The Early Universe

1. The Early Universe Thursday, January 24(planetarium show tonight: 7 pm, 5th floor Smith Lab)

2. Hot, dense, opaque objects emit light. Color (or wavelength) of light depends on object’s temperature.

3. Temperature is inversely proportional to wavelength. red-hot iron = 1000 K lightbulb filament = 2900 K Sun’s surface = 5800 K

4. Early universe was hot, dense, and opaque: it emitted light. In 1965, two astronomers (Penzias & Wilson) discovered faint “static” in their microwave antenna.

5. This static was the “leftover light” from hot, dense, opaque early universe. Intensity Its spectrum peaks at λ ≈ 1 mm; this is microwave radiation. Scientists call the “leftover light” the Cosmic Microwave Background (CMB).

6. Temperature implied by CMB spectrum is T ≈ 3 K. (That’s COLD!)

7. But…objects at T ≈ 3000 K produce visible & infrared light (think “lightbulb filament”), not microwave light. The universe became transparent at a temperature T ≈ 3000 K.

8. How did the cosmic background change from visible & infrared light (λ≈ 0.001 mm) to microwave light (λ≈ 1 mm)? How did its temperature drop from 3000 K to 3 K?

9. The universe is expanding. Distance between galaxies increases. Wavelength of light (distance between wave crests) increases.

10. Wavelength of cosmic background light has increased by a factor of 1000. 0.001 mm 1 mm Why? Because the universe has expanded by a factor of 1000 since the time it became transparent.

11. We now have two ways to think about a galaxy’s redshift. 1) The redshift is the result of a Doppler shift. 2) The redshift is the result of expansion stretching the wavelength.

12. Example: a galaxy has a redshift z≡ (λ-λ0)/λ0 = 0.01. Doppler explanation: • Radial velocity of the galaxy is 1% the speed of light: • v = 0.01 c = 3000 km/sec d = v/H0= 42.9 Mpc.

13. Expansion explanation: 2) The distance to the galaxy now is 1% greater than it was when the light we observe was emitted: dnow = 42.9 Mpc dthen= 42.9 Mpc / 1.01 = 42.5 Mpc

14. So, which way of thinking about redshift (Doppler or expansion) is The Right Way?? In the limit of small redshift (v << c) they are identical. Let’s see why!!

15. Light from galaxy has traveled daverage = 42.7 Mpc = 139 million light-years in a time t = 139 million years. During that time, distance to the galaxy has expanded by 0.01 daverage = 1.39 million light-years. Average radial velocity = 1.39 million light-years / 139 million years = 0.01 c.

16. Some galaxies have very high redshift. The arrowed galaxy at left has z = (λ-λ0)/λ0 = 5.7 For very distant galaxies, it’s best to think of redshift as being due to expansion (no guarantee of constant radial velocity!)

17. Astronomers are fascinated by galaxies at high redshift. “A telescope is a time machine.” Larger redshift → larger distance → longer light travel time.

18. Astronomers the Cosmic Microwave Background

19. The CMB has highest redshift of anything we can see (z = 1000). When we look at the CMB, we look at the surface of the glowing “fog” that filled the early universe!

20. When we look at the CMB, we see a message direct from the early universe. What is this message telling us? Messages are often (1) hard to read & (2) hard to interpret.

21. 1) “Reading” the CMB COBE WMAP Water vapor in Earth’s atmosphere absorbs microwaves: go above the atmosphere!

22. WMAP (Wilkinson Microwave Anisotropy Probe) is at the L2 point, beyond the Moon’s orbit.

23. 2) “Interpreting” the CMB Observation: Temperature of CMB is nearly isotropic (the same in all directions). Interpretation: early universe was nearly homogeneous (the same in all locations).

24. ← hotter cooler → Observation: Temperature of CMB is slightly hotter toward Leo, cooler toward Aquarius (on opposite side of sky). Temperature fluctuation = 1 part per 1000.

25. Interpretation: difference in temperature results from a Doppler shift. Earth orbits Sun (v = 30 km/sec) Sun orbits center of our galaxy (v = 220 km/sec) Galaxy falls toward center of Local Group (v ≈ 50 km/sec) Local Group falls toward Virgo Cluster (v ≈ 200 km/sec)

26. Net motion: toward Leo, with a speed v ≈ 300 km/sec ≈ 0.001 c. Cosmic light from direction of Leo is slightly blueshifted (shorter wavelength, higher temperature).

27. Observation: After subtracting the effect of our motion through space, CMB still shows hot & cold spots, about 1° across. Temperature fluctuation = 1 part per 105.

28. Interpretation: observed temperature fluctuations result from density fluctuations in the early universe. Regions that were compressed had higher density, but also higher temperature (gases heat up as they are compressed).

29. Hot spots in the CMB are higher in temperature than coldspots by only 1 part per 100,000. Why do we care about such tiny density fluctuations? Implication: the densityfluctuations in the early universe were also small (about 1 part per 100,000).

30. The Rich Get Richer, the Poor Get Poorer. A region that was only slightly denser than average will eventually become much denser than average; it’s compressed by its own gravity.

31. Great Oaks from Tiny Acorns Grow. A dense region that initially has a small mass will become more massive with time; its gravity attracts surrounding matter.

32. 2 1 3 4 Computer simulations of the growth of density fluctuations:

34. Gravity sucks. Since gravity sucks (and doesn’t spew), tiny density enhancements become dense, massive clusters of galaxies.

35. Tuesday’s Lecture: The VERY Early Universe Problem Set #3 due!! Reading: Chapter 4