SPU-22: The Unity of Science from the Big Bang to the Brontosaurus and Beyond

1. SPU-22: The Unity of Science from the Big Bang to the Brontosaurus and Beyond Lecture 7 19 February 2014 Science Center Lecture Hall A

2. Outline of Lecture 7 Cosmology: -Structure of universe: early 20th century thoughts - Implication of redshift-distance relation - Imputation of “Big Bang” - Relict black-body radiation - Discovery of cosmic microwave background (CMB) - Earth’s motion with respect to CMB - Inhomogeneities in CMB Concept of our place in universe

3. Early 20th Century Model Of Universe Milky Way in 1917 was universe and was static (stellar motions did not seem systematic) Einstein: Can’t be! Why? So he modified model Introduced Cosmological Constant into GTR: repulsive at large distances, not discernible at small distances. Why this combination?

4. Return Of Redshift-Distance Relation Glaring implication of this linear relation (redshift proportional to distance); purposely ignored last lecture Namely? Further galaxies from us, faster they move away from us: The UNIVERSE is EXPANDING, approximately uniformly: See demonstration for analogy Now look backwards in time. Implication also clear. UNIVERSE ever SMALLER in PAST!

5. Theoretical Advance And Verification Back in 1922, Alexander Friedman (1888 – 1925) in Russia (then part of USSR) and about same time Georges Lemaitre (1894 – 1966), Belgian priest, demonstrated mathematically: expanding universe possible solution to Einstein’s original equations of general relativity. (Did these results influence Hubble?) Given also expanding universe’s observational support by Hubble, Einstein abandoned his cosmological constant, making famous undocumented comment (“worst mistake of my scientific career”)

6. Concept Of “Big Bang” Extrapolate back to epoch of zero size. What then? Conclusion reached: Then huge explosion, giving rise from nothing to universe we now see and love. How, in detail? Not clear What would be supporting evidence??

7. Tell-Tale Trace Of Big Bang Accompanying putative phenomenally high- temperature explosion would be ultra-intense, extraordinarily high-energy light. (What is “high-energy” light? E=hν) In late 1940s, Ralph Alpher, George Gamov,and Robert Herman predicted that there would be “relict radiation” all around us today, remains from explosion now visible, but at FAR lower temperature, about 5 K (what is “K”?). Why?

8. Characteristics Of Suspected Relict Radiation Expansion of universe after Big Bang causes light to “cool” – its wavelengths increase and temperature decreases, dramatically by “our time” Its spectrum (what’s that?), however, would have same shape, but “appropriate” for lower temperature. What shape? As for black body

9. Black-Body Radiation (A Primer) Everything radiates, appropriately for its constitution and temperature Object that totally absorbs all radiation of all colors called “black body” (shouldn’t see it: no light reflected). Nonetheless, object will radiate with characteristic spectrum (energy vs. color or frequency). Spectrum has particular shape, one for each temperature of emitting object .

10. Black-Body Radiation: Deepening Our Knowledge Question: If object truly black body, how tell what kind of object it is, since emissions are same, save for temperature? Answer: For most objects, discern their different features through reflected light. When an object heated to high enough temperature, it approaches black-body status, and we lose sight of its individual features as when it’s in a furnace. (See demonstration)

11. Black-Body Radiation(Concluded) Key point: A furnace (or an explosion) has close to black-body spectrum (see next two slides). That’s way Nature behaves: (small) hole in furnace has intensity vs. color characteristic ofblack body – universal curve, dependent only on temperature in furnace Frequency (or wavelength) at which most energy emitted per unit wavelength, given by simple formula, called Wien’s law: λpeakT≈ 0.3 cm K (K= abs. temp.) (see third slide)

12. Black-Body Radiation:Relative Intensity Vs. Wavelength (units of 10-9 meter)

13. Curves Of Black-Body Radiation Characteristics of preceding curves: • Curves don’t cross. Intensity of radiation is higher at every wavelength for higher temperature • The peak of curve falls at lower wavelength for higher temperature (Wien’s law)

14. Temperature Relations K (Kelvin) = Temperature above “absolute” zero ≈ 273 + oC ≈ 459 + (5/9) oF

15. Prelude To Detection Of Light From Big Bang How to detect? Look for expected (weak) black-body radiation with peak intensity at color corresponding to wavelength of ~ 1 mm. After Alpher, Gamov, and Herman’s prediction in late 1940s, no one looked: Too hard then or everyone of relevance “too busy” or uninterested (again the Adams and Le Verrier experience?)?

