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Dark Matter
Mike Brotherton
Professor of Astronomy, University of Wyoming
Author of Star Dragon and Spider Star
The Father of Dark Matter
• In 1933, Fritz Zwicky checked out the Coma
Cluster. The galaxies were flying around too
fast (as measured by the Doppler effect) for
their visible mass to keep them together, so
he proposed dark matter was present.
The Mother of Dark Matter
• A few decades later, Vera Rubin
started to notice FLAT rotation
curves in spiral galaxies.
Flat Rotation Curves – so what?
•
Following Rieke, images from Bennett and Pryke
The Nature of Dark Matter
Can dark matter be composed of normal matter?
If so, then its mass would
mostly come from protons
and neutrons = baryons
The density of baryons right
after the big bang leaves a
unique imprint in the
abundances of deuterium
and lithium.
Density of baryonic matter is
only ~ 4 % of critical density,
total is 30%.
Most dark matter must be non-baryonic!
WIMPs???
• “WIMP” = Weakly interactive massive
particle
– Neutrinos?
• Seem to have mass, but too small.
– Axions?
• From Wikipedia, “The axion is a hypothetical
elementary particle postulated by Peccei-Quinn
theory in 1977 to resolve the strong-CP problem in
quantum chromodynamics (QCD).”
• As yet, not detected (axions are predicted to
change to and from photons in the presence of
strong magnetic fields, and this property is used for
creating experiments to detect axions)
MaCHOs
• “MaCHO” = Massive Compact Halo Object
– Just trying to explain flat rotation curves with
things like black holes, brown dwarfs, etc.
– These are “Baryonic,” made from
conventional stuff on the periodic table (like
people, planets, etc.)
– Can be probed via gravitational microlensing
Baryonic Dark Matter
Nature of baryonic dark matter still
very uncertain and speculative.
Brightness of background star
One component:
Massive Compact
Halo Objects =
“MACHOs”:
Small compact
objects (e.g., brown
dwarfs, small black
holes) acting as
gravitational lenses.
Time
Earth
MACHO
Distant background star
Are we sure Dark Matter is real?
• Astronomers have argued that dark matter
can explain the strange motions of
galaxies in clusters and stars in galaxies,
but that it can’t be normal stuff
• Could we just have gotten gravity wrong
on large scales?
– “MoND” = Modified Newtonian Dynamics
• Viable alternative, until 2006…
– Need to look more into the phenomenon of
gravitational lensing on larger scales
Probing Dark Matter with Distant
Quasars:
Gravitational Lensing
Light from a distant quasar is bent
around a foreground galaxy
 two images of the same quasar!
Light from a quasar behind a galaxy
cluster is bent by the mass in the cluster.
Use to probe the distribution of
matter in the cluster.
Gravitational Lensing of
Quasars
Gravitational Lensing
The huge mass of gas in a cluster of galaxies can
bend the light from a more distant galaxy.
Image of the galaxy is strongly distorted into arcs.
Hot Gas in Clusters of Galaxies
Space between galaxies is not empty, but
filled with hot gas (observable in X rays)
That this gas remains gravitationally bound,
provides further evidence for dark matter.
Visible light
Coma Cluster of Galaxies (From Horizons by Seeds)
X rays
The Bullet Cluster
• Given what we know about gravitational lensing
(tracing the total mass in blue), hot X-ray gas in
(the dominant baryonic mass, red), we can show
that dark matter exists in at least one system:
Images from
Clowe et al.
2006 and the
Chandra
press release
The Bullet Cluster
• Lensing of background
galaxies seen in the optical
images lets the mass
distribution be mapped.
• The X-rays trace the hot gas,
the dominant source of
baryons in this cluster merger.
• They don’t line up! Why?
Dark Matter seems to not
interact with itself the way
diffuse gas does during a
cluster collision.
The Bullet Cluster
Summary
• That Zwicky Bastard was right!
• Dark Matter does indeed seem to be real,
thank you Bullet Cluster.
• The majority of matter, dark or otherwise,
is “non-baryonic” exotic stuff, and we don’t
know for sure what it is. It’s likely flying
through this room right this instant in huge
amounts.
• WIMPs, not MaCHOs.