Score one more for Einstein. A new study has confirmed his
theory of general relativity works on extremely large scales.
The study was one of the first rigorous tests of this theory
of gravity beyond our solar system. The research found that even over vast
scales of galaxies and clusters of galaxies, the equations of general
relativity predict the way that mass pulls on other mass in the universe.
The new work also helps rule out a competing theory of
gravity that seeks to do away with the need for bizarre concepts like dark
matter and dark
energy that have irked some scientists. This research indicates those pesky
ideas may be here to stay.
What is relativity?
General relativity rocked the world of physics when Einstein
first published his paper on the subject in 1915. The theory built on the
traditional idea of
gravity based on Isaac Newton's laws, but added fundamentally new concepts
like the notion that mass deforms the shape of space-time. This means that
objects and even light that move through space near a large mass will travel on
a curved path. Furthermore, it means that mass can stretch or shrink time as
well. For example, someone watching a black hole from a distance would observe
a person falling into that black hole to fall extremely slowly.
To test this theory over distances up to 3.5 billion light-years
from Earth, researchers analyzed a survey of about 70,000 galaxies. The
scientists combined three different measurements. First, they calculated the
weak gravitational lensing caused by the galaxies – that is, they measured how
much the galaxies' mass was bending light from other galaxies around them by
noting the average distortion of the surrounding galaxies' shapes.
Then, they combined this data with measurements of the
galaxies' velocities to learn how the galaxies were moving toward and away from
one another. Finally, the astrophysicists calculated how clustered the galaxies
were together over various distances. All of these measurements combined
created a system to test theories of gravity independent of particular
parameters in the theories.
The scientists found that general relativity is consistent
with their observations of the universe at large scales. They also tested two competing
theories – the tensor-vector-scalar gravity (TeVeS) idea, and another
called f(R) (pronounced "f of r").
The quantities predicted by f(R) were somewhat different
from those observed, but still fell within the margin of error of the
measurements, so this theory is still a possibility. TeVeS, however, made
predictions that fell outside the observational error limits, so scientists
think they can probably eliminate this theory from consideration.
"It wasn't clear at the outset that our errors would be
small enough to be able to rule out other models – it was a nice
surprise," said study leader Reinabelle Reyes, a graduate student at
Princeton University in Princeton, N.J.
TeVeS was already looking doubtful based on recent
observations of a pair of colliding galaxy clusters called the bullet cluster,
which offered strong evidence for the existence of dark matter, Reyes said. The
new research offers another nail in its coffin.
Solid support
While general relativity was already pretty well accepted
among physicists, the new findings offer more solid support for the theory.
"It's good to know that general relativity is
consistent," Reyes told SPACE.com. "Now we have something to hold on
to saying the universe really works that way."
The study was detailed in the March 11 issue of the journal
Nature.
To further judge between Einstein's
theory and other ideas, including f(R), research on more galaxies will be
necessary to reduce the margins of error on the data.
"Reyes and colleagues' measurements are significant not
just because they are consistent within error with general relativity, but also
because they point the way to future high-precision tests that will better
distinguish between general relativity and some variant models," physicist
J. Anthony Tyson of the University of California, Davis, wrote in an
accompanying essay in the same issue of Nature. Tyson was not involved in the
study.
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