We have concentrated in this introductory text on the distinguishing features of radio astronomy, in the domains of both technique and astrophysics. In this concluding chapter we survey the prospects for astrophysical and cosmological advances through new and developing radio-astronomical observations, building on the attributes and techniques which are special to radio astronomy.

The Cosmic Century

In his masterly survey of the development of modern astronomy Longair (2006) gave this name to the twentieth century. Astronomy has been transformed both in terms of our conception of the Universe and regarding the ways in which we can observe it. Up to the middle of the century the term ‘telescopes’ needed no qualification: they are now ‘optical telescopes’, alongside infrared, X-ray, gamma-ray and radio telescopes. All have their own special techniques and their own particular domains of astrophysics and cosmology.

Radio astronomy provides unique information on phases of matter and radiation that are otherwise inaccessible. The low-temperature radiation of the cosmic microwave background has been studied from sub-millimetre to decimetre wavelengths, especially around its peak at millimetre wavelengths. Radio astronomy also encompasses radiation from electrons with cosmic-ray energies, often again in objects that are invisible or less accessible for observations at other wavelengths. Relativistic electron gas, detected by observing its synchrotron radiation, is characteristic of quasars, active galactic nuclei and many star systems. Magnetic fields in these synchrotron sources are revealed by the polarization of the synchrotron radiation.

All telescopes - radio, optical and X-ray - couple the electromagnetic radiation from sources in the Universe to the astronomer's measuring devices. Spacecraft can explore the solar system directly, but otherwise the Universe is accessible to us only by observing the distribution of electromagnetic radiation across the sky, including its variation with time, frequency and state of polarization. For the radio astronomer, the incoming radiation can be treated as a superposition of classical electromagnetic waves, whereas for the optical or X-ray astronomer, the radiation is arriving as photons, discrete quanta of energy. Infrared astronomy is between these extreme regimes; the ‘far’ infrared is close to millimetric wavelength radio in techniques, while the ‘near’ infrared is regarded as an extension of the optical regime. All astronomical observing starts with the telescope intercepting the incoming electromagnetic radiation. The received radiation goes to a radiometer, followed by a detection apparatus, which may be integral with the radiometer. The principal difference between radio astronomy and astronomy at other wavelengths is the use of low-noise amplifiers prior to signal detection and the consequent possibilities of using signal-processing techniques. The laws of quantum mechanics limit the use of amplifiers at shorter wavelengths in most cases.

This chapter is concerned with the properties of electromagnetic radiation, with an emphasis on fields rather than photons. Radio telescopes are treated in a generic sense, linking engineering and astronomical aspects.

A radio telescope intercepts the radiation coming from celestial sources, usually separating it into its two polarization components. The telescope sends the energy it receives through transmission lines to a receiving system where the signals are amplified and transferred to the detection system. The radio telescope must meet two basic requirements, sensitivity and angular resolution. The following three chapters are concerned with optimizing these two fundamental parameters. Sensitivity depends on having the largest collecting area possible, while minimizing the contributions of extraneous noise; this depends upon both the telescope design and the quality of the receiving system. The angular resolution is determined by the overall dimensions of the telescope.

In this chapter, we consider single-aperture telescopes, for which large area and high angular resolution go together. It is economically impossible to get the highest angular resolution by extending the size of a single aperture indefinitely, so it is necessary to use widely spaced single-aperture telescopes in an array. Such arrays, and the means by which their data are analysed, are discussed in Chapters 5 and 6. Arrays, which are now usually aperture-synthesis arrays, are made up of individual elements, usually paraboloidal radio telescopes, although they may themselves be arrays of dipoles if intended for long wavelengths. The individual elements instantaneously observe a patch of sky, determined by the beam pattern; this is called the field of view (FOV).When the elements are steerable, as they usually are for paraboloids, their design (or the horizon) defines the available sky.

The ‘alabaster curtain’ of the cosmic microwave background, the CMB, defines the limit of the directly observable Universe. Beyond that limit, the mean free path of photons is so short that the blackbody radiation field and the primitive plasma of ions and electrons are locked together in tight coupling. As expansion of the plasma proceeds, its density and temperature drop, and, when the Universe reaches an age that appears to be about 370 000 years, the era of decoupling is reached, the ions and electrons combine into atoms, and the photons escape through the nearly transparent neutral medium, to be observed as the CMB. Indirect paths of study of the Universe beyond the era of decoupling are few; one of these is the first era of nucleosynthesis, discussed in Chapter 14. There is the hope that neutrinos or more exotic particles might give another observation channel, reaching beyond the era of first nucleosythesis.

