91av

New York

THE ground is shifting under our feet. Fundamental properties of the Universe
are changing, and physicists can’t explain how or why. Now researchers say an as
yet undiscovered fifth force could be behind these mysterious changes.

Physicists combine supposedly unchanging physical properties, such as the
speed of light and an electron’s charge, into a number called the “fine
structure constant” that describes how our Universe hangs together. But in 1999,
astronomers analysing 10-billion-year-old light from distant quasars got a shock
when they found that it was different from what we’d expect to see today.

They concluded that the fine structure constant, or alpha, must have been
different 10 billion years ago. If so, the host of fundamental values tied to
alpha could be changing too: light may be slowing down, the electron’s charge
growing, and atomic nuclei losing mass.

If the Universe is four dimensional, then a fifth force is the only thing
capable of triggering these changes, say Gia Dvali and Matias Zaldarriaga of New
York University. The four fundamental forces known so far are gravity,
electromagnetism and the strong and weak nuclear forces. But the new force would
have a repelling effect, showing itself via tiny particles called “alpha ions”
slowly emitted from protons and neutrons. As the repelling alpha ions were lost,
atomic nuclei would lose mass, but atoms would become more strongly bound
together.

The force would be incredibly weak, 100,000 times fainter than gravity. But
it would work over long ranges, meaning it could serve as the mysterious
repulsive energy called quintessence, Latin for “fifth element”, that some
scientists say explains why the Universe is flying apart faster and faster.

“The possibility that you could find a [force] that is both making the fine
structure constant vary and causing the Universe to accelerate is
extraordinarily exciting. It’d be a fantastic discovery,” says theoretical
physicist Sean Carroll of the Enrico Fermi Institute at the University of
Chicago.

There are several theories to explain the variation in alpha, but they all
require some kind of fifth force, says John Barrow of the University of
Cambridge. “They would each have different consequences for observations very
close to the surface of neutron stars.” But Dvali and Zaldarriaga say there’s an
easier way to detect traces of alpha ions. They would be shed at different
energies depending on a particle’s mass, and that would make protons and
neutrons fall at very slightly different speeds.

A satellite programme called STEP is already being planned to test the
theory. Free-floating masses will be isolated in an ultra-high vacuum. If they
fall at different rates, it would support the existence of alpha ions.
Otherwise, we may have to look to higher dimensions for an answer. Dvali and
Zaldarriaga say that without a fifth force, that’s the only thing that could
explain why alpha is changing.