What is dark matter, and why are astronomers
so confident that it exists? The answer to
the first question—we don't know right know,
but astronomers have built up evidence that
some thing that has mass, but no light, exists
and is pervasive in the universe. This evidence
starts with the Milky Way. When we map out
the mass motions of the galaxy, it's disc,
towards the outer extremities, we see continued
high circular velocities, which have no explanation
in terms of Newtons law and Keplers laws of
motion. Basically, the light in the galaxy
is distributed far more centrally then the
mass, as indicated by the motions. There are
only 2 alternative explanations. One is that
our theory of gravity is simply wrong, and
that would be Newton's theory of gravity.
Astronomers have considered the possibility
that Newtonian gravity is incorrect, or needs
to be altered on very large scales. And it's
important to remember that Newtoneon theory
is only verified experimentally on solar system
scales. So on much larger scales like hundreds
or thousands or hundreds of thousands of lightyears,
we don't have a direct experimental
test of the theory. If the force law deviated
from inverse square, by only a few percent, you
could explain away dark matter—but that's
not a trivial thing to do. Newtoneon gravity
has passed every test put to it over hundreds
of years—except in the situation of very
strong gravity, where we need Einstein's superior
theory. On large scales, and for weak gravity, we
think Newton's theory is an excellent theory.
Astronomers haven't come up with anything
better, and they're loath to discard it. The
alternative, is that there is mass that doesn't
emit light on large scales in our galaxy, and
also in every other galaxy, and in the space
between galaxies. Evidence for this has slowly
and steadily accumulated over four decades
to the point where dark matter is now part
of the furniture of extragalactic astronomy.
Astronomers simply believe that it exists,
and they're trying to find ways of understanding
its physical nature. Let's look, in a little
more detail, at how astronomers reach this
conclusion—for our galaxy in particular—but
any spiral galaxy. One of the figures of merit
of a galaxy is its mass to light ratio. The
ratio of its stellar mass to its stellar luminosity.
Because we understand well how stars evolve,
the mean mass to light ratio of a stellar
population is a well determined number. What
we see in our galaxy is that the mass to light
ratio of the disc of the galaxy is much larger
than can be explained by any simple stellar
population. In terms of mass to light ratio,
the mass to light ratio of normal stellar
populations is higher than the value 1 that
it would have for the sun, because so many
stars are dim dwarf stars. Typical mass
to light ratios for all stellar populations
are between 5 and 10. The calculated mass
to light ratio for the Milky Way and other
spiral galaxies is almost 10 times larger.
If we went to estimate the mass of our galaxy,
we simply apply Kepler's Law, and we can do
this very robustly for the mass within the
solar orbit. Remember that the sun orbits the
disc of the Galaxy about 2/3's of the way
out, to the visible edge of the galaxy. Using
Kepler's Law, the stellar mass interior to
this orbit is about 2x10^11
solar masses, 200 billion times the mass of
the Sun. But the entire mass of the galaxy
seems to be ten times larger, 2 trillion solar
masses, and most of that mass is inferred
to be at larger scales than the sun's orbit.
So distributed to the periphery of the galaxy.
Even beyond its visible edge. When we're modeling the motions of galaxies, the indication for
dark matter is not any subtle or small effect
at the five or ten percent level. It's dramatic.
As you map the rotation speed of a galaxy
out beyond the solar radius to the edge of
the visible galaxy—and using gas clouds and
diffused stars you can go even beyond—remember:
galaxies don't have sharp edges—so there're
traces of the mass motions beyond where we'd
see the visible edge. If you do this, the
dichotomy is stark. The rotation speeds predicted
by Kepler's law should have declined by factors
of 3 or 4, moving beyond the visible edge.
In fact, they're constant to the visible edge,
and even beyond, into the thin gruel of intergalactic
space. So this factor of disagreement is very
large, and cannot be easily explained away by
experimental error. But rotation curves are
not the only reason astronomer think dark
matter exists. Other evidence has come from the
exquisite imaging of the Hubble space telescope
or other ground based observatories, able to make very sharp images of galaxies. Einstein's theory
of gravity says that mass bends or deflects
light. And the overall distribution of galaxies
in the universe, and the mass associated with
them, deflects light traveling through the
universe. Think of the universe as a fun house
mirror with slight undulations caused by mass
distributions that deviate light as it passes—subtly distorting images. This effect
can be easily measured with large telescopes;
It's quite subtle—just a few tenths of a
percent deviation of the average shape of
a galaxy caused by mass deflections along
the way. But the sum of those deflections
is another affirmation that some dark material
exists not only within those galaxies, but
in the space between them . Beautiful evidence
that dark matter—or something like it—exists
has come recently from observations from the
great observatories: the Hubble Space Telescope,
and the Chandra X-ray Observatory. There're
several situations in the distant universe—a few hundred million light years away—
where two clusters have approached each other
and passed through each other. The distance
between galaxies is such that when clusters
approach and pass through each other, the
galaxies rarely collide. They simply interact
by gravity and then continue on their way.