The Universe that we observe around us is clearly inhomogeneous, and that inhomogeneity had to grow from far smaller, unstable density fluctuations, urged on by the relentless force of gravity. After the discovery of the CMB, the search for spatial structure began almost immediately; it was generally recognized, by experimenters and theorists alike, that the testimony of those first perturbations must be present at some level in the apparently featureless facade of the CMB, motivating an intensive and difficult search.

The dynamical structure of our galaxy, the Milky Way system, is most simply described as a set of nested ellipsoids. The largest, approximately spherical halo, composed largely of dark matter, extends well beyond the stars of the Milky Way and is not yet understood. Within this there is a smaller, nearly spherical ellipsoid of old stars in orbits plunging through the plane of the Milky Way, appearing as the Population II high-velocity stars to Earth-bound observers, for our Sun is in an approximately circular orbit within the plane. The system of globular clusters, not as extended in distance from the centre, can be regarded as part of this old system. Within that, there is a quasi-ellipsoid of stars, denser near the Galactic centre, with an axial ratio of the order of 3 or 4 to 1; this includes the prominent bulge population of stars congregating about the Galactic centre in the central few kiloparsecs of the system. Finally, the Galactic plane is outlined by a thin disc of dust, gas and the youngest stars (Population I), stretching out to two or three times the Sun's distance from the centre, but with a thickness only about 1/100th the diameter of the disc. Much of Galactic radio astronomy concentrates on this extreme Population I region of the Milky Way. The stellar orbits are governed by the gravitational field of all the matter in the Milky Way system, with the Population I disc component in nearly circular motion.

The Michelson interferometer, whose basic properties were reviewed in the preceding chapter, was originally designed to measure the angular diameters of stars. It was first used in the radio domain by Ryle and Vonberg (1946) to find the angular diameter of sunspot radiation, and by McCready et al.(1947), who showed that interferometry could be used to make a map of the radio emission from the whole Sun. At radio wavelengths of the order of 1 m, no single aperture could map the Sun with enough angular resolution to be interesting, because of diffraction. The use of Michelson interferometry turned out to be an effective tool to obtain the necessary angular resolution. The technique was soon applied to study discrete radio sources, and, from these modest beginnings, large and complex interferometer systems have been built to map the distribution of brightness across small diameter radio sources, overcoming the limitations of diffraction that are inherent in single aperture telescopes. The resulting angular resolution now exceeds the resolving power of the largest optical telescopes.

The essential link between interferometer observations and the brightness distribution of a source is the Fourier transform, as the analysis in Section 5.4 has demonstrated: the amplitude and phase of the fringe visibility, defined by Equation (5.18), give one complex Fourier component of the brightness distribution. An array of radio telescopes, their outputs separately amplified and combined pairwise to form all possible interferometric combinations, is called an aperture-synthesis array.

In 1934 Baade and Zwicky suggested that the final stage of evolution of a massive star would be a catastrophic collapse, leading to a supernova explosion and leaving a small and very condensed remnant, a neutron star. This theory was immediately successful in explaining the observed supernovae, especially in relation to the Crab Nebula, which was identified as the remains of a supernova observed in the year 1054. Neutron stars seemed, however, to be hopelessly unobservable; they would be cold and only about the size of a small asteroid. More than 30 years later, neutron stars were discovered both in X-ray and in radio astronomy. As an X-ray source, a neutron star usually behaves as an intense thermal source of radiation; many are in binary systems, and the heating is then due to the accretion of matter from a binary companion. As a radio source, a neutron star is seen as a pulsar, when it behaves completely differently; it now radiates an intense beam of non-thermal radiation, rotating with the star, which is detected as a radio pulse as it sweeps across the observer. The intriguing and complex behaviour of the pulsars relates not only to condensed-matter physics but also to many aspects of stellar evolution, galactic structure and gravitational physics.

Early radio telescopes often use receivers with long integration times, which would not detect the periodic short pulses from pulsars.

