Caption: A cartoon the eternal inflation which is both a version of inflation and a version of the multiverse: a universe made up of some kind of pocket universes (see below).
Note eternal inflation and the multiverse are both highly speculative theories. The inflation paradigm in itself has a measure of robustness: it's resisted falsification since introduced in 1979 (see Wikipedia: Starobinsky inflation: History) though it has undergone considerable evolution.
Features:
We, of course, are one of those observers.
As aforesaid eternal inflation is also a version of inflation. But inflation and multiverse are different paradigms.
Some of the features of eternal inflation are rather undefined since eternal inflation probably has its own versions.
The universe is eternal and infinite---which solves the problem of before/after/outside of: i.e., there NO boundary conditions in spacetime to specify.
NO consensus theory tells us if pocket universes are spherical, if they have sharp edges, what their relative sizes are, what there relative separations are, or if they ever interact.
Recall, the observable universe is embedded in our pocket universe, and so we CANNOT directly observe outside of it.
Metastable means stable against small enough perturbations, but large enough perturbations will cause the state to change irreversibly at least on some time scale.
Two possible pieces of evidence for the
multiverse have been discussed
(see, e.g., Livio, M.
2013, How Can We Tell If a Multiverse Exists?):
In this case, we would be forced to conclude the
multiverse may well
exist even though direct observations may never be obtained.
Of course, maybe TOE
would rule out the multiverse
absolutely.
If so, observations in the future may detect the boundary or other
pocket universe.
This would be some proof of the multiverse.
However, at present all observations are consistent with the
observable universe
as having
homogeneity
and isotropy
when viewed on a sufficiently large scale: i.e., as obeying the
cosmological principle.
For an explication of the
cosmological principle,
see Cosmology file:
observable_universe_cosmological_principle.html.
You do have to look on a very large scale.
Circa 2022, the thinking is that the
cosmological principle scale ∼ 400 Mpc = 0.4 Gpc
(see Wikipedia:
Cosmological principle: Violations of homogeneity).
Note the
comoving radius of the observable universe = 14.25 Gpc = 46.48 Gly (current value)
according to the
Λ-CDM model (AKA the concordance model)
of the observable universe
(see Wikipedia: Observable universe).
So the cosmological principle scale ∼ 400 Mpc = 0.4 Gpc
is much smaller than the scale of the
observable universe.
Note, however, the
largest cosmic structures
are ∼ 1 Gpc
(see Wikipedia: List of
largest cosmic structures).
But these may be
statistical fluctuations
and NOT violations of the
cosmological principle scale ∼ 400 Mpc = 0.4 Gpc
(see Wikipedia:
Cosmological principle: Violations of homogeneity).
To digress, falsifiability
is the philosophy of science rule,
that a scientific theory should
make predictions that if they fail, the
scientific theory has be proven to be
wrong: i.e., it has be been falsified.
Actually, there has been a lot of discussion about
falsifiability, but most
scientists probably agree that
treated with a great deal of flexibility, it is a useful rule.
Flexibility: To incompletely explicate flexibility:
What most scientists probably agree on
is that a theory
whose proponents explain away any failed prediction with
ad hoc hypotheses
or false arguments is
a theory which is no longer
a scientific theory
in practice even if it once was.
Such a theory is now fruitless
and even a fraud or
a crank's delusion.
For example,
philosopher of science
Karl Popper (1902--1994)
argued that Marxism as
it had come to be promoted was no longer
a scientific theory
(see
Wikipedia: Karl_Popper: Falsifiability and the problem of demarcation).
Yours truly's short answer is that
the multiverse has passed one
significant falsification test,
but may be the only one anyone can ever think of.
So multiverse is in
scientific Limbo.
For an under construction discussion of
the multiverse
and falsification,
see Cosmology file:
inflation_eternal_falsifiable.html.
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Note that NEITHER of these two possible evidences are
sufficient to prove the multiverse
to most people's satisfaction.