Announcements Final exam is Monday, May 9, at 7:30 am. Students with last names A-K go to 225 CB. Students with last names L-Z go to 300 CB. All students must bring their ID. 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 Problems with the Big Bang The solutions to the problems The horizon problem The flatness problem The solutions to the problems

Review Questions 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? How can fluctuations in the cosmic microwave background be used to determine the geometry of the Universe? What are the major components and their fraction of the total mass/energy in the Universe?

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?

Temperature variations in the Cosmic Microwave Background (CMB) are observed to be about 0.0003 K. The expected physical size of the hot/cold regions can be calculated.

Contents of the Universe

Whether the Universe will continue to expand forever or eventually collapse is determined by a competition between Density and the rate of Hubble expansion Gravity and redshift Density and redshift 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.

The Horizon

Horizon Problem

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

Inflation makes the Universe flat

Inflation Whole observable universe came from a tiny region. Size [cm] Time [seconds] Whole observable universe came from a tiny region.

Inflation and cosmology The Universe is very uniformity and very close to flat. A new theoretical idea, inflation, gives an explanation for the uniformity and flatness of the Universe. Inflation still needs to be tested. NASA plans to fly a satellite to measure the polarization of the cosmic microwave background in order to test inflation.

Review questions Why is it surprising that the microwave background has almost exactly the same temperature in all directions on the sky? Why is it surprising that the geometry of the universe is so close to flat? What is the best explanation to date of why the Universe is uniform and flat?