For astronomers prior to 1925, the stars of the Milky Way seemed to form a Universe of immense size, but cosmology was transformed from philosophy to science in that year, when Edwin Hubble announced that he had identified Cepheid variables in the Andromeda nebula, M31. Through the known period-luminosity relation of Cepheids, he proved that M31, contrary to majority opinion, was a giant system of stars, comparable to the Milky Way. The stars of the Milky Way represented only a tiny fraction of a Universe populated by other galaxies fully as rich as our own. The climax of the Hubble revolution was reached in 1929 when, through measuring the Doppler shifts of the external galaxies, he demonstrated that the entire system of galaxies was expanding. The more distant the galaxy, the higher its radial velocity. This fitted nicely with cosmological models derived from solutions of Einstein's general theory of relativity (GR), whose cosmological interpretation of GR had puzzled theorists for over a decade, but it was now applied to Hubble's discovery and provided the intellectual framework within which a consistent cosmology could be developed. The ‘standard cosmological model’ of a Universe of galaxies, expanding from an initial state of high density, became the principal focus of theoretical cosmology for 20 years.

The second revolution in cosmology took place 35 years after the discovery of the expanding Universe, not from conventional optical observations, but from radio astronomy.

The role of radio observations in astronomy

Jansky's discovery of radio emission from The Milky Way is now seen as the birth of the new science of radio astronomy. Most astronomers remained unaware of this momentous event for at least the next decade, and its full significance became apparent only with the major discoveries in the 1950s and 1960s of the 21-cm hydrogen line, the quasars, the pulsars and the cosmic microwave background. These are now fully assimilated into astronomy, and radio is now regarded as one among the several tools available to astronomers in their pursuit of the astrophysics of our Galaxy, or of neutron stars, black holes or cosmology. Nevertheless, radio astronomy has its own distinctive character, not least in its techniques and the particular expertise which they demand. It also has several fields of application in which it is uniquely useful: there is no other way of exploring the cosmic microwave background, it allows spectroscopic investigation of molecular clouds in The Milky Way and it reveals a previously unseen Universe through the synchrotron emission of high-energy particles in stars, galaxies and quasars.

Radio wavelengths are hundreds to millions of times longer than optical wavelengths. Consequently, all single-aperture radio telescopes are hindered by severe diffraction effects, and their angular resolution is crude by optical standards. The application and development of radio interferometry, building on the rapidly developing arts of electronics and signal processing, overcame this handicap. In their 1947 studies of the Sun, Pawsey, McCready and Payne-Scott recognized that an interferometer's response to an extended source amounted to determining a particular value of the Fourier transform of the source brightness distribution. This insight was broadly recognized in the radio-astronomy community, and informed much of the work in Sydney, Cambridge and Manchester.

As the art of interferometry progressed, interferometers were used with multiple spacings to develop approximate Fourier transforms of extended sources, which could be inverted to give maps of the brightness distribution. Fourier concepts, reviewed in Chapter 3, became the natural language for discussing the brightness distribution across sources. Finally, Ryle formulated Earth-rotation synthesis, in which the rotation of the Earth is used to vary the orientation and length of interferometer baselines, yielding an extensive sampling of the Fourier transform of the sources each day. The aperture-synthesis arrays, such as the Westerbork Synthesis Radio Telescope in the Netherlands, the Merlin array in the UK, the Australia Telescope and the Very Large Array in the USA, all use this principle, with each possible pair of array elements forming a separate two-element interferometer.

The strongest discrete radio source in the sky, and the first to be discovered, is the Sun. At metre wavelengths, however, where the discovery was made, it was not the photospheric surface but the very extensive solar atmosphere, the corona, whose radio emissions were being observed. The visible discs of stars are generally best studied at optical and infrared wavelengths, but stars at a late stage of evolution may develop very large atmospheres, which are the seat of some of the most spectacular of radio emissions. These stars have left the main sequence, having exhausted hydrogen burning in their cores, cooling and expanding to form red giants. This chapter is concerned with the surfaces of the Sun and the planets in our solar system, and with thermal radiation from stellar atmospheres. The widest significance for astronomy comes from the circumstellar masers, which occur both in the out flowing stellar winds and in infalling accretion.

Surface brightness

At millimeter wavelengths the radio Sun is closely similar to the visible Sun; it approximates to a disc at a temperature not much higher than 6000 K, which is the temperature of the photosphere. At metre wavelengths it is larger, brighter and more variable, with occasional flaring outbursts; here we are observing the solar corona rather than the photosphere. In either case, however, stars like the Sun would not be observable at the distance of the nearest stars, where the inverse square law would reduce the observed flux density by a factor of 1010 or more.

