Earlier
this year, Scientific American published a blistering critique
of the theory of cosmic inflation originally advanced by Alan Guth
(not to be confused with the more general Big Bang theory). The
theory of cosmic inflation (which arose in 1980) is a kind of baroque
add-on to the Big Bang theory that arose decades earlier. The Big
Bang theory asserts the very general idea that the universe began
suddenly in a state of incredible density, perhaps the infinite
density called a singularity; and that the universe has been
expanding ever since. The cosmic inflation theory makes a claim
about less than one second of this expansion – that during only the
first second of the expansion, there was a special super-fast type of
expansion called exponential expansion.
The
article in Scientific American criticizing the theory of
cosmic inflation was by three scientists (Anna Ijjas, Paul J.
Steinhardt, Abraham Loeb), one a Harvard professor and another a
Princeton professor. It was filled with very good points that should
be read by anyone curious about the claims of the cosmic inflation
theory. But now the article has been half-censored, for Scientific
American has put the article behind a paywall. But don't worry,
you can still read the article on a Harvard web site here. Or you
can go to this site by the article's authors, summarizing their
critique of the cosmic inflation theory.
The
Scientific
American
article by the three scientists provoked an unusual response. The
main supporters of cosmic inflation theory (including Alan Guth and
Andrei Linde) along with about 30 other cosmologists have published a
rebuttal article
called “A Cosmic Controversy.” It is kind of an authoritarian
power play, designed to impress the reader by listing authorship by
some of the top names in cosmology. The list of authors is very
impressive, but there are some factually inaccurate claims in the
article.
The
article claims that the cosmic inflation theory has been empirically
successful. Referring to variations of the theory called slow-roll
models, Guth claims that “many models in this class continue to be
very successful empirically,” and later refers to “the dramatic
observational successes of inflation.” These claims are not
accurate.
The
cosmic inflation theory makes two main claims: (1) that there is or
was something called an inflaton field; (2) that the universe
underwent a period of exponential expansion during part of its first
second. Various versions of the cosmic inflation theory have also
claimed that there exist or once existed other universes (sometimes
called “bubble universes”), or that something called grand
unification theories (GUT theories) are in some sense correct. We
have not a single observation verifying any of these claims.
The
claim that the universe underwent an exponential expansion during
part of its first second is one that can never be confirmed, because of
physical limitations (the recombination era issue) that will always
prevent us from looking back to the universe’s first second with
our telescopes. No evidence has been produced for this so-called
inflaton field, and it has not been detected by the Large Hadron
Collider. No evidence has been produced that there is any universe
other than our own, and there are very strong reasons for thinking
that no such evidence ever could be produced (anything that we might
observe would always be an observation of our universe, not some
other universe). No evidence has turned up for the idea of grand
unification theories (GUTs), which have been one of the more
embarrassing failures of modern physics.
Given
this situation, in which not a single one of its key claims has been
backed up by observations, it is pure baloney for Guth and
his allies to be calling their theory an empirical success. Their
claim is based on the very shaky idea that various versions of cosmic
inflation theory have made some predictions about a few
things that have been consistent with observed reality. In my post
here I give several paragraphs explaining why this type of claiming
success based on imprecise general predictions is fallacious. Among
the reasons I gave is that there are very many versions of the cosmic
inflation theory (each predicting a wide range of things, as each
version allows a range of input parameters); so even if the theory is
bunk we should not be surprised if some of the resulting predictions
matched reality.
Let
us suppose that some theory claimed that green monsters from Alpha
Centauri have landed on our planet and invested in the bond market;
consequently next year the bond market will go up and the stock
market will go down. If next year the bond market did go up and the
stock market did go down, it would still not at all be accurate to
claim that this weird theory was empirically successful. It would
only be correct to say that if the green monsters were actually
observed. Similarly, no theory of cosmic inflation can be called
empirically successful until one of its core central claims (unique
to the theory) has been observed. No such thing has happened.
There
is another huge reason why the cosmic inflation theory cannot be
called empirically successful. The reason is that the theory is
inconsistent with observations of anomalies in the cosmic background
radiation (also called the cosmic microwave background, or CMB).
