Is Einstein's General Relativity the correct theory for gravitational phenomena? Can these phenomena be used to discover new fundamental fields?

In the paper published today in Nature Astronomy, Leonardo Gualtieri, Susanna Barsanti and Paolo Pani of the Department of Physics of Sapienza University of Rome, together with researchers from the Gran Sasso Science Institute (GSSI), the International School for Advanced Studies (Italian acronym SISSA) and the University of Nottingham, show that observations of gravitational waves by the Laser Interferometer Space Antenna (LISA) will be able to reveal the presence of new fundamental fields with great accuracy.

New fundamental fields associated with gravity, particularly scalar fields, form the basis of theoretical models developed to explain various physical scenarios. They could, for instance, provide clues to the accelerated expansion of the Universe or to dark matter or be low-energy manifestations of a consistent and complete description of gravity and elementary particles.

To date, observations of astrophysical objects characterised by weak gravitational fields and small space-time curvatures have shown no indication of the existence of these fields. However, several models suggest that deviations from General Relativity, or interactions between gravity and new fields, are more relevant when space-time curvature is very large. For this reason, the observation of gravitational waves - which has opened a new window on the strong gravitational field regime - represents a unique opportunity to discover new fundamental fields.

LISA, the space-based gravitational wave detector developed to observe gravitational waves from astrophysical sources, will make it possible to study new families of astrophysical sources, different from those observed by Virgo and LIGO, such as Extreme Mass Ratio Inspirals (EMRI).

EMRIs, binary systems in which a compact stellar-mass object - a black hole or neutron star - orbits a black hole millions of times larger than our Sun, are indeed among the sources expected to be observed with LISA and represent a precious arena for studying the strong gravitational field regime. The smallest body in an EMRI completes tens of thousands of orbital cycles before falling into the supermassive black hole, thus emitting long-lived signals that allow us to measure even the smallest deviations from the predictions of Einstein's theory and the Standard Particle Model.  

The authors of the study have developed a new approach to modelling the signal emitted by EMRIs, studying rigorously for the first time whether and how LISA can detect the existence of scalar fields coupled to the gravitational interaction and measure the scalar charge, a quantity that quantifies the field associated with the smallest body in the binary system.

The newly developed approach is "agnostic" with respect to the theory that predicts the existence of the scalar field since it does not depend on the origin of the charge or the nature of the compact object. The analysis also shows how future measurements of scalar charge can be translated into very tight constraints on deviations from General Relativity or the Standard Model.

LISA, which will start as an ESA-NASA mission in 2037, will operate in orbit around the Sun, in a constellation of three satellites millions of kilometres apart, observing gravitational waves emitted at low frequencies in a band not accessible to terrestrial interferometers due to ambient noise. LISA's visible spectrum will open a new window on the evolution of compact objects in a wide variety of astrophysical systems in our Universe.

References:

Detecting fundamental fields with LISA observations of gravitational waves from extreme mass-ratio inspirals - Andrea Maselli, Nicola Franchini, Leonardo Gualtieri, Thomas P. Sotiriou, Susanna Barsanti, Paolo Pani - Nature Astronomy  https://doi.org/10.1038/s41550-021-01589-5

Further Information 

Leonardo Gualtieri
Department of Physics
leonardo.gualtieri@uniroma1.it

Paolo Pani
Department of Physics
paolo.pani@uniroma1.it