Astronomers are now closer to understanding how gravity works across the universe, thanks to a 21-year study of a pair of stars, a pulsar and a white dwarf.
Pulsars are rapidly spinning, superdense remains of massive stars that burst as supernovas. 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.
Because they are phenomenally dense and massive, yet comparatively small, just 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 ideal cosmic laboratories to study the fundamental nature of space, time, and gravity.
Gravity is one of the four fundamental forces of nature. It appears surprisingly constant across the Universe, according to the decades-long study. This research helps in answer a long open question in cosmology, whether the force of gravity the same everywhere and at all times.
Astronomers, using the National Science Foundation’s Green Bank Telescope in West Virginia and its Arecibo Observatory in Puerto Rico, conducted the study to precisely measure the steady beat of a pulsar known as PSR J1713+0747.
Uncommonly Wide Orbit
PSR J1713+0747 is around 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 the pulsar 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 since 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 disrupt the accuracy of the pulsar timing experiment. Lead author Weiwei Zhu, an astronomer formerly with the University of British Columbia, said:
“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. This is an observation that has important implications in cosmology and some of the fundamental forces of physics.”
Said Scott Ransom, an astronomer with the National Radio Astronomy Observatory in Charlottesville, Va:
“Gravity is the force that binds stars, planets, and galaxies together. 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.”
Unchanging Gravitational Constant
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.
Co-author Ingrid Stairs, from the University of British Columbia, added:
“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. 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.”
Illustration: “Crab Nebula pulsar x-ray” by Smithsonian Institution