What it means is that most professionals do not believe that the universe started with the big bang. That is why they use phrases like “the universe as we know it today”. We know this because of the observed smoothness of temperatures and densities including the Cosmic Microwave Background that the big bang was preceded and set up by a previous phase of the universe. This previous phase is generally but not universally believed to be “cosmic inflation”. It had an unknown length, perhaps lasting billions or trillions of years. We do not know when or how the previous phase started. But we know that it did exist because we can extrapolate backwards to the final fraction of a second of that previous phase, which is now included in the timeline of the standard model.

I tried to find it but got lost in all the words. So if the tldr can have a eli5 I would be very happy.

But from the little I believe I managed to understand. It was something about how a measurement of the event horizon, and new data about it. Doesn’t line upp with the theory of nothing then big bang than universe, but lines upp more with it going in a cycle. Like universe, big bang, universe.

But there was a lot of big complicated words. So I can definitely gotten something wrong.