# ASTR100 (Spring 2008) Introduction to Astronomy The Fate of the Universe Prof. D.C. Richardson Sections 0101-0106.

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ASTR100 (Spring 2008) Introduction to Astronomy The 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. old older 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 desire I hold with those who favour fire. But if it had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And 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. 10 12 yr: all matter consists of dead stars or cold lumps (planets, meteorites, used spaceships, etc.). 10 27 yr: Extremely rare events become important. Stellar collisions  galaxy evaporation and growth of central black hole. 10 31 yr: gravitational radiation  galactic black holes merge to form 10 15 M Sun black holes.

Universe 10 40 yr: protons decay—all atomic matter disintegrates into radiation and subatomic particles (electrons, neutrinos, etc.). 10 67 yr: stellar-mass BH’s evaporate. 10 100 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 Astronomy The 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.

Photons converted into particle-antiparticle pairs and vice-versa: E = mc 2

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 sec Temp: > 10 32 K No theory of quantum gravity. All forces may have been unified.

GUT Era Time: 10 -43 – 10 -38 sec Temp: 10 32 – 10 29 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 sec Temp: 10 29 – 10 15 K Era ended when all four forces became distinct.

Particle Era Time: 10 -10 – 0.001 sec 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!)

Era of Nucleosynthesis Time: 0.001 sec–5 min Temp: 10 12 – 10 9 K Began when matter annihilates remaining antimatter at ~ 0.001 sec. Nuclei began to fuse.

Era of Nuclei Time:5 min–380,000 yr Temp: 10 9 – 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 years Temp: 3,000 – 20 K Atoms formed at age ~380,000 years. Background radiation is released.

Era of Galaxies Time: ~1 billion years – present Temp: 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.

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