ASTR100 (Spring 2008) Introduction to AstronomyThe Fate of the Universe

Prof. D.C. Richardson

Sections 0101-0106

Will the universe continue expanding forever?

Does the universe have enough kinetic energy to escape its own gravitational pull?

Fate of universe depends on the amount of dark matter.

Critical density of matter

Not enough dark matter

Lots of dark matter

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

Not enough dark matter

But expansion appears to be speeding up!

Dark Energy?

Not enough dark matter

Estimated age depends on both dark matter and energy.

oldolder

oldest

Thought Question

Suppose that the universe has more dark matter than we think there is today — how would that change the age we estimate from the expansion rate ?

A.Estimated age would be older.B.Estimated age would be the same.C.Estimated age would be younger.

Thought Question

Suppose that the universe has more dark matter than we think there is today — how would that change the age we estimate from the expansion rate ?

A.Estimated age would be older.B.Estimated age would be the same.C.Estimated age would be younger.

Is the expansion of the universe accelerating?

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

Accelerating universe is best fit to supernova data.

What is the fate of the Universe?

Fire & Ice

Some say the world will end in fire,Some say in ice.From what I’ve tasted of desireI hold with those who favour fire.But if it had to perish twice,I think I know enough of hateTo say that for destruction iceIs also greatAnd would suffice.

—Robert Frost

Solar System

Few Myr: Moon moves away from us. Day becomes longer.

Few Gyr: Sun ascends giant branch.

5 Gyr: inner solar system consumed. Sun becomes white dwarf.

10’s of Gyr: Passing star dislodges remaining planets.

Galaxy

100 Myr: LMC & SMC merge with MW. 3 Gyr: MW & Andromeda collide.

Together may become elliptical galaxy.

10’s of Gyr: MW runs out of gas… Star formation ceases. Most material locked up in WD, NS, BH.

Galaxy becomes fainter & redder.

Universe

Expands forever. 1012 yr: all matter consists of dead stars or cold lumps (planets, meteorites, used spaceships, etc.).

1027 yr: Extremely rare events become important. Stellar collisions galaxy evaporation and growth of central black hole.

1031 yr: gravitational radiation galactic black holes merge to form 1015 MSun black holes.

Universe

1040 yr: protons decay—all atomic matter disintegrates into radiation and subatomic particles (electrons, neutrinos, etc.).

1067 yr: stellar-mass BH’s evaporate.

10100 yr: supermassive BH’s evaporate.

Universe ends as just photons and subatomic particles separated by enormous distances—this is the end of time…

(not with a bang but a whimper).

ASTR100 (Spring 2008) Introduction to AstronomyThe Big Bang

Prof. D.C. Richardson

Sections 0101-0106

What were conditions like in the early universe?

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

The early universe must have been extremely hot and dense.

Early universe was full of elementary particles and radiation because of its high temperature.

Early universe was full of elementary particles and radiation because of its high temperature.

Photons converted into particle-antiparticle pairs and vice-versa:

E = mc2

What is the history of the universe according to the Big

Bang theory?

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.

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

Thought Question

Which of the four forces keeps you from sinking to the center of the Earth?

A.Gravity.B.Electromagnetism.C.Strong Force.D.Weak Force.

Thought Question

Which of the four forces keeps you from sinking to the center of the Earth?

A.Gravity.B.Electromagnetism.C.Strong Force.D.Weak Force.

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

Do forces unify at high temperatures?

Yes!(Electroweak)

Maybe (GUT)

Who knows?(String Theory)

Planck Era

Time: < 10-43 secTemp: > 1032 K

No theory of quantum gravity.

All forces may have been unified.

GUT Era

Time: 10-43 – 10-38 secTemp: 1032 – 1029 K

Era began when gravity became distinct from other forces.

Era may have ended with sudden burst of inflation (more later).

Electroweak Era

Time: 10-10 – 10-10 secTemp: 1029 – 1015 K

Era ended when all four forces became distinct.

Particle Era

Time: 10-10 – 0.001 secTemp: 1015 – 1012 K

Amounts of matter and antimatter are nearly equal.

(Roughly one extra proton for every 109 proton–antiproton pairs!)

Era of Nucleosynthesis

Time: 0.001 sec–5 minTemp: 1012 – 109 K

Began when matter annihilates remaining antimatter at ~ 0.001 sec.

Nuclei began to fuse.

Era of Nuclei

Time:5 min–380,000 yrTemp: 109 – 3,000 K

Helium nuclei formed at age ~3 minutes.

The universe became too cool to blast helium apart.

Era of Atoms

Time: 380,000 years – 1 billion yearsTemp: 3,000 – 20 K

Atoms formed at age ~380,000 years.

Background radiation is released.

Era of Galaxies

Time: ~1 billion years – presentTemp: 20 – 3 K

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

Primary Evidence

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.

How do we observe the radiation left over from the

Big Bang?

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

Background radiation from the Big Bang has been freely streaming across the universe since atoms formed at temperature ~3,000 K: visible/IR.Creation of the Cosmic Microwave Background

Expansion of universe has redshifted thermal radiation from that time to ~1,000 times longer wavelength: microwaves.

Background has perfect thermal radiation spectrum at temperature 2.73 K

Full sky in all wavelengths

WMAP gives us detailed baby pictures of structure in the universe.

How do the abundances of elements support the Big Bang

theory?

Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old.

Big Bang theory prediction: 75% H, 25% He (by mass).

Matches observations of nearly primordial gases.

Abundances of other light elements agree with Big Bang model having 4.4% normal matter—more evidence for WIMPS!

Thought Question

Which of these abundance patterns is an unrealistic chemical composition for a star?

A.70% H, 28% He, 2% other.B.95% H, 5% He, less than 0.02%

other.C.75% H, 25% He, less than 0.02%

other.D.72% H, 27% He, 1% other.

Thought Question

Which of these abundance patterns is an unrealistic chemical composition for a star?

A.70% H, 28% He, 2% other.B.95% H, 5% He, less than 0.02%

other.C.75% H, 25% He, less than 0.02%

other.D.72% H, 27% He, 1% other.