Research Highlights

doi:10.1038/nindia.2014.172 Published online 18 December 2014

Suitability of binary system for testing general relativity

Researchers have developed a theoretical model that describes a hypothetical binary system consisting of a neutron star and a black hole1. They claim that this binary system could be used to determine whether there are any deviations from the theory of general relativity.

A pulsar is a rapidly spinning neutron star. When a pulsar is paired with another neutron star or a white dwarf, it influences the gravity of the companion star; the companion star also affects the gravity of the neutron star. Astrophysicists have long predicted that a binary system consisting of a neutron star and a black hole could be used to test the theory of general relativity.

To investigate this, the researchers considered a hypothetical binary system of a neutron star and a black hole and compared various parameters (orbital period, orbital eccentricity, individual masses and line of sight) of this binary system with those of two other binary systems — a neutron star–neutron star system and a neutron star–white dwarf system.

The researchers found that a neutron star–black hole system can theoretically test the theory of general relativity. Given sufficient data, such a system could even be used to test cosmological phenomena that cannot be explained by the theory of general relativity.

They found that a neutron star–black hole system cannot easily verify violations of the strong equivalence principle, which states that the laws of gravity are independent of the velocity and location of any moving body. This system is also inadequate for finding potential variations in the gravitational constant, which determines the strength of the gravitational pull between two bodies.

“The discovery of a neutron star–black hole system would enable astrophysicists to test alternative theories to the theory of general relativity and evolutionary models of binary systems, including the first unambiguous mass measurement of a black hole,” says researcher Manjari Bagchi.


References

1. Bagchi, M. et al. In what sense a neutron star-black hole binary is the holy grail for testing gravity? Journal of Cosmology and Astroparticle Physics (2014) doi: 10.1088/1475-7516/2014/08/055