Ruminations on multiverses: excellent review (Introduction)

by David Turell , Thursday, July 26, 2018, 18:57 (2100 days ago) @ David Turell

Why this figment of a theory exists, based on what we know:

https://www.forbes.com/sites/startswithabang/2018/07/17/what-is-and-isnt-scientific-abo...

"According to the leading ideas of theoretical physics, however, our Universe may be just one minuscule region of a much larger multiverse, within which many Universes, perhaps even an infinite number, are contained. Some of this is actual science, but some is nothing more than speculative, wishful thinking. Here's how to tell which is which.

"In the 1980s, a large number of theoretical consequences of inflation were worked out, including:
what the seeds for large-scale structure should look like,
that temperature and density fluctuations should exist on scales larger than the cosmic horizon,
that all regions of space, even with fluctuations, should have constant entropy,
and that there should be a maximum temperature achieved by the hot Big Bang.
In the 1990s, 2000s and 2010s, these four predictions were observationally confirmed to great precision. Cosmic inflation is a winner.

"Inflation tells us that, prior to the Big Bang, the Universe wasn't filled with particles, antiparticles and radiation. Instead, it was filled with energy inherent to space itself, and that energy caused space to expand at a rapid, relentless, and exponential rate. At some point, inflation ends, and all (or almost all) of that energy gets converted into matter and energy, giving rise to the hot Big Bang. The end of inflation, and what's known as the reheating of our Universe, marks the start of the hot Big Bang. The Big Bang still happens, but it isn't the very beginning. (my bold)

***

"If you then require inflation to have the properties that all quantum fields have:
that its properties have uncertainties inherent to them,
that the field is described by a wavefunction,
and the values of that field can spread out over time,
you reach a surprising conclusion.

***

"Inflation doesn't end everywhere at once, but rather in select, disconnected locations at any given time, while the space between those locations continues to inflate. There should be multiple, enormous regions of space where inflation ends and a hot Big Bang begins, but they can never encounter one another, as they're separated by regions of inflating space. Wherever inflation begins, it is all but guaranteed to continue for an eternity, at least in places.

"Where inflation ends for us, we get a hot Big Bang. The part of the Universe we observe is just one part of this region where inflation ended, with more unobservable Universe beyond that. But there are countlessly many regions, all disconnected from one another, with the same exact story.

"That's the idea of the multiverse. As you can see, it's based on two independent, well-established, and widely-accepted aspects of theoretical physics: the quantum nature of everything and the properties of cosmic inflation. There's no known way to measure it, just as there's no way to measure the unobservable part of our Universe. But the two theories that underlie it, inflation and quantum physics, have been demonstrated to be valid. If they're right, then the multiverse is an inescapable consequence of that, and we're living in it.

"So what? That's not a whole lot, is it? There are plenty of theoretical consequences that are inevitable, but that we cannot know about for certain because we can't test them. The multiverse is one in a long line of those. It's not particularly a useful realization, just an interesting prediction that falls out of these theories.

"So why do so many theoretical physicists write papers about the multiverse? About parallel Universes and their connection to our own through this multiverse? Why do they claim that the multiverse is connected to the string landscape, the cosmological constant, and even to the fact that our Universe is finely-tuned for life?

***

"As I've explained before, the Multiverse is not a scientific theory on its own. Rather, it’s a theoretical consequence of the laws of physics as they’re best understood today. It’s perhaps even an inevitable consequence of those laws: if you have an inflationary Universe governed by quantum physics, this is something you’re pretty much bound to wind up with. But — much like String Theory — it has some big problems: it doesn't predict anything we either have observed and can't explain without it, and it doesn't predict anything definitive we can go and look for.

"Because even though it's obviously a bad idea, they don't have any better ones.

***

"But when people then contend that they can draw conclusions about fundamental constants, the laws of physics, or the values of string vacua, they're no longer doing science; they're speculating. Wishful thinking is no substitute for data, experiments, or observables. Until we have those, be aware that the multiverse is a consequence of the best science we have available today, but it doesn't make any scientific predictions we can put to the test."

Comment: Multiverse may be worthless, but the article is a great review of what we know.