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Read more: Instant Expert: The unseen universe

Radio and microwave telescopes expose the cold and quirky cosmos 鈥� from the chilled-out radiation of the big bang to extreme pulsars and quasars

Radio and microwave telescopes study the longest electromagnetic wavelengths 鈥� anything longer than about a millimetre. Some of these emissions are produced by the coldest objects in the cosmos, such as the 2.7-kelvin background radiation from the big bang.

Most, however, are generated as 鈥渟ynchrotron radiation鈥�, given off when electrons spiral through magnetic fields at close to the speed of light. Identifying the sources of this radiation has revealed some of the universe鈥檚 most extreme objects, such as pulsars and quasars.

鈥淚dentifying the sources of radio waves reveals some of the universe鈥檚 most extreme objects鈥�

Quasars

The first isolated celestial source of radio waves, Cyg A in the constellation Cygnus, was identified as a distant galaxy in 1954. By 1962 astronomers at the University of Cambridge had in the northern sky.

A few of these were remnants of supernovae in our galaxy, including an object 鈥� now known to be a pulsar 鈥� at the heart of the Crab nebula, the remains of a supernova explosion seen by Chinese astronomers in AD 1054. Most, however, were within distant galaxies. Some were associated with objects that looked like stars, and became known as quasi-stellar radio sources, or quasars. What these luminous, compact objects were was long controversial. Today we believe them to be supermassive black holes at the centre of distant galaxies, with masses ranging from a million to a billion times that of the sun.

We now suspect that most galaxies, including our own, have a black hole at their heart, and that in radio galaxies and quasars this black hole is swallowing up the surrounding gas. As the gas spirals in towards the hole, magnetic field lines in the gas get wound up too, accelerating electrons and producing radio waves. More than 200,000 quasars are now known.

Galactic interactions

Regular galaxies are suffused with hydrogen gas. As hydrogen atoms emit radio waves with a wavelength of 21 centimetres, radio telescopes can map this gas. Often it extends far beyond a galaxy鈥檚 visible boundary and can even link objects that appear separate. An example is the around 12 million light years away (pictured below). In an optical telescope these galaxies seem distinct, but radio observations show a web of hydrogen connects them, through which they tug at each other gravitationally.

We can get a wealth of information on the internal dynamics of galaxies by looking at other spectral lines from interstellar gas molecules, for example in the microwave band, which lies between the radio and the infrared. Such observations reveal that dense molecular clouds have a rich chemistry, much of it based on carbon: more than 140 molecules have been identified, with carbon monoxide the most abundant after hydrogen.

Pulsars

In 1967, Jocelyn Bell and Antony Hewish (above) were studying emissions from quasars with a new radio antenna on the edge of Cambridge, UK, when . It was the first of a new class of radio sources known as pulsars. These rapidly rotating neutron stars, the remnants of massive supernovas, have stupendous magnetic fields which can reach 10 gigateslas; Earth鈥檚 field, by comparison, is a puny 50 microteslas. As they spin, pulsars emit synchrotron radiation in jets that sweep through space like a lighthouse beam, resulting in the pulsing signal seen by our telescopes.

Radio telescopes have found thousands of pulsars with periods ranging from a millisecond to several seconds. In 1974, the orbit of a pulsar in a binary system with an ordinary, non-pulsing neutron star was seen to be slowing down exactly as it would if it were emitting gravitational waves 鈥� the only indirect evidence we have so far of this key prediction of Einstein鈥檚 general theory of relativity (see Instant Expert 1, 鈥淕eneral relativity鈥�, 快猫短视频, 3 July).

Cosmic microwave background

In 1965, while trying to make the first microwave observations of the Milky Way, Arno Penzias and Bob Wilson of Bell Labs in Holmdel, New Jersey, (below) found their instruments coming from all directions in the sky. This turned out to be one of the most important astronomical discoveries of the 20th century: the radiation left over from the big bang, known as the cosmic microwave background or CMB.

This radiation has a spectrum exactly like that of a body with a temperature of 2.73 kelvin, a spectacular confirmation of what the big bang theory predicts. Its strength is virtually identical no matter where you look: disregarding a systematic 1 in 1000 variation caused by our galaxy鈥檚 motion through the cosmos, its intensity varies by no more than 1 part in 100,000.

These tiny fluctations are nonetheless important, as they provide a wealth of information about the abundance of different types of mass and energy in the universe. Measurements of the CMB by the suggest just 4 per cent of the universe is ordinary matter, while 23 per cent is unseen dark matter, presumed to be made of unknown particles, and 73 per cent is the even more perplexing dark energy, whose nature remains a mystery.

The European Space Agency鈥檚 , launched in 2009 on the same rocket as the infrared telescope, will map the CMB in still more exquisite detail than WMAP, perhaps even detecting the fingerprint of gravitational waves left over from the early stages of the big bang.