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The Big Bang

The Big Bang. Where do we come from, where are we going?. 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?. Hubble’s Law. Recall: All galaxies are moving away from us.

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The Big Bang

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  1. The Big Bang Where do we come from, where are we going?

  2. 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?

  3. Hubble’s Law • Recall: All galaxies are moving away from us. • The farther away the faster they go. • V = Ho x D

  4. 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.

  5. 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?

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

  7. Cosmological Redshift • Another way to think of it: • Galaxies aren’t zooming through a fixed Universe. • Expanding Universe is carrying galaxies along. • Galaxy redshifts: • Aren’t Doppler shifts. • Are cosmological shifts.

  8. 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 “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

  9. 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 helped solve the problem: Cepheids.

  10. Hubble Space Telescope • Use HST to find Cepheids in other galaxies.

  11. Hubble’s Constant • In recent decades, Ho = 50 – 100 km/s/Mpc. • Difference in distances by factor of 2. • Difference in age of Universe by factor of 2. • Recent HST results:Ho = 65 km/s/Mpc Recall: T = 1/ Ho T = 15 billion years • Is this older than the oldest stars? • Recent HST result says the oldest white dwarfs are 13 billion years old!

  12. 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

  13. Dividing line is the critical density. Wo is the ratio of measured density to the critical density. If Wo > 1then closed, Wo < 1 then open. Critical Density

  14. 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!

  15. 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.

  16. Lookback Time • We see everything as it once was. Old Young

  17. 90s 80s 70s 60s 50s 40s Baby Boomer Universe Farther away we look, further back in time we see!

  18. What We See

  19. Nearby Galaxies

  20. Hubble Deep Field

  21. V=HoD V Present Past Distance (Lookback time) 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). Slowing Accelerating

  22. Slowing Accelerating Are We Slowing Down? • Unseen mass making stars move fast: Dark Matter • Unseen energy accelerating galaxies: Dark Energy

  23. Homework #18 • For Friday read Bennett Ch23: • Do Ch23: • Problem 4, 8

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