Novel tests of gravity using nanoHertz stochastic gravitationalwave background signals
Abstract
Gravity theories that modify General Relativity in the slowmotion regime can introduce nonperturbative corrections to the stochastic gravitationalwave background (SGWB) from supermassive blackhole binaries in the nanoHertz band, while not affecting the quadrupolar nature of the gravitationalwave radiation and remaining perturbative in the highlyrelativistic regime, as to satisfy current postNewtonian (PN) constraints. We present a modelagnostic formalism to map such theories into a modified tilt for the SGWB spectrum, showing that negative PN corrections (in particular 2PN) can alleviate the tension in the recent pulsartimingarray data if the detected SGWB is interpreted as arising from supermassive binaries. Despite being preliminary, current data have already strong constraining power, for example they set a novel (conservative) upper bound on theories with timevarying Newton's constant (a 4PN correction) at least at the level of Ġ/G ≲ 10^{5} yr^{1} for redshift z = [0.1÷1]. We also show that NANOGrav data are best fitted by a broken powerlaw interpolating between a dominant 2PN or 3PN modification at low frequency, and the standard generalrelativity scaling at high frequency. Nonetheless, a modified gravity explanation should be confronted with binary eccentricity, environmental effects, nonastrophysical origins of the signal, and scrutinized against statistical uncertainties. These novel tests of gravity will soon become more stringent when combining all pulsartimingarray facilities and when collecting more data.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 April 2024
 DOI:
 10.1088/14757516/2024/04/056
 arXiv:
 arXiv:2307.11665
 Bibcode:
 2024JCAP...04..056C
 Keywords:

 gravitational waves / theory;
 Gravitational waves in GR and beyond: theory;
 modified gravity;
 General Relativity and Quantum Cosmology;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 Astrophysics  High Energy Astrophysical Phenomena;
 High Energy Physics  Phenomenology
 EPrint:
 7 pages, 4 figures. Matches the published version