There is no denying the fact that science
has solved many mysteries of nature, from
demonstrating that the Sun and not the Earth
is at the center of the solar system, to discrediting
the long-held stork theory of where babies
came from.
This success gives scientists tremendous pride
in their accomplishments and is the reason
that a recent poll by the National Academy
of Sciences showed that scientists are generally
highly respected by the American public.
However, there is also no doubt that there
are unsolved mysteries and one of the biggest
mysteries in modern physics originates from
the fact that we scientists can’t conclusively
identify 85% of the matter of the universe.
So here’s the deal.
The most modern research says that the kind
of matter that makes up you and me constitutes
only about 5% of the mass and energy of the
universe.
It seems that about 25% is made of a substance
called dark matter and 70% is made of an even
more elusive substance called dark energy.
Earlier videos of mine describe both of these
possible dark substances.
However, today I want to talk about dark matter,
which seems to be about five times more prevalent
than ordinary matter.
The simplest picture that scientists have
of dark matter is a subatomic particle with
a mass between one and ten thousand times
the mass of a proton.
This particle has no electrical charge and
interacts with ordinary matter via gravity
and possibly involving forces weaker than
we’ve discovered so far.
For this reason, scientists call this particle
a WIMP, for weakly-interacting massive particle.
Using the WIMP idea, scientists can explain
many mysteries, from why galaxies rotate faster
than can be explained from the known laws
of physics and visible matter in the universe,
to the motion of vast clusters of galaxies.
Because of the success of the theory, the
WIMP idea is very well regarded in the scientific
community.
However, while the WIMP theory solves many
mysteries in astronomy, it doesn’t solve
all of them.
For instance, when scientists use WIMP theory
to simulate the formation of large galaxies
like the Milky Way, the simulations predict
many, perhaps as many as 500, small satellite
galaxies buzzing around the periphery of the
Milky Way.
When astronomers look, they find only about
twenty.
Also, the simulations predict that the small
galaxies should be arrayed uniformly around
the Milky Way, while the observed satellite
galaxies are preferentially located in a plane
perpendicular to the Milky Way’s orbital
plane.
These discrepancies and a few others have
pointed to possible weaknesses of the WIMP
theory.
These weaknesses are not enough to entirely
throw away the idea of the WIMP dark matter
and, truth be known, scientists debate just
how important these discrepancies really are.
It‘s entirely possible that with improved
simulations the disagreement between the model
and observations will be smaller.
Further, astronomers may still find more satellite
galaxies.
This recently happened when the researchers
using data taken by the Dark Energy Survey
collaboration found several additional satellite
galaxies at the Milky Way.
This discovery demonstrates just how fluid
our understanding of the situation really
is.
Still, it may be that some tweaks to the WIMP
theory might be needed.
While there are a number of proposed solutions,
one idea is particularly cool.
One of the key properties of dark matter is
that it does not absorb, nor emit, electromagnetic
radiation.
Basically, visible light, radio waves, infrared,
ultraviolet, none of them interact with dark
matter.
This is because dark matter carries no electric
charge.
But what if dark matter had a different kind
of charge- a dark charge so to speak.
If it did, we could imagine that, in analogy
to familiar electromagnetism, that particles
carrying dark charge could emit dark photons
and feel the force of dark electric fields.
Because ordinary matter doesn’t carry this
hypothetical dark charge, these dark photons
zip by ordinary matter without interacting
at all.
We just wouldn’t see them.
Now this sounds like it might be a kind of
silly proposal, but is it?
I mean, after all, ordinary matter is quite
complex, with its quarks and leptons and at
least five forces that govern its behavior.
Why couldn’t dark matter be equally complicated?
While we admit that we have no real reason
to believe this dark charge idea, we can ask
ourselves “Given existing data, is there
any chance this new theory could be true?”
It turns out that measurements of ordinary
matter can tell us both what is possible and
impossible for this new idea.
For instance, observations of the rotation
of galaxies tell us that dark matter is not
concentrated in a disk like the matter of
the Milky Way.
Instead, most of it is located in a big sphere
surrounding most galaxies.
This sets limits on both the strength of dark
matter charge and how dense the dark matter
is.
The reason is relatively simple.
If the dark charge were the same as ordinary
electric charge, it could radiate away energy
like familiar matter and the dark matter distribution
would be disk-like and not sphere-like.
Using ideas like this, scientists are able
to rule out some possible properties of dark
matter that interacts strongly with itself.
However, there is one proposed idea of self-interacting
dark matter that is very interesting.
This model suggests that dark matter is even
more complicated.
In this model, in addition to the regular
WIMP particle, there are two forms of charged
dark matter, with one of the dark matter particles
being heavy and the other being light.
This is kind of like imagining a dark matter
proton and a dark matter electron that could
conceivably bind together to make dark matter
atoms.
I should caution you against making too much
of that analogy, but it helps give you a mental
image of the idea.
The reason I caution you against this analogy
is that if one postulated dark atoms, it’s
easy to let your imagination get away from
you and start thinking about dark molecules
and then dark planets, dark stars and an entire
dark matter Milky Way living alongside of
ours, forever invisible except through gravitational
interactions between the dark and the ordinary
worlds.
However we know this can’t be true.
In the past, before the scientific community
settled on the dark matter hypothesis, astronomers
looked for other explanations for why galaxies
rotate so fast.
The simplest hypothesis was that rogue planets,
brown dwarfs, black holes and similar dense
bodies of ordinary matter were to be found
in the Milky Way.
The idea was that scientists had simply underestimated
the amount of matter in the galaxy.
However, these experiments set pretty strict
limits on the existence of these compact objects.
Because of the way that these measurements
were done, we can rule out the existence of
stars made solely of dark matter.
The reason is simple.
If dark matter stars existed, they’d have
been detected in these searches for dark objects
made of ordinary matter.
However, it’s still possible that smaller
compact dark objects could exist- say dark
asteroids, for instance.
So what do existing measurements still allow?
The result is actually surprising.
Under the idea that dark matter consists of
a WIMP and two particles carrying dark charge,
dark matter could consist of a big cloud of
WIMPs with a smaller disk of dark matter held
together with dark electromagnetism.
In fact, given existing measurements, the
disk of strongly interacting dark matter could
contain as much mass as we see in ordinary
matter.
This is a surprising conclusion and gives
us a very different picture of the structure
of galaxies.
Now while I have told you about this interesting
hypothesis, I must add a word of caution.
No matter how interesting this idea is, it’s
just that- an idea- and most ideas in science
turn out to be wrong.
However, for the moment, it’s a cool possibility
that lives in the gap between ideas that are
still possible and those that have been ruled
out completely.
Only time, and experiments, will tell us in
what category this idea will finally end up.
