Assignments

• Read Sec. 26-28 of DocOnotes-cosmology.pdf

• HW7.pdf, due 5 p.m. Friday May 10• Term project due 5 p.m. Friday May 17• Final exam 10:30-12:30 Friday May 24

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Unification of Forces

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Recombination epoch

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Recombination epoch• Universe immersed in microwave radiation.• Unleashed at the end of the Era of Nuclei.

• 380,000 years after Big Bang, Universe had cooled to 3000 K• Free electrons became bound into atoms of H & He• “Recombination epoch” • without free electrons to scatter photons, Universe became transparent

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

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.

How do the abundances of elements support the Big Bang theory?

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

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

CMB Blackbody Spectrum

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Matter vs. Radiation

transition between radiation-dominated &matter-dominated

Universe today is matter dominated

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But: number photons/number atomsis constant!

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Clicker Question

Which of these abundance patterns is an unrealistic chemical composition for a star?A.70% H, 28% He, 2% otherB.95% H, 5% He, less than 0.02% otherC.75% H, 25% He, less than 0.02% otherD.72% H, 27% He, 1% other

Clicker Question

Which of these abundance patterns is an unrealistic chemical composition for a star?A.70% H, 28% He, 2% otherB.95% H, 5% He, less than 0.02% otherC.75% H, 25% He, less than 0.02% otherD.72% H, 27% He, 1% other

Planck CMB

CMB angular structure spectrum

Big Bang Nucleosynthesis

The Creation of Matter• The early Universe was filled with radiation & subatomic particles.• We’ve seen matter converted to energy…

• but Einstein’s famous equation is a two-way street!

E = m c2

If T > 1012 Kp+

p-

n

n

e-

e+

matter

antimatter

Challenges for the Big Bang Model• Above is evidence in support of the Big Bang theory. • But prior to 1980, cosmologists had identified three major

questions that the basic theory was unable to answer: 1. Where does structure come from? 2. Why is the large-scale Universe so smooth? 3. Why is the density of matter almost critical?

• In 1981, physicist Alan Guth realized that the Grand Unified Theories could hold the answers to these questions.

• When the strong force froze out of the GUT force… • it released enough energy to expand the Universe by

1030 times in less than 10–36 sec!! • We call this dramatic expansion inflation

Mysteries Needing Explanation

1. Where does structure come from?

2. Why is the overall distribution of matter so uniform?

3. Why is the density of the universe so close to the critical density?

An early episode of rapid inflation can solve all three mysteries!

Inflation can make structure by stretching tiny quantum ripples to enormous sizes.

These ripples in density then become the seeds for all structure in the universe.

How can microwave temperature be nearly identical on opposite sides of the sky?

Regions now on opposite sides of the sky were close together before inflation pushed them far apart.

The overall geometry of the universe is closely related to total density of matter and energy.

Density = Critical

Density > Critical

Density < Critical

The inflation of the universe flattens the overall geometry like the inflation of a balloon, causing overall density of matter plus energy to be very close to critical density.

How can we test the idea of inflation?

Patterns of structure observed by WMAP show us the “seeds” of the universe.

Observed patterns of structure in the universe agree (so far) with the “seeds” that inflation would produce.

“Seeds” Inferred from CMB• Overall geometry is flat

— Total mass + energy has critical density • Ordinary matter ~5% of total • Total matter is ~30% of total

— Dark matter is ~25% of total — Dark energy is ~70% of total

• Age of 13.7 billion years

In excellent agreement with observations of present-day universe and models involving inflation and WIMPs!

Big Bang Nucleosynthesis