The
long-awaited dust analysis of the Planck team has finally arrived,
and it's bad news for those who claimed last March that the BICEP2
study had produced evidence for cosmic inflation. Last March such
people had claimed to have found a “smoking gun” that finally
provided evidence for the theory of cosmic inflation: evidence of
b-mode polarization caused by gravitational waves produced at the
dawn of time. Subsequent studies suggested that the BICEP2 study was
a false alarm, and that the results can be explained as being the
result of ordinary dust. Now an analysis by a large team of
scientists (using the Planck space satellite) shows that the “clean
window” claimed by the BICEP2 team (a particular area of the sky
where there's supposedly little dust) is more like a dirty window
that has lots more dust than the BICEP2 team thought. It now looks
like the “evidence for cosmic inflation” claimed by the BICEP2
study is no such evidence at all. The BICEP2 observations can be
explained as being the products of dust and gravitational lensing,
without any need for cosmic inflation. Yesterday Physics World summarized
the situation with a story having the headline “BICEP2
gravitational wave results bites the dust thanks to new Planck data.”
This
new Planck result will get a little coverage, but much less than the
BICEP2 news coverage last March, when it seemed like almost every
cosmologist was popping a champagne cork in premature
self-congratulation. It was a huge orgy of unwarranted credulous
enthusiasm over a study with quite a few problems, problems I pointed
out in a very skeptical blog post the day after the BICEP2 study was
released (at a time when I seemed like a rare doubter, with few
others voicing similar doubts at that time). My skepticism about
BICEP2 was apparently warranted.
Given
the new Planck result, it may be a good time to look at a basic
question: should we actually believe in the theory of cosmic
inflation? I will argue in this post that we should not.
The
Difference Between the Big Bang Theory and the Cosmic Inflation
Theory .
Before
giving my case against the cosmic inflation theory, let me clarify
the difference between the cosmic inflation theory and the Big Bang
theory. No doubt many people get the two mixed up, because the
concepts are fairly similar.
The
Big Bang theory originated around the middle of the twentieth
century. The Big Bang theory is the theory that about 13 billion
years ago the universe originated from an extremely hot and dense
state. It's basically the idea that the universe “exploded into
existence” billions of years ago (or began to expand from a state
so hot and dense that it was just as if the universe had exploded
into existence). The theory was dramatically substantiated by the
discovery of the cosmic background radiation around 1965, believed to
be the “relic radiation” of the Big Bang. The Big Bang theory is
also supported by the simple fact that the universe is expanding.
When you “rewind the film” on an expanding universe all the way
to the beginning, you are stuck with something like the Big Bang.
The
cosmic inflation theory did not originate until about 1980. The
cosmic inflation theory is actually a theory about a tiny fraction of
the universe's first second. The theory maintains that when the
universe was a fraction of a second old, the universe underwent
exponential expansion, which is a type of expansion vastly quicker
than the type of expansion we now observe. According to the cosmic
inflation theory, this phase of super-fast exponential expansion
lasted only a fraction of a second. The diagram below illustrates
the cosmic inflation theory.
You
can believe in the Big Bang theory without believing in the cosmic
inflation theory, which is exactly what cosmologists generally did
during the decade of the 1970's. However, you cannot believe in the
cosmic inflation theory without believing in some form of the Big
Bang theory.
The
Reasons Many Cosmologists Believe in the Cosmic Inflation Theory
.
The
rationale given for believing in the cosmic inflation theory is that
it supposedly solves
some
cosmic mysteries. The main such mystery is what is called the
flatness problem. The
flatness problem is a fine-tuning problem involving the Big Bang.
According to cosmologists, when the universe began, it started to
expand at just the right rate. If the universe had started to expand
at a tiny bit faster rate, it would have expanded so quickly that
galaxies would not have formed from gravitational contraction. If the
universe had started to expand at a tiny bit slower rate, the
gravitational attraction from the universe's matter would have caused
the universe's matter to form into super-dense black holes rather
than galaxies.
