The CMB: The Early Years
Bruce Partridge
Haverford College
bpartrid@haverford.edu
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 observationsAn 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 largescale 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”
-- Ho (rate of expansion)
-- qo, the deceleration parameter, rate of gravitational slowing of
expansion
-- qo = +0.5 for flat Universe
-- qo = -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 qo
radio sources
Excess
implies
evolution
He reports qo = 1.2 ± 0.4
inconsistent with SST (which has
qo = -1.0)
whic
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 elements from previous cycles
Need T > 109 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 radiometer
λ = 7 cm.
With clean optics
Puzzling “excess noise”
equivalent to 3.5 K
More or less same in
all directions (not Galactic)
Cyclic Universe
Need to burn up heavy elements from previous cycles
Need T > 109 K
so present T ~ 1-10 K
Started search for with low
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
1965-67: 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
1965-67: 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 systematics: 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
The Princeton “Isotropometer” – First Results
Note scale
By 1967, showed ΔT/T < 0.002: Isotropy test passed
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 perturbations (Zel’dovich)
Near scale-invariant
perturbations (Zel’
dovich)
Early work on Hot Big
Bang & CMB (Doroshkevich & 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.
1970 -- ~ 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 experiments: angular scale; atmosphere; detector sensitivity
~ 1977 - 1992
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 ~ 10-4
Limitations of these experiments:
range of angular scale;
foregrounds made blatantly visible,
but not yet limiting
1992-2003
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 microKelvin2 in power
spectrum)
Therefore expected B mode amplitudes ~ 1 microKelvin2 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