Dark
matter is a hypothetical substance postulated by scientists to help
explain astronomical observations. When astronomers peer out at
distant objects, they find certain clusters and galaxies behaving in
a way that we cannot seem to account for by using merely the
gravitational attraction of visible matter. So astronomers speculate
that a large fraction of the universe's matter is some completely
invisible form of matter that they call dark matter.
Despite
some astronomers who speak as if dark matter is fact, there is so far no
conclusive evidence that dark matter actually exists. This has not
stopped physicist Lisa Randall, who has written a highly readable new
book entitled Dark Matter and the Dinosaurs. Like many of her
colleagues, Randall is a fearless speculator, not afraid to spin a
web of speculations that seems to be built of gossamer threads.
In
Chapter 16 of the book, readers will learn one of the reasons why
dark matter isn't actually on very solid ground: the fact that scientists
have not identified any particles of which dark matter is made up of.
The triumphant Standard Model of Physics tells us nothing at all
about dark matter particles. In Chapter 17 of the book, Randall
discusses attempts to observe dark matter, which have come up short,
despite a few false alarms and ambiguous hints.
In
Chapter 18 of the book, Randall discusses how computer simulations
using dark matter have come up short:
Explaining
why density profiles look flat or cored and not cuspy, as per dark
matter predictions, is an important challenge to the simplest dark
matter models. This, along with the missing
satellite problem (fewer dwarf galaxies than predicted
orbiting around bigger central galaxies) and the too
big to fail problem (a related issue in which the
predictions for the densest, most massive galaxies do not agree with
observations), possibly point to inadequacies of the standard cold
dark matter paradigm.
But
Randall has a solution that seems to satisfy her: make the model more
complicated to try to explain these discrepancies. Much more
complicated. While scientists often claim that they will cheerfully
give up a theory when it falls short, it seems that usually when
observations conflict with a scientist's favored theory, he or she
will prefer not to give up the theory but just to make it more
complicated to try to explain away the discrepancies.
What
Randall proposes is a model she calls “partially interacting dark
matter.” Below are some of her speculations (page 320-321):
I'll
call the force that is experienced by the interacting dark matter
dark light, or more generally I'll call it dark
electromagnetism....It would be an entirely different influence
acting on particles charged under a distinct additional force that is
communicated by an entirely different new type of particle – a dark
photon if you will...Two types of electrically charged particles in
the same place that don't interact with each other is really not so
mysterious.
What
Randall proposes is that dark matter has some type of
electromagnetism physics similar to the electromagnetism physics of
regular matter, but one that only works with dark matter. This is,
of course, wild speculation. One would think that the likelihood of
such a thing would be extremely small. It would seem far more likely
that dark matter might follow its own weird rules rather than
duplicating the rules followed by regular matter. The chance of the
two sets of rules matching would seem like the chance of marriage
customs on some alien planet matching the marriage customs in
America.
Another
problem is this: the idea of dark matter was introduced so scientists
could avoid having to speculate about “dark forces” –
undiscovered laws of nature that might supplement gravitation. But
why propose a combination of dark matter and dark forces, when dark
forces alone could explain any observational discrepancy?
Randall
also proposes that there is a “dark disk” of dark matter that
occupies our galaxy, occupying much of the space of our galaxy's
disk. She proposes that this “dark disk” somehow had some role in
causing a comet to stray from the Oort Cloud that surrounds our solar
system, and that such a comet may have wiped out the dinosaurs.
The Oort Cloud (Credit: NASA)
This
is all speculation run rampant, and there is no good evidence for
such a “dark disk” in
our galaxy. I would estimate that the likelihood of dark matter
existing is only about 50%, that the likelihood of such a “dark
disk” existing in our galaxy is only about 10%, and that the
likelihood that dark matter had anything to do with the demise of the
dinosaurs is no greater than 1%.
One
almost wonders whether the idea for Randall's book wasn't hatched by
some book publisher looking to boost sales. You can imagine a
conversation like this:
Joe:
We can't call it All
About Dark Matter. No one will
buy that.
John:
What about the combo approach? We can call it Dark Matter
and Jesus.
Joe:
No, those markets don't mesh. What about Dark Matter and
Lady Gaga?
John:
How can you write a book with that title?
Joe:
I've got it! Dark Matter and the Dinosaurs.
John:
That's it!
Randall's
book is well-written and entertaining, but I am puzzled by why the
Midtown branch of the New York Public Library has purchased seven
copies of this not terribly important book, and placed them right in
the physics section, next to textbooks of physics. It would have been
more appropriate to have the book placed in the section with equally
speculative books about things like ancient astronauts.
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