The
physicist Paul Davies puts it this way:
For
a given density of cosmic material, the universe has to explode from
the creation event with a precisely defined degree of vigor to
achieve its present structure. If the bang is too small, the cosmic
material merely falls back again after a brief dispersal, and
crunches itself to oblivion. On the other hand, if the bang is too
big, the fragments get blasted completely apart at high speed, and
soon become isolated, unable to clump together to form galaxies.
How
finely balanced did this expansion rate have to be in order for there
to be a universe like ours, in which galaxies exist? Scientists say
that it had to be balanced to at least one part in 10 to the
thirtieth power (1 part in
1,000,000,000,000,000,000,000,000,000,000). In other words, if the
universe had expanded at a rate only
.00000000000000000000000000000001 faster or slower, it would not have
galaxies, and would not have life.
The
calculation given here is not some oddball conclusion made by only
one or two scientists.A
statement like the statement above has been made in innumerable scientific
books and papers.
The
cosmic inflation theory was created mainly to solve this problem. It
seems that if the universe underwent the exponential phase of
expansion imagined by the cosmic inflation theory, the universe's
expansion would not need to be so fine-tuned.
Another
reason given for believing in the cosmic inflation theory is that it
offers an answer for what is called the horizon problem, which is
basically the problem of why opposite ends of the universe have
identical thermodynamic attributes, as viewed in the cosmic
background radiation.
A
third reason given for believing in the cosmic inflation theory is
that it solves some “missing monopole” problem, although this is
not a compelling reason because the problem only arises for those who
believe in some family of theories called grand unification theories
(and there seems to be no particular necessity in believing in such a
theory).
Why
Cosmic Inflation is Not a Good Way of Explaining These Problems .
The
theory of cosmic inflation offers a way of explaining the flatness
problem and a way of explaining the horizon problem. But both of
these problems are examples of more general phenomena.
The
flatness problem is an apparent case of cosmic fine-tuning, and the
horizon problem is an example of cosmic uniformity. The weakness in
trying to solve these problems with a theory of cosmic inflation is
that we have many other apparent cases of cosmic fine-tuning and many
other cases of astonishing cosmic uniformity – but the cosmic
inflation theory only offers a solution to one of the many cases of
cosmic fine-tuning, and only one of the many cases of cosmic
uniformity.
Cases
of apparent cosmic fine-tuning are discussed here and here (in a blog
post that includes a handy color-coded chart). Among the many
astonishing cases of cosmic fine-tuning are the flatness problem, the
fine-tuning of the Higgs to 1 part in 100,000,000,000,000.000, the fine-tuning of the cosmological constant to 1 part in 10 to the
sixtieth power (or 10 to the 120th
power, depending on how you look at it), the fine-tuning of the
strong nuclear force, the fine tuning of atomic resonances, the
fine-tuning of fundamental constants related to stellar nuclear
reactions, and the fine-tuning of the proton charge and the electron
charge (involving a match to one part in
1,000,000,000,000,000,000,000). There are also many similar cases.
Now, how many of these cases of cosmic fine-tuning does the cosmic
inflation theory claim to explain? Exactly one: the flatness problem.
In this sense, the cosmic inflation theory is rather like a theory
that tries to explain the origin of animal species, but only explains
the origin of tigers rather than explaining the origin of the rest of
the animals.
If
we are to try to explain cosmic fine-tuning, we need a more general
explanation – some particular principal or assumption that will
explain all the cases (or most of the cases) of fine-tuning, rather
than jumping on some “one-trick pony” that explains just one
example of cosmic fine-tuning.
