10B The Big Bang Where do we come from, where are we going?

10B Goals What’s the cosmological principle? Where did things begin? Where will things end? What’s the temperature of space?

10B Hubble’s Law Recall: All galaxies are moving away from us. The farther away the faster they go. V = H o x D

10B You Are Here Large Scale Structure Farther away we look, further back in time we see!

10B Cosmological Principle Isotropy – The view from here is the same in all directions. Homogeneity – We live in an average place the same as any other. This is the cosmological principle. Implies: –Universe has no edge! –Universe has no center!

10B Olber’s Paradox Why is it dark at night? In an infinite and unchanging universe: Every line of sight should end at the surface of a star. Universe is neither.

10B Expanding Universe If galaxies are all moving away, then at some point they were all much closer. Hubble’s Law implies the Universe is expanding.

10B Age of the Universe Since all galaxies are moving away from us, how long has it been since all galaxies were together? time = distance / velocity velocity = H o x distance time = distance / (H o x distance) time = 1/H o Measured H o ~ 65 km/s/Mpc T ~ 15 billion years

10B Age Disagreements Until recently, much disagreement on the value of H o and therefore, the age of the Universe. Need to know the distance to some galaxies in order to know the slope of velocity versus distance. Different methods yielded different distances. Some values of H o yielded an Universe younger than some of its stars. HST has helped solve the problem: Cepheids.

10B The Big Bang Big Bang: the explosion out of which the Universe began expanding. Into what did the Universe expand? Nothing. Where was the Big Bang? Everywhere. Where is the center of the Universe? Nowhere.

10B The End of the Universe Will the universe expand forever? Depends on the density of the Universe. Too big: Big Crunch –Closed Universe –Bound Universe Too small: Big Freeze –Open Universe –Unbound Universe

10B Critical Density Dividing line is the critical density.  o is the ratio of measured density to the critical density. If  o > 1 then closed,  o < 1 then open.

10B Density of the Universe Add up all the mass we see and  o = 0.01 But we know there is some dark matter in galaxies and clusters. How much? Think ~10 x more dark matter than “light” matter. Cosmologists think  o < 0.3 Result: Open Universe  Big Freeze!

10B Are we Slowing down? In our experience, things slow down over time. Is the Universe slowing down at all? Use supernovae as “standard candles.” –Get distance to distant supernovae. Plot distance versus velocity. Slowing Accelerating Dark Energy

10B Observational Clues Instant of the Big Bang: –Very Dense –Very hot Universe one giant blackbody. –Temperatures  gamma rays As space expands, wavelengths of light expand with it  Cosmological Redshift

10B 3 Kelvin Recall: p  1/T Since longer wavelengths mean cooler blackbodies then space should be cooling. Today, it should be ~3 K

10B COBE Satellite measured the intensity of energy from space at a range of frequencies. 2.7 K

10B Isotropy Amazingly uniform temperature in all directions. Deviations < Kelvin 10 x more energy in background than everywhere else.

10B Big Bang Questions 1.The Horizon problem: Two widely separated areas are the same temperature (isotropy). But at the time these regions formed, light did not have time to get from one to the other.

10B Big Bang Questions 2.Flatness Problem: The density of the Universe now is close to critical density. At Universe beginning, if not exactly at critical density, quickly becomes very far from critical density as it evolves. How is Universe so close to, but not equal to, critical density now?