The CMB: The Early Years Bruce Partridge Haverford College

Thanks… Start with thanks: To funding agencies for supporting CMB research To ICISE To the organizers for giving me a chance to relive the early days of CMB science To you, the audience, for listening to history not science, memories not results To the people of Viet Nam for the many kindnesses they have shown me this trip and earlier

“Philosophy” of this Talk “Cosmology 50 Years after the Discovery of the CMB” Thus I will begin by reminding us all what cosmology was like 50 years ago Then – mostly for the CMB folks – talk informally about the early years of CMB science As I go – mainly for those not in CMB field -- try to highlight the experimental problems and the crucial contributions of CMB science

50-60 Years Ago….Cosmology Was a Field on the Fringe of Physics Examples: 1959: Text for my first course on astronomy: “the Universe” mentioned only once, wrongly identified with the Milky Way (H. N. Russell one author) 1968: “Cosmology is mostly a dream of zealots….” Willie Fowler Sandage, 1970 When considered at all, cosmology was a set of mathematical models, at best loosely linked to observations- An exception – the “California” group, Hubble  Sandage -- took astronomical observations seriously

Cosmology in the Early 1960’s The prominent debate: Big Bang (Gamow, Alpher and Herman) -- necessarily a hot initial state Versus Steady State (Hoyle, Gold and Bondi) – constant density, exponential expansion, no large-scale evolution (required matter creation) Note that all were physicists

Cosmology in the Early 1960’s The prominent debate: Big Bang versus Steady State– constant density, exponential expansion, no large- scale evolution (required matter creation) How to decide? Two methods: 1.Direct evidence for a Hot Big Bang 2.Disproof of Steady State by showing evidence for evolution

Big Bang or Steady State? Direct Evidence for a Hot Big Bang Hot Big Bang allowed nucleosynthesis Interest in this approach died away because it was shown that Big Bang Nucleosynthesis could not produce all elements heavier than Hydrogen -- the mass = 5 problem (Turkevitch et al.) -- Burbidge, Burbidge, Fowler and Hoyle explain stellar nucleosynthesis Note that the large abundance of Helium remained unexplained… Any possibility of (observable) left-over heat ignored/forgotten

Big Bang or Steady State? Emphasis instead on second approach: Disproof of Steady State Clear example: Martin Ryle’s counts of radio sources implied a higher density in the past than the present ( any evolution is incompatible with Steady State) Indirect example: Alan Sandage’s “Search for two numbers” -- H o (rate of expansion) -- q o, the deceleration parameter, rate of gravitational slowing of expansion -- q o = +0.5 for flat Universe -- q o = -1.0 for Steady State -- measured by plotting luminosity vs. redshift for galaxies

Disproofs of a Steady State Universe Ryle’s number counts of Sandage’s value of q o radio sources He reports q o = 1.2 ± 0.4 inconsistent with SST (which has q o = -1.0) whic Excess implies evolution

Disproofs of a Steady State Universe Ryle’s number counts of radio sources “Local hole” counter-argument as example of Steady State counter argument

1964: A Crucial Year Bell Labs (NJ) Low noise radiometer λ = 7 cm. With clean optics Puzzling “excess noise” equivalent to 3.5 K More or less same in all directions (not Galactic)

1964: A Crucial Year Princeton (NJ) Cyclic Universe Need to burn up heavy ele- ments from previous cycles Need T > 10 9 K so present T ~ 1-10 K Started search for with low noise radiometer λ = 3 cm. With clean optics

1964: A Crucial Year Bell Labs (NJ)Princeton (NJ) Low noise radiometerCyclic Universe λ = 7 cm.Need to burn up heavy ele- With clean optics ments from previous cycles Puzzling “excess noise”Need T > 10 9 K equivalent to 3.5 K so present T ~ 1-10 K More or less same inStarted search for with low all directions (not Galactic) noise radiometer λ = 3 cm. With clean optics

Late 1964 – Connection Made Telephone call from Bell Labs to Princeton: Bob Dicke, “Well, boys, we’ve been scooped.” Dicke, Peebles, Roll and Wilkinson (1965) offer an explanation of the Bell Labs results:

1965: The Importance of a Second Measurement Incredibly audacious, and incredibly important, graph! The background IS COSMIC!

