The Book of Nothing by John D. Barrow

Author:John D. Barrow
Language: eng
Format: mobi, epub, pdf
ISBN: 9780307554819
Publisher: Knopf Doubleday Publishing Group
Published: 2000-10-15T10:00:00+00:00

LAMBDA – A NEW COSMIC FORCE

“If an elderly but distinguished scientist says that something is possible he is almost certainly right, but if he says that it is impossible he is very probably wrong.”

Arthur C. Clarke

When Albert Einstein first began to explore the cosmological consequences of his new theory of gravity, in 1915, our knowledge of the scale and diversity of the astronomical universe was vastly smaller than it is today. There was no reason to believe that there existed galaxies other than our own Milky Way. Astronomers were interested in stars, planets, comets and asteroids. Einstein wanted to use his equations to describe our whole Universe but they were too complicated for him to solve without some simplifying assumptions. Here he was very fortunate. He assumed something about the Universe that certainly makes life easy for the mathematician but which might well not have been an appropriate assumption to make about the real Universe. The observational evidence simply did not exist. Einstein’s simplifying assumption was that the Universe is the same in every place and in every direction at any moment of time. We say that it is homogeneous and isotropic. Of course, it is not exactly so. But the assumption is that it is so close to being so that the deviations from perfect uniformity are too small to make any significant difference to the mathematical description of the whole Universe.17

As Einstein continued he found that his equations were telling him something very peculiar and unexpected: the Universe had to be constantly changing. It was impossible to find a solution for a universe which contained a uniform distribution of matter, representing the distant stars, which remained on average the same for long periods of time. The stars would attract one another by the force of their gravity. In order to avoid a contraction and pile-up of matter in a cosmic implosion, there would need to be an outward motion of expansion to overcome it – an ‘expanding’ universe.

Einstein didn’t like either of these alternatives. They were both contrary to the contemporary conception of the Universe as a vast unchanging stage on which the motions of the celestial bodies were played out. Stars and planets may come and go, but the Universe should go on for ever. Faced with this dilemma of a contracting or an expanding universe, he returned to his equations and searched for an escape clause. Remarkably, he found one.

To see how this happened we must first see something of what led Einstein to his original equations. His equations relating the geometry of curved space to the material content of space have a particular form:

{geometry} = {distribution of mass and energy}.

All sorts of formulae describing the shapes of surfaces are possible in principle on the left-hand side of this equation. But if they are going to be equated to realistic distributions of matter and radiation, with properties like density, velocity and pressure, then they must reflect the fact that quantities like energy and momentum have to be conserved in Nature.

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