Transcript Document
The Big Bang Where do we come from, where are we going? 16 Goals • • • • What’s the cosmological principle? Where did things begin? Where will things end? What’s the temperature of space? 16 Hubble’s Law • Recall: All galaxies are moving away from us. • The farther away the faster they go. • V = Ho x D 16 Large Scale Structure Farther away we look, further back in time we see! You Are Here 16 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! 16 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. 16 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. 16 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 = Ho x distance time = distance / (Ho x distance) time = 1/Ho • Measured Ho ~ 65 km/s/Mpc T ~ 15 billion years • Recall: Recent HST result says the oldest white dwarfs are 13 billion years old! 16 Age Disagreements • Until recently, much disagreement on the value of Ho 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 Ho yielded an Universe younger than some of its stars. • HST has helped solve the problem: Cepheids. 16 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. 16 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 16 Critical Density • Dividing line is the critical density. Wo is the ratio of measured density to the critical density. • If Wo > 1 then closed, Wo < 1 then open. 16 Density of the Universe • Add up all the mass we see and Wo = 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 Wo < 0.3 • Result: Open Universe Big Freeze! 16 Are we Slowing down? Slowing Accelerating Dark Energy • 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. 16 Observational Clues • Instant of the Big Bang: – Very Dense – Very hot • Universe one giant blackbody. – Temperatures gamma rays • As time goes on the Universe should cool. 16 3 Kelvin • Recall: lp a 1/T • Since longer wavelengths mean cooler blackbodies then space should be cooling. • Today, it should be ~3 K 16 COBE • Satellite measured the intensity of energy from space at a range of frequencies. 2.7 K 16 Isotropy • Amazingly uniform temperature in all directions. • Deviations < 0.0002 Kelvin 10 x more energy in background than everywhere else. 16