For centuries, astronomers have wondered how the
galaxies and large-scale structures in our universe were formed. In
the second half of the 20th century, cosmologists realized that
these events had a witness: a hot bath of light, now cooled to a few
kelvin above absolute zero, which is the afterglow of the big bang.
The sky-pervading cosmic microwave background (CMB) radiation (1)
was released just before matter began to get structured. About 10
years ago, tiny variations discovered in its effective temperature
provided information on the size of the primordial seeds that led to
the nascent galaxies, after eons of gravitational evolution.
Now, another piece of evidence shows how these primordial seeds
were moving some 400,000 years after the big bang. With a radio
telescope at the South Pole, scientists from the DASI collaboration
have measured the minute level of orientation, or polarization, that
these microwaves received when they emerged from the seething
plasma--a signal that only the peculiar dynamics of the seeds
present at that epoch can generate (5,
Most light around us is unpolarized. Its many individual waves
oscillate in different planes as it propagates. But unpolarized
light becomes polarized whenever it is scattered or reflected, as in
sunglasses or in the surface of a lake. In these cases, most of the
intensity of the scattered light is concentrated in one plane along
the line of propagation, resulting in linearly polarized light.
Early on, when the universe was hot enough, matter was ionized
and the free electron density was so high that photons could not
propagate freely without colliding with electrons. But as the
universe expanded and the ambient temperature decreased, the
energetic collisions became less frequent. The relatively low-energy
photons that ensued could not destroy the increasing number of
neutral particles (essentially hydrogen and helium) that began to
form through combination of protons, neutrons, and electrons. Soon
after this "recombination" period, the CMB was released. According
to theory, it is at this precise time, nearly 14 billion years ago,
that the CMB became polarized.
CMB polarization was first proposed 35 years ago by Rees (7).
However, there was no evidence of its existence until the DASI
detection late last year. Polarization is an important probe for
cosmological models and for the more recent history of our nearby
universe. It arises from the interaction of the cosmic background
radiation with free electrons; hence, CMB polarization can only be
produced at the time of its last scattering, because afterwards no
free electrons exist. Unlike temperature fluctuations, polarization
is largely unaffected by inhomogeneities in the growing distribution
of matter after recombination.
To understand how the CMB becomes polarized, two points should be
clear. First, the oscillating electric field of the incoming
radiation will push the electron to also oscillate; the latter can
then be seen as electric dipole radiation, emitted preferentially
perpendicularly to the direction of oscillation (see the first
figure). Second, after interaction with the electron, the resulting
radiation field will be polarized with the same orientation as the
incident electromagnetic wave. These rules help to understand why
the CMB should be linearly polarized.
Playing tricks with
light. (Left) An electromagnetic linearly
polarized wave (red) oscillates in a given plane (pink). Reaching an
electron (orange ball), the wave induces the electron to also
oscillate, making it emit radiation (green). This resulting wave is
concentrated essentially in the (green) plane orthogonal to the
movement of the electron and is polarized like the incident wave.
(Right) Nonpolarized light can be decomposed into
the sum of two linearly polarized waves: one along the line of sight
(pink), the other along a perpendicular direction (green). Scattered
radiation due to the first wave is contained in the plane orthogonal
to the line of sight and cannot be detected. Only the second
component (green) reaches the observer and is polarized like the
Before the recombination epoch, the radiation field was
unpolarized. In unpolarized light, the transverse electric field can
be decomposed into two directions (pink and green arrows in the
first figure) that are orthogonal to the line of propagation. The
electric field component along the vertical direction (green arrow)
will make the electron oscillate also vertically. Hence, the dipolar
radiation will be maximal over the horizontal plane. Looking from
the side, we can easily detect this component. However, the incident
component oriented along our line of sight (pink arrow) will not
reach us, because this component causes the electron to also
oscillate along our line of sight, emitting the scattered dipole
radiation on the orthogonal plane. From our position, we cannot
perceive this radiation. Thus, it appears as if only the vertical
component of the incoming electric field has caused the radiation we
perceive. Recalling now the second rule, the resulting radiation
reaching the observer should have the same polarization as the
incident vertical component (green arrow). In conclusion, the
observer only receives one part of the incident radiation, and this
fraction is linearly polarized.
