arXiv:0902.3796v1 [astro-ph.CO] 22 Feb 2009
The Origin of the Universe as Revealed Through the Polarization
of the Cosmic Microwave Background
(Dated: February 22, 2009)
Scott Dodelson, Richard Easther, Shaul Hanany, Liam McAllister, Stephan Meyer, Lyman Page, Peter Ade, Alexandre Am-
blard, Amjad Ashoorioon, Carlo Baccigalupi, Amedeo Balbi, James Bartlett, Nicola Bartolo, Daniel Baumann, Maria Beltran,
Dominic Benford, Mark Birkinshaw, Jamie Bock, Dick Bond, Julian Borrill, Franois Bouchet, Michael Bridges, Emory Bunn,
Erminia Calabrese, Christopher Cantalupo, Ana Caramete, Carmelita Carbone, Sean Carroll, Suchetana Chatterjee, Xingang
Chen, Sarah Church, David Chuss, Carlo Contaldi, Asantha Cooray, Paolo Creminelli, Sudeep Das, Francesco De Bernardis,
Paolo de Bernardis, Jacques Delabrouille, F.-Xavier Dsert, Mark Devlin, Clive Dickinson, Simon Dicker, Michael DiPirro,
Matt Dobbs, Olivier Dore, Jessie Dotson, Joanna Dunkley, Cora Dvorkin, Hans Kristian Eriksen, Maria Cristina Falvella,
Dave Finley, Douglas Finkbeiner, Dale Fixsen, Raphael Flauger, Pablo Fosalba, Joseph Fowler, Silvia Galli, Evalyn Gates,
Walter Gear, Yannick Giraud-Heraud, Krzysztof Gorski, Brian Greene, Alessandro Gruppuso, Josh Gundersen, Mark Halpern,
Jean-Christophe Hamilton, Masashi Hazumi, Carlos Hernandez-Monteagudo, Mark Hertzberg, Gary Hinshaw, Christopher
Hirata, Eric Hivon, Richard Holman, Warren Holmes, Wayne Hu, Johannes Hubmayr, Kevin Huffenberger, Howard Hui,
Lam Hui, Kent Irwin, Mark Jackson, Andrew Jaffe, Bradley Johnson, Dean Johnson, William Jones, Shamit Kachru, Kenji
Kadota, Jean Kaplan, Manoj Kaplinghat, Brian Keating, Reijo Keskitalo, Justin Khoury, Will Kinney, Theodore Kisner,
Lloyd Knox, Hideo Kodama, Alan Kogut, Eiichiro Komatsu, Arthur Kosowsky, John Kovac, Lawrence Krauss, Hannu Kurki-
Suonio, Jean-Michel Lamarre, Susana Landau, Charles Lawrence, Samuel Leach, Louis Leblond, Adrian Lee, Erik Leitch,
Rodrigo Leonardi, Julien Lesgourgues, Andrew Liddle, Eugene Lim, Michele Limon, Marilena Loverde, Philip Lubin, Enrico
Lunghi, Joseph Lykken, Carolyn MacTavish, Antonio Magalhaes, Davide Maino, Victoria Martin, Sabino Matarrese, John
Mather, Harsh Mathur, Tomotake Matsumura, Pieter Meerburg, Alessandro Melchiorri, Laura Mersini-Houghton, Amber
Miller, Michael Milligan, Kavilan Moodley, Michael Neimack, Hogan Nguyen, Alberto Nicolis, Ian O’Dwyer, Angela Olinto,
Luca Pagano, Enrico Pajer, Bruce Partridge, Timothy Pearson, Hiranya Peiris, Marco Peloso, Francesco Piacentini, Michel
Piat, Lucio Piccirillo, Elena Pierpaoli, Davide Pietrobon, Giampaolo Pisano, Levon Pogosian, Dmitri Pogosyan, Nicolas Pon-
thieu, Lucia Popa, Clement Pryke, Christoph Raeth, Subharthi Ray, Christian Reichardt, Sara Ricciardi, Paul Richards,
Antonio Riotto, Graca Rocha, John Ruhl, Benjamin Rusholme, Robert Scherrer, Claudia Scoccola, Douglas Scott, Carolyn
Sealfon, Emiliano Sefusatti, Neelima Sehgal, Michael Seiffert, Leonardo Senatore, Paolo Serra, Sarah Shandera, Meir Shi-
mon, Peter Shirron, Jonathan Sievers, Joe Silk, Kris Sigurdson, Robert Silverberg, Eva Silverstein, Suzanne Staggs, Glenn
Starkman, Albert Stebbins, Federico Stivoli, Radek Stompor, Naoshi Sugiyama, Daniel Swetz, Andrea Tartari, Max Tegmark,
Peter Timbie, Maxim Titov, Matthieu Tristram, Mark Trodden, Gregory Tucker, Jon Urrestilla, Marcella Veneziani, Licia
Verde, Joaquin Vieira, Terry Walker, David Wands, Scott Watson, Steven Weinberg, Rainer Weiss, Benjamin Wandelt, Bruce
Winstein, Edward Wollack, Mark Wyman, Amit Yadav, Ki Won Yoon, Olivier Zahn, Matias Zaldarriaga, Michael Zemcov,
This white paper was assembled by Scott Dodelson with input from many of the
cosigners. It is part of the efforts of NASA’a Primordial Polarization Program Def-
