CMB Suggested Reading: Ryden, Chapter 9
1934, Richard Tolman, blackbody radiation in an expanding universe cools but retains its thermal distribution and remains a blackbody 1941, Andrew McKellar, excitation of interstellar CN doublet absorption lines gives an effective temperature of space of ~2.3K Flux McKellar, PDAO, 7, 251 (1941) Wavelength (Å)
1946, Gamow, to match observed abundance, nuclei should be built up out of equilibrium in hot early universe (high expansion rate, assume matter domination) 1948, Gamow, T~10 9 K when deuterium formed, argues for radiation domination in early universe; the existence of CMB 1948, Alpher, Bethe, & Gamow (αβγ paper), element synthesis in an expanding universe; calculations based on previous ideas 1948, Alpher & Herman, make corrections to previous results; state that present radiation temperature should be ~5K (close! but largely a coincidence; incorrect assumptions - neutron dominated initial state); no mention of the observability.
1957, Shmaonov, horn antenna at 3.2cm, find the absolute effective temperature of radio emission background 4±3K, independent of time and direction Early 1960s, Zel dovich, Doroshkevich, Novikov, estimate expected background temperature from helium abundance; realize Bell Labs telescope can constrain 1964, Hoyle & Tayler, essentially correct version of primordial helium abundance calculation (no longer pure neutron initial state; weak interaction for neutron vs proton) 1965, Dicke, Peebles, Roll, & Wilkinson, realize oscillating or singular universe might have thermal background; build detector to search; then they hear about the discovery of...
1965, Penzias & Wilson, antenna has isotropic noise of 3.5±1.0K at wavelength of 7.35cm; careful experiment (e.g., shooed away pigeons roosted in the antenna; cleaned up the usual white dielectric generated by pigeons); explanation could be that of Dicke et al. Nobel Prize in Physics (1978)
1965, Roll & Wilkinson, detect the radiation background at 3.2cm, with amplitude consistent with Penzias & Wilson for blackbody spectrum; isotropic to 10% (fl V) QJ I- e CL lo '4- K O O ~lo l6 ta 0 E lo-is XxtO O pg) Z~ 0 O-20 l P R I NC E TON (3.5 (3.i Roll & Wilkinson, PRL (1965) l lo' IO IO I WAVELENGTH (c m )!0 ' FIG. 2. Measurements to date of the microwave background radiation. The galactic radio background is extrapolated with a spectral index of n =0.5. This figure due to P. J. E. Peebles.
1966-1967, Field & Hitchcock, Shklovsky, Thaddeus & Clauser, Thaddeus (following a suggestion by Woolf) independently show that the excitation of interstellar CN is caused by CMB (McKellar s 1941 observation explained!) 1970s, 1980s, ground, balloon, satellite observations 1990, NASA s COsmic Background Explorer (COBE) satellite confirms CMB as nearly perfect isotropic blackbody and discovers the anisotropies. John Mather & George Smoot Nobel Prize in Physics (2006)
CMB - Main Observational Results 1. nearly perfect blackbody spectrum of temperature T=2.73K Blackbody spectrum of CMB measured by COBE (1990) [error bars enlarged by 400x!]
CMB - Main Observational Results 1. nearly perfect blackbody spectrum of temperature T=2.73K Scott 1999 CMB dominates the energy density of radiation backgrounds Blackbody spectrum of CMB measured by COBE (1990) [error bars enlarged by 400x!]
CMB - Main Observational Results 2. isotropic, better than ~10-3
CMB - Main Observational Results 3. anisotropy, 10-3 level, dipole, kinetic effect v~370km/s our motion w.r.t. the CMB frame
CMB - Main Observational Results 4. anisotropy, 10-5 level, primordial Planck Satellite COBE WMAP (Wilkinson Microwave Anisotropy Probe)
CMB - Main Observational Results 5. weak polarization, 10-7 level primordial (grav. waves) [not detected yet/bicep2?]+reionization COBE WMAP (Wilkinson Microwave Anisotropy Probe)