The general theory of relativity is a remarkably successful model for gravity. However, many of the best tests for it don't push its limits: they measure phenomena where gravity is relatively weak. Some alternative theories predict different behavior in areas subject to very strong gravity, like near the surface of a pulsar—the compact, rapidly rotating remnant of a massive star (also called a neutron star). For that reason, astronomers are very interested in finding a pulsar paired with another high-mass object. One such system has now provided an especially sensitive test of strong gravity.
The system is a binary consisting of a high-mass pulsar and a bright white dwarf locked in mutual orbit with a period of about 2.5 hours. Using optical and radio observations, John Antoniadis and colleagues measured its properties as it spirals toward merger by emitting gravitational radiation. After monitoring the system for a number of orbits, the researchers determined its behavior is in complete agreement with general relativity to a high level of precision.
The binary system was first detected in a survey of pulsars by the Green Bank Telescope (GBT). The pulsar in the system, memorably labeled PSR J0348+0432, emits radio pulses about once every 39 milliseconds (0.039 seconds). Fluctuations in the pulsar's output indicated that it is in a binary system, though its companion lacked radio emissions. However, the GBT's measurements were precise enough to pinpoint its location in the sky, which enabled the researchers to find the system in the archives of the Sloan Digital Sky Survey (SDSS). They determined the companion object was a particularly bright white dwarf, the remnant of the core of a star similar to our Sun. It and the pulsar are locked in a mutual orbit about 2.46 hours in length.
Following up with the Very Large Telescope (VLT) in Chile, the astronomers built up enough data to model the system. Pulsars are extremely dense, packing a star's worth of mass into a sphere roughly 10 kilometers in radius—far too small to see directly. White dwarfs are less extreme, but they still involve stellar masses in a volume roughly equivalent to Earth's. That means the objects in the PSR J0348+0432 system can orbit much closer to each other than stars could—as little as 0.5 percent of the average Earth-Sun separation, or 1.2 times the Sun's radius.
The pulsar itself was interesting because of its relatively high mass: about 2.0 times that of the Sun (most observed pulsars are about 1.4 times more massive). Unlike more mundane objects, pulsar size doesn't grow with mass; according to some models, a higher mass pulsar may actually be smaller than one with lower mass. As a result, the gravity at the surface of PSR J0348+0432 is far more intense than at a lower-mass counterpart, providing a laboratory for testing general relativity (GR). The gravitational intensity near PSR J0348+0432 is about twice that of other pulsars in binary systems, creating a more extreme environment than previously measured.
According to GR, a binary emits gravitational waves that carry energy away from the system, causing the size of the orbit to shrink. For most binaries, the effect is small, but for compact systems like the one containing PSR J0348+0432, it is measurable. The first such system was found by Russel Hulse and Joseph Taylor; its discovery won the two astronomers the Nobel Prize.
The shrinking of the orbit results in a decrease in the orbital period as the two objects revolve around each other more quickly. In this case, the researchers measured the effect by studying the change in the spectrum of light emitted by the white dwarf, as well as fluctuations in the emissions from the pulsar. (This study also helped demonstrate the two objects were in mutual orbit, rather than being coincidentally in the same part of the sky.)
To test agreement with GR, physicists established a set of observable quantities. These include the rate of orbit decrease (which is a reflection of the energy loss to gravitational radiation) and something called the Shapiro delay. The latter phenomenon occurs because light emitted from the pulsar must travel through the intense gravitational field of the pulsar when exiting the system. This effect depends on the relative orientation of the pulsar to us, but alternative models also predict different observable results.
In the case of the PSR J0348+0432 system, the change in orbital period and the Shapiro delay agreed with the predictions of GR, placing strong constraints on alternative theories. The researchers were also able to rule out energy loss from other, non-gravitational sources (rotation or electromagnetic phenomena). If the system continues as models predict, the white dwarf and pulsar will merge in about 400 million years—we don't know what the product of that merger will be, so astronomers are undoubtedly marking their calendars now.
The results are of potential use for the Laser Interferometer Gravitational-wave Observatory (LIGO) and other ground-based gravitational-wave detectors. These instruments are sensitive to the final death spiral of binaries like the one containing PSR J0348+0432. The current detection and observation strategies involve "templates," or theoretical models of the gravitational wave signal from binaries. All information about the behavior of close pulsar binaries helps gravitational-wave astronomers refine those templates, which should improve the chances of detection.
Of course, no theory can be "proven right" by experiment or observation—data provides evidence in support of or against the predictions of a particular model. However, the PSR J0348+0432 binary results placed stringent constraints on any alternative model to GR in the strong-gravity regime. (Certain other alternative models focus on altering gravity on large scales to explain dark energy and the acceleration expansion of the Universe.) Based on this new data, only theories that agree with GR to high precision are still standing—leaving general relativity the continuing champion theory of gravity.