Believed
to date from early in the universe, the cosmic background radiation
is a type of radiation pervading all of space. The cosmic inflation
theory predicts that this radiation should be very smooth and
isotropic (just as the theory that your friend spent 20 minutes
stirring his bowl of pancake batter predicts that the pancake batter
should be very smooth, and without lumps).
The
first satellite to observe in detail the cosmic background radiation
was the WMAP satellite launched in 2001. This satellite detected some
very strange anomalies in the cosmic background radiation, anomalies
that came as a surprise to scientists. One was an anomaly called the
cosmic cold spot. Another was an anomaly that is technically known
as the hemispherical variance asymmetry. Then there is an anomaly
called the quadrupole-octopole alignment. There are nine other
anomalies in the cosmic background radiation that are summarized in a
table in this scientific paper. The table is below:
The
p-values here give us a rough idea of the probability of finding such
anomalies if standard ideas of cosmology (including cosmic inflation
and dark matter) are correct. This all presents a huge problem for
cosmic inflation theorists such as Guth. These are all things that
we should not expect to be finding in the cosmic background radiation
if the cosmic inflation theory is correct.
Years
after the WMAP satellite was launched, scientists launched another
satellite called the Planck satellite. It was predicted that the
troubling anomalies in the cosmic background radiation would go away
after the more powerful Planck satellite did its work. But that did
not happen. The Planck team reported the same anomalies. The table
above is from a paper entitled, “CMB Anomalies After Planck.”
When the Planck team reported its results on these anomalies, they
buried their findings in a dense technical document, as if they were
trying to make it as hard as possible for anyone to discover the
truth about this matter. But the “CMB Anomalies After Planck”
paper gives us some of the straight talk that the Planck paper
lacked.
Referring
to the hemispherical asymmetry it reported, the paper says, “An
inflationary theory could, in principle, accommodate models that
produce hemispherical asymmetry, but such a model would have to be
multi-field and involve, for example, a large-amplitude superhorizon
perturbation to the curvaton field.” Which is a fancy say of
saying the simpler versions of the cosmic inflation theory are not compatible
with this CMB anomaly – only more baroque and implausible versions
are compatible with it. Requiring a multi-field cosmic
inflation theory (imagining not just one but more than one
undiscovered fundamental field involved in primordial cosmic
inflation) is kind of like it being that your theory of crop circles requires not just UFO's but also cooperation by Bigfoot creatures. In a similar vein, this paper states that the most popular version of the
cosmic inflation theory (called slow-roll inflation) is not
compatible with the CMB anomalies:
Measurements
of CMB temperature fluctuations by the Wilkinson Microwave Anisotropy
Probe (WMAP) indicate that the fluctuation amplitude in one half of
the sky differs from the amplitude in the other half. We show that
such an asymmetry cannot be generated during single-field slow-roll
inflation without violating constraints to the homogeneity of the
Universe.
But
this “slow-roll” version of the inflation theory is the very one
that Guth called “very successful empirically” in his Scientific
American article. Apparently it is no such thing.
A
2010 paper states the following about the anomalies in the cosmic
background radiation:
While
not all of these alignments are statistically independent, their
combined statistical significance is certainly greater than their
individual significances. For example,
given their mutual alignments, the conditional probability of the
four normals lying so close to the ecliptic, is less than 2%; the
combined probability of the four normals
being both so aligned with each other and so close to the ecliptic is
less than 0.4% × 2% = 0.008%. These are therefore clearly
surprising, highly statistically significant
anomalies — unexpected in the standard inflationary theory and the
accepted cosmological model.
This
is a probability of less than 1 in 10,000 under the assumptions of
the cosmic inflation theory and the accepted cosmological model.
That's hardly what we would find if the cosmic inflation theory
really was “empirically successful” as Guth claims.
Using
the phrase “in tension” to mean “conflict with,” Stephon H.
Alexander (a professor of physics at Brown University) writes the
following about these anomalies in the cosmic background radiation,
and their relation to the cosmic inflation theory:
By
suggesting that it may be time for “alternative theories of the
early universe,” Alexander is clearly suggesting these CMB
anomalies are in conflict with the cosmic inflation theory.
