Comments from Kirk Durston and Rob Sheldon added. From Natalie Wolchover at Quanta:
But in the past few years, a growing number of cosmologists have cautiously revisited the alternative. They say the Big Bang might instead have been a Big Bounce. Some cosmologists favor a picture in which the universe expands and contracts cyclically like a lung, bouncing each time it shrinks to a certain size, while others propose that the cosmos only bounced once — that it had been contracting, before the bounce, since the infinite past, and that it will expand forever after. In either model, time continues into the past and future without end.
With modern science, there’s hope of settling this ancient debate. In the years ahead, telescopes could find definitive evidence for cosmic inflation. More.
Well, if we have not yet found definitive evidence for cosmic inflation, maybe it isn’t that hot an idea. Politically correct, yes. Hot, no. In science, there may still be a difference (though maybe not for long).
Part of the issue here is that the Big Bang is politically incorrect. Implying that the universe has a beginning constrains a number of ideas, as any type of boundary will, and, worse, it provides support for theism. No non-Big Bang theory has shown much promise, but we can expect to hear about a lot more of them anyway.
So now comes the pitch:
Cosmic inflation also has a controversial consequence. The theory — which was pioneered in the 1980s by Alan Guth, Andrei Linde, Aleksei Starobinsky and (of all people) Steinhardt, almost automatically leads to the hypothesis that our universe is a random bubble in an infinite, frothing multiverse sea. Once inflation starts, calculations suggest that it keeps going forever, only stopping in local pockets that then blossom into bubble universes like ours. The possibility of an eternally inflating multiverse suggests that our particular bubble might never be fully understandable on its own terms, since everything that can possibly happen in a multiverse happens infinitely many times. The subject evokes gut-level disagreement among experts. Many have reconciled themselves to the idea that our universe could be just one of many; Steinhardt calls the multiverse “hogwash.”More.
It’s not just hogwash; it is science’s assisted suicide
But at the Big-Bang, the entropy of the universe had to be very low and it increased thereafter. In the bounce model, wouldn’t entropy keep increasing with each bounce? What kind of universe could there be after the first (or is it the second) bounce? After each bounce the universe would have larger entropy and at some point (maybe after even one bounce) the entropy would be so large as to make the universe very uninteresting.
But answering those questions is not nearly s important as keeping them in play, never seriously addressed.
Biophysicist Kirk Durston comments,
I am not sure what the utility of a bouncing universe is. Even if we grant that the universe oscillates like a giant perpetual motion machine, there cannot be a countably infinite number of bounces in the past leading up to this particular bounce. In other words, a perpetual motion, oscillating universe still does not avoid a beginning to the bouncing.
Note: Durston has also written on this question: Why we know the universe has a beginning
Physicist (and our physics color commentator) Rob Sheldon says
It is my understanding that the current models of the Big Bang — high temperature, homogeneous, isotropic (essential 1-d) are already at MAX entropy. Which is to say, current models already violate the LOW entropy universe that we see today–clusters, galaxies, stars, planets, life. This has led to weird theories that think gravitational energy is negative, so perhaps expansion of the universe PRODUCED low entropy–e.g. created information.
If you can slip that one past the thermodynamics censors, then bouncing is relatively trivial. Yes, we are long overdue for a better cosmology model.
See also: Cosmic inflation theory loses hangups about the scientific method
The Big Bang: Put simply, the facts are wrong.
The multiverse is science’s assisted suicide
Big Bang exterminator wanted, will train