Cosmologists have achieved a first
detection of a long-sought component in the Cosmic Microwave Background
(CMB). This component, known as B-mode polarisation, is caused by
gravitational lensing, the bending of light by massive structures as it
travels across the Universe. The result is based on the combination of
data from the South Pole Telescope and ESA's Herschel Space Observatory.
This detection is a milestone along the way to the possible discovery
of another kind of B-mode signal in the polarised CMB - a signal
produced by gravitational waves less than a second after the Universe
began.
E-modes and B-modes in the CMB polarisation
(left and right panels, respectively) and the gravitational potential
of the large-scale distribution of matter that is lensing the CMB
(central panel) from SPT and Herschel data. Credit: Image from D. Hanson, et al., 2013, Physical Review Letters, 111, 141301
The Cosmic Microwave Background is the most ancient light that has
travelled almost unimpeded across the Universe, and it contains a wealth
of information about the origin and nature of the cosmos. During their
journey, photons from the CMB have encountered a multitude of galaxies
and galaxy clusters and have been deflected by these large
concentrations of matter.
This phenomenon, known as gravitational lensing, imprints a subtle
distortion on the pattern of the CMB that encodes details about the
large-scale distribution of structure in the Universe. In recent years,
cosmologists have detected the signature of gravitational lensing on the
CMB temperature using data from ground-based and space-borne
experiments, including the
first all-sky image of this effect achieved using ESA's Planck satellite.
Credit: ESA and the Planck Collaboration
A small portion of the CMB is polarised,
and gravitational lensing also affects this part of the signal. In fact,
the polarised CMB is an additional and even richer treasure trove than
the unpolarised signal to use to explore the Universe's past. Now a team
of cosmologists studying the polarised CMB has detected in it the
signature of gravitational lensing, opening new and exciting
possibilities to study the distribution of matter across the cosmos.
This result is also the first detection of the elusive second component
of the CMB polarisation – the long-sought B-modes.
"The CMB is partially polarised: this means that it carries
additional directional information, like the light that can be observed
using polarised glasses," explains Joaquin Vieira from the
California Institute of Technology in Pasadena and University of
Illinois at Urbana-Champaign, USA. Vieira led the Herschel survey that
enabled this result.
"The pattern we observe in polarised light can be split in two
distinctive components: we call these E-modes and B-modes. In the case
of CMB polarisation, these two components carry very different and
complementary information about both the early and the late Universe."
The CMB is the glow from the early Universe, when it first became transparent to radiation, about 380 000
years after the Big Bang. There are fluctuations in both the
temperature of the CMB and its polarisation, which represent tiny
differences in density and pressure at that epoch. The polarisation of
the CMB has a distinctive pattern of E- and B-modes that dates back to
the early Universe. But this pattern, and in particular the intensity of
the B-mode component, underwent substantial changes as the polarised
CMB propagated across the Universe.
"When gravitational lensing distorts the polarised CMB photons, it transforms part of the E-modes into B-modes," explains Vieira.
Only a small fraction of the CMB is polarised, so it is a very weak
signal and extremely difficult to detect. The E-mode component of CMB
polarisation, which has a stronger intensity than the B-mode one, was
first observed in 2002 with the ground-based Degree Angular Scale
Interferometer (DASI), and with a variety of other experiments in the
following years. The B-modes are an extremely weak signal and, until
now, had remained undetected.
"In our study, we combined the polarised CMB observed by SPT with
independent data from Herschel. This technique allowed us to finally
spot the B-modes induced by gravitational lensing," comments Vieira.
The cosmologists detected the B-mode signal due to gravitational
lensing in the data from SPT. To make their detection more robust, they
added complementary observations from Herschel to trace the large-scale
distribution of galaxies that cause the lensing.
Joint observation from the South Pole Telescope (left panel) and Herschel (right panel)
Credit: Image from G. Holder et al., 2013,
The Astrophysical Journal Letters, 771, L16
"Herschel offers us a good data set to reconstruct the gravitational potential of the galaxies that are distorting the CMB," says Vieira.