inition Team (PPPDT), Shaul Hanany chair, and of a NASA award to Steve Meyer
and colleagues entitled ”A study for a CMB Probe of Inflation” (07-ASMCS07-0012).
Modern cosmology has sharpened questions posed for millennia about the origin of our
cosmic habitat. The age-old questions have been transformed into two pressing issues primed
for attack in the coming decade:
• How did the Universe begin?
The current cosmological paradigm successfully explains how the majestic structure
observed in the Universe today grew out of small ripples in the density of matter. What
is the physical origin of the primordial seeds which are ultimately responsible for the
existence of galaxies, stars, planets, and people in the Universe? It is natural to expect
(and many theories predict) that whatever produced the density ripples also produced
gravity waves – undulations in the fabric of space-time which travel at the speed of
light. Does the Universe contain a spectrum of primordial gravity waves produced by
the same mechanism which produced the ripples in the density?
• What physical laws govern the Universe at the highest energies?
All explanations for the seeds of structure rely on physics at energies far beyond those
probed by, e.g., CERN’s Large Hadron Collider. Experiments probing these seeds
therefore may provide information about new particles, forces, or perhaps even extra
dimensions of space that are visible only at the highest energies.
The clearest window onto these questions is the pattern of polarization in the Cosmic
Microwave Background (CMB), which is uniquely sensitive to primordial gravity waves. A
detection of the special pattern produced by gravity waves would be not only an unprece-
dented discovery, but also a direct probe of physics at the earliest observable instants of our
Universe. Experiments which map CMB polarization over the coming decade will lead us
on our first steps towards answering these age-old questions.
I. HOW DID THE UNIVERSE BEGIN?
Over the course of billions of years, perturbations in the early Universe were amplified
by gravitational instability, transforming an almost perfectly smooth Universe into one with
planets, stars, galaxies, and galaxy clusters. This cosmic evolution has been quantitatively
confirmed: the small initial perturbations encoded in the CMB have just the right amplitude
to produce the structure observed in the Universe today. We are emboldened to seek an
understanding not only of the origin of the primordial perturbations which seeded structure
in the Universe, but ultimately of the origin of the Universe itself.
Beyond their amplitude, the initial perturbations present several distinctive features .
They are nearly scale-invariant: perturbations at all wavelengths have nearly the same
amplitude. They are almost exactly Gaussian, in that their statistical properties conform to
a classic Gaussian random field to at least one part in 1000. Most strikingly, measurements
of the CMB indicate that the perturbations were synchronized at early times: when the
perturbations are decomposed into Fourier modes, one finds that every mode began with the
same temporal phase.
This early synchronization is particularly puzzling since it was locked in when the relevant
spatial scales were apparently larger than the distance light traveled since the beginning of
time (the horizon). This discovery of the last decade sharpens the classic horizon problem:
why does radiation arriving from opposite ends of the Universe share the same temperature?
The problem is now even more profound: how were the initial perturbations, with their
puzzling synchronization, produced? What physical mechanism could have possibly planted
these primordial seeds?
