verified (although there is some issue with lithium, which could have been produced later)
This lead to the 'hot' Big bang theory. based on what we can observe, this describes the early universe very well from the time when nuclear reactions started to predominate to the present. By those models, this would have been from the first second or so to the present.
Now, it turns out that general relativity predicts the existence of 'singularities' in many situations. So it is important to understand what a singularity is and what it means. In the case of the Big Bang, the singularity happens as we approach the 'starting point'. But all that it means to be a singularity is that some important quantity goes unbounded. In the case of the BB, the density of the universe, as well as the curvature of spacetime is what gets large. Another aspect is that it is impossible to *define* time past a certain stage.
In this sense, asking what is 'before the Big Bang' is similar to asking what is 'south of the South Pole'. The geometry of the situation itself prevents the question from having an answer: there simply isn't a 'south of the south pole'. Nor, in the standard BB model is there a 'before the BB'.
The singularity is NOT A THING. it is a description of what happens as we approach a certain situation. In the case of the BB, the singularity is what happens as we approach the 'south pole' of the BB.
Next, there is no 'edge' to the universe. There is a limit to what we can *see* because light doesn't travel infinitely fast and the universe had a beginning (in the above sense). But in the BB model, every place in the universe 'looks like' every other place: the density is uniform in all directions everywhere.
When people ask where the 'center of the universe' is, thinking that the Big Bang happened there, they are making a fundamental error in understanding. ALL points are similar. ALL points are 'centers of expansion'. And, in fact, the expansion is exactly what is required so that the distance-redshift relation we see is exactly what *any* other galaxy anywhere would see. NO point is the center. There is no 'edge'.
An analogy of sorts is to imagine that latitude on the Earth represents time: more northern latitudes are later times. And a latitude circle represents 'space at that time'.
The South Pole in this analogy would be like the Big Bang: there is no 'before' and space is expanding (the latitude circles are growing) as we move north of the north pole. Of course, on the Earth, the equator would be a 'maximum expansion' time and the North Pole would represent a 'Big Crunch'. If the universe were denser than it is a similar Big Crunch would be in our future.
To modify the analogy slightly, imagine spacetime to be a cone. The vertex of the cone is the 'Big Bang' singularity. As we move away from that vertex, the circular cross sections expand and continue to do so. this represents the expansion of space.
Now, the actual BB model has three dimensions of space and one of time instead of one of each, but the geometrical aspects are analogous.
Now, it turns out that the cosmic background radiation was actually discovered decades after it was predicted. It has some *very* specific properties, including uniformity across the sky and a very precise temperature (Black Body radiation). This radiation is amazingly uniform: to one part in 100,000 across the sky in all directions. This is one thing the hot BB model predicted that no other model has been able to explain. Nothing else we know can produce that level of uniformity in all directions.
Furthermore, even though it is amazingly uniform, there *are* small variations in this background radiation. And *those* small fluctuations *also* match the predictions. You see, the background radiation is a type of 'afterglow' of the time when the universe cooled enough to become transparent. Since that time the light has red-shifted by a factor of 11,000.
The fluctuations we see in that background are the exact size needed to product the large scale structure of the universe, like galaxy clusters. We are seeing the small variances that grew to be large structures like our own Milky Way.
And, again, these variations precisely match the previously postulated models of a hot Big Bang.
I can go on for quite a while, but this seems like a good place to stop for now.
