From Ethan Siegel at Forbes:
The Universe began not with a whimper, but with a bang! At least, that’s what you’re commonly told: the Universe and everything in it came into existence at the moment of the Big Bang. Space, time, and all the matter and energy within began from a singular point, and then expanded and cooled, giving rise over billions of years to the atoms, stars, galaxies, and clusters of galaxies spread out across the billions of light years that make up our observable Universe. It’s a compelling, beautiful picture that explains so much of what we see, from the present large-scale structure of the Universe’s two trillion galaxies to the leftover glow of radiation permeating all of existence. Unfortunately, it’s also wrong, and scientists have known this for almost 40 years.
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The conclusion was inescapable: the hot Big Bang definitely happened, but doesn’t extend to go all the way back to an arbitrarily hot and dense state. Instead, the very early Universe underwent a period of time where all of the energy that would go into the matter and radiation present today was instead bound up in the fabric of space itself. That period, known as cosmic inflation, came to an end and gave rise to the hot Big Bang, but never created an arbitrarily hot, dense state, nor did it create a singularity. What happened prior to inflation — or whether inflation was eternal to the past — is still an open question, but one thing is for certain: the Big Bang is not the beginning of the Universe!More.
Actually, they do not know that. It is the latest contestable cosmology fad. And a universe infinite in the past is simply not tenable.
See also: The Big Bang: Put simply, the facts are wrong.
Cosmic inflation theory loses hangups about the scientific method
Big Bang exterminator wanted, will train
and
Durston and Craig on an infinite temporal past