Imagine: 300 000 years after the universe was created darkness reigned. Then, suddenly, the first rays of light burst through the shadows. The cosmos has retained a trace of this original moment in the form of background radiation. As such, it constitutes a unique memory, providing elements of a response to questions of the birth of the universe, its expansion and its future.
Arno Penzias and Robert Wilson in front of the Horn Antenna built by the Bell Telephone laboratories in the town of Holmdel (New Jersey). Bell discovered that the device was capturing a strange signal and – fortunately – let the astrophysicists know. Thus began the story of cosmic background radiation.
In 1965, two engineers employed by the Bell Telephones laboratories – Arno Penzias and Robert Wilson – were working in New Jersey. They were constructing a huge metal antenna in the shape of a horn, known as the Holmdel Horn Antenna. It was to serve as a relay between the Earth and Telstar, the first communication satellite. They soon noticed that their antenna was systematically picking up a strange signal whose level remained constant whichever way they turned the reception antenna and for which they had no explanation. However much they looked at possible electronic interference and cleaned the antenna from top to bottom this undesirable interference remained.
Fortunately, Penzias and Wilson decided to discuss the problem with astrophysicists which led to the realisation that the origin was background radiation of cosmological origin. Such a phenomenon had been conceptually predicted back in 1948 by Georges Gamow when working on the hypotheses of the Big Bang and the expanding universe. What is more, two Princeton scientists, Robert Dicke and James Peebles, were actively searching for signs of it at the very time. This discovery was of major importance to cosmology and earned the two Bell engineers the Nobel Prize for physics in 1978. Subsequently, it became the subject of many experiments, which have continued to the present and will continue well into the future. In 2007, for example, the European Space Agency (ESA) will be devoting the Planck mission to the phenomenon, the results of which are eagerly awaited by the experts (see box).
Background radiation is like a ‘fossil’, preserving traces of a universe in its infancy between around 300 000 and 400 000 years after the Big Bang. It bears witness to the very first rays of light to penetrate the universal darkness freely. We know that light is emitted by matter in the form of electromagnetic waves or particles known as photons (the famous wave-corpuscle duality of quantum mechanics). Before this stage, as soon as a photon appeared, it was immediately reabsorbed by matter due to the latter’s enormous density – rather like an individual trying to get through a crowd only to bump into another person. However, from the original instant of the Big Bang, the universe began to expand like a gradually inflating balloon and, as its volume rose, so its temperature and the density of its matter decreased over time. After thousand of years of darkness, this density became sufficiently weak for a photon to be able finally to launch itself without encountering an obstacle. At this stage in its development we believe the universe was 1 000 times smaller than it is today and with a temperature of 3 000 degrees Kelvin(1).
This fossil radiation as captured by Penzias and Wilson is, thus, seen as the physical proof of the Big Bang and of the existence of an extremely hot and dense state at the universe’s genesis. Its remarkable characteristic is the unity of its temperature in all directions. Although it has cooled considerably since its creation, reaching the very low temperature of 2.73 degrees Kelvin, it has retained this characteristic feature of uniformity.
Universal inflation This uniformity fascinated the scientists. The observable universe is made up of ‘causally disjointed’ regions. This means regions that are so distant from one another that they should normally not share the same temperature. The most commonly accepted explanation of the fact that all the background radiation in all these regions gives off a temperature of 2.73 degrees Kelvin is that, in the early life of the universe, these regions were causally linked and, thus, could have been at the same temperature. The expansion of the universe then accelerated dramatically, causing it to grow exponentially by a factor of 60 – referred to by cosmologists as the inflation hypothesis (2) – before entering a phase of deceleration. The inflationary period was brief but sufficient to break the unique causality of the universe, of which the background radiation is the ‘memory’.
However, the determined efforts of astrophysicists to retrace an extraordinary past subsequently showed that this homogeneity of temperature on in the background cosmos is not so perfect after all. In 1992, NASA’s Cobe (Cosmic Background Explorer) satellite discovered small fluctuations around this pivotal value of 2.73 Kelvin. Like fossils, these were the mark of fluctuations in the density of matter when the universe was about 300 000 years old. They revealed the existence of ‘seeds of matter’ out of which the galaxies and other major cosmic structures would develop.
In 2001, NASA launched the Wilkinson Microwave Anisotropy Probe (WMAP) to observe more precisely the subtle nuances of this cosmic background that was such a rich field of inquiry for researchers seeking to delve into the literally universal memory. Compared with the Cobe instrument, which was unable to detect details separated by less than 7° in the sky, this probe made it possible to obtain much more precise data, with a separation of under 0.3°.
