This
week scientists announced the result of a three-year search for dark
matter using the most sensitive instruments. The result: a big fat
nothing.
“We
built an experiment that has delivered world-leading sensitivity in
multiple new results over the last three years,” said a scientist.
“We gave dark matter every opportunity to show up in our
experiment, but it chose not to.”
Why,
then, are scientists continuing to be so dogmatic about dark matter?
The first way in which scientists are dogmatic about dark matter is
the way in which they assert its existence unconditionally.
Matter-of-fact assertions that something exists should not be made by
scientists unless there is direction observational evidence. When
asked what we know about dark matter, a typical physicist will answer
that we know it exists. No, we don't. There are some astronomical
observations that dark matter might be convenient in explaining, but
such a situation does not justify matter-of-fact assertions that dark
matter exists. Dark matter may be one way of explaining puzzling
astronomical observations, but there are always many different ways
of explaining a puzzling observation.
The
second way in which scientists are dogmatic about dark matter is in
the way they claim we know what percent of the universe is dark matter.
We hear scientists making claims such as the universe is 26.8% dark
matter and 68.3% dark energy. Given that neither dark matter nor dark
energy has been directly observed, such claims are rather like claiming that
the population of heaven is 23% angels and 12% archangels.
If
you do a Google search for “evidence for dark matter,” you will
find various web sites that argue for the existence of dark matter.
But the reasons given are not very compelling. One common reason
goes like this: scientists have compared what is called the
gravitational mass of galaxies with the mass inferred from the
luminosity of stars. It seems that these two figures do not match
with each other, and that dark matter might resolve the discrepancy.
This is not a very convincing reason, because it is all too possible
that scientists may have underestimated or overestimated either of
these two things. It is also all too possible that various
non-luminous forms of regular matter might resolve this discrepancy –
things such as dust, brown dwarfs and black holes (all of which are
either hard to observe or hard to quantify). Since we are faced with
at least 1000% uncertainty when estimating the total mass
contribution from dust, brown dwarfs and black holes, we can't
reliably calculate the total amount of mass from regular matter in
the galaxy.
Another
reason given for believing in dark matter has to do with what are
called galactic rotation curves. Without any assumption of dark
matter, scientists expect stars around the outer edges of our galaxy
to be rotating the galaxy more slowly than stars closer to the center
of our galaxy. But instead they found that the velocity of stars not
close to the galactic center remained roughly constant, regardless of
how far they are from the galactic center.
There
is a straightforward way to interpret such observations. We can
assume that there is some principle that is causing stars to rotate
the galaxy at the same speed, perhaps some principle that is related
to the fact that if stars did not rotate the galaxy at the same
speed, spiral galaxies would not preserve their structure, and we
would no longer have our beautiful spiral galaxies. But instead of
evoking such a principle, astronomers advance the weird idea that
there is a large outer halo of invisible dark matter, which happens
to be arranged in such a way so that (just coincidentally) stars
rotate the galaxy at the same speed. Now if astronomers wish to
advance such a contrived explanation, they may do so. But such an
explanation (which is far from straightforward) is only one of many
ways of explaining the fact that stars rotate galaxies at the same
speed. It is not at all correct to cite such a fact as strong
evidence for dark matter, although it may be evidence that there is
something out there interested in preserving the universe's beautiful
spiral galaxies (such a power might be a transcendent power, or perhaps something like billion-year-old extraterrestrials with godlike powers).
Web
sites arguing for the existence of dark matter also tend to cite the
cosmic background radiation. We will be shown some picture of the
cosmic background radiation, one of those maps that exaggerates the
differences in this radiation that is uniform to one part in 10,000.
We are told that the darker spots on the map are “dark matter
concentrations.” But, in fact, the cosmic background radiation
provides no support at all for the concept of dark matter. A
realistic map of the cosmic background radiation will use only one
color, since the radiation is uniform to one part in 10,000.
Some
writers also claim that something called the Bullet Cluster provides
evidence for dark matter. These claims are based on a statement of
Doug Clowe of the University of Arizona,
who in 2006 did a study on the Bullet Cluster. “"These
results are direct proof that dark matter exists,” Clowe assured us. But his results were no such thing. Certainly not direct proof,
his results were neither proof nor compelling evidence for dark
matter.
Clowe
merely studied a funny-looked cluster, and provided a complicated
dark matter story as an explanation. But there are always 101 ways to
explain some strange-looking astronomical object. Some funny-looking
cluster that might be explained by dark matter is not proof for dark
matter. See here for an explanation of the Bullet Cluster that does
not involve dark matter.
Some
gullible NASA personnel uncritically quoted Clowe's unwarranted
claims. People were fooled by a caption on a visual showing the
Bullet Cluster. The visual showed blue areas and pink areas. Captions
to the visual told us that the blue areas were dark matter and the
pink areas were regular matter.
The Bullet Cluster (which doesn't look like this to the eye)
You
can realize here the misleading silliness when you remember that dark
matter is not believed to be blue, and is not even believed to be
visible. The image in
question was a composite image, made from combining one image from
one regular telescope and one image from a radio telescope. The blue
areas were simply areas emitting different amounts of radiation.
The claim that the blue areas were dark matter is an unproven speculation. There
is a rather similar cluster called the Train Wreck cluster which is
very hard to explain under dark matter assumptions.
The central triumph of modern physics is what is known as the Standard Model of physics. Dark matter has no place in that model.
When
dealing with dark matter, our scientists are plagued by what is known
as confirmation bias. When confirmation bias occurs, someone may
eagerly scout for anything that might be interpreted as evidence for
something that he wants to believe in, but also ignore any evidence
that is inconsistent with the thing he wants to believe in. The
scientific paper here presents quite a bit of evidence that is
inconsistent with dark matter assumptions.
The paper above quotes Karl
Popper giving this description of confirmation bias:
For
if we are uncritical we shall always find what we want: we shall look
for, and find, confirmations, and we shall look away from, and not
see, whatever might be
dangerous to our pet theories. In this way it is only too easy to
obtain what appears to be overwhelming evidence in favor of a theory
which, if approached critically, would have been refuted.
I
am not claiming that the existence of dark matter is very unlikely.
I merely claim that nothing we have learned warrants dogmatic
assertions claiming that dark matter definitely exists. Something
unproven should not be represented as something proven, and speculations should be candidly described as speculations.
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