A multi-decade analysis of a distant pulsar is affirming the longstanding notion that the gravitational constant—one of four fundamental forces of nature—is the same everywhere in the universe.
“Gravity is the force that binds stars, planets, and galaxies together,” noted study co-author Scott Ransom from the National Radio Astronomy Observatory in Charlottesville, Va. in a statement. “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 new study, which now appears in the latest issue of the Astrophysical Journal, suggests these alternative theories are nothing more than a wild goose chase. As Albert Einstein surmised a century ago, the gravitational constant is a universal constant—an immutable law of the cosmos that permeates every region of space, regardless of time or distance.
The researchers analyzed 21 years worth of pulsar timing data collected by the NSF’s Robert C. Byrd Green Bank Telescope. Pulsars are the “lighthouses” of the universe—rapidly rotating neutron stars that generate super-concentrated beams of electromagnetic radiation that can only be detected by an observer situated directly in their path. Because of their diminutive size—they only measure about 20 to 25 kilometers (12 to 15 miles) across—they spin with a rate of precision that rivals the best atomic clocks on Earth. This consistency allows astronomers and cosmologists to study fundamental aspects of space, time, and gravity.
For the study, the researchers analyzed a pulsar binary, dubbed J1713+0747, located some 3,750 light-years away. This pulsar is special, not only because of its brightness, but because of its companion, a white dwarf, that’s located reasonably far enough away for the researchers to measure gravitational effects between the pair, and gravitational radiation in particular. Gravitational radiation, predicted by Einstein, is the steady conversion of orbital velocity to gravitational waves.
The researchers demonstrated that the gravitational constant we observe here on Earth is the same in a distant star system. As the authors write in their study:
These measurements, when combined with those of other pulsars, significantly improve the pulsar timing limit on the rate of change of the gravitational constant, G. Although the pulsar constraint is not better than the best solar system ones, it is nevertheless an independent test using extrasolar binary systems thousands of light-years away.
Indeed, previous gravity studies between the Earth and Moon found the same consistency over time. The new analysis, though not as accurate as these local ones, shows that gravity behaves identically in a stellar system far, far away.
The researchers say that the “uncanny consistency” of this pulsar provides intriguing evidence in support of the assertion that the fundamental force of gravity is “rock-solid” throughout space, adding that this “is an observation that has important implications in cosmology and some of the fundamental forces of physics.”
At the same time, the study rules out, with a reasonable degree of confidence, the presence of “special” times or locations in the universe where gravity behaves differently.
Read the entire study at Astrophysical Journal: “Testing Theories of Gravitation Using 21-Year Timing of Pulsar Binary J1713+0747”. A pre-print version can be found at arXiv.