The dramatic improvements in astrophysical observation technologies opened up a new era. The initial paradigm of the Big Bang and the inflationary cosmos had to accommodate not only new knowledge but also new questions. In particular, the WMAP probe made it possible to estimate the age of the universe at 13.7 billion years – a figure compatible with the oldest objects observed by astronomers.
The next meeting of these ‘cosmic archaeologists’ will be in 2007, during the much-awaited launch of the European Planck mission. This will not only make it possible to measure cosmological background radiation with unequalled precision but, above all, to study its polarisation and the ability of a light wave to vibrate in a plane, an aspect that is crucial to our understanding of the origin and the future of the universe.
(1) The measurement of temperature in degrees Kelvin is based on the physical state of matter at ‘absolute zero’ (minus 273°C), below which it is impossible to descend. (2)The inflationary paradigm is today expressed in the form of various competing theories. Most take into account the presence of scalar fields (a mathematical function that associates a number with each point in space and time – the temperature of a room, for example) representing bosons predicted by certain theories of particle physics (super symmetry, Higgs particles, etc). They all consider that the universe experienced an accelerated and excessively rapid period of expansion, known as inflation. After this event, the potential energy of scalar fields was transferred to elementary particles in a spate of fusions and decreases, the scenarios of which vary from one model to another.
Planck: while awaiting the next stage The future Planck satellite instrument, designed under ESA auspices, will be launched jointly with the Herschel Space Observatory in 2007. The mission’s principal scientific objective is to enter a new stage in the study ...
From Hubble to Herschel The Herschel Space Observatory is the fourth cornerstone of the ESA’s Horizon 2000 programme. The mission initially bore the name FIRST and was renamed in December 2000 in honour of the astronomer William Herschel who discovered infrared ...
Planck: while awaiting the next stage The future Planck satellite instrument, designed under ESA auspices, will be launched jointly with the Herschel Space Observatory in 2007. The mission’s principal scientific objective is to enter a new stage in the study of the background cosmic radiation and to provide some answers to two fundamental questions: how has the universe evolved to date and how might it evolve in the future?
Planck is the fruit of intense European co-operation, from conception to launch, which is reflected in every one of its technological components. One of its most precious tools is the High Frequency Instrument (HFI), the result of a collaborative effort between some 20 partners and coordinated by the Institut d’Astrophysique Spatiale d’Orsay (FR). The HFI will be studying background radiation at wavelengths of between 1/3 and 3 millimetres inclusive. The array of 50 bolometric detectors(1) that make up the device must be cooled to the extremely low temperature of 0.1 degrees above absolute zero.
"This will enable measurements of cosmological background radiation to achieve a level of sensitivity dictated by the fundamental laws of physics. In a way, it is the perfect instrument,” enthuses Jean-Michel Lamarre, director of the Lerma (a laboratory at the Paris Observatory) and one of the men who inspired the choice of this technology. "We expect Planck to achieve a sensitivity 30 times higher than the WMAP, a better angular resolution and, above all, a clear improvement in measuring the background radiation polarisation. It is this that holds the proof of the existence of this famous inflation that is, today, a simple hypothesis awaiting experimental validation.”
Planck will also be equipped with the Low Frequency Instrument (LFI), manufactured by a consortium of 22 partners under the management of the Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri of Bologna (IT). With its 56 radio receivers operating at 20 degrees Kelvin, the LFI will pick up background radiation at wavelengths of between three and ten millimetres. The probe will also be equipped with three large mirrors of between one and two metres in diameters, which will be precise, rigid and light. These mirrors are made of carbon fibre and were designed and built under the auspices of the Danish Space Research Institute.
(1) The bolometer was invented by the American Samuel Pierpont Langley, in 1881, to study sunrays. It makes it possible to convert radiation into heat. But it has only been within the past 20 years that this technology has become sufficiently developed, thanks to continuous refrigeration techniques, to permit the detection of low-level radiation.
From Hubble to Herschel The Herschel Space Observatory is the fourth cornerstone of the ESA’s Horizon 2000 programme. The mission initially bore the name FIRST and was renamed in December 2000 in honour of the astronomer William Herschel who discovered infrared radiation in 1800. It is hoped that the launch of this impressive space telescope, equipped to explore the submillimetric/infrared wavelength range, will lead to some spectacular astrophysical discoveries, similar to those achieved by the Hubble telescope. http://sci.esa.int/herschel/