It seems that Douglas Adams was right after all: the answer to Life, the Universe and everything, is 42.

Cambridge astronomers have found that 42 is the value of an essential scientific constant - one which determines the age of the universe.

In his novel The Hitch Hiker's Guide to the Galaxy (1979) Mr Adams describes how an alien race programs a computer called Deep Thought to provide the ultimate answer to "Life, the Universe and Everything". After seven and a half million years' calculation, back came the answer - 42.

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In slightly less time - two years- a team at the Cavendish Laboratory has managed the same feat, using a new technique to estimate the value of the "Hubble Constant". This measures how quickly objects in the universe are receding from each other - a natural outcome of the Big Bang that created the universe. Dr Richard Saunders, who led the research, sounded a trifle abashed by the result. "We have taken two measurements for the constant, and the average of them is, well, it's 42," he said. But he insisted this is "entirely fortuitous" - though thousands of fans of the Hitch Hiker novels might disagree.

Mr Adams said yesterday that when he wrote the novel 20 years ago he chose the number especially for its bathetic nature: "I wanted a nice, ordinary number, one that you wouldn't mind taking home and introducing to your parents."

But later he realised that the choice was no accident: when he was working for John Cleese's film company, Video Arts, as a "prop borrower", he and the other writers picked 42 for its amusing qualities as a punchline.

The Hubble Constant indicates the age of the universe because if we know how quickly everything is flying apart, we can work out how long ago it was all together at the same point - like working out how long a film has been running by measuring the film and knowing how many frames per second it shows.

Astronomers have bickered for decades about the constant's value, calculating it to be anywhere between 20 and 80. But large values imply that the universe is younger than its oldest stars - a logical conundrum which the new value avoids, said Dr Saunders, as it puts the universe's age at about 16 billion years.

The Cambridge team produced the measurement by combining data from X- ray telescopes with information about cosmic background radiation, leftover energy in space from Big Bang. Dr Saunders insists future revisions will alter the value of the constant from its present, resonant value. That would suit Mr Adams: "It does come up awfully often," he said.