II. NEW LAWS OF PHYSICS
Over the next decade, the era during which the seeds of structure were produced – perhaps
10−35seconds after the Big Bang – will join nucleosynthesis (3 minutes) and recombination
(380,000 years) as windows into the primordial Universe that can be explored via present-
day observations. However, recombination and nucleosynthesis depend on the well-tested
details of atomic and nuclear physics respectively, while the energy scale at which the seeds
were laid down is likely to be so high that the fundamental constituents of the universe and
the laws of nature at that time are currently unknown. Our ability to see through this new
window will turn the early universe into a laboratory for ultra-high energy physics  at
scales entirely inaccessible to conventional terrestrial experimentation.
Is the new physics associated with the Grand Unified Scale at which the three low-
energy forces – weak, electromagnetic, and strong – become one?Supersymmetry is a
theory of particle physics which explains why the electroweak scale is so different from
the scale associated with gravity. Is the new physics part of a supersymmetric theory?
Are there other particles or fields that can be discovered which are related to those which
generated the primordial perturbations? Almost all models for these seeds predict an epoch of
acceleration in the early universe. Did some early form of dark energy drive this acceleration?
A number of models rely on extra dimensions. Does the universe have more than three spatial
dimensions? Almost all models rely on assumptions about the laws of physics at energies
close to the Planck scale, the scale at which quantum-mechanical fluctuations render general
relativity unstable. The underlying complete theory that describes physics at the Planck
scale – perhaps a string theory, or perhaps some theory not yet conceived – then dictates
the amplitude of the gravitational waves produced. In particular, the symmetries of this
fundamental theory can leave traces in the primordial gravity wave signal, so that a detection
of, or constraints on, primordial gravity waves could provide the first observational clue as
to the nature of quantum gravity.
The general considerations outlined above are most easily illustrated in the context of the
most-studied model of the early Universe: inflation – the idea that the Universe expanded
nearly exponentially rapidly very early in its history. Inflation resolved several classical
problems in cosmology and correctly predicted the observed features of the primordial per-
turbations. The early accelerated expansion drove small regions that had been in causal
contact far away from one another. Quantum fluctuations, usually observed only on micro-
scopic scales, were stretched to astronomical sizes and promoted to cosmic significance as
the seeds of large scale structure. The wavelengths of these fluctuations became so large –
larger even than the horizon – that the perturbations froze at a constant amplitude. When
they re-entered the horizon much later, all modes were therefore synchronized to have the
same temporal phase. Most models of inflation are driven by an almost constant energy
density (similar to the models for dark energy today), so perturbations in the small wave-
length modes which left the horizon latest were generated under the same conditions that
existed when large wavelength modes left the horizon. Hence, the spectrum of perturbations
is nearly scale-invariant, in agreement with observations. Additionally, the huge growth
eliminated curvature, in full agreement with today’s percent-level measurements that the
Universe is flat.
All models of inflation make predictions for the shape of the density spectrum, the ampli-
tude and shape of the gravity wave spectrum, and the level of deviations from Gaussianity.
Many of the simplest models predict an appreciable gravity wave signal but no detectable de-
viations from Gaussianity, while alternatives to inflation seem to predict a Universe with no
detectable primordial gravity waves but often appreciable non-Gaussianity. The amplitude
of primordial gravity waves therefore provides a way to distinguish between simple models
of inflation and alternative proposals for the dynamics of the early Universe.
Moreover, the gravity wave amplitude is directly tied to the energy scale during inflation,
so a detection can be translated into clues about the new physics responsible for the origin of
structure in the Universe. The amplitude of the gravity wave spectrum is expressed relative to
that of the density perturbation spectrum by the parameter r. Current experiments constrain
r < 0.3, and in the coming decade values of r at least as low as 0.01 will be attainable.
This amplitude of gravity waves represents a crucial target: theoretical models with r >
0.01 are qualitatively different from those with small r. Particle physicists have recently
made progress understanding the symmetries underlying these two classes of theories ,
so detection of or constraints on r will provide information about the underlying principles
governing the physics operating at ultra-high energies.
Summarizing the reasons why the hunt for primordial gravity waves is so compelling, a
• Rule out alternatives to inflation,
• Pinpoint the energy scale at which inflation took place,
• Provide clues about the symmetries underlying new physics at the highest energies.
