Credit: NRAO/AUI/NSF
Gravity, one of the four fundamental forces of nature, appears reassuringly constant across the Universe, according to a decades-long study of a distant pulsar. This research helps to answer a long-standing question in cosmology: Is the force of gravity the same everywhere and at all times? The answer, so far, appears to be yes.
Astronomers
using the National Science Foundation’s (NSF) Green Bank Telescope
(GBT) in West Virginia and its Arecibo Observatory in Puerto Rico
conducted a 21-year study to precisely measure the steady
"tick-tick-tick" of a pulsar known as PSR J1713+0747. This painstaking
research produced the best constraint ever of the gravitational constant
measured outside of our Solar System.
Pulsars are the rapidly
spinning, superdense remains of massive stars that detonated as
supernova. They are detected from Earth by the beams of radio waves
that emanate from their magnetic poles and sweep across space as the
pulsar rotates. Since they are phenomenally dense and massive, yet
comparatively small – a mere 20–25 kilometers across – some pulsars are
able to maintain their rate of spin with a consistency that rivals the
best atomic clocks on Earth. This makes pulsars exceptional cosmic
laboratories to study the fundamental nature of space, time, and
gravity.
This particular pulsar is approximately 3,750
light-years from Earth. It orbits a companion white dwarf star and is
one of the brightest, most stable pulsars known. Previous studies show
that it takes about 68 days for the pulsar to orbit its white dwarf
companion, meaning they share an uncommonly wide orbit. This separation
is essential for the study of gravity because the effect of
gravitational radiation – the steady conversion of orbital velocity to
gravitational waves as predicted by Einstein – is incredibly small and
would have negligible impact on the orbit of the pulsar. A more
pronounced orbital change would confound the accuracy of the pulsar
timing experiment.
"The uncanny consistency of this stellar
remnant offers intriguing evidence that the fundamental force of gravity
– the big 'G' of physics – remains rock-solid throughout space," said
Weiwei Zhu, an astronomer formerly with the University of British
Columbia in Canada and lead author on a study accepted for publication
in the Astrophysical Journal. "This is an observation that has important implications in cosmology and some of the fundamental forces of physics."
"Gravity
is the force that binds stars, planets, and galaxies together," said
Scott Ransom, a co-author and astronomer with the National Radio
Astronomy Observatory in Charlottesville, Va. "Though it appears on
Earth to be constant and universal, there are some theories in cosmology
that suggest gravity may change over time or may be different in
different corners of the Universe."
The data taken throughout
this experiment are consistent with an unchanging gravitational constant
in a distant star system. Earlier related research in our own Solar
System, which was based on precise laser ranging studies of the
Earth-Moon distance, found the same consistency over time.
"These
results – new and old – allow us to rule out with good confidence that
there could be 'special' times or locations with different gravitational
behavior," added Ingrid Stairs, a co-author from the University of
British Columbia in Canada. "Theories of gravity that are different from
general relativity often make such predictions, and we have put new
restrictions on the parameters that describe these theories."
Zhu
concluded: "The gravitational constant is a fundamental constant of
physics, so it is important to test this basic assumption using objects
at different places, times, and gravitational conditions. The fact that
we see gravity perform the same in our Solar System as it does in a
distant star system helps to confirm that the gravitational constant
truly is universal."
This work was part of the North American
Nanohertz Observatory for Gravitational Waves (NANOGrav), a Physics
Frontiers Center funded by the NSF.
The GBT is located in the
National Radio Quiet Zone, which protects the incredibly sensitive
telescope from unwanted radio interference, enabling it to study pulsars
and other astronomical objects.
The National Radio Astronomy
Observatory is a facility of the National Science Foundation, operated
under cooperative agreement by Associated Universities, Inc.
Contact:
Charles E. Blue
(434) 296-0314
Email: cblue@nrao.edu
