radio background radiation
radio background radiationSee interstellar medium; radio source.
Radio Background Radiation
(residual cosmic radio radiation, relict radio radiation), electromagnetic radiation that fills the observable part of the universe. Such radiation already existed in the early stages of the expansion of the universe. Such radiation already existed in the early stages of the expansion of the universe. It has played an important role in the evolution of the universe and is a unique source of information about the universe’s past history. The intensity and spectrum of the radio background radiation corresponds to the radiation from the blackbody with a temperature of 2.7°K.
The radio background radiation was detected in 1965 in the radio band of the electromagnetic spectrum at a wavelength of 7.35 cm. At centimeter and decimeter wavelengths, observations of the radiation are carried out from the earth’s surface with radio telescopes. At millimeter and submillimeter wavelengths, emission from the earth’s atmosphere prevents observations of the radiation; therefore, broadband bolometers installed in balloons and rockets and lifted above the atmosphere are used. Observations at wavelengths of 50 cm to 0.5 mm show that the background radiation is evenly distributed over the celestial sphere and is the main component of the sky’s brightness at decimeter, centimeter, millimeter, and submillimeter wavelengths (see Figure 1). The background radiation fixes the energy density of the electromagnetic radiation in the universe at approximately 0.25 electron volts/cm3 and the photon density at approximately 400 per cm3. For each atom of the universe, there are more than 100 million photons of the background radiation.
The discovery of the background radiation confirmed a hypothesis proposed by G. A. Gamow in 1946 (the so-called “big bang” model of the universe), according to which the universe was characterized in the early stages of its expansion not only by a high density but also by a sufficiently high temperature to permit the nuclear fusion that results in the production of light elements. At these high temperatures, the plasma was in thermodynamic equilibrium with the radiation. In the course of the universe’s subsequent expansion, the temperature of the matter and of the radiation diminished adiabatically, protons and electrons recombined, and the equilibrium between the matter and the radiation was destroyed. However, the thermal radiation was preserved up to the present time and can be detected as the radio background radiation.
Studies of the radio background radiation yield valuable material for cosmogonic and cosmologic theories. Thus, from the absence of any marked anisotropy in the background radiation, one can draw conclusions about the large-scale properties of the universe, such as the universe’s isotropy and homogeneity. The detection of small-scale fluctuations in the temperature of the background radiation over the celestial sphere would make it possible to draw conclusions about the initial perturbations in the density and velocity of matter and to estimate when galaxies and clusters of galaxies were formed as a result of the increase in these perturbations. Detection of the departures of the background radiation from the laws governing the blackbody radiation would reveal sources of energy release that occurred when the radiation was cooling.
The background radiation has a substantial effect on a number of processes occurring also at the present time in the universe. Thus, the radiation determines the lifetime of relativistic electrons and superhigh-energy cosmic rays in intergalactic space; electrons, in scattering photons of background radiation, impart energy to them and are themselves braked. The energy of the photons is thus increased many times. This mechanism is possibly the cause of background X-radiation. When the photons of background radiation collide with ultrahigh-energy protons, pi-mesons are generated, and the protons quickly lose energy. Under certain conditions, the collision of these photons with cosmic-ray nuclei can lead to the splitting of the nuclei. The background radiation affects the population of the low energy states of molecules of interstellar matter. This is the basis, in particular, for an indirect method of determining the temperature of the radiation. The temperatures obtained using this method agree closely with those obtained from direct radio observations.
REFERENCESZel’dovich, la. B., and I. D. Novikov. Reliativistskaia astrofizika. Moscow, 1967.
Zel’dovich, la. B., and I. D. Novikov. Stroenie i evoliutsiia Vselennoi. Moscow, 1975.
Longeir, M. S., and R. A. Siuniaev. “Elektromagnitnoe izluchenie vo Vselennoi.” Uspekhifizicheskikh nauk, 1971, vol. 105, issue 1.
R. A. SIUNIAEV