1. Announcements Pick up graded tests Homework 12 due today Pick up Homework 13 Second project is due a week from Friday If you’re following our syllabus, you’ll notice that I’ve interchanged today’s topic and Wednesday’s.

2. The Expanding Universe 20 November 2006

3. Today : Doppler shifts of galaxies The age of the (known) universe The cosmological distance ladder Some recent cosmological results

4. Doppler Shifts of Galaxies 1914: V.M. Slipher (Lowell Observatory) measures Doppler shifts of 15 “spiral nebulae”. Typical speeds are a few hundred km/s. 11 of the 15 “nebulae” are moving away from us. 1923: Edwin Hubble demonstrates that spiral nebulae are “island universes”, that is, galaxies like our own. 1929: Hubble announces that the more distant galaxies are moving away from us faster.

5. Doppler Shifts of Galaxies Hubble, 1929

6. The Expanding Universe Except for a few nearby galaxies (like Andromeda), ALL the galaxies are moving away from us. Generally, the recession speed of a galaxy is proportional to its distance from us; that is, a galaxy that’s twice as far away is moving twice as fast (aside from local motions within galaxy clusters). This implies that on large scales, ALL the galaxies are moving away from each other (not just away from us).

7. The Andromeda Galaxy

8. Virgo Cluster

9. History of the Universe In the future, the galaxies will be farther apart. Will this expansion continue forever? Or will gravity eventually stop the expansion? In the past, the galaxies must have been closer together. Was there a time when the universe was so dense that there were no galaxies, or stars, or even atoms? The answer to the second question is YES (evidence coming next week). For now, let’s just calculate how long ago this time would have been: the “age of the known universe”!

10. Calculating the Age of the Universe A galaxy moving away at 30,000 km/s (10% of light speed) is currently about 1.3 billion light-years away. If it were moving AT the speed of light, it would have taken 1.3 billion years to get that far. Moving only 1/10 as fast, it would have taken 10x as long: 13 billion years. A galaxy moving twice as fast is also twice as far, so the time comes out the same. ALL the galaxies were in about the same place, roughly 13 billion years ago. “Age of the (known) universe” = 13 - 14 billion years

11. Calculating the Age of the Universe Sources of Uncertainty Galaxies also have “peculiar velocities,” orbiting around in clusters and pulled toward their neighbors. This effect averages out over many galaxies, large distances. Distances are hard to measure! To measure such huge distances we must “climb the cosmological distance ladder,” on which each rung inherits the uncertainties from lower rungs. The rate of expansion could be changing! You’d think that gravitational attraction would slow the galaxies down over time. Conceivably, there could be some other effect that causes them to speed up. In the last five years, astronomers have measured this!

12. The Cosmological Distance Ladder Secondary “standard candles”: Type-I supernovae, or entire galaxies. Primary “standard candles”: Cepheid variable stars, or other star types that can be identified somehow and have known luminosity. With Hubble Space Telescope, Cepheids can be identified as far away as the Virgo Cluster (60 million light-years). Parallax: Using earth’s orbit as a baseline, measure distances to stars by triangulation. With Hipparcos satellite, this is accurate to about 1% at 50 light-years, 10% at 500 light-years. Radar: Distances in inner solar system are measured by bouncing radar signals off planets or asteroids, and measuring the time until the echo is received. Very accurate!

13. Hubble Space Telescope can detect Cepheids out to 60 million light-years (Virgo Cluster)

14. Beyond 50 million light-years, the best standard candles are type-I supernovas

15. We can use redshifts to map the universe… You are here!

16. Recent result from latest supernova data: The expansion is accelerating!!! Velocity --> Very distant galaxies are moving slower than expected according to Hubble’s law. We see these galaxies as they were, billions of years ago. Conclusion: The rate of expansion was slower in the distant past. Alex Filippencko, U.C. Berkeley