"Including the Herschel data in our analysis made the SPTpol data
less sensitive to instrumental effects and was key to isolating the
lensing-induced B-mode signal."
With its wide spectral coverage ranging from far-infrared to
sub-millimetre wavelengths, Herschel is sensitive to the Cosmic Infrared
Background (CIB). In contrast to the CMB, which is the diffuse light
from the early Universe, the CIB is a cumulative background, and arose
with the formation of stars and galaxies, which started several
hundreds of millions of years after the Big Bang.
Whilst stars shine primarily at ultraviolet wavelengths, over the
entire age of the Universe roughly half of this energy has been absorbed
by cosmic dust within galaxies; this cold dust reradiates starlight at
longer, far-infrared wavelengths. For this reason, the CIB encapsulates
the cosmic history of star formation.
Galaxies tend to group in galaxy clusters, which are embedded in dark
matter halos, and these large concentrations of dark and normal matter
are what causes the gravitational lensing of the CMB. For this reason,
there is a very strong correlation between the gravitationally-lensed
CMB and the CIB detected by Herschel, as the latter traces the lenses
responsible for the deflection. By locating points in the sky where more
(or fewer) galaxies are present, the extra information contained in the
Herschel data allowed the team to see the gravitational lensing effect
more clearly.
Credit: Image from G. Holder et al., 2013,
The Astrophysical Journal Letters, 771, L16
This first result opens a new era in the study of the
gravitationally-lensed CMB. So far, cosmologists have successfully
studied gravitational lensing on the CMB temperature, but this signal is
subject to a large degree of intrinsic noise, and it will be extremely
difficult to improve significantly on the best current results. Studying
the effect of gravitational lensing in the polarised CMB, instead, is
expected to provide a much cleaner probe of the underlying distribution
of matter causing the lensing.
"Polarisation holds the key to the future of gravitational lensing studies of the CMB," comments Duncan Hanson from McGill University in Montréal, Canada, who is first author of the paper reporting the discovery.
"This field is in its early stages right now, but as we collect
more and more data, we will be able to study the large-scale
distribution of matter with ever greater precision."
The study was based on observations with SPTpol together with Herschel
data of a large patch of the sky, measuring 100 square degrees, that
overlaps with the survey performed with SPTpol.
"It is great to see this ingenious use of Herschel data in
achieving the first detection of B-modes in the CMB polarisation, which
are fluctuations at a level of one in about ten million," comments Göran Pilbratt, Herschel Project Scientist at ESA.
"This work displays yet another use of the treasure trove of the available Herschel data," he adds.
Apart from its application to gravitational lensing, the discovery of
B-modes is a milestone because it proves that it is possible to detect
such a signal. Worldwide, cosmologists are still searching for a
different type of B-modes, those created by primordial gravitational
waves, using experiments including SPT and Planck.
Cosmologists believe
that the Universe began with a very early phase of accelerated expansion
known as inflation. During this very rapid phase, which boosted the
size of the Universe exponentially, it is thought that gravitational
waves were also generated.
"Gravitational waves are ripples in the fabric of space-time, and
we think that those produced during inflation left an imprint in the
B-mode component of the CMB polarisation," explains co-author Stephen Hoover from the Kavli Institute for Cosmological Physics at the University of Chicago, USA.
Finding such a signal would provide crucial information to study the
very early Universe and inflation. Detection of B-modes induced by
primordial gravitational waves, however, may prove even more complex as
they are expected to have very different properties to those caused by
gravitational lensing. Since primordial B-modes become apparent on much
larger angular scales than those probed in this study, cosmologists will
need to scan and analyse the signal on larger portions of the sky.
Besides, cosmologists are still in the dark as to the amplitude and
shape of the signal they are looking for, due to the many theoretical
uncertainties that are still plaguing inflation.
