The beginning of the era of exact cosmology

The MAP satellite recently entered an orbit from which it will measure the cosmic background radiation - the key to determining the fundamental values ​​of the universe

By: Yoel Rafaeli

The American (NASA), European and Japanese space agencies have so far launched many research satellites to measure radiation from planets, stars and galaxies, in order to deepen the knowledge of the celestial bodies. On June 30, 2001, NASA launched a second satellite to study the cosmic background radiation. Microwave Anisotropy Probe The purpose of this launch is particularly grandiose: the high quality of the MAP measurements should enable a very precise determination of the nature of the universe, its size, age and material-energy composition. Then the great cosmic riddles will be solved and the era of precise cosmology will begin.

The cosmic background radiation in the microwave frequency range is a cold remnant from the universe's fiery past. Radiation is the central pillar upon which rests the accepted model in the theory of the universe - the Big Bang model. According to this model, the primordial universe was extremely dense and hot in such a way that the matter and radiation in it were in a physical state known as thermodynamic equilibrium. In this situation the radiation field, which can be described by the dependence of the radiation intensity on the wave frequency (ie, the range), is fully determined by the identical temperature of the material and the radiation. The range then has a universal shape (technically known as the Planck curve of a "black body"). An interesting fact is that this shape does not change even when the expanding universe becomes thin and cold, and is no longer in equilibrium. Furthermore, according to the accepted cosmological theory, the radiation field in the very early universe was extremely uniform, so that the intensity of the radiation - and therefore the temperature - in different directions in the sky was the same, to a tremendous degree of accuracy. At the same time, to explain the creation of stars and galaxies, and the visible structure of the universe, cosmological theory requires a small degree of non-uniformity (initially), so that the radiation temperature is slightly different in different directions in it.

Radiation is found in the entire universe and is ancient: with its help we can learn about the primordial universe, when it was only about 300 thousand years old, a very small fraction of its age today, 20-13 billion years. During this period, known as the "reconnection era", the radiation particles (photons) were created for the last time, when the electrons and protons connected to form hydrogen atoms. Until then there was a close connection between matter and radiation in such a way that photons were generated and absorbed frequently. In the age of reconnection, the coupling between matter and radiation was eliminated, and this age constitutes a limit to our ability to "see" the early universe. Since the effect of the material on the radiation is mainly through its scattering, the area where the radiation detached from the material and has been moving undisturbed ever since is also called the "last scattering surface".

The processes that took place in the age of connection leave a unique and characteristic mark on the spatial distribution of radiation. This situation is somewhat similar to the emission of light from the face of the sun. Analyzing the properties of sunlight makes it possible to determine the temperature on its surface, as well as to estimate the density of the material in the outer shell of the sun. Similarly, it is possible to learn about the properties of matter and radiation in the universe at the stage when the radiation was detached from the matter and moved freely without absorption or scattering, all the way to the measuring device on the surface of the earth. While the mapping of the spatial distribution of the radiation over large areas makes it possible to learn about the cosmic background in which the radiation travels to us - for example, about the curvature of the cosmic space - measuring the distribution over a small area, an area that was the entire visible universe in the age of reconnection, makes it possible to determine the density of ordinary matter and other important sizes.

COBE, the first cosmological satellite, was launched by NASA in 1989. One of the measuring devices on the satellite measured the radiation range with incredible precision. The measured distribution exactly fit the Planck curve. This result is extremely important, even if not surprising. More important was the measurement of the spatial distribution of the radiation across the sky by another instrument (DMR). This instrument measured the radiation temperature in different directions in the sky and discovered, for the first time, that there is a small non-uniformity - at the rate of one in a hundred thousand - in the temperature value in different directions. This sensitive measurement was made possible by mapping the entire sky for about four years. Measurements over such a long period make it possible to minimize the various "measurement noises" and to deduce the size of the deviation from uniformity statistically with a very high degree of significance.

The importance of these COBE measurements is very great: their results fundamentally and significantly substantiate the Big Bang theory. When the first results of the data processing were published in 1992 and it was clear that the expected non-uniformity in the spatial distribution of the radiation had finally been discovered, the enthusiasm among physicists was so great that some hailed the discovery as one of the most important scientific results of the twentieth century. The great success of COBE led to a very impressive development in research in cosmology in general, and in the study of background radiation in particular. Since the beginning of the nineties, many measurement systems (telescopes equipped with sensitive detectors) have measured the spatial structure of the radiation in an ever-expanding frequency range, and more importantly - with a much higher spatial resolution (resolution) than that of COBE.

COBE measurements focused on characterizing the range and spatial distribution of radiation across the surface
large areas in the sky. In order to determine in more detail the composition of matter in the universe and the basic sizes that determine how it evolved from a state of almost complete uniformity in the primordial universe to its visible structure today, it is necessary to measure the distribution of radiation over much smaller areas. Direct observational evidence of the physical processes that took place during the transition of the universe from a state in which the normal matter in it was hot and ionized to a state in which the matter turned into atoms of hydrogen and helium - processes that occur mainly in regions whose angular diameter is small - will allow us to determine much more precisely what is the density of the normal matter, the dark matter, and other possible contributions to the mass-energy density in the universe. The importance of knowing these densities is not purely technical: Einstein's theory of general relativity fully and accurately encompasses the universe's geometric nature (flat or curved space), size (finite or infinite) and rate of expansion (deceleration or acceleration) of the universe in its material-energetic content.

As mentioned, there has been a great development in the degree of accuracy of background radiation measurements since COBE, mainly by raising measurement systems on balloons to an altitude of several tens of kilometers. In measurements from these heights, the absorption of radiation by the atmosphere is much smaller than at the surface of the ground. The most important results were obtained by two similar measurement systems - MAXIMA and - BOOMERANG which measured the radiation in a wide range of frequencies. BOOMERANG measurements were made over 10 days (in early January 1999) as the balloon circled over Antarctica around the South Pole. According to the data from these measurements, it is possible to determine in a much more detailed manner the values ​​of most of the important cosmological quantities - the density of dark matter, the contribution of the cosmological constant to the energy density, the density of normal matter, the nature of the initial non-uniformity range, and other quantities. The basic conclusion from these and other results is particularly important: the universe is infinite, and its space is flat.

This success raises the question of whether it was necessary to launch MAP, whose cost is much higher, of course, than the cost of launching a balloon, if it is possible to achieve the same scientific goals in this way. The answer is that it is indeed possible to achieve some of the research goals using balloons, but there is still a great need for measurements over a longer period of time - several years, as opposed to tens of days - and a much more extensive and complete mapping of the sky. These are the main advantages of MAP compared to measurements from a balloon.

About two months ago, on October 1, MAP reached a point about 1.5 million km from the Earth on the other side of the Earth-Sun line (see figure). The satellite enters an orbit around this point in such a way that its distance from each of these bodies is constant (this point is called Lagrange L2). Movement in this orbit has very important advantages: the measuring devices can always be directed away from the sun, the earth and the moon together, and the fixed distance from these bodies (the intensity of radiation from which is much higher than that of the cosmic background radiation) allows measurements to be made in a constant radiation environment, and under uniform temperature conditions . This is the first time that a NASA satellite is in an orbit that is so far from Earth. The satellite instruments have already started initial measurements of the background radiation. The measurements are planned to last about 27 months, with the satellite completing a full mapping of the sky every six months.

Prof. Rafaeli is the chairman of the astrophysics department at Tel Aviv University