16. Fast-Forward To 1960s Robert Dicke, at Princeton University, tried to detect black-body radiation from Big Bang, now called cosmic microwave background (CMB), as in second slide. Why called CMB? Very difficult measurement: Signal expected to be very weak, background “noise” expected to be very strong. How to distinguish? CMB was expected to be isotropic - same intensity in every direction- key property. Why so expected?

17. The Unawares Competition Nearby to Princeton, two (nearly) newly minted PhDs in physics, Arno Penzias and Robert Wilson, were working at Bell Labs in its heyday. Their project was to analyze noise detected by latest, sophisticated horn-shaped antenna from Bell. Why important? Wanted to track down sources of all radio noise to determine what could be eliminated, reduced, or avoided, so that space communications set up by AT&T, the parent company, could be as efficient and cost effective as feasible.

18. The Smoking Gun Astronomicalmeasurements at radio wavelengths of 7.5 cm. Penzias and Wilson found antenna ~3 K hotter than expected. Even removing pigeon droppings from horn antenna did not change anything. No variation over sky, after celestial sources of radio radiation accounted for. Somehow, this extra radiation was “real,” not noise in Bell Labs’ antenna system

19. The Key Connection Astronomical grapevine, working at long distances through MIT, connected two neighboring Princeton groups. Pair of papers prepared, one based on Bell Labs observations and other on Princeton motivation. Results submitted to The Astrophysical Journal and from there, somehow, leaked to New York Times. Result? Long front-page story. Another example of how observation of predicted effect changes nature of game in everyone’s eyes. Again, shades of Adams and Le Verrier

20. The 20 foot Horn-Reflector

21. Wilson on left Penzias on right Picture taken after 1978 award of Nobel Prize in Physics

22. Confirmations Tumbled Out Precursor observations resurrected from late 1930s –early 1940s: Then not understood excitation of CN molecules, at temperature of ~2.5 K Radio observations from other observatories at other wavelengths, all longer than that of peak of blackbody curve came from many different groups Real advance came from space in early 1990s

23. Results From CMB Observations Made In Space First measurements from space, made in early 1990s, approximated black-body curve (next slide) better than anything ever measured on earth. Space-measured values, at each frequency, had uncertainties far smaller than size of any visible point that could be placed on curve.

24. CMB Results From COBE

25. What About Us? CMB provides a reference frame for universe: How, if at all, does sun move with respect to this frame? How can we tell? Doppler shift tells all: Our motion would be disclosed by temperature differences we would detect in different directions. Result: We’re moving about 600 km/sec in certain direction, further degrading any claim we had to special place in universe. (Note: 600 km/sec is 2x10-3c.)

26. Structure In CMB Progression from degree accuracy, through millidegree accuracy, to nearly microdegree accuracy in under half century Cosmology thus transformed in relatively short time from mostly pure speculation to highly quantitative subject based on data in addition to theory, as opposed to theory (or speculation) alone.

27. The scale is about ± 200μK centered at 2.725K

28. Latest Planck Map Of Sky

29. What Do Small Fluctuations In CMB Signify? If CMB exactly identical in all directions (no fluctuations at all), we would not be here to observe: Why? There would be nothing to disturb “sameness.” Universe would remain without structure. Because small fluctuations existed in CMB, structures could form which led, eventually, to us, through long intricate process of evolution, both physical and biological (or so present best scientific story goes)

30. Evolution Of Concepts Of OurPlace In Universe Earth at center Sun at center Sun at center of Milky Way Sun far from center of Milky Way Milky Way one of ≈ 1011 galaxies - All in c. 1,000 years