When
we look at examples of cosmic uniformity, we find a very similar
situation. There is not just one amazing case of cosmic uniformity
(the horizon problem), but many others. Among the main cases of
cosmic uniformity are the uniformity of fundamental constants in
opposite regions of the universe. Scientists have determined that
some fundamental constants such as the fine-structure constant are
the same in opposite regions of the universe separated by a distance
of more than twenty billion light-years (ten billion light-years in
one direction, plus ten billion in another direction). This is
particularly amazing because it is not just a uniformity over a
vastness of space but also a uniformity over a vastness of time equal
to almost the age of the universe. Other examples of cosmic
uniformity are the uniformity of the universe's laws. The universe is
like a vast machine that keeps on following the same set of rules
(which we call the laws of nature), obeying those laws to the letter, with slavish
obedience eon after eon.
If
we were to list all of the cases of cosmic uniformity, it would be a
long list. But how many on that list does the theory of cosmic
inflation purport to explain? Exactly one: the horizon problem.
Again, in this sense the cosmic inflation is like a theory of the
origin of species that only explains the origin of tigers, without
explaining the origin of any other species. If we are to start
trying to explain cosmic uniformity, we need a more general
explanation, rather than jumping on some “one-trick pony” that
explains just one example of cosmic uniformity.
How
the Cosmic Inflation Theory Robs Peter to Pay Paul .
The
advocates of the cosmic inflation theory neglect to explain that as
the price of explaining one example of cosmic fine-tuning (the
flatness problem), the cosmic inflation theory requires its own
fine-tuning, in not just one place, but multiple places. One has to
imagine various types of fine-tuning to create a theory of cosmic
inflation compatible with observations. You have to do fine-tuning so
that the cosmic inflation can start at just the right instant, and
more fine-tuning so that the cosmic inflation can end at just the
right time (or else you end up with a universe that keeps inflating
exponentially, which we know did not happen). It is not clear at all
that when you add up all these types of fine-tuning needed for cosmic
inflation to work, that you end up with less cosmic fine-tuning than
if you don't believe in the theory. It's basically a case of robbing
Peter to pay Paul.
The
False Prediction of the Cosmic Inflation Theory
Before
justifying my assertion that the cosmic inflation theory makes a
false prediction, I must declare an interesting and important
principle that is sometimes overlooked. This is the principle that we
should never ignore the “gross predictions” of a theory, and
never try to subtract counterfactual predictions, thereby judging a
theory only on a set of “net predictions.”
Here
is what some people think our procedure should be when evaluating a
theory:
(1)
Start with the “gross predictions” of a theory – everything it
seems to predict, regardless of known facts.
(2)
Subtract from these “gross predictions” anything known to be
false.
(3)
Then evaluate the theory on a smaller set of “net predictions.”
I
think such an approach is badly mistaken. Rather than discarding
“gross predictions” of a theory that are clearly counterfactual,
we should in fact pay great attention to such predictions, because
they are often very important indicators that the theory is false.
It's rather like this: suppose a theory predicts that a factory makes
only red phones, and you open a package from the factory, seeing a
blue phone. What does the theory predict now? Exactly the same thing
it predicted before you opened the package: that the factory makes
only red phones.
So
let's look at exactly what are the predictions are of the cosmic
inflation theory, without discarding any counterfactual “gross
predictions.” The predictions of the cosmic inflation theory are
as follows:
(1)
The universe is spatially flat, or very close to being spatially
flat.
(2)
There is a relatively small amount of what is known as cosmological
non-Gaussianity.
(3)
Our universe is a lifeless
“small bubble” universe that is way too young and small for any galaxies to
have formed in it.
The
cosmic inflation theory actually makes the third of these predictions
because it predicts that each universe that undergoes exponential
expansion produces many other “bubble universes,” and that each
of these bubble universes themselves produce many other bubble
universes, and so on and so forth. According to the predictions of
the theory, the number of these bubble universes too small to contain
any galaxies (and any life) should be billions and trillions and
quadrillions of times larger than the number of bubble universes
large enough for galaxies to form. As
cosmic inflation proponent Alan Guth describes here (in a discussion of this “youngness paradox”),
“The
population of pocket universes is therefore an incredibly
youth-dominated society, in
which the mature universes are vastly outnumbered by universes that
have just barely begun to evolve.”
