Chapter 20Dark Matter, Dark Energy, and

the Fate of the Universe

Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence

Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

Unseen Influences

• “Normal” Matter: ~ 4.4%– Normal Matter inside stars: ~ 0.6% – Normal Matter outside stars: ~ 3.8%

• Dark Matter: ~ 25%• Dark Energy ~ 71%

Contents of Universe

What is the evidence for dark matter in galaxies?

Newton’s Description of Gravity Yields

Where m is the mass contained inside of the radius r

Orbital velocity curve if most of the mass was at the center of the

galaxy.

Radius

Orb

ital

vel

ocit

y

𝑣=√𝐺𝑚𝑟

The Mass of the Milky WayIf all mass was concentrated in the center, Rotation curve would follow a modified version of Kepler’s 3rd law.

Rotation Curve = orbital velocity as function of radius.

The Mass of the Milky Way (II)

Total mass in the disk of the Milky Way:

Approx. 200 billion solar masses

Additional mass in an extended halo:

Total: Approx. 1 trillion solar masses

Most of the mass is not emitting any radiation:

dark matter!

Mass within Sun’s orbit:

1.0 x 1011 MSun

Total mass:

~1012 MSun

The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H

Astronomers calculate a mass to light ratio.

Galaxy luminosity is measured to estimate the amount of mass in the stars.

Total mass is found from motion. Orbital velocities of the gas clouds and stars. If total mass is larger than that attributed to stars difference must be due to dark matter.

Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter :10x as much dark matter as normal or 90% of mass is dark

Remember the motion of stars in elliptical galaxies is more random than in spirals so you cannot get an orbital velocity curve. There is also very little H2 in elliptical galaxies.

Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting. Different parts of the galaxy have similar spectral broadening.

These galaxies also have dark matter

Slowest

Fastest

What is the evidence for dark matter in clusters of galaxies?

Fritz Zwicky did this in the 1930s

• Treating galaxy clusters as orbiting the common center of mass

• Measures speed of the whole cluster• Found individual speeds of the galaxies

around the center of mass• Calculated the mass of clusters• He found dark matter to be ~50x that of

normal matter

Result was too radical for others to accept.

Now with the spiral galaxy data and other independent methods confirming this result it has been accepted.

Clusters contain large amounts of X-ray emitting hot gas

Temperature of hot gas (particle motions) tells us cluster mass:

85% dark matter 13% hot gas 2% stars

Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

All three methods of measuring cluster mass indicate similar amounts of dark matter

Does dark matter really exist?

Our Options

1. Dark matter really exists, and we are observing the effects of its gravitational attraction

2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

Because gravity is so well tested, most astronomers prefer option #1

What might dark matter be made of?

Baryons: protons and neutrons

Normal matter is sometimes called baryonic matter

Nonbaryonic matter is the other stuff. What ever it may be.

A bit of terminology

• Ordinary Dark Matter (MACHOS)– Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

• Extraordinary Dark Matter (WIMPS)– Weakly Interacting Massive Particles:

mysterious neutrino-like particles

Two Basic Options

The Best Bet

MACHOs occasionally make other stars appear brighter through lensing

MACHOs occasionally make other stars appear brighter through lensing

… but not enough lensing events to explain all the dark matter

• There’s not enough ordinary matter

• WIMPs could be left over from Big Bang

• Models involving WIMPs explain how galaxy formation works

Why Believe in WIMPs?

What is the role of dark matter in galaxy formation?

Gravity of dark matter is what caused protogalactic clouds to contract early in time

WIMPs can’t contract to center because they don’t radiate away their orbital energy

Dark matter is still pulling things together

After correcting for Hubble’s Law, we can see that galaxies are flowing toward the densest regions of space

What are the largest structures in the universe?

Maps of galaxy positions reveal extremely large structures nearby: superclusters and voids; and a near uniform distribution on the larger scale.

Models show that gravity of dark matter pulls mass into denser regions – universe grows lumpier with time

Time in billions of years

0.5 2.2 5.9 8.6 13.7

Size of expanding box in millions of lt-yrs

13 35 70 93 140

Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs

Will the universe continue expanding forever?

Two Options• The density of the universe is large enough

that gravity will cause the universe to collapse back on itself.

• The universe will expand forever.

• Normal matter only contributes about 0.5% of the matter density needed to cause the universe to collapse

Critical density of matter

Not enough dark matter

Fate of universe depends on the amount of dark matter

Lots of dark matter

Amount of dark matter is ~25% of the critical density suggesting fate is eternal expansion

Not enough dark matter

But expansion appears to be speeding up!

Not enough dark matter

Dark Energy?

Brightness of distant white-dwarf supernovae tells us how much universe has expanded since they exploded

An accelerating universe best fits the supernova data, and puts the age of the universe just under 14 billion years old.