Precise measurements of the cosmic background radiation reveal new details about the structure of the universe

The two parts of the article are presented here: the helium balloon under a satellite and the return of the cosmic constant

The pair of researchers Arno Panzias and Robert Wilson. This radiation, known as cosmic background radiation, hits the Earth all the time. It is similar to the radiation we use in our home microwave ovens, except that its intensity is weaker and it comes from every direction we look in space; It is impossible to point to a specific source that emits it.

In the photo: The Boomerang Project balloon against the background of a section of a map depicting the intensity of the background radiation from different directions in the sky

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Penzias Wilson, employees of the Bell Laboratories of the T&AT telephone company in the USA, used a huge radio antenna to test the intensity of background noise in the field of microwave radiation, as part of a project to examine the use of communication satellites. It turned out that the antenna picks up very faint background noise whose source cannot be pinpointed. The intensity of the noise did not depend on the place where the antenna was aimed.

Despite prolonged efforts, Penzias and Wilson were unable to locate any source of the weak radiation in the receiving equipment, antenna or its surroundings. The two researchers had to determine somewhere that the radiation they found was coming from space. But where in space? What is the astronomical source of radiation? The two had no answer to these questions. In their search for an explanation, they consulted with a group of cosmologists from Princeton University in the United States. The conclusion of the cosmologists was extremely surprising: the source of the radiation that was discovered is at the very beginning of the universe - in the Big Bang itself.

The currently accepted theory, according to which the universe was created in a huge primordial explosion known as the "big bang", predicts the existence of background radiation identical to the one discovered. It was an amazing discovery that earned Penzias Wilson the Nobel Prize in Physics. The communication project that Nahal originally worked on is also a success: microwaves are currently used as a main means of communication with communication satellites in space.

The study of the cosmic background radiation continued to yield exciting scientific results even in the XNUMXs and XNUMXs. It turned out that this radiation is very uniform: its intensity is the same in every direction. This property reflects the properties of the primordial matter shortly after the Big Bang: it appears that the matter was very uniform, with almost identical properties everywhere. However, the world we live in today is not uniform at all: stars and galaxies represent areas where the density of matter is very high, and between them are separated empty spaces where there is almost no matter.

The differences in density today must be due to small changes in the density of the primordial material. These changes should be reflected in the background radiation. Therefore, the researchers assumed that there must be differences, even if small, between the background radiation coming from different directions. If such changes are not found, the Big Bang theory will be in serious trouble. To measure the background radiation with great precision, NASA launched the COBE satellite in 1989. The observations of this satellite, and in particular the discovery of a tiny unevenness in the background radiation, in a manner consistent with the predictions of the Big Bang theory, are considered one of the most important scientific achievements of the end of the century.

And this is not the only information that can be extracted from the background radiation. Scientists have calculated that an accurate measurement of the typical rate of change of radiation intensity between points visible to an observer from Earth as close to each other in space may teach many more things. In fact, if we accept the assumptions accepted by most cosmologists, it is possible to deduce from the background radiation almost all the details about the structure of the universe: the density of matter and its average energy, the composition of its characteristic matter, its rate of expansion and its geometric shape. The COBE satellite was not equipped with instruments precise enough for such measurements. However, technological improvements made since its launch now make it possible to measure the distribution of radiation coming from space with ten times greater accuracy or more.

Two groups of researchers have recently published new results based on measurements of the cosmic background radiation using new and improved detectors installed in inflated balloons. The Earth's atmosphere contains large amounts of water vapor and other substances that make it almost impervious to microwaves. This is why accurate measurements of the microwave background radiation were made only after the launch of the COBE satellite outside the atmosphere. Launching research satellites involves a large financial investment and long years of waiting in line for launch by NASA. That's why the researchers turned to methods that were developed even before the launch of COBE: launching the scientific equipment using balloons filled with helium. The balloon ascends to the uppermost layers of the atmosphere, above the layers that most interfere with microwave observations.

An international research group, known as "Project Boomerang", launched a balloon to study the cosmic background radiation from a US military base at the South Pole. Two main advantages motivated the people of the group to go as far as Antarctica. The first is the dry atmosphere above the pole, where there is very little water vapor. The second advantage is the wind that blows in the polar region. For long periods of the year, a circular wind blows there. With its help, the balloon released above the launch base is carried around the pole and lands, after about two weeks, in almost the same place from which it was launched. The prolonged stay of the balloon in the air allowed the "Boomerang" people to reach measurements that are considered the most accurate measured to date of the background radiation.

Another research group, "Project Maxima", one of whose senior researchers is the Israeli Shaul Hanani, preferred to concentrate on further development of the detector technology. Their system, mounted on a balloon, was launched from the town of Palestine, Texas. The weight of the charming balloon can reach more than a ton. A meeting between him and a human could end in disaster. Since it is impossible to predict the exact landing place of the balloon, the researchers were required to perform statistical calculations and determine the chances that the balloon will fall, God forbid, in a settlement. In the end, the balloon flight was in an isolated area, where the chance of it hitting a person is less than one in a million. The researchers call these areas One MicroDeath Areas.

The Maxima balloon provided measurements that are similar to those of the "Boomerang" project. The analysis of the results of the two projects was published in recent months. It turns out that from the measurements it is indeed possible to learn about the most fundamental properties of the universe.

