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Does the value of Newton’s constant, G, oscillate rapidly? If it does,
say two physicists in the US, it might resolve the dilemma of the Universe’s
‘missing mass’. Ninety per cent of the matter in the Universe is thought
to be in an invisible or ‘dark’, form. But recently, cosmological models
involving missing mass in the form of cold dark matter have run into conflict
with observations (New Å®ÉúСÊÓƵ, Science, 23 February).

Paul Steinhardt and Frank Accetta of the University of Pennsylvania
have developed a new approach to the problem of matching up observations
of the amount of matter in the Universe with theoretical predictions. It
involves rapid changes in the constant in Newton’s law of gravity.

Cosmologists have compelling reasons for believing that the Universe
is ‘flat’ – that is, there is enough matter around to gravitationally brake
the expansion of the Universe so that it sits on the boundary between one
that expands forever and one that one day will re-collapse into a black
hole. The problem is that astronomers see only about 10 per cent of the
matter required to do the job.

Theorists have proposed various more or less bizarre ideas to resolve
the discrepancy. These include the possibility that 90 per cent of the mass
of the Universe is in the form of dark matter, and the suggestion that a
‘cosmological constant’ can modify Einstein’s theory of gravity – general
relativity. This might flatten the Universe, even without the extra mass.
Steinhardt and Accetta’s thinking is the most lateral yet.

The two physicists point out that in many standard forms of unified
theory, gravity can be treated as a rapidly varying phenomenon. The classical
value of G, measured by all experiments, is simply the time-averaged value.
The oscillations occur at a frequency of 10 ¹² hertz – which
is too rapid to show up in everyday experiments.

The amplitude of these oscillations would have been large during the
big bang. But, by now, the amplitude would have declined so that it is less
than one part in 10 ³⁰ of the traditional value of G.

With these constraints, the variable gravity would have no effect on
observable features of the Universe today, such as the evolution of stars
or the fall of an apple. But much larger oscillations during the big bang
would flatten the Universe without requiring any more matter than we can
see.