Well, one would say, there is nothing to test anymore in the field of the general theory of relativity. It could sound ‘almost perfectly logical’, as Albert Einstein outlined the principles of this theory quite a long time ago and most of predictions seem to be working when investigating the interactions between the gravity and the spacetime. But as with any theory, there is always a possibility of some ‘tiny’ error in the construction of the overall apparatus of equations and scientific assumptions. The general relativity is not an exception: the aim is not to prove that Einstein was wrong, but to further advance our understanding of the Universe.

So how do we test the predictions of general relativity? This theory has already successfully passed a number of observational and experimental tests, as those involving measurements of deflection of light by the Sun, gravitational redshift of light, gravitational lensing, and quite a large number of modern precision tests. However, most of these tests involved weak gravitational fields and many of strong-field general relativity predictions still remain to be verified.

It does not mean that there were no attempts to investigate the effects of strong gravity. Astrophysical systems involving collapsed objects, such as black holes and neutron stars, provide new ways to explore relativistic principles. The matter accreting onto black holes and neutron stars emits strong radiation which can be analyzed according to the aspects of general relativity.

“The motions of this accreting matter also hold the potential to study the properties of gravity in the strong field mode and to verify some of the yet untested and key predictions of the general relativity”, write the scientists behind a new study published on arXiv.org. In this study, they suggest that quasi-periodic oscillations (QPOs) observed in the X-ray flux emitted by accreting black holes could become a very useful predictor of strong-field gravity effects. According to the team, the only thing required is the capability to measure these oscillations with very high precision.

The phenomenon of quasi-periodic oscillations and related X-ray flux variability is not an entirely new thing. Different QPO modes have been studied in a number of binary neutron stars as well as in black hole X-ray binaries. Almost as a rule, theoretical models developed to interpret this phenomenon involve the fundamental frequencies of motion of matter under the impact of super-strong gravity. Here is the point where such frequency observations do not exactly match approximations provided by the general relativity.

“The signals generated by quasi-periodic oscillations can provide a powerful diagnostic of strong gravitational fields; black holes are especially promising in this respect, by virtue of their simplicity”, the authors of the study write. They also list the keypoints for this ‘simplicity’: according to the general relativity, the spacetime of a stationary black hole depends only on its mass and angular momentum; also, differently from neutron stars, black holes do not generate stable magnetic fields and do not contain ‘hard’ surface or boundary layer which could effectively alter the dynamics of inflowing matter.

The problem is that the precision of QPO frequency measurements is limited to >1-2%, which also makes it difficult to detect the so-called simultaneous modes of quasi-periodic oscillations which are needed to make more accurate theoretical assumptions. However, the team explains that with the development of new very high throughput X-ray instruments QPOs are expected to be detected in a variety of black holes, with accuracy high enough to perform quantitative tests of the general relativity predictions in the strong gravity field regime.

The authors cite a recently proposed ESA X-ray satellite LOFT as a measure that could offer very good prospects for the QPO-based tests in the not-so-far future. In their study, the team discuss fragments of the fundamental science describing quasi-periodic oscillations of accreting black holes and resulting variations in the emitted flux of X-rays. Based on subsequent mathematical derivations, the authors also show that sensitivity of the proposed ESA M-class mission LOFT is sufficient to verify some of the untested aspects of the general theory of relativity.

*Written by Alius Noreika*