How do merging supermassive black holes pass the final parsec?

Rogue black holes and other intruders

• Some find the idea of ejected supermassive “rogue” black holes terrifying; others argue they’d be harmless and extremely far away.
• Several comments note such an object would be obvious long in advance via gravitational distortions and strong lensing of background stars.
• Small/“micro” black holes are seen as more worrisome because they could be hard to detect and might pass through a system with little warning.
• Clarification: “small” in the article means stellar-mass, not star-sized. A black hole with Earth’s mass would be ~cm-scale and could pass through the solar system with minimal, hard‑to‑attribute effects.
• Some argue an Earth-mass passerby would barely perturb orbits compared with undetected distant dwarf planets; others think there would be measurable disturbances, but brief and localized.

Mechanisms for black hole mergers and the last parsec

• Core issue: dynamical friction brings supermassive black holes to ~1 parsec; gravitational waves efficiently merge them only below ~0.1 parsec. How they bridge that gap on ~100 Myr timescales is debated.
• Standard mechanism: scattering of stars from specific “loss cone” orbits that carry away energy and shrink the binary. Problem: the cone can empty faster than it refills.
• Some work suggests triaxial (non-spherical) galaxies can keep refilling the loss cone; not universally accepted.
• Other proposed contributors: galactic tides shortly after mergers, stars injected from larger radii, and gas disks.
• Gravitational-wave energy loss is agreed to be real but negligible until very small separations; thus the puzzle is “why so fast,” not “how at all.”
• LISA and pulsar timing arrays are highlighted as promising tools to test these ideas.

Dark matter: explanation, epicycles, and alternatives

• One camp welcomes self‑interacting dark matter as a potentially testable way to extract angular momentum from binaries and unify multiple phenomena.
• Another camp sees this as piling on “kludges,” likening dark matter to epicycles: adjustable distributions per galaxy and ever more parameters.
• Disagreement over terminology: some use “dark matter” broadly for any non-luminous mass (e.g., black holes, brown dwarfs), others for new particles (WIMPs, etc.).
• Some insist dark matter remains the most useful framework until a better theory with evidence appears; others argue it’s depressing to tweak an already hypothetical component instead of questioning gravity itself.
• There is debate over whether this specific merger problem really demands new physics, with several stressing that galactic centers are messy and may be mis-modeled rather than fundamentally misunderstood.

Evidence, non-detection, and the search for dark matter

• Direct and indirect detection efforts are noted; each null result shrinks the allowed parameter space.
• One side emphasizes “absence of evidence is not evidence of absence” and sees systematic exploration of parameter space as normal science.
• Others counter with a Bayesian view: every failed search is weak evidence against existing dark matter models, akin to repeatedly failing to find a lost wallet at home.
• Discussion distinguishes “we have no way to detect it” vs. “we are trying but haven’t succeeded,” and notes both statements can simultaneously be true in practice.

Black hole properties, no-hair theorem, and causality

• One answer to “tidal heating” between black holes: classical black holes are externally described only by mass, charge, and spin, so there’s no surface to knead like a planet.
• Another participant objects that the no‑hair theorem strictly applies only to stationary solutions; real astrophysical black holes are dynamic.
• A thought experiment with infalling charge is used to argue that naively treating the black hole as an instantaneously updated point object can imply faster‑than‑light signaling, so care is needed.
• Follow-ups stress that from a distant observer’s frame, infalling matter appears to slow near the horizon, and changes to fields propagate at light speed, preserving causality; the subtleties remain somewhat unresolved in the thread.

Parsecs vs light-years

• Question: why use parsecs rather than light-years, which are more familiar?
• Replies: tradition in astronomy and the link to parallax measurements. A parsec directly encodes distance via the apparent shift of a star by one arcsecond when Earth moves by 1 AU.
• Some note parsecs also serve as a cultural shibboleth rather than offering a compelling practical advantage.

Aether, spacetime, and gravitational waves

• A long subthread compares historical aether theories with modern views of fields and spacetime.
• One position: modern fields and spacetime are effectively “aethers” (pervasive media supporting waves like light and gravitational waves), with LIGO as a scaled‑up successor to the Michelson–Morley interferometer.
• The opposing view: this is mostly semantic; classical aether was a moving, material medium meant to restore Galilean relativity, whereas modern spacetime/fields are Lorentz invariant and don’t constitute “stuff” in the same way.
• Debate touches on whether spacetime can meaningfully be said to “move,” issues of Lorentz invariance, warp bubbles, and how to define a “chunk” of spacetime; participants do not reach consensus.