Roll and Wilkinson (1965) Apparatus

: But Is the Background Truly Cosmic? A personal detour When I joined the Dicke group in 1965 Given two choices: to work on -- Microwave background with Dave Wilkinson -- Solar oblateness with Bob Dicke and Mark Goldenberg (I luckily chose the former) … and the bonus of theory with Jim Peebles

: But Is the Background Truly Cosmic? Needed to prove that the microwave background was truly cosmic -- not Galactic, or solar system, or the sum of emission from extragalactic sources -- All seriously proposed Two obvious tests: 1.The spectrum of the background (if cosmic, should be [close to] blackbody) 2.Isotropy (if cosmic, should be isotropic [except for small Doppler dipole] on both large and small scales) Dave Wilkinson and I set out to test both, with help from Paul Boynton and Bob Stokes

The White Mountain Spectrum Experiment: 1967 Scaled radiometers at several wavelengths λ = 3, 1.6 and 0.86 cm High altitude site to limit atmospheric emission -- and a means of measuring it (by varying zenith angle) Careful attention to syste- matics: cold load; ground screens

The White Mountain Experiment: 1967 Cryogenic comparison source (with less loss) Clean optics (Parallels construction of Planck LFI instrument)

The White Mountain Experiment – and Extension Spectrum showed curvature at λ < 1 cm expected for a Planck (blackbody) spectrum T refined to ~ 2.66 ± 0.12 K (George Smoot will give more details) Spectrum test passed

The Other Test: The Princeton “Isotropometer” Comparative measurements easier than absolute Sky near North Pole as reference; slow switching (not ideal) Scanned [near] celestial equator, so dipole would generate a 24h period

The Princeton “Isotropometer” – First Results

By 1967, showed ΔT/T < 0.002: Isotropy test passed Note scale

Second Generation Isotropometer – Yuma, Arizona

Additional Early Support for Cosmic Interpretation Several people recall old measurement of excitation of cyanogen molecule (CN) at λ = 2.3 mm: T ~ 2-3 K required Long wavelength measurements (Cambridge) consistent with T = 3 K Small scale isotropy rules out radio sources (Smith and Partridge 1969) Most crucially, re-animation of Big Bang Nucleosynthesis to explain He abundance (Waggoner and Peebles) Beautiful example of the power of correct scientific models: Hot Big Bang explains CMB and excess He -- and the two together constrain density of baryons, number of neutrino families, neutron half-life…..

But Also Some External Discrepancies… Rocket observations (Shivanandan et al., PRL 1968) suggest greybody not blackbody spectrum (and huge energy density in radiation) Later withdrawn (and shown to be wrong by Muehlner and Weis ) By ~1970, cosmic interpretation generally accepted

Crucial Involvement from Moscow

Crucial Contributions from Moscow Inflation (Linde) Quantum fluctuations are the seeds of all present cosmic structure (Chibisov and Mukhanov) Adiabatic pertur- bations (Zel’dovich) Near scale-invariant perturbations (Zel’ dovich) Early work on Hot Big Bang & CMB (Dorosh- kevich & Novikov) The S-Z effect (Zel’ dovich & Sunyaev) “Top down” structure formation (Zel’dovich) CN (Shklovsky) ….

A Commercial Break The story of these early years is told by many of the people involved in a volume called Finding the Big Bang (Cambridge Univ. Press) Highlights Sections on the Russian contributions particularly interesting. Sense of how much fun the experimentalists had in the early days comes through clearly.

