|
An international team of
researchers taking part in the Archeops experiment has just obtained the
most precise measures ever obtained over a large range of angular scales
of the fossil radiation emitted by the Universe, a short time after the
Big Bang. This team, with a major French contribution from CNRS(1)
and from CEA(2) , is led by Alain Benoît, from
the Centre de recherche sur les très basses températures
(Very Low Temperature Research Center, CRTBT, CNRS). The results have
been obtained from measurements taken during a flight made on February
7, 2002 by CNES*, from the Base of Kiruna (Sweden).
They confirm that the Universe is spatially flat and give details on the
ordinary matter content of the Universe, perfectly in tune with the theory
of the Big Bang.
Measurement of the fossil radiation
is essential to obtain precise information on the evolution of the Universe:
density, expansion rate, age, etc. It is performed by means of a telescope
equipped with very sensitive detectors making it possible to take measurements
at temperatures close to absolute zero (0.1 Kelvin(3))
. The telescope is suspended from a stratospheric balloon making it possible
to avoid a large proportion of the interference radiation from the atmosphere.
The Archeops experiment is designed as a prototype of the high-frequency
instrument that is to be installed on board the European Space Agency’s
future satellite Planck, whose launch is scheduled for 2007.
Fossil radiation
Fossil radiation was emitted about 300,000 years after the Big Bang.
At that time, the Universe was made up of a gas that was hot (about 3,000
Kelvin) and homogenous. The light emitted by that gas (at a temperature
close to the temperature of the surface of the Sun) was thus visible light
with a wavelength of about one micron. As a result of the expansion of
the Universe, the wavelength of that light was increased to its current
value of close to one millimeter. The background of the sky thus appears
as radiation of radio waves from a black body at a temperature in the
vicinity of 3 Kelvin.
The results of Archeops in the international context
The first observations of that radiation were achieved by radio techniques
and at a wavelength in the vicinity of one centimeter by Penzias and Wilson
in 1965, for which they were awarded a Nobel Prize in 1978. The Cobe satellite
(1992) showed that, although the radiation was very isotropic, relative
variations in intensity existed, of a few millionths of the mean intensity
on scales of a few degrees. Since then the American and European experiments
BOOMERanG (2000) and Maxima (2000), and the Amercian experiment Dasi (2001)
have confirmed these measurements on angular scales of a few tens of arc
minutes, which has enabled them to establish one of the most outstanding
results in cosmology of the Twentieth Century.
By means of the results presented today, Archeops makes it possible, with
unrivaled precision, to confirm that the Universe is spatially flat at
very large scale. This result implies that the matter-energy density contained
in the Universe is extremely close to the critical value separating an
“open” (spatially infinite) universe from a “closed”
universe. Only a portion (about 5%) of said density is due to ordinary
matter, or baryonic matter, 25% being associated with other exotic forms
of matter whose nature is still unknown; one of the consequences of these
measurements is to confirm the acceleration of the expansion of the Universe
that had been shown by the observations using the I-a type supernovae
as distance indicators.
The Archeops
instrument
Archeops was born out of the thinking that took place when the high-frequency
instrument was being defined for the European Space Agency’s Planck
satellite. It is designed for mapping a large portion of the sky, which
gives it unprecedented sensitivity at very large angular scales. For this
purpose, the telescope (a mirror that is 1.5 m in diameter) is aimed upwards
at 49° to the vertical, and the entire assembly is suspended from
a balloon. An original mapping strategy has made it possible to observe
30% of the celestial vault in only 12 hours of observations. The only
drawback with this technique is the need to fly during a long moonless
night in order to avoid the radiation from the Sun and from the Moon.
That is why Archeops flew during the polar night in Kiruna to the north
of the Arctic Circle. The flight, which lasted 19 hours in all, made it
possible to collect measurements for 12 hours at an altitude of 33 km,
corresponding to covering 30% of the sky. In the wavelength range of 400
microns to 2 mm, the most sensitive detectors are bolometers : the rise
in temperature of a crystal cooled to a temperature of one tenth of a
degree above absolute zero (0.1 Kelvin) is measured. A very sophisticated
cooling system has been developed for space applications at the Centre
de recherches sur les très basses températures (CRTBT-CNRS),
in Grenoble. That system, designed for cooling the detectors of the ESA’s
Planck satellite, was successfully tested for the first time during the
flights of the Archeops balloon.
A precursor of the Planck satellite
The Archeops experiment has made it possible to test for the first time,
and with success, a certain number of cutting-edge elements that are to
be used on the Planck satellite. The results of Archeops were obtained
with two detectors that operated for 12 hours. In comparison, Planck will
take data on the sky for one year with 94 detectors, which will enable
it to obtain precision 100 times better than the precision obtained with
Archeops.