Between the stars, the thin mixtures of gas, dust and high-energy particles that make up the interstellar medium have a strong influence on observations despite the rarefied density. At optical wavelengths, the dust absorbs starlight; at infrared wavelengths the dust emits thermal radiation; at sub-millimetre and millimetre wavelengths the gas molecules emit a rich array of spectral lines; at 21 cm the strong hyperfine line of interstellar atomic hydrogen is a tracer of galactic dynamics; at centimetre and metre wavelengths, there is thermal bremsstrahlung from the ionized hydrogen component; as the hydrogen atoms recombine, the high-quantum-number transitions give rise to recombination lines throughout the radio spectrum. In this chapter, the emphasis will be on the physical properties of the gaseous component as revealed by spectroscopic studies, and on the electron component as revealed by its effect on radio propagation. The following chapter will address the application of atomic and molecular radio spectroscopy to large-scale aspects of galactic structure. By general convention, ‘interstellar medium’ is usually abbreviated to ISM.

Atoms and molecules

Some atomic species, notably neutral hydrogen, are observable by virtue of transitions between high-quantum-number orbital states. These spectral lines, which are a continuation of the Balmer and other series in the visible spectrum, are known as recombination lines. They occur when hydrogen atoms are ionized by ultraviolet photons and recombine with the electron, initially in orbits with high quantum number, cascading down to lower orbital energies.

Both quasars and radio galaxies have large radio luminosities, and the most luminous of these can be detected at very large redshifts. Indeed, one of the complications encountered in determining the distribution of their intrinsic luminosities (the radio luminosity function) is that the objects are so faint optically that the largest telescopes are needed to obtain the spectra from which redshifts can be derived. At the same time, this raises the expectation that radio sources can serve as probes of the large-scale geometry of the Universe. The first indication of cosmological evolution was provided by the statistical relation between numbers and flux density of radio sources, the source counts; a similar test of cosmologies, the luminosity-volume testwas applied to visible quasars. These tests immediately ruled out the steady-state model of the Universe, but did not contribute to the precision cosmology which later emerged from the WMAP measurements of the CMB. The extension of the source counts in surveys of extreme sensitivity has, however, contributed dramatically to the astrophysics of radio-source evolution.

Another cosmological test, the relation between apparent source diameter and luminosity, emerges from the geometry outlined in Chapter 14. This again proves to be more of interest regarding source evolution than for cosmology itself. A further observational field opened by the geometrical theory is gravitational lensing, which was discovered as a radio phenomenon and is now observed through most of the electromagnetic spectrum.

Astronomy makes use of more than 20 decades of the electromagnetic spectrum, from radio to gamma rays. The observing techniques vary so much over this enormous range that there are distinct disciplines of gamma-ray, X-ray, ultraviolet, optical, infrared, millimetre and radio astronomy, often concentrated in individual observatories. Modern astrophysics depends on a synthesis of observations from the whole wavelength range, and the concentration on radio in this text needs some rationale. Apart from the history of the subject, which developed from radio communications rather than as a deliberate extension of conventional astronomy, there are two outstanding characteristics that call for a special exposition. First, the astrophysics: long-wavelength radio waves are most often observed as a continuum in which the interaction with matter follows classical electrodynamics. High-energy electrons are involved; they are created in a variety of circumstances, and their radiation as they circulate in magnetic fields gives evidence of new phenomena, often showing a close link to the phenomena observed in X-rays and gamma-rays. At the shorter wavelengths the low quantum energy gives access to spectral lines from atomic and molecular species at comparatively low temperatures. Second, the techniques: radio astronomy takes account of the phase as well as the intensity of incoming radio waves, allowing the development of interferometers of astonishingly high angular resolution and sensitivity

The electromagnetic signals that give information about the Universe have the characteristics of random noise. More specifically, in the radio part of the spectrum, the signals are composed of Rayleigh, or Gaussian, noise, the result of an assemblage of many random oscillators with random frequency and phase. As one moves to shorter wavelengths, through the infrared and into the optical, ultraviolet and X-ray bands, the discrete character of photons becomes increasingly dominant, and the random noise obeys Poisson statistics, sometimes called shot noise. Throughout the spectrum, the process of detecting and measuring the signals gathered by a telescope is almost always electronic; for the optical astronomer the eye and the photographic plate are not sensitive enough, and at both the radio and X-ray ends of the spectrum electronic means have always been essential. The device that measures the power of the incoming signal is a radiometer; when it measures power as a function of frequency, it is a spectrometer.

At wavelengths shorter than about 100 μm, immediate detection of the received power is almost always forced on the observer because the laws of quantum mechanics require any amplifier to add extraneous noise. For the radio astronomer, the incoming signal is amplified before its power is measured in a detector, and the construction of low-noise amplifiers has become an art.