~ 1977 The epoch of upper limits (Bob Dicke’s influence) Using available equipment and facilities Flavor: “What can we do with what we have?” Rate of publications slowed down… Hunt for the dipole: definitive results by 1977 (the U2 expt.) But earlier strong hint (Conklin and Bracewell, 1971) Limitations of these experi- ments: angular scale; atmos- phere; detector sensitivity

~ Retrenchment Purposeful experimental design (and new detectors) Well-grounded theoretical results (But too little interaction between the two before COBE) Flavor for experiments: “How can we do this better?” Culmination in 1992: COBE DMR detection of statistical evidence for ΔT/T ~ Limitations of these experiments: range of angular scale; foregrounds made blatantly visible, but not yet limiting

CMB anisotropy science matures Theory influences experimental design The power spectrum as THE representation of anisotropies Search for and detection of first acoustic peak Evidence for Silk damping in the high-ell tail Flavor: “We’re doing the experiments right; now to beat the systematics and foregrounds…” Limitations: foregrounds; sensitivity of single detectors; instrumental systematics Culmination: the 2003 WMAP power spectrum

1993 The WMAP Power Spectrum I actually show the 5 year results

Need for Smaller Angular Scale Measurements Limited angular resolution of WMAP  need for measurements at smaller scales; the goal of ground-based instruments like SPT and ACT

2003 – 2015: WMAP to Planck New directions New infusion of innovation/cleverness from theorists Flavor: “How can we wring more cosmological and physical results out of the data?” Polarization (EE first detected by DASI, 2001) The richness of the damping tail (the province of SPT and ACT) Exploiting the CMB for physics as well as cosmology Limitations: now clearly foregrounds; use of arrays gets around limits on sensitivity of individual detectors (makes focal plane and optics harder); instrumental systematics

The following material was not presented in Quy Nhon, since I ran out of time.

Modern Detectors Arrays of bolometers (~1000) TES devices Work near quantum limit “Spider web” to reduce cosmic ray hits Now polarized

The Future A few observations: 1. Polarization EE and TE polarization will be firmly nailed down BB depends on which model of Inflation (a good thing) – and on control of foregrounds (a hard thing) 2. Lensing The CMB will increasingly be used as a background (or backlight) for gravitational lensing 3.Clever exploitation of the data Theorists will find new ways to use the observations

A Bit More on Polarization Polarization induced by Thomson scattering of an anisotropic radiation filed Decomposition of polarization field into 2 components: E modes (divergence-like) – produced by scalar perturbations B modes (curl-like) – produced ONLY by tensor perturbations Tensor perturbations are a natural prediction of (most) theories of inflation Hence detection of B modes is a crucial test of inflation

E Modes First detected by DASI group in 2001 Now well characterized by Planck and many other experiments

The E Mode Pattern Observed Hot and cold spots in CMB have opposite sign: Top: models Lower: stacked Planck results

On to the B Modes Amplitude (relative to E modes) depends on model of inflation Specified by scalar/tensor ratio r For most inflation models r < 1, and typically 0.1 to 0.01 E modes are themselves faint (~10-30 microKelvin 2 in power spectrum) Therefore expected B mode amplitudes ~ 1 microKelvin 2 or less Both astrophysical foregrounds and instrumental systematics impose severe limits

On to the B Modes: Before 2014, only Upper Limits

The BICEP2 Detection, March 2014 Hot and cold spots in CMB have opposite sign, as expected

But Are the BICEP2 B-modes Cosmic? In 1965, needed to show the microwave background was cosmic ; In 2014, needed to show the B mode signal was cosmic Unfortunately, much of it due to polarized emission from Galactic dust, as shown by joint BICEP2-Planck analysis

Conclusions Motivations for early experimental work very different from today’s goals of characterizing the power spectrum of temperature and polarization fluctuations. Scope and cost of CMB experiments has mushroomed (as have size of teams) Ingenuity of theorists (and data analysts) more and more crucial The expansive nature of CMB science: cosmology, structure formation, fundamental physics (CMB will do better on neutrino mass than lab experiments), and much, much more. Let’s hope the next 50 years are as rich