Chris Choi (최민영)

Pulsar timing arrays (PTAs) probe the stochastic gravitational wave (GW) background through the angular cross-correlations of timing residuals. For a GW background in general relativity (GR), the expected overlap reduction function (ORF) is given by the Hellings–Downs (HD) correlation. We investigate how a nonzero graviton mass modifies this prediction through a massive dispersion relation and additional vector and scalar polarizations, producing a modified effective ORF. In the massive limit, the finite-distance pulsar terms survive angular averaging, multiplying the correlations of all three polarization sectors by a pulsar-dependent factor that can enhance, suppress, or reverse them. We adopt a tensor Earth–Earth normalization that preserves the continuous finite-distance effect. In the massless limit, we find that the vector ORF vanishes and the scalar ORF approaches a finite breathing mode. We propose the phenomenological prescription \Omega_S/\Omega_T=A_S(m_g/ω)^n_S, with scalar amplitude parameter A_S and suppression index n_S>0. These results provide a consistent framework for future PTA analyses of massive gravity.

Several observational phenomena suggest that the standard model of cosmology and particle physics requires revision. To address this, we consider the extension of general relativity known as massive gravity (MG). In this Letter, we explore the imprints of MG on the propagation of gravitational waves (GWs): their modified dispersion relation and their additional (two vector and one scalar) polarization modes on the stochastic GW background (SGWB) detected by pulsar timing arrays (PTAs). We analyze the effects of massive GWs on the Hellings- Downs curve induced by modification of the overlap reduction function. Our study consists of analyzing observational data from the NANOGrav 15-year dataset and the Chinese PTA Data Release I, and is independent of the origin of the SGWB (astrophysical or cosmological). By considering the bound on the graviton mass imposed through the dispersion relation, we scrutinize the possibility of detecting traces of MG in the PTA observational data. We find that massive GWs predict better fits for the observed pulsar correlations. Future PTA missions with more precise data will hopefully be able to detect the GW additional polarization modes and might be effectively used to constrain the graviton mass.

Convincing evidence of a stochastic gravitational wave (GW) background has been found by the NANOGrav Collaboration in the 15-year data set. From this signal, we can evaluate the possibility of its source being from the early Universe through the tensor perturbations induced by a massive spin-2 graviton field. We consider a time-dependent model of the minimal theory of massive gravity and find values of the graviton mass, mass cutoff time, and Hubble rate of inflation that amplify the energy spectra of primordial GWs sufficiently to reproduce the signal from the NANOGrav data within 1-3 standard deviation. However, a suppression mechanism for high-frequency modes must be introduced to conservatively obey the big bang nucleosynthesis (BBN) bound. While there are regions of the parameter space that reproduce the signal, it remains a challenge to simultaneously respect the BBN and cosmic microwave background bounds without making the graviton mass cutoff time too deep into the matter-dominated era.