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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 41: Dark Matter & Dark Energy

Readings: Ch 26, sec 26-8; Ch 28, sec 28-6 & 28-7

Key Ideas

Dark Matter
Matter we cannot see directly
Detected only by its gravity
Most of the matter in the Universe?

Dark Energy
Vacuum energy of the Universe
Responsible for the acceleration of the Universe's expansion

Galaxy Rotation Curves Revisited

Spiral Galaxies rotate such that the rotation speed:
Typical Galaxy Rotation Curve
Recall from our discussion of Spiral Galaxies that the total mass inside of a particular radius within a galaxy is approximately
Galaxy Mass-Rotation Formula

Mass Distribution in Galaxies

Most of the stars in a typical spiral galaxy are in the inner 10 kpc or so.

If stars provided all of its mass we would expect the following:

Observed vs. Predicted Keplerian Galaxy Rotation Curve
But, the observations show that the rotation speed stays roughly constant (a "flat rotation curve") at large radii! What is going on?

Dark Matter Halos

What is the source of the extra mass if it is not stars & gas?

Galaxies must have extended dark halos.

Properties of Galaxy Dark Halos:

Dark Matter in Galaxy Clusters

In 1933, Caltech astronomer Fritz Zwicky measured the motions of galaxies in the Coma cluster This led Zwicky to suggest that there must be a component of "dark matter" that adds extra gravity to hold the cluster together.

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

Together with galaxy rotation curves, this tells us that what we see (stars and gas) is not the whole picture.

Dark Matter

We call this mysterious extra matter Dark Matter because it cannot be detected directly using light of any kind.

Dark Matter can be detected astronomically only by its gravitational effects on stars and gas:

But, what is the Dark Matter made of?

Baryonic Dark Matter

Baryons are ordinary matter made of protons & neutrons.


All such forms of baryonic dark matter are collectively called

Microlensing Searches

If a MACHO passes between the Earth & a more distant background star: Such chance alignments are very rare:

A number of projects have undertaken multi-year monitoring of the LMC and SMC hoping to catch lensing by MACHOs in the Galactic Halo:

While interesting, the best limits to date are that MACHOs an only make up ~20% of the mass of the halo of our Galaxy.

Non-Baryonic Dark Matter

These are fundamental particles that only interact via gravitation and the weak nuclear force.

Candidate Dark Matter particles include:

Massive neutrinos:

Exotic new particles:

These are often collectively called Weakly Interactive Massive Particles, or WIMPs for short.

Particle Dark Matter Searches

Attempts to directly detect of dark matter particles.

These include:

So far, no convincing detections have been reported, but the searches go on.

Dark Energy

As we saw in the lecture on Fate of the Universe, galaxy distances measured using Type Ia Supernovae as standard candles suggest that we live in Universe that is

The extra expansion is due to something generically called Dark Energy

What is Dark Energy?

We know even less about Dark Energy than we do about Dark Matter.

Some forms of Dark Energy that are being discussed currently:

Cosmological Constant (L)

Generic Dark Energy

Future studies may (or may not) help resolve the question of the behavior of Dark Energy, which would provide us with clues as to its origins.

The Contents of the Universe

Whatever Dark Matter and Dark Energy are, we can at least for the present estimate their contributions to the total density of matter and energy of the Universe, W0:
Baryonic Matter: Wb
Stars: 0.4%
Gas: 3.6% (most of it very hot gas)
Based on assays of galaxies, clusters, and intergalactic gas.

Dark Matter: Wdm
Dark Matter: 26%
Based on Galaxy Rotation Curves & Galaxy Cluster dynamics

Dark Energy: WL
Vacuum Energy: 70%
Based on acceleration of the Universe from Type Ia Supernova measurements

Total: W0 = Wb + WdmWL
W0 = 1.0
All of these, of course, are subject to change as better data are acquired.

A (Very) Radical Suggestion:

Maybe Dark Matter and Dark Energy don't exist at all!

Is our theory of gravity wrong on large scales?


While still a possiblity that a few astronomers are exploring actively, so far this line of inquiry has been less fruitful than others.

Return to [ Unit 6 Index | Astronomy 162 Main Page ]
Updated: 2006 March 5
Copyright Richard W. Pogge, All Rights Reserved.