in short there is no evidence whatsoever for the beginning of the universe. people confuse universe expansion (which there is evidence for) with the beginning of the universe (no evidence).
Big Bang #1 the beginning of the universe [...] is what we find in the mathematics of Einstein’s general relativity if we extrapolate the current expansion of the universe back in time. The equations say that matter and energy in the universe becomes denser and hotter until, eventually, about 13.7 billion years in the past, both density and temperature become infinite. We cannot extrapolate any further back in time, so it’s fair to say that this event, if it happened, would be the beginning of the universe.
Einstein’s theory breaks down about 10-43 seconds before the mathematical singularity, a unit also known as the Planck time. Since physicists don’t believe the singularity is real, the phrase “Big Bang” has come to refer to whatever event might replace the singularity in the to-be-found theory of quantum gravity in this Planck time. Let’s call it just that—the Big Bang Event.
We have no evidence the Big Bang Event happened. We cannot look back in time anywhere near that long ago. The earliest direct observation we have is the formation of the cosmic microwave background, which was formed about 400,000 years after the hypothetical Big Bang Event. Be careful: If you Google for the time at which the microwave background was formed, you will get the answer that it was formed 13.7 billion years ago and that might look like it was formed at the Big Bang Event. But that’s because the figure is rounded.
The James Webb Space Telescope doesn’t tell us anything about the Big Bang Event.
We understand quite well how matter behaves at energy densities somewhat above those at which the microwave background must have formed, so we trust that our extrapolations back in time are correct until we reach an energy density that roughly corresponds to that which the Large Hadron Collider probes, which brings us to about 10-12 seconds before the hypothetical Big Bang Event. We know nothing about what matter does at higher energy density—even the density in neutron stars is lower than that.
Now, 10^-12 seconds isn’t much in human terms, but to get from there to the Big Bang Event, we’d still have to extrapolate over more than 50 orders of magnitude in energy density. That’s 50 orders of magnitude for which we have only speculation. This means the Big Bang Event might happen in our mathematics, but we have no observations that can tell us it happened in reality. Indeed, I think we will never have any observations that confirm the Big Bang Event. Some of my colleagues in astrophysics may disagree. But be that as it may, at least for now we just don’t know how the universe began.
That we have no evidence for (or against) the Big Bang Event is the reason why physicists have a large number of different hypotheses for the beginning of our universe. Besides the Big Bang Event, our universe could have been born out of a black hole; or it could have come about in a collision of higher-dimensional membranes; or it could have started as a big network in a non-geometric phase; or our universe could cycle through eons, as Roger Penrose has proposed. The most popular idea now is that our universe was born out of a fluctuation in a quantum field. All these alternative ideas to the Big Bang Event are possible because we cannot look back in time far enough to tell them apart.
The James Webb Space Telescope is an amazing instrument. It looks at young stars and galaxies that were formed long after the cosmic microwave background, at about 200 million years. That’s impressive, but it doesn’t tell us anything about the Big Bang Event, or its alternatives.
The problem has long been that the term Big Bang is used to refer to the expansion of the universe in general, and not to the event of the creation of the universe in particular. These are, however, two separate scientific hypotheses. We have overwhelmingly strong evidence that the universe expands (call it Big Bang #2), and we are confident about its history back to about the time of the electroweak phase transition, which is what the Large Hadron Collider probes. We have to date zero evidence for the beginning of the universe, whether it was a Big Bang Event or something else.
Historically, the first evidence for the expansion of the universe was Edwin Hubble’s observation that the light from other galaxies is systematically shifted to the red, indicating that they all recede from us. While this may have been the first evidence, the decisive evidence for the expansion of the universe was the discovery of the cosmic microwave background that ruled out the competing hypothesis, the “steady state” universe.