Gravity tests in the UltraViolet and the InfraRed with Pulsar timing (GUVIRP)

Progetto PRIN 2022 n. 20227XBYPX  “GUVIRP (Gravity tests in the UltraViolet and the InfraRed with Pulsar timing)” 

CUP: C53D23000910006

Descrizione del progetto

The GUVIRP project makes use of the extreme precision of pulsars in order to perform tests of gravity (such as General Relativity or alternative theories of gravity). "Pulsar timing" consists in the regular monitoring of the radio emission from pulsars at Earth's radio telescopes, and gives us very precise information about pulsars themselves (which are neutron stars), and in the case of pulsars with a companion, about orbital parameters of the binary. Several relativistic orbital parameters in compact binaries, the so-called Post Keplerian parameters, can be determined thanks to pulsar timing. In pulsar binaries for which several Post Keplerian parameters have been determined, tests of gravity are possible. For this project, we use real pulsar data to place constraints on gravity theories.

Finalita/Risultati attesi

The project is divided into different components. 

1) Tests of Einstein - Aether theories with pulsar timing 

Einstein-Aether theories are a specific subset of alternative theories of gravity (a deviation from General Relativity) which contain a preferred frame. Specific parameters determining these theories (such as "sensitivities", which are not present in General Relativity) can be constrained by doing a Bayesian analysis using pulsar timing data. The goal of this project is to use suitable pulsar binaries to constrain those parameters.

2) Propagation delay in pulsar - black hole binaries

Interesting pulsar binaries consist of pulsars (which are neutron stars) with a a compact object such as a neutron star, a white dwarf, or a black hole. No pulsar - black hole system has been discovered yet, but we may be really close. Since pulsars are extremely precise clocks (we receive pulses of radio emission at a very precise rate thanks to the stable rotation period of the pulsar), we can use them to study their companion, in this case, black holes, leading to new physics. When the pulsar is almost behind the companion, the radio emission will pass close to the companion before traveling toward the Earth. Passing closeby to the companion leads to a relativistic effect called the Shapiro delay, due to the effect of the gravitational potential from the companion. In pulsar timing, the Shapiro delay is generally assumed to be at the first Post Newtonian order. But close to a black hole, it has been calculated that this order was not sufficient. We thus explore the corrections needed for calculating the propagation delay in pulsar - black hole binaries.

In addition, some hypotheses (by Giddings) seeking to resolve the Black Hole information paradox imagine that because of the possibile non-locality of quantum mechanics, quantum fluctuations could be detectable outside the event horizon of a black hole. We study the possibility that the radio signal emitted by a pulsar passing by close to the event horizon of its black hole companion could be affected by quantum fluctuations. Pulsar timing in black hole binaries could therefore test quantum gravity in black hole environments.

Risultati raggiunti

1) We have successfully implemented a new Bayesian pipeline using "Vela" for the analysis of pulsar timing data. Thanks to this pipeline, we have studied the pulsar - white dwarf binary PSR J1738+0333 and provided tight constraints on Einstein - Aether theories. A paper by Vaglio et al. is about to be submitted.

2) We have studied the (theoretical) propagation delay in pulsar - black hole binaries. We looked at the different scenarios. When the companion is a supermassive black hole, the effect is important. One question was how important this effect is with intermediate and stellar-mass black hole companions. This was published in: Carleo, Perrodin, Possenti: "Towards an exact approach to pulsar timing" (PRD).

In addition, we have explored scenarios where quantum effects are present outside the event horizon. This work was the subject of Even Coquery's (from Ecole Normale Superieure-Lyon) masters internship project, and will be published shortly.