Unlike most of us, subatomic particles don't gain weight as
they get older. The mass of these tiny bits of matter has remained constant
over the last 6 billion years, recent astronomical observations indicate.
Believe it or not, but whether an electron was lighter or
heftier in the past is a question of fundamental importance. Variations in
particle masses and other so-called constants of nature, such as the
speed of light, may help explain the mystery of dark
energy and determine if hidden
dimensions exist.
"Some theorists claim the physical constants should
have varied over time," said Christian Henkel of the Max Planck Institute
for Radio Astronomy in Bonn, Germany. "This is something that is part of
the modeling of the universe."
Henkel and his colleagues have placed new limits on wavering
constants. By observing the absorption of radio waves by molecules in the
early universe, the researchers have shown that the mass ratio between two
particles — the proton and the electron — has not changed from its present
value by more than 2 parts in a million.
The results, reported in a recent issue of the journal Science,
call into question previous measurements that claim to have seen variations in
this mass ratio.
Sniffing out ammonia
Henkel came to study varying constants after he and some
colleagues detected ammonia in a galaxy "halfway across the
universe," he told SPACE.com. Such a faraway object offers a
glimpse back in time, because the light we see from it now left the galaxy
billions of years ago.
It was the first time this molecule had been seen so far
away. The team identified the ammonia by its absorption of radio waves from a bright
object called a quasar, located behind the galaxy. Only later did they realize
that this absorption could provide information about the fundamental physics in
the early universe.
"Ammonia is a molecule with a very special
structure," Henkel explained. It looks like a pyramid (or tetrahedron)
with three hydrogen atoms forming the base and one nitrogen atom on top.
Although most other molecules rotate faster when they absorb
the energy from radio waves, ammonia actually flips inside out, with the
nitrogen moving from above the hydrogens to below.
This flipping depends strongly on the ratio between the mass
of the proton and the electron. Knowing this, Henkel's team compared their
ammonia data to other molecules in the same galaxy and found the ammonia
absorption had not significantly shifted from where it was expected to be.
The implication is that 6 billion years ago, protons weighed
roughly 1,836 times more than electrons, just as they do now.
A changing world
"It is a puzzling result, but I am sure they did a
thorough job," said Wim Ubachs of Vrije University in Amsterdam, The
Netherlands.
Ubachs was one of the astronomers who wrote a 2006 research paper
that claimed to see a change (20 parts in a million) in the proton-electron
mass ratio. The experiment was similar to the current study, but in this case
the molecule was not ammonia, but hydrogen.
At face value, the two observations disagree, but Ubachs
pointed out that his group's results correspond to a much earlier time, more
than 11.5 billion years ago.
It is possible that the mass ratio changed in between the
two measurements. Indeed, there are theories that the fundamental constants
varied after the Big Bang but then stopped once dark energy (a hypothetical
energy that produces a force that opposes gravity) began accelerating
the expansion of the universe some 6 billion years ago. Some of these
models claim that dark energy is somehow responsible for variations in the
constants. Others, based on string theory, assume that extra spatial dimensions
(beyond the three we can see) cause the constants to fluctuate in value.
To prove any of these wild ideas will take a lot more data;
both Henkel and Ubachs have fresh observations that are in the process of being
analyzed.
"We hope that the upcoming results will shed more light
on the issue, which is very intriguing indeed," Ubachs said.
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