Lecture 43: Dark Matter

Key Ideas:

Dark Matter is matter we cannot see directly

• Detected only by its gravity
• >90% of the Mass of the Universe?

Evidence for Dark Matter:

• Flat Galaxy Rotation Curves
• Galaxy Motions & Hot Gas in Clusters

Baryonic & non-Baryonic Forms

Galaxy Rotation Curves revisited

Spiral Galaxies rotate such that the rotation speed:

• rises with radius from the center in the inner parts of the disk.
• becomes roughly constant (flat) with radius in the outer parts as far as can be measured in the disk.

Mass Distribution in Galaxies

Most of a galaxy's light is in the inner 10 kpc

If stars provided all of its mass we expect:

• rotation curve will rise in the inner parts to a maximum speed
• fall steadily with radius outside ~10 kpc

But the curve stays flat!

• Need more mass at large radii than is observed in the stars alone...

Dark Matter Halos

Q: Where is the extra mass if it is not coming from stars?

A: Galaxies have extended "dark halos"

Properties:

• Contains >90% of the galaxy's mass
• more extended than the starlight component
• orbits of satellite galaxies suggest it may extend out as far as 200 kpc or more!

Fritz Zwicky's "Missing Mass"

1933: Fritz Zwicky measured the peculiar velocities of galaxies in the Coma cluster

• Found peculiar velocities of ~1000 km/sec.
• This is greater than the escape velocity computed if you just add up the light of the galaxies in the cluster.

Zwicky suggested that "missing mass" adds extra gravity to hold the cluster together.

Dark Matter in Clusters

Subsequent observations showed that galaxy clusters are 90-99% dark matter.

More evidence:

• Clusters are filled with hot X-ray emitting intergalactic gas. Without dark matter, this gas would dissipate away.
• Gravitational lensing of background galaxies by clusters requires dark matter in clusters.

Dark Matter

It is "Dark" because it cannot be detected directly using light (of any kind).

Detected only because of the effects of its gravity on its environment:

• It makes the outer parts of galaxies rotate faster than expected from their starlight.
• It makes galaxies in clusters orbit faster than expected from the total starlight of the galaxies.

What is it?

Baryonic Dark Matter?

• Ordinary matter ("baryons") made of protons and neutrons?

Non-Baryonic (Particle) Dark Matter?

• Exotic subatomic particles that only interact with matter via Gravitation (or maybe the Weak Force)?

Some mixture of the two?

Baryonic Dark Matter

Some ideas:

• Brown Dwarfs and Jupiter-sized planets.
• Diffuse gas clouds
• cold stellar remnants (black holes, neutron stars, and white dwarfs)
• Frozen hydrogen snowballs

Problem:

• All are hard to keep from making light.

Microlensing Searches

What if a Brown Dwarf, Jupiter, or dark stellar remnant in the halo passes between Earth and a more distant star?

• GR predicts: the dark object's gravity bends the starlight of the more distant star.
• This makes a "Gravitational Microlens" that magnifies the star momentarily as it passes.

Gravitational Microlensing

• Chance alignment between the dark object and the distant star makes a gravitational lens.
• Motion of both the lens and the distant star makes it sweep over the Earth.
• As the lens sweeps past, we see the distant star brighten suddenly at all wavelengths, then fade to normal brightness.

Microlensing Collaborations

Chance alignments are very rare.

Need to monitor millions of stars for many years.

• A number of large international collaborations are monitoring the LMC and Galactic Bulge.
• So far, they have collectively found ~ a dozen microlensing events.

Non-Baryonic Dark Matter

Fundamental particles which only interact via gravitation or the weak force.

Massive neutrinos:

• Produced in large numbers in the Big Bang
• Moving very fast ("Hot" dark matter)

Exotic new particles:

• Predicted by speculative particle theories.
• Massive "cold" dark matter particles.

Particle Dark Matter Searches

Searches for direct detection of dark matter particles.

These include:

• Attempts to measure the mass of the neutrino
• Particle Collider experiments to create new fundamental massive particles in collisions.
• Searches for "cold dark matter" particles hitting the Earth from space.

A (Very) Radical Suggestion:

Some have suggested that Dark Matter does not exist, but rather our theory of gravity is wrong on large scales!

Problems:

• None of the alternative theories of gravity have survived tests of detailed predictions.
• Hard to reconcile these theories with the observed gravitational lensing.

How much Dark Matter is there?

D and He from Primordial Nucleosynthesis predicts W in baryons alone is

Galaxy halo masses from rotation curves gives

Implications:

• Some of the dark matter is non-Baryonic (??)
• The Universe is Open if all dark matter is only in galaxy halos.