However, there is hot gas in a cluster, and
the gas is viscous and so the gas belonging
to 2 clusters heats ups and emits X-rays and
crashes into one pile. In at least 2 clusters,
we've been able to observer a pile up of
gas that emits X-rays, while the dark matter
has continued and sailed on through in each
of their clusters. The dark matter is measured
by the gravitational lensing effect that I
just referred to. In these cases, it's excellent
evidence that something like dark matter—which
interacts extremely weakly with electro-magnetic
radiation—exists, and forms most of the mass
of each of these clusters. There's another
reason that astronomers are fairly confident
that dark matter exists, and it's based on
theory and simulation. As computers have
grown in their capabilities, we've been able
to simulate larger and larger volumes of the
universe. In particular, how—starting from smooth
initial conditions—the clusters and galaxies
could have formed in the first place, by simple gravitational effects.
If in a computer simulation you fail to put in
dark matter as an ingredient at the beginning
of the simulation, you fail to produce anything
that looks like the real universe we live
in. Basically, the matter will crunch down
onto small scales, and produce too much structure
early on. Dark matter mediates the formation
of galaxies in clusters, and produces the
delicate filamentary structure of clustering that
we observed in the actual universe. So without
dark matter as an ingredient in simulation,
the simulation fails to describe what we see.
Which is reason that theorists are confident
that dark matter exists. So we have fairly
impressive evidence that some form of matter
that interacts very weakly with electromagnetic
radiation, and exceeds the visible mass in
the stars and all the galaxies exists. But
what is it? So far astronomers have only been able to play the game of elimination, ruling things
out. We can think of a detective mystery where
in the final seen all the possible killers
in the stately home are gathered together,
and one by one their alibis are inspected
and they are ruled out of deliberation and
we find out who the culprit was. Well, the
game doesn't work that neatly in science. We
may not be able to think of all the possibilities.
But the number that have been ruled out is
actually quite impressive. Let's start at
the high end; What about black holes? Black
holes of course by definition are black, if
they're truly isolated in space perhaps the
universe and galaxies are riddled with black
holes, and that's the dark matter. That's
actually very easy to rule out. Imagine that
there was a party in your house. Teenagers
left alone over the weekend. You may not have
been there for the party, but you'll sure
see the damage they left behind. Well, any
time a black hole forms there was a party
before hand in the form of a supernova—
a violent explosion ejecting gas, causing
high energy emission, emitting X-rays in a
supernova remnant for millennia afterwards.
So it's impossible to form black holes without
seeing evidence of the violence that proceeded
them as a massive star dies. And to have enough
black holes in the universe to account for
dark matter you'd have to have vast numbers
of massive stars dying at some point through
history. We see no evidence of that. So stellar
mass black holes are actually the easiest
things to rule out for dark matter. The next
possibility is interesting. Recall that as
you go down the mass spectrum of collapsing
gas clouds there is a mass corresponding to 8 percent of the mass of the sun, below
which a star does not form. The gas cloud
simply becomes hot, but but never hot enough
for fusion to occur. Nature will probably
collapse gas clouds that are less than that
number. They may be 5% the mass of the
Sun, 1%, .1%, and so in theory there's a set
of collapesd objects out there that are warm,
but not radiating by fusion. And if they're
small they might actually be quite dark and
not visible in light. These have been called
massive compact halo objects. If the halo
of our galaxy was filled with them they could
account for the dark matter. We already have
an indication that there are many more low
mass stars than high mass stars. So it's reasonable to hypothesize an extremely large number of
sub-stellar objects ranging down to Jupiters,
and even below exist in free space. Think
of them as free floating planets if you like.
This is harder to rule out but, lensing—
the affect where mass bends light—has actually
done so. Experiments in the mid to late 1980's
look for gravitational lensing, taking place
throughout the halo of our galaxy. In terms
of the momentary amplification of the light of a background star from a foreground star. That
same amplification could occur if the foreground object was a massive compact halo object: a substellar
object, or a Jupiter for example. The statistics
of that search were very impressive. And they
easily ruled out the halo being composed primarily
of such objects. MACHOs, as they're called, may
exist, but thy don't form the majority of
the dark matter. With normal stars—such as
red dwarfs—and the remnants of massive stars
ruled out, and also substellar objects,
the last possibility is even smaller physical
objects, say, asteroid size raging down to
house size or even down to dust grain size.