Contrary
to the claims of Guth and his clique, the cosmic inflation theory is not empirically
successful. It has enjoyed another type of success: sociological
success. The history of modern science culture shows repeatedly that
a theory that is not empirically successful may become sociologically
successful and become popular due to a bandwagon effect and
groupthink. Once this snowball effect gets rolling, the theory may become a speech custom of an insular academic subculture,
and a little piece of tribal folklore has been born. The adherents
of the theory will in effect place gold medals around their own
necks, congratulating themselves on what they think is their
brilliant explanatory triumph. But such gold medals may be very
undeserved.
But
why did I use the term “shocking” in this blog post's title to refer
to these anomalies in the cosmic background radiation? There's one
CMB anomaly that is quite shocking. It seems that something called
the quadrupole – octopole alignment aligns with the plane of our
solar system. Since the cosmic background radiation has often been
described as something that looks the same everywhere in the
universe, we should not expect to find any such correlation involving
our solar system. One paper states the following:
Particularly
puzzling are the alignments with solar system features. CMB
anisotropy should clearly not be correlated with our local habitat.
While the observed correlations
seem to hint that there is contamination by a foreground or perhaps
by the scanning strategy of the telescope, closer inspection reveals
that there is no obvious way to explain the observed correlations.
Physicist Lawrence Krauss has this to say about this topic:
But when you look at CMB map, you also see that the structure that is observed, is in fact, in a weird way, correlated with the plane of the earth around the sun. Is this Copernicus coming back to haunt us? That's crazy. We're looking out at the whole universe. There's no way there should be a correlation of structure with our motion of the earth around the sun — the plane of the earth around the sun — the ecliptic. That would say we are truly the center of the universe. The new results are either telling us that all of science is wrong and we're the center of the universe, or maybe the data is simply incorrect, or maybe it's telling us there's something weird about the microwave background results and that maybe, maybe there's something wrong with our theories on the larger scales.
There is a small band of
cosmology-following geocentrists who believe that the Earth is the
center of the universe, and that this quadrupole – octopole
alignment supports their claim. But we know the earth revolves
around the sun. A less outrageous claim would be that the solar
system may be somehow in some kind of privileged position, and that
the quadrupole – octopole alignment supports this claim. It would
be premature to make even this less outrageous claim based on this
limited evidence. But it is interesting that this quadrupole –
octopole alignment may suggest one of the key assumptions of modern
cosmology (the Copernican principle, that there is nothing special
about the position of the solar system) may be wrong.
Postscript: For more information on this topic, do a Google search for "lopsided universe," "axis of evil (cosmology)", "CMB cold spot," and "CMB anomalies." In a previous post I had mentioned the cold spot as a thorn in the side of cosmic inflation theory, but from the table above it is clear that the hemispherical variance asymmetry is a ten times bigger problem for that theory.
Postscript: Theoretical physicist Sabine Hossenfelder has a post entitled "Is the Inflationary Universe a Scientific Theory? Not Anymore." After belittling some of the reasons given for the theory, she states this:
It is this abundance of useless models that gives rise to the criticism that inflation is not a scientific theory. And on that account, the criticism is justified. It’s not good scientific practice. It is a practice that, to say it bluntly, has become commonplace because it results in papers, not because it advances science.
Postscript: For more information on this topic, do a Google search for "lopsided universe," "axis of evil (cosmology)", "CMB cold spot," and "CMB anomalies." In a previous post I had mentioned the cold spot as a thorn in the side of cosmic inflation theory, but from the table above it is clear that the hemispherical variance asymmetry is a ten times bigger problem for that theory.
Postscript: Theoretical physicist Sabine Hossenfelder has a post entitled "Is the Inflationary Universe a Scientific Theory? Not Anymore." After belittling some of the reasons given for the theory, she states this:
It is this abundance of useless models that gives rise to the criticism that inflation is not a scientific theory. And on that account, the criticism is justified. It’s not good scientific practice. It is a practice that, to say it bluntly, has become commonplace because it results in papers, not because it advances science.
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