Gravitational-Wave Propagation Through the Axiverse

Abstract

We study the effects of oscillating, ultralight scalar and pseudoscalar fields on the propagation of gravitational waves (GWs). We consider two potential couplings of the (pseudo)scalars to gravity; a parity-even Gauss-Bonnet coupling, and parity-odd Chern-Simons coupling. We find several effects at both the population and individual GW event level, characterized by oscillatory features controlled by the (pseudo)scalar mass. In the parity-even case, this feature can be seen in the observed GW redshift and speed distributions, as well as in the dispersion relation and phase of individual events. We use the observation of the GW170817 multimessenger binary neutron star event to place constraints on the parity-even scalar-graviton coupling. In the parity-odd case, the effects are birefringent, but we find an overall washout of polarization at the population level. Oscillatory features can be seen in the observed GW amplitude and inclination distributions. Finally, we find that continuous, monochromatic GW sources are a promising target to observe these effects. The presence of a (pseudo)scalar field imprints a modulation of the GW waveform in the time domain, which can potentially be observed with space-based detectors such as LISA.

Figures (8)

• Figure 1: Joint constraint on $\alpha$ and $m_\varphi$ at fixed $\delta=0.1$ due to the observed GW speed from GW170817 (upper) and on $\alpha_{\hbox{\tiny PI}}$ and $\delta$ at fixed $m_\varphi = 10^{-22} \, {\rm eV}$ (lower). The shaded region in each figure shows the ruled out parameter space.
• Figure 2: Oscillation in GW speed as a function of redshift.
• Figure 3: Modulation of the observed redshift distribution at a frequency of $m_\varphi/H_0$. The amplitude is set by the coupling parameter.
• Figure 4: Parity-violating waveform modification for GWs at fixed mass and varying redshift (top) and fixed redshift with varying mass (bottom). The dotted lines correspond to propagation in vaccuum with no coupling to an ultralight field/ In the upper panel, we show the modification at fixed $m_\varphi = 10^{-22}$ eV at three different redshifts, and the lower panel shows the effect of varying $m_\varphi$ at fixed redshift, keeping $m_\varphi \alpha_{\hbox{\tiny PV}}= 10^{49}$ constant across all three cases.
• Figure 5: Parity violating oscillation effect in the observed GW population, approximated by the BBH merger rate. We assume that the vacuum distribution has equivalent contributions in right- and left-handed GWs.