In the interstellar medium, and in the space
between planets there are of course a lot
of rocks. And those rocks only emit infrared
radiation. Perhaps space is filled with such
dust, or particles. It turns out this was
ruled out 20 or 25 years ago by the IRAS satellite
of NASA which looked across the sky at far
infrared wavelengths. Even if you hypothesize
that space if filled with tiny particles or
boulders, or house sized objects or asteroids,
those objects do not exist in isolation from
radiation. They're radiated by the light from
near by stars or galaxies, and they reach
an equilibrium temperature sufficient to pump
out infrared radiation. IRAS was a sensitive
mission that would have seen the diffuse
dim glow of far infrared radiation from such
hypothetical objects. They simply don't exist.
Also that large an amount of dust in interstellar
space would dim and redden the light of distant
galaxies in a way that we simply don't observe.
If we summarized this, astronomers have essentially ruled out dark objects ranging from black
holes down to dust grains one micron in size.
All that's left is the only current viable
explanation for dark matter: a subatomic
particle. So we're left with one viable explanation,
in physics or astrophysics for dark matter.
A subatomic and fundamental particle. Remember
the constraints. This particle has to dominate
the number of normal particles protons, neutrons,
and electrons by a factor or several. So it's
a ubiquitous particle. It has to interact
very weakly with electromagnetic radiation,
which puts a strong constraint on the type
of particle it might be. In general these
particles are called weakly interacting massive
particles, WIMPS the acronym. We think that
that's what the dark matter is, but not such
particle has yet been detected. Although searches are underway. What king of particle
might this be, even in principle? Physicists
are aware that the standard model of particle
physics is incomplete. One of the favorite
extensions of the standard model of particle
physics is called supersymmetry,where the
fermions—particles with half integer spin, and
bosons—particles with integer spin, which have
quite different properties in the laboratory,
are actually unified—in a sense—by range of
shadow particles for each of them. This of
course doubles the number of particles, cause each fermion has a super symmetric twin, and each
boson has a super symmetric twin. For these
to be unobservable, currently, these would
have to be high mass particles only available
at very high energies—perhaps beyond our
current accelerators. So this is speculative
theorising that unifies physics in an
interesting way, but produces effects that
are difficult to observe in the laboratory.
It turns out that high energy physicists, such as at the Large Hadron Collider at CERN, are actively
looking for the lightest supersymmetric particles. And the lightest of them, or the least massive of them,
could indeed have the properties such as to
be dark matter. This is an exciting convergence
between a desire of physics to explain physics
beyond the standard model, and astronomers
to explain one of their biggest enigmas. Unfortunately, supersymmetry is not one theory, it's a suite
of theories and with very little experimental
guidance, it's been hard to discriminate between
these theories. The slightly disappointing news from CERN in the last year or so, is that hints
of supersymmetry have not been seen in the LHC running at it's highest energies. And this has
already ruled out some of the simplistic
and most attractive supersymmetry theories.
So although supersymmetry is desired by physicists, and would provide a neat potential explanation
of dark matter for astronomers, there is no
verification of this theory at present. Dark
matter remains an enigma. High energy physics is trying to find supersymmetric particles
by high energy collisions. But there are other
ways to find dark matter. So there's another
set of lab experiments designed specifically
to detect a weakly interacting massive particle
or WIMP. Most of these are situated deep down mine shafts because the major contaminating
noise, for any search for dark matter particles is interactions of cosmic rays with the detector. The detectors,
in these cases, are usually large ultra pure
lumps of solid state material, such as a silicon
or germanium. A series of these experiments is underway, where the shielded detector, shielded
from cosmic waves, and other interfering particles is simply observed carefully for a long period
of time for the very occasional interaction
of the detector particles with the passing
dark matter particle. The weak interaction
strength of the dark matter particle means
that a large massive detector—ultra-pure—has
to be observed essentially for years to find
sufficient interactions. Current experiments
are in a regime where in 3-5 years they should
detect dark matter particles, if they are indeed
weakly interactive massive particles, the
kind predicted by supersymmetry. At the moment it's tantalizing. No detection's have been
made, and within a couple of years if the
detectors still find nothing, then we'll be
back to square one. Most of the supersymmetric theories or plausible explanations of dark
matter for particle physics would have been
ruled out. The evidence for dark matter is
convincing. Based initially on rotation curves of spiral galaxies, it's been found in essentially
every spiral and elliptical galaxy inspected
even down to dwarf galaxies. Additional evidence
comes from gravitational lensing, which shows that dark matter not only exists in halos
around galaxies, but it permeates intergalactic
space. As for what dark matter is—so far,
we've eliminated everything from black holes
down to small dust grains in space. The only
remaining viable option is a subatomic fundamental particle—as yet unobserved in a physics lab
or with accelerators, and representing an
extension to the standard model of particle
physics. The answer may be in within a couple of years, based on lab and accelerator experiments
as to whether this dark matter particle actually exists.
