Effect of ultralight dark matter on compact binary mergers

TL;DR

This work investigates how an ambient ultra-light dark matter (ULDM) environment can bias compact-binary merger statistics. By extending a baseline astrophysical merger model with ULDM-induced accretion and dynamical-friction dissipation, the authors study both individual binaries and binary-population evolution, linking environmental density to merger timescales and rates. The analysis shows that ULDM with densities above roughly $10^4\ \mathrm{GeV/cm^3}$ can significantly accelerate mergers and shift the merger-probability distribution to higher redshifts, with the strongest consistency with GWTC-3 data occurring for $\rho$ around $10^{12}\ \mathrm{GeV/cm^3}$ up to $z_m \lesssim 2$. These results illustrate the potential of gravitational-wave observations to constrain ULDM properties, while acknowledging the simplifications and degeneracies in the toy model. Future work with more realistic DM densities and baseline astrophysics could sharpen these constraints and reveal robust ULDM signatures in merger statistics.

Abstract

The growing catalogue of gravitational wave events enables a statistical analysis of compact binary mergers, typically quantified by the merger rate density. This quantity can be influenced by ambient factors, following which, in this work we have investigated the impact of dark matter environment on the merger statistics. We construct a baseline astrophysical model of compact binary mergers and extend it by incorporating a model of ultra light dark matter, which affects the orbital evolution of binaries through accretion and dynamical friction. Our analysis of the merged population of binary progenitors demonstrates that, compared to the baseline model, ULDM can significantly alter the merger statistics when its ambient density becomes larger than 104GeV/cm3. A comparison with the gravitational wave data from the GWTC-3 catalogue provides insight into potential observational signatures of the ULDM in merger events, leading to possible constraints on the existence and density of dark matter distribution in galaxies.

Figures (5)

• Figure 1: The figure depicts the evolution of the binary separation parameter $a$ (in units of solar radius $R_{\odot}$) of a given progenitor as a function of the redshift $z$. We assume that the progenitors start out at $z = 0.2$ and consists of binary main sequence stars with $M_{\rm T}^{\rm P}=45 M_{\odot}$ and $q^{\rm P}=0.8$. The left most region of the plot corresponds to $z\sim 0.2$, while the rightmost region is $z\sim 0$. The initial colour shading (brown) corresponds to the star-star phase, the next colour shading (green) corresponds to the star-compact object phase and the final colour shading refers to the situation when both are compact objects. We show our results with ($\rho > 0$) and without ($\rho = 0$) the presence of ULDM. The units of DM density $\rho$ are in $\textrm{GeV}/\textrm{cm}^{3}$. See text for more details.
• Figure 2: Same configuration as in Fig. \ref{['fig:progmerg0p2']}, except for the fact that the progenitors start out at $z=0.8$. In this case, the objects merge at a higher redshift (z=0.4) in the presence of ULDM environment, while in vacuum they merge much later, close to $z=0$. The '0' tick on the x-axis is just to guide the eye.
• Figure 3: Plot showing the composition of merged and unmerged final states of all possible combinations of binary main sequence star progenitors beginning at $z=0.5$. The grey shaded regions are off-limits in progenitor configuration in our modelling (in those regions, either masses are larger than $80M_{\odot}$, or smaller than $10M_{\odot}$). We show the $z$ distribution for the merged cases (green shades) and the binary separation $a$ in units of solar radius $\mathrm{R}_\odot$ at $z=0$ for the unmerged cases as a function of the total progenitor masses in the binary and assuming a mass transfer, for different cases of dark matter density. The red line is the separatrix between the unmerged and merged populations at $z=0$. See text for further details.
• Figure 4: Plot showing merger rate densities $\mathcal{R}_m(z_m)$ in blue as a function of redshift $z$ for our choices of ULDM density $\rho$. The green shaded region represents our current uncertainty about the total local merger rate density $R_0$ of binary compact objects inferred from GWTC-3 data KAGRA:2021duu. The red dashed line show the inferred behaviour of the exponent $\kappa$ of $\mathcal{R}_m^\mathrm{inf}(z_m)$ from GWTC-3 data along with its error bars. See text for more details.
• Figure 5: Plot showing the merger probability distribution as a function of the redshift $z$ for four different choices of the ULDM density $\rho$. As evident, increasing $\rho$ leads to more merger happening at larger redshifts, leading to a shift of the peak of the merger distribution towards higher redshifts. See text for more details.