Chapter 16 Dark Matter, Dark Energy, and the Fate of the Universe

Normal matter: The kind of matter we are familiar with, which is made of atoms and responds to the electromagnetic force and gravity. 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

Contents of Universe

• Normal matter: ~ 4.9% • Dark matter: ~ 26.8% • Dark energy: ~ 68.3%

1 Rotation curve

A plot of orbital speed versus orbital radius

Solar system’s rotation curve declines because Sun has almost all the mass.

Rotation Curve of the Solar System

The rotation curve of the Milky Way stays flat with distance.

Mass must be more spread out than in the solar system.

Rotation Curve of a Spiral Galaxy

The mass in the Milky Way is spread out over a larger region than the stars.

Most of the Milky Way’s mass seems to be dark matter!

Encircled Mass as a Function of Distance for a Spiral Galaxy

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

We can measure orbital velocities in other spiral galaxies using the Doppler shift of the 21-cm line of atomic H.

3 We can measure the velocities of galaxies in a cluster from their Doppler shifts.

The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!

Clusters contain large amounts of X ray–emitting hot gas.

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

85% dark matter 13% hot gas 2% stars

4 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?

5 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.

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?

6 How dark is it?

How dark is it?

… not as bright as a star.

Two Basic Options

• Ordinary Matter (MACHOs) — Massive Compact Halo Objects: dead or failed stars in halos of galaxies

• Exotic Particles (WIMPs) — Weakly Interacting Massive Particles: mysterious neutrino-like particles

7 Compact starlike objects occasionally make other stars appear brighter through lensing…

Compact starlike objects occasionally make other stars appear brighter through lensing…

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

Why WIMPs?

• There’s not enough ordinary matter.

• WIMPs could be left over from the Big Bang.

• Models involving WIMPs explain how galaxy formation works.

8 Will the universe continue expanding forever?

Fate of universe depends on the amount of dark matter.

Lots of Critical Not enough dark density of dark matter matter matter

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

Not enough dark matter

9 But expansion appears to be speeding up!

Dark energy? Not enough dark matter

old older oldest Estimated age depends on both dark matter and dark energy.

The brightness of distant white dwarf supernovae tells us how much the universe has expanded since they exploded.

10 An accelerating universe is the best fit to supernova data.

Chapter 17 The Beginning of Time

The universe must have been much hotter and denser early in time.

Estimating the Age of the Universe

11 The early universe must have been extremely hot and dense.

Photons converted into particle–antiparticle pairs and vice versa. E = mc 2

The early universe was full of particles and radiation because of its high temperature.

Defining Eras of the Universe

• The earliest eras are defined by the kinds of forces present in the universe.

• Later eras are defined by the kinds of particles present in the universe.

12 Four known forces in universe:

Strong Force

Electromagnetism

Weak Force

Gravity

13 Do forces unify at high temperatures? Four known forces in universe:

Strong Force

Electromagnetism

Weak Force

Gravity

Do forces unify at high temperatures? Four known forces in universe:

Strong Force

Electromagnetism

Weak Force

Gravity

Yes! (Electroweak)

Do forces unify at high temperatures? Four known forces in universe:

Strong Force

Electromagnetism

Weak Force

Gravity

Yes! Maybe (Electroweak) (GUT)

14 Do forces unify at high temperatures? Four known forces in universe:

Strong Force

Electromagnetism

Weak Force

Gravity

Yes! Maybe Who knows? (Electroweak) (GUT) (String Theory)

Planck Era

Time: < 10 -43 s Temp: > 10 32 K

No theory of quantum gravity

All forces may have been unified

15 GUT Era

Time: 10 -43 –10 -38 s Temp: 10 32 –10 29 K

GUT era began when gravity became distinct from other forces.

GUT era ended when strong force became distinct from electroweak force.

This was accompanied by inflation.

Electroweak Era

Time: 10 -38 –10 -10 s Temp: 10 29 –10 15 K

Strong force and electroweak force now separate

Particle Era

Time: 10 -10 –0.001 s Temp: 10 15 –10 12 K

Amounts of matter and antimatter are nearly equal.

(Roughly one extra proton for every 10 9 proton– antiproton pairs!)

16 Era of Nucleosynthesis

Time: 0.001 s–5 min Temp: 10 12 –10 9 K

Began when matter annihilates remaining antimatter at ~ 0.001 s.

Nuclei began to fuse.

Era of Nuclei

Time: 5 min–380,000 yrs Temp: 10 9–3000 K

Helium nuclei formed at age ~3 minutes.

The universe became too cool to blast helium apart.

Radiation could not travel far

The universe was “opaque”

Era of Atoms

Time: 380,000 years– 1Gy Temp: 3000–20 K

Atoms formed at age ~380,000 years.

The universe became “transparent”.

Cosmic background radiation is released.

17 Era of Galaxies

Time: ~1Gy–present Temp: 20–3 K

The first stars and galaxies formed by ~1 billion years after the Big Bang.

Primary Evidence for the Big Bang

1. We have detected the leftover radiation from the Big Bang.

2. The Big Bang theory correctly predicts the abundance of helium and other light elements in the universe.

The cosmic microwave background — the radiation left over from the Big Bang— was detected by Penzias and Wilson in 1965.

18 Background radiation from the Big Bang has been freely streaming across the universe since atoms formed at temperature ~3000 K: visible/IR.

Background has perfect thermal radiation spectrum at temperature 2.73 K.

Expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves.

Protons and neutrons combined to make long-lasting helium nuclei when the universe was ~5 minutes old.

19 Big Bang theory prediction: 75% H, 25% He (by mass) Matches observations of nearly primordial gases