Announcements

• Observing next week will count for the fourth exam.

• The final exam will be cumulative.

• The final will be 40 questions, 10-15 will be on cosmology, the remainder on material covered earlier in the semester.

Cosmology

• Why is the night sky dark?

• Problems with the Big Bang

Review Questions

• Give three pieces of evidence for the Big Bang model.

• What will happen to the Universe if the density is less than the critical density?

• A few seconds after the big bang did hydrogen atoms exists? Why or why not?

Three pieces of evidence for the Big Bang model

• Hubble expansion: galaxies are moving away from us with speed proportional to distance.

• The ratio of Helium to Hydrogen in gas clouds unaffected by stars.

• The cosmic microwave background: a 2.7 K glow seen in all directions.

What will happen to the Universe if the density is less than the critical density?

C0

A few seconds after the big

bang did hydrogen

atoms exists?

Olber’s Paradox

• Why is the night sky dark?

Assume:

• Universe is infinite

• Universe has always existed

• Universe has a flat geometry

• Universe is homogeneous

Olber’s Paradox

Which is the solution to Olber’s Paradox

• Stars have finite size and hide behind each other.

• Dust blocks light from stars

• The Universe has a finite age

Whether the Universe will continue to expand forever or eventually collapse is determined by a competition between

1. Density and the rate of Hubble expansion

2. Gravity and redshift

3. Density and redshift

4. Iowa and Michigan

Problems with the Big Bang

• The horizon problem

• The flatness problem

Cosmic Microwave Background

The Universe glows at 2.7 K in every direction.

The temperature is the same to < 0.1%.

Observable Universe

We can only see the parts of the Universe from which light has had time to travel to us.

Horizon Problem

Flatness problem

Any tiny deviation from the critical density is amplified over time.

Inflation solves flatness problem

Inflation solves horizon problemSize[cm]

Time [seconds]

Whole observable universe came from a tiny region.

Problems with Big Bang

• Problems point to an incomplete model, not an incorrect one.

• Problems can be solved by a new theoretical idea, inflation, but this new idea still needs to be tested.

Review questions

• Why is it surprising that the microwave background has almost exactly the same temperature in all directions on the sky?

• Why is the night sky dark?