The steady-state theory suggests that the universe is always expanding but maintaining a constant average density, matter being continuously created to form new stars and galaxies at the same rate that old ones become unobservable as a consequence of their increasing distance and velocity of recession. A steady-state universe has no beginning or end in time; and from any point within it, the average density and arrangement of galaxies is the same. Galaxies of all possible ages are intermingled. This hypothesis was first put forward in 1920 by the cosmologist Sir James Jeans.
After the war Bondi - then a fellow of Trinity College, Cambridge, working with Hoyle and Gold - developed the theory further. In 1952 he published a seminal book, Cosmology, in which he set out the theory in full. Later, in 1959, with Dr R A Lyttleton, he suggested that the outward movement of galaxies could be due to the action of electrical charges sufficient to cause their repulsion away from one another.
During the 1950s the steady-state theory stimulated a great deal of productive research in cosmology. However, it fell out of favour when it was discovered, from research into radio galaxies and quasars in the far universe, that the universe had been more dense, and hotter, in the past. The theory encountered further difficulties in 1965, following the discovery, by Arno Penzias and Robert Wilson, of microwave background radiation, which suggested that the Big Bang theory - the creation of the universe from a single cataclysmic event - was a more likely explanation.
Bondi was sufficiently objective to realise that Big Bang was the more credible of the two theories, and revised his opinions. However, Hoyle and Gold refused to abandon the steady-state theory, still insisting that the universe was infinitely old.
Hermann Bondi was born of Jewish parents on November 1 1919, in a hospital in a remote part of Austria for Russian prisoners of war run by his father, a doctor. His early upbringing in Vienna was rationalist and socialist. He was educated at the Vienna Realgymnasium, where he was a popular pupil despite his Jewish background and precocity. During the early 1930s, however, as civil war broke out and anti-semitism became more intense, Bondi became increasingly unhappy. He loved skiing and climbing, but hated having to exercise bare-chested under the supervision of crypto-Nazis who made no effort to hide their anti-semitism.
His mathematics, however, was effortless. He once completed a two-hour exam in 10 minutes, then tackled the alternative paper in another 10. By the age of 12 he was on to calculus, and by 15 he was studying undergraduate textbooks on theoretical physics.
Within a couple of years there was nothing to keep him in Austria. He felt drawn to England by its devotion to democracy and by the tremendous scientific standing of Cambridge University. So, encouraged by the astronomer Sir Arthur Eddington, he applied to Trinity. When he arrived in 1937, he said it was like moving from a lower civilisation to a higher one. Six months later, as Hitler's troops marched into Austria, the rest of his family fled to Switzerland in response to a frantic telegram from Cambridge telling them to leave "immediately ". They eventually settled in New York.
In 1940, shortly before the Battle of Britain, Bondi was interned on the Isle of Man and later in Canada. On his first night in internment camp, he met his future lifelong friend, Thomas Gold, another Viennese Jew studying at Trinity. Released from internment after 15 months, in 1942 Bondi became number two to Fred Hoyle, the astrophysicist who was working on radar at the Admiralty Signals establishment. The two men found themselves holed up in the cafˇ on the top of Snowdon, waited upon by a batman and supplied with stores brought up by the rack and pinion mountain railway. There they tested equipment designed to spot enemy submarines from the air.
By 1945 Bondi was back at Trinity on a research fellowship, and the scientific papers flowed out. He wrote not only on the steady-state theory, but also on fluid motion and electromagnetism, the sun's corona and geophysics. In 1946 he became a naturalised British citizen. "I never had any doubt that this was where I wanted to live for the rest of my days," he said. Two years later he became a university lecturer at Cambridge; finally, in 1954, he took up a chair of mathematics at King's College, London.
Bondi was a inspired lecturer. He spoke, usually without notes, giving the impression of off-the-cuff brilliance, something which is unusually difficult to carry off in mathematics.
He often spiced his lectures with jokes: "Never throw rocks down a mountain," he would say, "even when you know there's someone underneath"; or, "Do you know the worst curse a wicked fairy could pronounce at the birth of a Jewish child? May you be the only rich member of a poor family."
So wide-ranging were Bondi's scientific interests that, after he was appointed a Fellow of the Royal Society in 1959, he was set to work by the government on a range of projects about which, he had to confess, he knew very little.
These included feasibility studies for the Thames Barrier (which he persuaded the government to pursue) and a Severn Barrage (which never came to fruition); the Anglo-Australian telescope; rocket launchers; communications satellites; ecology and energy policy. He continued to write prolifically on subjects ranging from the origin of the universe to the exponential growth of algae on the Norfolk Broads.
In 1967 Bondi became Director-General of the European Space Research Organisation, commuting weekly to Paris. By the time he left, in 1971, to become chief scientific adviser to the Ministry of Defence, the agency had launched three new satellites, developed a good tracking network and was well advanced with plans to launch a further satellite. In 1971 he became chief scientific adviser at the Ministry of Defence, where he was an unwavering supporter of Britain's independent nuclear deterrent.
He also made himself popular with the services by opposing the Defence Secretary Denis Healey's insistence on cost-effectiveness, observing that, according to this principle, private lavatories, occupied on average for only two per cent of the time, would never be installed in homes; public conveniences would satisfy the nation's needs more cost-effectively. In 1977 he became chief scientific adviser to Tony Benn at the Department of Energy. He became chairman and chief executive of the Natural Environmental Research Council in 1980, and three years later was elected Master of Churchill College, Cambridge.
Bondi held visiting professorships at a number of universities throughout the world and won many international prizes and medals. In 1983 he became the second British scientist to win the Einstein gold medal.
He always considered himself to have been lucky in life: "Sometimes it seems to me that I have been walking through life with a wide open mouth and roast ducks have come flying in with monotonous regularity," he said.
Yet Bondi was no mere theoretical scientist. At his home near Cambridge he had designed the family kitchen, heating and plumbing, and was proud that, for 28 years, he had managed his own septic tank.
A humanist throughout his life, he claimed never to have "felt the need for religion".
He was president of the British Humanist Association in 1982 and of the Rationalist Press Association from 1982. He worked under a portrait of his hero, Albert Einstein.
Hermann Bondi was knighted in 1973.
He married, in 1947, Christine Stockman, whom he met while she was working as an astrophysics research student with Fred Hoyle. They had two sons and three daughters.