IV. CMB POLARIZATION: THE ULTIMATE GRAVITY WAVE DETECTOR
Primordial gravity waves leave a unique imprint on the microwave background
FIG. 1: Any polarization field can be de-
composed into two modes. Positive (neg-
ative) E-modes surround hot (cold) spots.
B-modes cannot be produced by ordinary
perturbations to the density but are pro-
duced by gravity waves.
as they stretch and squeeze the space in which the
electrons and photons interact. A quadrupole inten-
sity anisotropy in the radiation field produces observ-
able polarization in the CMB via Compton scattering.
When gravity waves are the source of the anisotropy,
the ensuing polarization pattern has a handedness, de-
picted as the B-modes in Figure 1. On the other hand,
density perturbations sourcing the anisotropy produce
only E-mode polarization patterns. On large angular
scales, the most plausible cosmological sources of a B-
mode signal are primordial gravity waves, so the am-
plitude of the B-mode signal is a direct measure of the
gravity wave background, and thus the energy scale of
inflation. A detection would be not only an unprece-
dented discovery, but also a direct probe of physics at
the earliest observable instants of our Universe.
Figure 2 depicts the expected angular spectra of
the two modes of CMB polarization. E-modes have
been detected and a number of experiments are on the
verge of pinning down their spectrum, thereby further constraining cosmological parameters.
FIG. 2: Predicted spectra of E- and B-modes. The blue solid curves representing B-modes labeled r=0.3 and
r=0.01 correspond to amplitudes just below current limits and within reach of a satellite mission dedicated
to polarization, respectively. The hatched region and the dashed curve labeled “EPIC” show the noise levels
projected for two possible implementations of this mission . The dashed curves labeled “WMAP” and
“Planck” correspond to the statistical noise limits for these satellites after 9 years and 1 year, respectively.
All noise curves are averaged over bins of width ∆l = 0.3l.
The primordial B-mode spectrum has a characteristic double-humped shape, the first bump
on large angular scales produced at the end of the Dark Ages and the second on degree
scales produced during electron-photon decoupling around the time of recombination. The
amplitude of the B-mode spectrum is unknown since inflationary models make a range of
predictions for the amplitude of the primordial gravity waves. There are no known technical
limitations  to achieving the sensitivity necessary to detect r down to 10−3. Astrophysical
foregrounds will likely degrade this sensitivity, but a variety of simulations using multiple
techniques shows that a robust detection of r down to a level of 0.01 – a key threshold
delineating the theoretical models – is achievable with a future satellite mission .
Beyond this principal science, CMB polarization measurements will also impact upon
non-inflationary science.These measurements will determine the gravitational potential
along the line of sight to the last scattering surface , thereby constraining models of dark
energy and possibly detecting the decaying gravitational potentials produced by massive
neutrinos. CMB polarization will also constrain reionization, which heralds the end of the Download full-text
Dark Ages , and will provide information about the distribution of magnetic fields in and
outside our Galaxy .
Cosmic microwave background polarization offers an extraordinary opportunity to gain a
first glimpse into the physics that shaped our Universe. Experimentalists have demonstrated
that a coordinated attack on this problem over the coming decade will likely detect primordial
gravity waves – thereby providing extensive information about new physics at ultra-high
energy scales – or severely constrain the scenario responsible for the origin of the Universe.
 D. Baumann et al. [CMBPol Study Team Collaboration], “CMBPol Mission Concept Study:
Probing Inflation with CMB Polarization,” arXiv:0811.3919 [astro-ph].
 J. Bock et al., “Task Force on Cosmic Microwave Background Research,”(2006),
 J. Dunkley et al., “CMBPol Mission Concept Study: Prospects for polarized foreground re-
moval,” arXiv:0811.3915 [astro-ph].
 J. Bock et al., “The Experimental Probe of Inflationary Cosmology (EPIC): A Mission Concept
Study for NASA’s Einstein Inflation Probe,” arXiv:0805.4207 [astro-ph].
 K. M. Smith et al., “CMBPol Mission Concept Study: Gravitational Lensing,” arXiv:0811.3916
 M. Zaldarriaga et al., “CMBPol Mission Concept Study: Reionization Science with the Cosmic
Microwave Background,” arXiv:0811.3918 [astro-ph].
 A. A. Fraisse et al., “CMBPol Mission Concept Study: Foreground Science Knowledge and
Prospects,” arXiv:0811.3920 [astro-ph].