Chapter 16

Dark Matter And The Fate Of The Universe

The Case for Dark Matter

Rotation Curve for the Solar System (This is what we expect to see from the laws

of Gravity applied to all the visible mass)

Rotation Curve for the Milky Way Galaxy (unexpected)

What we expect from only the visible mass

• The matter lying farther than our Sun from the galactic center orbits the galaxy at unexpectedly high speeds.

• Conclusion: Much of our galaxy’s mass lies beyond in the outskirts of the galaxy, yet most of the light comes from the stars close to the galactic center.

Dark Matter distribution relative to luminous matter

Dark Matter in Galaxies• Light from luminous matter can be measured

and attributed to stellar masses, emission nebulae, etc. (matter that is emitting detectable radiation)

• From the Rotation curves of spiral galaxies and measuring the Doppler shifts, a good estimate of the total mass of the galaxy can be obtained.

• Astronomers have found that the luminous mass is only a small fraction of the total mass of the galaxy. ( ~ 10% of the total mass)

Measuring the rotation of a spiral galaxy with 21-cm line of atomic hydrogen.

Blue shifted lines show how fast that

side is rotating toward us

Red shifted lines show how fast that

side is moving away from us

A rotation curve shows the orbital velocities of stars or gas clouds at different distances from a galaxy’s center

Actual Rotation Curves of Four Spiral Galaxies

Dark Matter in Galactic Clusters• Galaxies occur in clusters and superclusters.

• The masses of these clusters can be measured by three methods:

1. Measure the speeds of galaxies orbiting the center of a cluster.

2. Measure and analyze the X-ray emission from hot gas between the galaxies in a cluster.

3. Observing how light is deflected by the cluster’s gravity.

The Coma Cluster of galaxies seen in visible light

A map of X-ray emission from the Coma Cluster, showing the intra-

cluster medium

Intra-Cluster Medium – X-ray observations of hot gas that fills the intergalactic space between galaxies in a cluster

Gravitational Lensing - Hubble Space Telescope picture of a galaxy cluster acting as a gravitational lens.

Yellow elliptical galaxies are cluster members

Blue ovals are multiple images of a single galaxy behind the cluster’s center

How gravitational lensing works

A Cluster’s powerful gravity bends light from background galaxies.

This produces multiple images.

HST- Cluster Abell 2218.The thin elongated galaxies are the images of

background galaxies distorted by the cluster’s gravity

What is Dark Matter?• We do not yet know, however some

suggested possibilities are:

– Massive Compact Halo Objects (MACHOs)• trillions of faint red stars, brown dwarfs, Jupiter

sized objects left over from the galaxies formation and residing in the halo.

– Weakly Interacting Massive Particles (WIMPs)• Hypothetical particles that have yet to be

discovered, which are believed to be similar to neutrinos but considerably more massive.

Structure Formation

• Since 90% of the matter in the universe is now believed to be in the form of “Dark Matter”, it is likely that the overall structure of the universe was/is significantly influenced by it.

The Great Wall

400 Mpc survey H = 65km/s/Mpc

“Peculiar Velocities” of galaxies flowing into superclusters

Slices show galaxies in thin fan-like swaths.

Each dot represents a galaxy

Earth-Milky Way

Six frames from a supercomputer simulation of structure formation for a region the size of our Local Group

Fate of the Fate of the

UniverseUniverse

A finite universe, oscillating A finite universe, oscillating universe, and infinite universeuniverse, and infinite universe

finitefinite Oscillating infinite

As density increases, deceleration As density increases, deceleration increasesincreases

Accelerating Universe?

• The expanding universe probably The expanding universe probably originated in an explosion called the originated in an explosion called the Big BangBig Bang between 12 and 18 billion between 12 and 18 billion years ago.years ago.

• Will the universe end?Will the universe end?– Present observations suggest that it will Present observations suggest that it will

expand forever.expand forever.

• What caused the Big Bang? What caused the Big Bang?

• Where did the energy come from?Where did the energy come from?

• Why did it happen?Why did it happen?

Unknown…Unknown…

End of Section