"The fact that we were able to detect B-modes in the CMB
polarisation at all is a great experimental success. We're all eager to
find out whether this will be followed by the even more exciting
discovery of primordial gravitational waves," concludes Vieira.
Background information
The study is based on data from SPTpol, the polarisation-sensitive
instrument of the National Science Foundation's South Pole Telescope
(SPT), and from the SPIRE instrument on board ESA's Herschel Space
Observatory.
The Cosmic Microwave Background (CMB) originated when photons last
scattered off electrons at the epoch of recombination, when the Universe
was about 380 000 years old. Since the scattering process polarises
light, a small fraction of the CMB (less than ten per cent) is
polarised.
Polarised light carries additional, directional information, and its
pattern has two geometrical components. One way to split the polarised
fraction of the CMB in its two components is into E-modes and B-modes,
which are defined in a way that resembles the patterns of electric (E)
and magnetic (B) fields in electromagnetism.
The pattern of E-modes is aligned either tangentially or radially; it
would look the same when reflected in a mirror – something that
scientists call 'even parity'. In contrast, the pattern of B-modes is
aligned at an angle of 45 degrees with respect to that of E-modes: this
creates a 'handedness', meaning that the pattern changes orientation
when reflected in a mirror.
These two components of the CMB polarisation carry different,
complementary information. Of the two components, E-modes have a higher
intensity than B-modes.
Gravitational lensing, the bending of light caused by massive objects,
also affects the CMB as it propagates across the large-scale
distribution of structure that started populating the Universe a few
hundred million years after the Big Bang. Massive bodies such as
galaxies and galaxy clusters act as lenses and deflect the path of
photons, causing distortions to the image of distant sources. The effect
of the distortion on the CMB polarisation is a mixing of E- and
B-modes: part of the signal contained in E-modes is transferred to the
B-modes.
Like ordinary glass lenses, a gravitational lens is most effective when
located half way between the source of light and the observer. In a
cosmological context, the galaxies that most contribute to lens the CMB
are those located at a redshift z~2. These galaxies are best probed
through the longest-wavelength band on the SPIRE instrument on Herschel,
which is centred on 500 microns.
The data from Herschel contained information about the distribution of
the galaxies that are distorting the CMB (and its polarised component)
via gravitational lensing. As such, they provided extra leverage to make
the detection of the B-modes induced by gravitational lensing on the
CMB polarisation more robust.
Herschel is an ESA space observatory with science instruments provided
by European-led Principal Investigator consortia and with important
participation from NASA.
The SPIRE instrument contains an imaging photometer (camera) and an
imaging spectrometer. The camera operates in three wavelength bands
centred on 250, 350 and 500 µm, and so can make images of the sky
simultaneously in three sub-millimetre colours; the spectrometer covers
the wavelength range between 194 and 671 μm. SPIRE has been developed by
a consortium of institutes led by Cardiff Univ. (UK) and including
Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ.
Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial
College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL,
NHSC, Univ. Colorado (USA). This development has been supported by
national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS
(France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC, UKSA (UK);
and NASA (USA).
Herschel was launched on 14 May 2009 and completed science observations on 29 April 2013.
The SPT is a 10-metre diameter telescope located at the Amundsen-Scott
South Pole Station, Antarctica. It is supported by the National Science
Foundation, the Kavli Foundation and the Betty Moore Foundation.
Contacts
Joaquin Vieira
California Institute of Technology
Pasadena, CA, USA
and University of Illinois at Urbana-Champaign
Urbana, IL, USA
Email: vieira@caltech.edu
Phone: +1-949-887-5795
Phone: +1-514-398-6517
Stephen Hoover
Kavli Institute for Cosmological Physics
University of Chicago
Chicago, IL, USA
Email: hoover@kicp.uchicago.edu
Phone: +1-773-834-2103
Göran Pilbratt
Herschel Project Scientist
Research and Scientific Support Department
Science and Robotic Exploration Directorate
ESA, The Netherlands
Email: gpilbratt@rssd.esa.int
Phone: +31-71-565-3621
