Most people today believe in The Big Bang theory when it comes to the origins of the cosmos. Can we be certain that the universe had a beginning? The history of the universe involves various stages. At very early times, it went through an extraordinarily rapid period of accelerating expansion when it became hugely bigger in a very short time; this is called inflation. At the end of inflation, that expansion had caused all the matter and radiation to dilute to almost zero, but then the field that had caused inflation decayed into very hot matter and radiation that continued expanding, but at a slower rate; that was the start of what we call the Hot Big Bang Era. The physical processes that occurred during this era are well understood, and all cosmologists agree on what happened then.
What we do not know is what happened before inflation began. The universe may or may not have had a beginning in that pre-inflationary era. The singularity theorems that Stephen Hawking developed do not apply, because the required energy conditions are now known to not be satisfied at that pre-inflationary time. In any case, a theory of quantum gravity is expected to apply at early enough times, but we don’t know what that theory is. To sum up: we do not know if the universe had a start, but we do know there was a Hot Big Bang.
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