Given
such a situation (in which small bubble universes vastly outnumber
bubble universes large enough for galaxies to form), and given that
predicting one thing is trillions of times more likely than another
thing is equivalent to predicting the first thing, it must be said
that the cosmic inflation theory predicts that our universe is one of those
smaller, lifeless universes. It is not legitimate at all to subtract
this counterfactual prediction because of some principle that we are
allowed to subtract counterfactual predictions, reducing a set of
“gross predictions” to a set of “net predictions.”
So
how can an advocate of the cosmic inflation theory explain how we got
lucky enough to be living in one of the rare life-compatible “bubble
universes,” when it is almost infinitely more likely (under cosmic
inflation theory) that our universe would be one of the young “bubble
universes” too small for galaxies to form in it? He must resort to
a “blind luck” explanation. But the luck needed is greater than
the luck needed to have a successful universe without
cosmic inflation. So nothing is accomplished, and the “miracle”
of our existence is not made any less miraculous. In fact, the
cosmic inflation theory seems to make our existence even more
miraculous. How can such a result be described as scientific
progress?
Cosmic
Inflation: A “Cash Cow” for Lazy Cosmologists?
If
the case for cosmic inflation is so weak, why do so many cosmologists
support it? One answer can be found in groupthink effects, the tendency of modern cosmologists to travel in a herd because of sociological “go with the crowd” reasons. But another reason is
that for decades the cosmic inflation theory has seemingly been an easy “meal
ticket” for lazy cosmologists.
Producing
a new paper on cosmic inflation is a cinch for a modern cosmologist.
He merely has to write some new paper juggling some astrophysical
numbers (perhaps thinking of some minor new speculative tweak), and
doing the same type of calculations done by many earlier
cosmologists. Since the cosmic inflation theory was introduced,
cosmologists have published thousands of new papers discussing
different flavors of the theory. A large fraction of this work has
been funded by university grants or federal research grants. So for a
cosmologist who is not particularly innovative, the cosmic inflation
theory is a wonderful “cash cow.” Think of how easy it is: just
produce “yet another cosmic inflation paper” (something like
“cosmic inflation paper number 5,678”) without any real
originality, and with zero risk that anyone will ever prove you
wrong; and let the taxpayers or your university foot the bill. I'm
reminded of the phrase in that Gershwin song: nice
work if you can get it.
Given
the existence of this convenient “cash cow” that pays well for
easy speculative work, cosmologists are reluctant to bite the hand
that feeds them, and admit how weak the cosmic inflation theory is.
That would be like up ripping up their meal ticket.
Conclusion
There seems to be good evidence for the Big Bang and the expansion of the universe, so we should keep believing in such theories. There is no good evidence for the theory of cosmic inflation (the theory of exponential expansion in the universe's first second), and it should be dumped, in the sense of being relegated to a mere possibility rather than asserted as a likelihood. Scientists Ijjas, Steinhardt, and Loeb recently wrote
a paper giving some powerful objections to the cosmic inflation theory.
Rather
than embracing a theory that claims to explain only one case of
cosmic fine-tuning (when there are many such cases to explain), we
should look for a more general explanation. Rather than embracing a
theory that claims to explain only one case of cosmic uniformity (when
there are many such cases to explain), we should also look for a
more general explanation. If scientists cannot think of such a more
general explanation, they should simply say that they do not
understand the explanation for the flatness problem and the horizon
problem that originally motivated the cosmic inflation theory. It is
an intellectual sin to claim to understand a cosmic mystery that you
do not really understand. For 1000 years, astronomers embraced the
Ptolemaic theory, and claimed to understand why the solar system
behaves as it does, before they really understood that mystery.
There's a lesson to be learned from such a long mistake: don't claim
to understand a cosmic mystery based on some weak theory. Much better
to simply candidly say: I don't understand this cosmic mystery.