I
have no problem with the title of the recent BBC article on the
multiverse. It is “Why there might be many more universes beside
our own.” Sure, there might be – anything's possible. My problem
is with the subtitle of the article, which is: “The
idea of parallel universes may seem bizarre, but physics has found
all sorts of reasons why they should exist.” This is not accurate,
and the article fails to provide any reasons why “parallel
universes” should exist.
The
first reason the article gives for thinking that there might
be multiple universes is that our universe might have an infinite
amount of matter, and if so, there might be duplication – such as
some other galaxy exactly like our galaxy in all respects (including
a double of you). But that's not really imagining another universe –
it's imagining an infinite universe with large-scale duplication.
There is no possibility that we could ever verify that such a
large-scale duplication is likely, because we have no prospect of
ever verifying that the universe does have an infinite amount of
matter. Whatever observations we make in a finite span of time, they
will never be able to justify a conclusion that the amount of matter
in the universe is infinite. Even if you verified the existence of a
trillion trillion trillion trillion galaxies, you would have no basis
for assuming that there was more than a trillion trillion trillion
trillion trillion
galaxies. So the very idea of a universe with infinite matter is not
provable, and not scientific. It certainly does not count as a
“physics reason” for believing in a multiverse.
The
article then describes the cosmic inflation theory, which is actually
a large group of theories with a few similarities. Exaggerating the
case for this theory (as many scientists do whenever discussing a
theory that fits their ideological inclinations), the article claims
that the match between predicted variations in the cosmic background
radiation and those predicted by the inflationary theory are “almost
unbelievably good,” suggesting that the theory is correct.
But
that's misleading. Cosmic inflation theory (not to be confused with
the more general Big Bang theory) started out around 1980, when we
already knew pretty much how small the variations in the cosmic
background radiation were. During the past 35 years (as our knowledge
of such variations has slightly increased) physicists have
continually fiddled with different varieties of the cosmic inflation
theory, trying to get some version that matches observations. Most
versions of the cosmic inflation theory do not match observations,
and any that do are those that have been fiddled with and tweaked to
try to match observations as those observations came in. Given such a
situation, the evidence value of such a match between “prediction”
and observation is minimal. We do not at all have a situation where
the theory predicted something very surprising, which was much later
confirmed to be exactly true.
In
fact, there are many problems with the cosmic inflation theory, such
as its excessive requirements for fine-tuning (the theory was created
to help get rid of some fine-tuning, but may require more fine-tuning
than it gets rid of). Far from predicting the cosmic background
radiation variations exactly, as the BBC article suggests, the theory
actually is hard to reconcile with one of those variations – the
feature known as the Cold Spot. A cosmologist quoted here puts it
this way:
[The inflationary
model] “predicts that today’s universe should appear uniform at the largest scales
in all directions. That uniformity should also characterize the distribution of
fluctuations at the largest scales. But these anomalies, which Planck
confirmed, such as the cold spot, suggest that this isn’t the case… This is very strange.
And I think that if there really is anything to this, you have to question
how that fits in with inflation…. It’s really puzzling.
The
BBC article then tries to make the leap from the general idea of the
cosmic inflation theory to a particular variation of that theory
called the eternal inflation theory, which imagines many bubble
universes. This eternal inflation theory is not at all verified nor
well supported, so such an idea does not qualify as a “physics
reason” for believing in a multiverse. We have zero evidence for
the existence of any other “bubble universe” outside of our own
(or any type of universe outside our own).
The
BBC article then attempts to blend from an inflationary multiverse
(the idea of lots of little bubble universes) to what is called the
string theory landscape – the completely speculative idea that
there are many different universes which each have a different
version of string theory physics. There is no factual basis for such
a leap, as there is currently no evidence that string theory is a
correct theory.
Strange galaxy in an alternate universe
The
article implies that such an idea of a multiverse consisting of many
universes may be helpful in explaining the fine-tuned features of our
universe. But it isn't. This is because one does not increase the
likelihood of success on any one trial by increasing the number of
trials. If a universe as fine-tuned as ours is a zillion-to-one shot,
it's still exactly the same zillion-to-one shot if you assume there
are a zillion other universes. Increasing the number of universes
may increase the chance that some universe may be accidentally
habitable, but we are not interested in the probability of some
universe being accidentally habitable. We are interested in the
probability of our universe being accidentally habitable. That
probability is not increased by even 1 percent by assuming other
universes.
As
physicist V. Palonen states in this scientific paper:
The
overall result is that, because multiverse hypotheses do not predict the
fine-tuning for this universe any better than a single universe
hypothesis, the
multiverse hypotheses fail as explanations for cosmic fine-tuning.
Conversely, the fine-tuning data does not support the multiverse
hypotheses.
Next
the BBC article touches on Lee Smolin's theory of cosmological natural
selection. The article describes it like this:
The theory in question is a heap of crazy speculations. We have no basis for concluding that black hole collapses lead to new universes, nor is there any basis for concluding that such a new universe would have different laws and physical constants from our universe. Even if such a theory were true, it would not at all explain our universe's fine-tuning, contrary to Smolin's claims. This is because in order for you to have a universe in which black holes are forming in the first place, a universe needs to already have an incredible amount of fine-tuning – the fine tuning needed for atoms and for stars (the predecessors of black holes) to exist. There are specific reasons why we should not expect stars to exist in a millionth of universes with random laws and fundamental constants. You can't explain the universe's fine-tuning if you start out with a universe that is fine-tuned in the first place. See here for some other reasons for rejecting Smolin's theory.
The
BBC article then touches on M-theory, a version of string theory.
This is also speculation for which there is no evidence. Its main
purpose currently seems to be to create busy work for mathematicians who
can play around with speculative equations because they can't think
of something more productive to do.
Finally,
as if building up to some muddling climax of confusion, the BBC
article touches on the “many worlds” theory of parallel
universes. This is the crazy idea that all physically possible
realities are actualized – so there must be some universe in which
your dog is the ruler of America. Thankfully there is not the
slightest evidence for this morally ruinous theory, which tells us
that any absurdity you can imagine is just as real as the news you
watch on television (an idea that is a perfect prescription for moral
indifference).
In
short, the BBC article fails to provide any substantiation for its
subtitle claim: “The idea of parallel universes may seem bizarre,
but physics has found all sorts of reasons why they should exist.”
The article fails to produce a single physics reason why they
“should exist.” The article tacitly admits such a thing near its
end, when it says “these ideas lie on the border of physics and
metaphysics” and then attempts to rank the plausibility of
multiverse scenarios “in the absence of any evidence.” Yes, we
finally have a confession that we are in the realm of groundless
speculation – drifting off into metaphysics “in the absence of
any evidence.” So why did the article appear in the Science
section?
Postscript: See page 39 of this paper for a discussion of how only a certain small range of the fundamental constants will allow for the existence of stars. If we assume that the gravitational force can have any value between its current value and a value as strong as the strong nuclear force, then "the stable star-permitting region" of parameter space occupies less than a trillionth of a trillionth of a trillionth of the total parameter space.
Postscript: See page 39 of this paper for a discussion of how only a certain small range of the fundamental constants will allow for the existence of stars. If we assume that the gravitational force can have any value between its current value and a value as strong as the strong nuclear force, then "the stable star-permitting region" of parameter space occupies less than a trillionth of a trillionth of a trillionth of the total parameter space.
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