But so far, we are leaving aside the fact that, in the real case,
the target electron will be hit by radiation waves coming from all
possible directions. Thus, to convey the total effect, we need to
sum up all these contributions. Each of these waves will be
scattered to the observer in the form of linearly polarized
radiation, but each one with a different orientation. If the
incoming radiation were completely isotropic in intensity, the net
outgoing radiation would be unpolarized: In a spherically symmetric
configuration, no direction is privileged.
But the CMB is not perfectly isotropic. It has a tiny "quadrupole
anisotropy," first discovered by experiments on the COBE satellite
Hence, from any point of view, the orthogonal contributions to the
final polarization will be different, leaving a net linear
polarization in the scattered radiation. Theorists believe that this
is how the CMB polarization detected by DASI, and recently confirmed
with data from the Wilkinson Microwave Anisotropy Probe (WMAP)
There is one last point to emphasize. Before recombination,
ionized matter, electrons, and radiation formed a single fluid. In
this fluid, inertia was provided by massive nucleons while the
pressure was that of radiation. And this fluid supported sound
waves: The gravitational clumping tendency of the effective mass in
the perturbations was resisted by the restoring radiation pressure,
leading to acoustic oscillations in both fluid density and local
velocity. Now, the polarization field responds to the local
quadrupole moment at recombination, and this quadrupole is mainly
due to the Doppler shifts induced by the velocity field of the
This is why we know with certainty that the CMB polarization shows
the uncontaminated dynamics of the primordial seeds at
However, the polarized fraction of the temperature anisotropy is
small, because only those photons that scattered at the very last
instant of the decoupling process possess a sufficiently large
quadrupole moment that has not been lost in subsequent interactions.
The reason is that the scattering that generates polarization also
suppresses the quadrupole from which polarization arises. As a
result, the polarized fraction of the temperature anisotropy is no
more than 10%. Because the temperature anisotropies are at the
10-5 level, the polarization signal was expected to be at
the 10-6 level or just a few microkelvin--a phenomenal
The level of polarization detected by the DASI (see the second
figure) and WMAP collaborations was compared (5,
with predictions of one of the currently most favored theoretical
models, the so-called concordance model. This model best fits the
bulk of current astrophysical observations, and contains 5% of
ordinary matter, 22% of dark matter, and the rest in dark energy in
the form of Einstein's cosmological constant: a truly preposterous
universe. Comparing their results with the prediction of the
concordance model, scientists claim that polarization has been
detected with a 95% confidence level.
A measure of
motion. Intensity and polarization of the cosmic
microwave background (CMB) radiation measured with the DASI
telescope. The small temperature variations of the CMB are shown in
false color, with yellow indicating hot and red cold. The
polarization at each spot in the image is shown by a black line. The
length of the line shows the strength of the polarization; its
orientation indicates the direction in which the radiation is
polarized. The size of the white spot (lower left) approximates the
angular resolution of the observations.
As more independent experiments confirm and extend these
findings, the CMB polarization will become the next gold mine of
cosmology. A new window on the physics of the early universe is
opening wide in front of us. Through it, we expect to obtain key
information on the fundamental parameters of cosmology--and perhaps
even the mechanism behind the formation of large-scale structures in
References and Notes
- W. Hu, S. Dodelson, Annu. Rev. Astron. Astrophys.
40, 171 (2002) [ADS].
- G. F. Smoot et al., Astrophys. J.
396, L1 (1992) [ADS].
- DASI (Degree Angular Scale Interferometer) home page: astro.uchicago.edu/dasi/.
- E. Hivon. M. Kamionkowski, Science
- E. M. Leitch et al., Nature
420, 763 (2002) [Medline].
- J. Kovac et al., Nature
420, 772 (2002) [Medline].
- M. Rees, Astrophys. J. 153, L1
- C. L. Bennett et al., in preparation; see map.gsfc.nasa.gov.
- M. Zaldarriaga, D. Harari, Phys. Rev. D
52, 3276 (1995) [APS].
- The author is supported by Consejo Nacional de Investigaciones
Científicas y Técnicas (CONICET), the University of Buenos Aires,
and Fundación Antorchas (Argentina).
The author is in the Physics Department, University of Buenos Aires,
and at the Institute for Astronomy and Space Physics/CONICET, Ciudad
Universitaria, 1428 Buenos Aires, Argentina. E-mail: email@example.com
Number 5611, Issue of 28 Feb 2003, pp. 1333-1334.
Copyright © 2003 by The American Association for the
Advancement of Science. All rights reserved.