1. Lecture 14: The Expanding Universe Astronomy 1143 – Spring 2014

2. Key Ideas: Light & Motion -- wavelengths of light can be changed by Motion of source away from /towards observer Expansion or contraction of space Leads to redshifts and blueshifts Hubble’s Law: Galaxies are receding from us. Recession velocity gets larger with distance. Galaxy z’s + Cosmological Principle = Expanding Universe Cosmological Principle: Universe is Homogeneous & Isotropic on Large Scales. No special places or directions. Hubble Parameter :H 0 Present-day rate of expansion of the Universe.

3. Doppler Shift The frequency & wavelength of waves (sound, light, etc) change depending on the motion of the source Examples: sirens, train whistles Used in radar detectors and Doppler radar for weather observations This effect is called the Doppler effect or Doppler shift for light Detect motions toward or away from observer

4. Doppler Shift Shift in the observed wavelength when the source is moving relative to the observer. Amount of the shift and its sign depends on relative speed of the source & observer direction (towards or away) Requires knowing the emitted wavelength Each element has a unique pattern of lines Measure their wavelengths in a stationary laboratory

5. Stationary Source Same Pitch

6. Moving Source

7. Doppler Effect in Sound High Pitch (short waves) Low Pitch (long waves)

8. The Doppler Effect in Light Works same as it does for sound Light moving away from the observer Wavelength gets longer: REDSHIFT Light moving toward the observer Wavelength gets shorter: BLUESHIFT

9. Doppler Effect in Practice Traffic Radar Guns: Bounce microwaves of known frequency off moving cars Measure frequency of reflected microwaves The Doppler Shift gives the car’s speed and direction...

10. . Cosmological Shifts

11. As Space Expands or Contracts: Wavelength of light changes Light Wave Recession Velocity

12. Cosmological Shifts The expansion or contraction of space will cause the wavelengths of light to change Not caused by motion through space, but by the change in space itself This means that galaxies along the same line- of-sight will experience some of the same expansion/contraction of space

13. Discovery of Expansion 1914-22: Vesto Slipher, working at Lowell Observatory Measured radial velocities from spectra of 25 galaxies. Found: 21 of the 25 galaxies show a redshift. speeds of some  2000 km/sec Most of these galaxies appear to be rapidly receding away from us.

14. Hubble’s Discovery 1929: Edwin Hubble measured the distances to 25 galaxies: Used Cepheids in Andromeda & Local Group Used brightest stars in the other galaxies Compared distances and recession velocities Discovered: Recession velocity gets larger with distance. Systematic expansion of the Universe.

15. Increasing Distance

16. Hubble’s Data

17. In Class Demonstration:The Slinky Universe Put your paper clips on ½ of the rubber band in this order Yellow – green – red – gold – white – pink -- blue

18. Systematic Expansion Universe 2x larger Galaxies are 2x further apart

19. Refinement The 1929 work was very suggestive of an effect, but the uncertainties were large. Set about collecting more data: By 1931: Added 8 more galaxies with good distances The most distant galaxy had a recession velocity of nearly 20,000 km/sec! Showed a stronger, tighter trend with distance If we assume that the Milky Way is not special, then the Universe must be expanding

20. The Cosmological Principle “The Universe is Homogeneous and Isotropic on the Largest Scales.” Critical assumption underlying Cosmology. Homogeneous: No special places in the Universe. Isotropic: No special directions Largest Scales: Average out small-scale details

21. 100 Mpc

22. Isotropy When viewed on the largest scales: The Universe looks the same to all observers. The Universe looks the same in all directions as viewed by a particular observer. Does not apply locally: We see different numbers of local objects in different directions.

27. Modern Hubble diagram: Cepheid distances only Freedman et al. 2001

28. Beyond Cepheids To make an accurate measurement of the rate of expansion of the Universe, we need to have distances to galaxies beyond the Virgo clusters “Peculiar” velocities of galaxies can be a large part of their redshifts at close distances Peculiar velocities are actual motions of galaxies through space Larger distances – more space to expand – larger velocities – cleaner signal

29. Freedman et al. Hubble Space Telescope Key Project H 0 = 72  8 km/sec/Mpc

30. v = recession velocity in km/sec d = distance in Mpc H 0 = expansion rate today (Hubble Parameter) In words: The more distant a galaxy, the faster its recession velocity. Hubble’s Law

31. Interpretation Hubble’s Law demonstrates that the Universe is expanding in a systematic way: The more distant a galaxy is, the faster it appears to be moving away from us. Hubble Parameter: Rate of expansion today. Comments: Empirical result - based only on data Not an exact law

32. Nature of the Expansion General Expansion of Space: All observers in different galaxies see the same expansion around them. No center - all observers appear to be at the center. What is the recession velocity? NOT motions through space... Expansion of space: galaxies carried along.

33. Hubble Parameter: H 0 Measures the rate of expansion today: H 0 = 72 ± 8 km/sec/Mpc Based on Hubble Space Telescope observations of Cepheids in nearby galaxies to calibrate distant galaxy indicators H 0 is hard to measure: Recession speeds are easy to measure from the shifts of spectral lines. But distances are very hard to measure. Galaxies also have extra motions.