An international collaboration
The Archeops experiment is, in France, the fruit of the collaboration
of numerous research laboratories; it has enjoyed the support of the French
national cosmology program, of the CNES, and of the Rhône-Alpes
Region. This international collaboration brings together researchers from
French, Italian, British, and American laboratories. The cooling system
was developed at the CRTBT in Grenoble; the cold optics was manufactured
at the University of Cardiff; the stellar sensor was built at the University
of Rome, La Sapienza; the bolometers were supplied by the NASA/JPL laboratory
in collaboration with Caltech; the telescope was developed at the University
of Minnesota; the integration and the calibration of the instrument, the
launch campaigns, and the data analysis have been performed by all of
the French laboratories coming from three communities: solid physics,
astrophysics, and particle physics.
(1)
Département des sciences physiques et mathématiques (Department
of Physical Sciences and Mathematics), Institut national des sciences
de l’Univers (National Institute for Sciences of the Universe), Institut
national de physique nucléaire et de physique des particules (National
Institute of Nuclear and Particle Physics).
(2)DAPNIA
(3)0 Kelvin = -273.15°C
Further information (illustrations and commentaries) is available
on the Web Page of the Archeops experiment:
www.archeops.org
LIST OF LABORATORIES TAKING PART IN ARCHEOPS
Centre de recherches sur les très basses températures de
Grenoble (CNRS)
Centre d’étude spatiale des rayonnements de Toulouse (CNRS-université
Joseph Fourier)
Département d’astrophysique, de physique des particules, de
physique nucléaire et de l’Instrumentation associé
du CEA (CEA-Dapnia)
Institut d’astrophysique de Paris (CNRS-université Paris VI)
Institut d’astrophysique spatiale d’Orsay (CNRS-université
Paris 11)
Institut des sciences nucléaires de Grenoble (CNRS-université
Joseph Fourier)
Laboratoire de l’accélérateur linéaire d’Orsay
(CNRS-université Paris 11)
Laboratoire d’astrophysique de l’observatoire de Grenoble (CNRS-université
Joseph Fourier)
Laboratoire d’astrophysique de Toulouse (CNRS-université Paul
Sabatier)
Laboratoire de physique corpusculaire et cosmologique du Collège
de France de Paris (CNRS-Collège de France-université Paris
7)
University of Rome 1, La Sapienza
University of Florence
University of Cardiff, JPL/Caltech de Pasadena
University of Minnesota.
The balloon program of the Centre National d'Etudes
Spatiales (French National Center for Space Studies, CNES)
The CNES implements a balloon launching service for scientists or other
users. The balloons constitute a relatively inexpensive way of conducting
scientific space experiments or technological demonstrations.
They are capable of flying at altitudes of up to 45,000 meters for times
of a few hours to several months. The stratospheric balloon is today the
only means of exploring all of the layers of the stratosphere, and of
staying there long enough to perform various types of measurement.
The fields of scientific use are astronomy, study of space plasmas, physics
of the globe, and above all study of the atmosphere. Several types of
balloon are available, including Infrared Hot-Air Balloons (Montgolfières
InfraRouge, MIRs) and Open Stratospheric Balloons (Ballons Stratosphériques
Ouverts, BSOs).
Researcher
contacts:
SPM (Physical Sciences and Mathematics Department): Alain Benoît,
Research Director at CNRS and coordinator of the experiment, Centre de
recherche sur les très basses températures (CNRS-Grenoble).
e-mail: benoit@grenoble.cnrs.fr
IN2P3 (Nuclear and Particle Physics Department): Yannick Giraud-Héraud,
Research Director at CNRS, Laboratoire de physique corpusculaire et cosmologie
(Particle Physics and Cosmology Laboratory, Collège de France,
Université Paris7, CNRS – Paris)
e-mail: giraud-heraud@cdf.in2p3.fr
INSU (National Institute for Sciences of the Universe): François-Xavier
Désert, Laboratoire d'astrophysique de l'Observatoire des sciences
de l’univers de Grenoble (Astrophysics Laboratory of the Grenoble
Observatory of Sciences of the Universe, CNRS, Université Grenoble
1)
e-mail: Francois-Xavier.Desert@labs.polycnrs-gre.fr
CEA (French Atomic Energy Authority)/Dapnia: Dominique Yvon, Engineer
at CEA, Dapnia/SPP
e-mail: yvon@hep.saclay.cea.fr
Press contacts:
CNRS - Martine Hasler
Tel : +33 1 44 96 46 35
e-mail : martine.hasler@cnrs-dir.fr
CNES - Sandra Laly
Tel: +33 1 44 76 77 32 - 06 08 48 39 31
CEA: Anne Guichard
Tel: +33 1 40 56 21 56
e-mail: anne.guichard@cea.fr
|