Regarding the claim made by hooberus of why the multi-verse theory was proposed and regarding his claim of "Other universes aren’t testable", the Discovery magazine article (from the year 2008) which he provided a link to (on page 7 of this topic) says the following.
'And in the late 1970s, Linde, then a
professor at the prestigious Lebedev Physical Institute in Moscow, ..was trying to understand the physics of the Big Bang.
Linde and other researchers knew that something was missing from the
conventional theory of the Big Bang, because it couldn’t explain a key
puzzling fact about the universe: its remarkable uniformity.
In the standard version of the Big Bang, they couldn’t.
... MIT physicist Alan Guth found a viable,
but flawed, solution to the puzzle in 1981. Linde shored up that work
shortly thereafter, making improvements to overcome those flaws. In a
nutshell, Guth and Linde proposed that the universe underwent a colossal
growth spasm in the first instants of its existence, a phenomenon
called inflation.
Today widely accepted as the standard version of the Big Bang theory,
inflation holds that regions of the universe that are currently
separated by many billions of light-years were once close enough to each
other that they could exchange heat and reach the same temperature
before they were wildly super-sized. Problem solved.
By the mid-1980s Linde and Tufts University physicist Alex Vilenkin
had come up with a dramatic new twist that remains nearly as
controversial now as it was then. They argued that inflation was not a
one-off event but an ongoing process throughout the universe,
where even now different regions of the cosmos are budding off,
undergoing inflation, and evolving into essentially separate universes.
The same process will occur in each of those new universes in turn, a
process Linde calls eternal chaotic inflation.
Linde
has spent much of the past 20 years refining that idea, showing that
each new universe is likely to have laws of physics that are completely
different from our own. The latest iteration of his theory provides a
natural explanation for the anthropic principle. If there are vast
numbers of other universes, all with different properties, by pure odds
at least one of them ought to have the right combination of conditions
to bring forth stars, planets, and living things.
... Rees, an early supporter of Linde’s
ideas, agrees that it may never be possible to observe other universes
directly, but he argues that scientists may still be able to make a
convincing case for their existence. To do that, he says, physicists
will need a theory of the multiverse that makes new but testable
predictions about properties of our own universe. If experiments
confirmed such a theory’s predictions about the universe we can see,
Rees believes, they would also make a strong case for the reality of
those we cannot. String theory is still very much a work in progress,
but it could form the basis for the sort of theory that Rees has in
mind.
“If a theory did gain
credibility by explaining previously unexplained features of the
physical world, then we should take seriously its further predictions,
even if those predictions aren’t directly testable,” he says. “Fifty
years ago we all thought of the Big Bang as very speculative. Now the
Big Bang from one millisecond onward is as well established as anything
about the early history of Earth.”
... “If you measure something which confirms
certain elaborations of string theory, then you’ve got indirect evidence
for the multiverse,” says Bernard Carr, a cosmologist at Queen Mary University of London.
Support for the multiverse might also
come from some upcoming space missions. Susskind says there is a chance
that the European Space Agency’s Planck satellite, scheduled for launch
early next year, could lend a hand. Some multiverse models predict that
our universe must have a specific geometry that would bend the path of
light rays in specific ways that might be detectable by Planck, which
will analyze radiation left from the Big Bang. If Planck’s observations
match the predictions, it would suggest the existence of the multiverse.
When
I ask Linde whether physicists will ever be able to prove that the
multiverse is real, he has a simple answer. “Nothing else fits the
data,” he tells me. “We don’t have any alternative explanation for the
dark energy; we don’t have any alternative explanation for the smallness
of the mass of the electron; we don’t have any alternative explanation
for many properties of particles.
“What I am saying is, look at it with open
eyes. These are experimental facts, and these facts fit one theory: the
multiverse theory. They do not fit any other theory so far. I’m not
saying these properties necessarily imply the multiverse theory is
right, but you asked me if there is any experimental evidence, and the
answer is yes. It was Arthur Conan Doyle who said, ‘When you have
eliminated the impossible, whatever remains, however improbable, must be
the truth.’” '
The article also says the following which seems very weird to me, but which at the same time seems consistent with some other things I have read (including of pantheism in the sense of the entire universe being one conscious entity, of which its component parts [including human minds] are interconnected in way) and with some experiences of mine pertaining to the idea of the so-called "Law of Attraction" (also known as "The Secret"). 'As for Linde, he is especially interested
in the mystery of consciousness and has speculated that consciousness
may be a fundamental component of the universe, much like space and
time. He wonders whether the physical universe, its laws, and conscious
observers might form an integrated whole. A complete description of
reality, he says, could require all three of those components, which he
posits emerged simultaneously. “Without someone observing the universe,”
he says, “the universe is actually dead.” '
https://www.popularmechanics.com/science/a36329671/is-the-universe-conscious/ has an article called "Some Scientists Believe the Universe Is Conscious:Sounds like a bad trip ... but what if it's true?"
