Numerical simulations of Scalar Dark Matter Around Binary Neutron Star mergers

TL;DR

This work probes wave-like dark matter around binary neutron star mergers by modeling DM as a minimally coupled complex scalar field and embedding it in full GR hydrodynamics simulations. Using two initial DM profiles and two binary masses, the study shows that the scalar field forms a bound, corotating cloud around the binary and can induce a measurable GW dephasing and reduce postmerger ejecta at high densities, though realistic DM densities yield small effects. The results highlight a robust DM retention around BNSs and identify gradient-pressure effects that resist remnant compression, delaying collapse and altering ejecta morphology, while noting substantial degeneracies with the neutron-star EOS and other baryonic physics. Overall, while wave DM can influence merger dynamics under favorable conditions, detecting such signatures with current or near-future GW detectors remains challenging, and additional DM couplings or self-interactions could amplify observable effects, motivating further coupled Einstein–Klein–Gordon studies.

Abstract

Binary neutron star mergers provide a laboratory for probing fundamental physics through their gravitational- wave emission and electromagnetic counterparts. In particular, they may allow us to explore signatures of physics beyond the Standard Model in strong-gravity regimes, such as those of dark matter. In this work, we investigate the dynamics of light dark matter, modeled as a minimally coupled scalar field, surrounding a binary neutron star system. Our primary focus is to assess whether the scalar field remains bound to the binary over the late inspiral-merger timescales and to determine its potential impact on observable signatures. We find that, in a range of scenarios, the scalar field forms a common cloud around the binary that does not disperse. At sufficiently high densities, this leads to measurable effects, including a dephasing of the binary inspiral, a less compact post-merger remnant, and suppression of the dynamical ejecta. For densities motivated by astrophysical considerations, however, these effects remain small and are unlikely to be detectable with current or next-generation gravitational-wave observatories.

Figures (12)

• Figure 1: 2D schematics showing the comparison of the energy densities for the two scalar field initial profiles. Top row: For the Uniform profile (UP) at different times starting from early-inspiral till the post-merger phase. Bottom row: Energy densities snapshots for the Overdense profile (OP) at the same time stamps of the simulation, showing localised enhancement close to the binary even at $t=150$. The UP is effectively a nearly homogeneous field initially, as the cut-off is set at a large radius. At later times, a co-rotating configuration develops, giving rise to characteristic spiral-arm features in the energy density.
• Figure 2: Noether charge conservation plot for the $2.7$ total mass binary for the Uniform profile with the scalar field mass $\mu = 0.085$, extracted at a radius of $\rm R=40$. The red dotted lines represent the time derivative of the charge ($\int Q \ dV$), the blue dashed line is the flux ($\int F \ dA$), both are oscillatory, and the residual is shown in a black solid line and is conserved throughout the simulation. The axis on the right shows the residual on a log scale.
• Figure 3: Noether charge distribution around a single star part of the binary for different scalar field profiles extracted at a radius of $\rm R=10$. Top left: $M_{\rm tot} = 2.7$, for the Uniform profile and $\mu = 0.17$ ($\rm UP^{2.7}_{0.17}$). Top right: For the Uniform profile and a lower field mass $\mu = 0.085$ ($\rm UP^{2.7}_{0.085}$). Bottom left: $M_{\rm tot} = 3.6$ and Uniform profile ($\rm UP^{3.6}_{0.17}$). Bottom right: $M_{\rm tot} = 3.6$ and Overdense profile ($\rm OP^{3.6}_{0.17}$). The black dashed line indicates the merger time. We note that the scalar field oscillations form a beating pattern for the $\mu = 0.17$, and are smoother for the lower mass case on the top right owing to the gradient pressure. For the higher mass binary, the oscillation frequency is higher for both profiles.
• Figure 4: 2D snapshots at $t=2000$ of the scalar–field energy density $\rho_{\varphi}$. Top-left: Uniform profile, $M_{\rm tot}=2.7$, $\mu=0.17$. Top-right: Overdense profile, $M_{\rm tot}=3.6$, $\mu=0.17$. Bottom-left: Uniform profile, $M_{\rm tot}=2.7$, $\mu=0.085$. Bottom-right: Uniform profile, $M_{\rm tot}=3.6$, $\mu=0.17$. The lower-mass field ($\mu=0.085$) shows markedly reduced central accumulation, consistent with a stronger gradient pressure. The $3.6$ mass binary simulations exhibit broader, more extended spiral-arm features, potentially due to higher oscillations of the scalar field and a deeper gravitational potential well.
• Figure 5: Left: The real part of the GW strain (top panel) and the corresponding phase differences (bottom panel) for different scalar field profiles extracted at a radius $\rm R = 1000$. The Uniform (UP) and Overdense profiles (OP) both show the same dephasing with the vacuum case (NF) of roughly $0.3$ rad close to the merger. Middle: The real part of the GW strain (top panel) and the corresponding phase differences (bottom panel) for different scalar field profiles with a total mass of $M_{\rm tot} = 3.6$, along with the NF of the same $M_{\rm tot}$. Only a negligible dephasing is observed between the vacuum case and the configurations with the scalar field. Right: The real part of the GW strain (top panel) and the corresponding phase differences (bottom panel) for two different masses, $\mu = 0.17$ and $\mu = 0.085$. The lower scalar field mass exhibits a smaller accumulated dephasing relative to the vacuum case close to the merger, owing to the pressure term, as shown in the inset.