{Appeared in Haaretz newspaper, 28/8/2000}

Part II: Cosmic radiation, the cosmological constant and the inflationary model of the universe

The observations of the "Boomerang" and "Maxima" research groups, made using helium balloons, confirm the predictions of a popular cosmological theory based on the assumption of the big bang, and known as the "inflationary model". The cosmological model accepted today holds that the universe was created in an initial explosion, the "big bang", and since then it has been expanding and going. This model of the universe is based on many observations, primarily measurements showing that the galaxies in the universe are moving away from each other - that is, the universe today is expanding and expanding.

The first to point out this phenomenon was the famous American astronomer Edwin Hubble. Over the years, many researchers have pointed out various paradoxes and problems that exist in the model in its simplest form. In the 10s, the American physicist Alan Goth proposed an elegant solution to many of these problems, according to which the rate of expansion of the universe immediately after its formation is 40 times faster to the power of XNUMX than the big bang theory suggests. The rapid expansion process was called "swelling" (inflation), and the model based on this process is called the "inflationary model of the universe".

If this inflationary model is correct, the average density of all matter and energy in the universe must equal a certain critical value, which results from the solution of Einstein's equations of general relativity for the entire universe. When the model was proposed for the first time, it was known that the sum of all the ordinary matter (that is, particles like atoms, protons and electrons) and the energy we know (for example, light radiation) is not enough to reach the required critical value. The assumption was that there is "dark matter", which together with normal matter will bring the average density to the required critical value.

Dark matter is a substance whose properties differ from the matter we are familiar with in that it does not emit any electromagnetic radiation (like light). However, it can be distinguished with the help of the effect of its gravity on the environment, and the fact of its existence is based on various astronomical observations. In recent years it has become clear that even after adding all the amount of dark matter required to explain the various astronomical observations, the average density of the universe reaches at most one third of the critical density required for inflation to exist.

Two ways have been proposed to overcome this problem in the model. The first is the presentation of an inflationary model that does not require that the density reach the critical density. Such a model was proposed by the famous British physicist Stephen Hawking, but was highly criticized by other theorists because it was very forced, complicated and specially constructed to fit a set of observations that was known in advance.

A second way to settle the average density problem of the inflationary model is to assume that there is an additional component, apart from normal matter and energy and dark matter, that contributes to the density. Such a candidate was already known: energy density resulting from the existence of a "cosmological constant". The cosmological constant appeared on the stage of history immediately after the publication of general relativity by Einstein. It turned out then, that solving Einstein's equations for the entire universe shows that the universe cannot be stable; that it must expand or contract. Einstein, who believed that the universe is not expanding, added a constant term to his equations, representing a uniform energy density in space and constant in time. This additional element was called the "cosmological constant". Why was the cosmological constant needed? In Einstein's original equation, a member appeared representing the density of matter and energy in the universe, which create the force of attraction that exists between any two bodies. The cosmological constant represented an energy density that causes the presence of a kind of "repulsive force" that balances the gravitational force. Einstein thought that this addition would result in his equations describing a stable universe.

Shortly thereafter, in 1929, Edwin Hubble published his observations that the universe was expanding. These observations made Einstein admit that he was wrong; He even called the cosmological constant "the biggest mistake of my life". In retrospect it became clear that even in the presence of the cosmological constant the universe is unstable, so the reason for adding it is nullified in the first place. However, once the cosmological constant was released into the world's air, it could no longer be eliminated, and it reappeared from time to time for various reasons.

When it became clear that the normal forms of matter and energy (including dark matter) are not enough to reach the critical value that allows inflation, the possibility was raised that the missing energy is the one embodied in the cosmological constant. Until about two years ago, the two models existed side by side: the model according to which there is no cosmological constant, the average density is less than the critical density and the simple inflation model does not hold, and the model according to which the cosmological constant makes up the missing energy to the critical value, and the inflation model is valid.

In the recently presented article, background radiation researchers analyze the results of the two new projects "Boomerang" and "Maxima" in an integrated manner. The researchers compared the exact distribution of the intensity of the background radiation across the sky with the distribution that was expected according to various cosmological models. The new and accurate maps of the background radiation contain so much information that it is possible to rule out with their help most of the models tested, and draw conclusions about the physical properties of the universe.

The main results from these experiments are three: 1. The background radiation distribution corresponds to the predictions of the inflationary model; 2. The average density of matter and energy in the universe is approximately equal to the critical density; 3. The average density of normal matter (that is, one composed of particles familiar to us, such as protons, electrons and atoms, as opposed to "dark matter") is very small: only about three percent of the critical density. This implies that most of the matter and energy in the universe is hidden from our eyes in forms that are not quite clear to us: dark matter and the energy embodied in the "cosmological constant".

However, the background radiation measurements of the projects carried by balloons were not accurate enough to exhaust all the information stored in the background radiation. Also, there are certain contradictions in some of the measurements of the two projects. It seems that the final understanding of the information contained in the background radiation about the universe will have to wait until the results of additional, even more precise measurements, which will be carried out by two satellites: MAP, which should be launched next year by NASA, and Planck, which should be launched by the European Space Agency in about five years.
{Appeared in Haaretz newspaper, 29/8/2000}