Dark MatterDark MatterMike Brotherton

Professor of Astronomy, University of WyomingAuthor of Star Dragon and Spider Star

The Father of Dark MatterThe 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 MatterThe Mother of Dark Matter• A few decades later, Vera Rubin

started to notice FLAT rotation curves in spiral galaxies.

Flat Rotation Curves – so what?Flat Rotation Curves – so what?

• Following Rieke, images from Bennett and Pryke

The Nature of Dark MatterThe Nature of Dark MatterCan 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???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)

MaCHOsMaCHOs

• “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 MatterBaryonic Dark MatterNature of baryonic dark matter still

very uncertain and speculative.

One component: Massive Compact

Halo Objects = “MACHOs”:

Small compact objects (e.g., brown dwarfs, small black

holes) acting as gravitational lenses.

Earth

Bri

ghtn

ess

of b

ack

gro

un

d st

ar

Time

Distant background starMACHO

Are we sure Dark Matter is real?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

Probing Dark Matter with Distant Quasars:

Gravitational Lensing

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.

Light from a distant quasar is bent around a foreground galaxy

two images of the same quasar!

Gravitational Lensing of Quasars

Gravitational Lensing of Quasars

Gravitational LensingGravitational 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 GalaxiesHot Gas in Clusters of Galaxies

Coma Cluster of Galaxies (From Horizons by Seeds)

Visible light X rays

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.

The Bullet ClusterThe 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 ClusterThe 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 ClusterThe Bullet Cluster

SummarySummary

• 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.