Presentation on theme: "Ch1 0 The Big Bang Where do we come from, where are we going?"— Presentation transcript:
Ch1 0 The Big Bang Where do we come from, where are we going?
Ch10 Goals Where did the Universe come from? Where is it going? How can we see the past? How can we learn about the future from seeing the past?
Ch10 Hubble’s Law Galaxies are moving away from us. The farther away the faster they go. V = H o x D
Ch10 Mapping the Universe V = H o x D V/H o = D If you know V and H o then you get D. Map the Universe!
Ch10 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.
Ch10 The Big Bang Big Bang: the event from which the Universe began expanding. Into what did the Universe expand? Where was the Big Bang? Where is the center of the Universe?
Ch10 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 “An expanding universe does not preclude a creator, but it does place limits on when he might have carried out his job.” -Steven Hawking, A Brief History of Time
Ch10 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 helped solve the problem: Cepheids.
Ch10 Standard Candles “Standard Candle” = A class of object where every member has the same luminosity. For Cepheid variables: –Pulsation period is proportional to average luminosity –Observe the period find the luminosity
Ch10 Luminosity and Distance Brightness goes as 1/D 2. Move light: –2x farther away, one quarter as bright. –3x farther, one ninth as bright –10x farther, one hundredth as bright. If you know: –How bright it looks –How bright it SHOULD be –You know how far away it must be. Standard candles yield distances!
Ch10 Hubble Space Telescope Use HST to find Cepheids in other galaxies.
Ch10 Hubble’s Constant In recent decades, H o = 50 – 100 km/s/Mpc. Difference in distances by factor of 2. Difference in age of Universe by factor of 2. HST Cepheid result: H o = 65 km/s/Mpc Recall: T = 1/ H o T = 15 billion years Is this older than the oldest stars? Recent HST result says the oldest white dwarfs are 13 billion years old!
Ch10 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
Ch10 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.
Ch10 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!
Ch10 The Future from the Past Is the Universe: –Slowing down? –Speeding up? –Staying the same velocity? In the past, was the Universe: –Going faster? –Going slower? –Going the same velocity? The Universe is a time machine.
Ch10 Lookback Time We see everything as it once was. Young Old
Ch10 60s 50s Baby Boomer Universe 40s Farther away we look, further back in time we see! 90s 80s 70s
Ch10 What We See
Ch10 V Distance (Lookback time) PresentPast Are We Slowing Down? In our experience, things slow down over time. Is the Universe slowing down at all? Plot distance versus velocity. Use supernovae as “standard candles.” –Distant supernovae (large lookback time). Accelerating Slowing V=H o D
Ch10 Are We Slowing Down? Unseen mass making stars move fast: Dark Matter Unseen energy accelerating galaxies: Dark Energy Slowing Accelerating