Assess the competition between retrograde inspiral and disk tilting

Determine whether supermassive black hole binaries with astrophysically realistic Mach-number retrograde circumbinary disks can reach the gravitational-wave regime through retrograde evolution before disk tilting reorients them into a prograde configuration.

Background

The paper estimates that retrograde circumbinary disks can undergo a tilting instability on timescales of roughly 10710^7 years, with the timescale shortening at higher binary eccentricity. This instability can transfer the system from a retrograde to a prograde configuration before the binary reaches the gravitational-wave regime.

The simulations use Mach numbers substantially larger than those expected in realistic systems. Higher-Mach simulations are therefore needed to establish the relative timescales of orbital inspiral and disk tilting and to determine whether retrograde evolution alone can produce gravitational-wave-driven coalescence.

References

Simulations of retrograde disks involving astrophysically realistic Mach numbers is a clear priority for several reasons. First, the race between orbital inspiral and disk tilting will dictate whether or not binaries will reach the gravitational wave regime via retrograde evolution alone, or whether a flip to prograde orientation intervenes before gravitational wave-driven inspiral.

— Electromagnetic Emission and Orbital Evolution of Eccentric Supermassive Black Hole Binaries in Retrograde Disks  (2609.09314 - ONeill et al., 8 Sep 2026) in Section 5.4, Outlook