36B solar mass black hole at centre of the Cosmic Horseshoe gravitational lens

Original Article ↗ Hacker News Discussion ↗

Black hole mass limits

  • Several comments ask whether there is any theoretical upper limit on black hole mass.
  • One source (via Wikipedia) is cited as giving a maximum of ~270 billion solar masses for luminous accreting supermassive black holes, rising from ~50 billion for “typical” ones to 270 billion only for maximally spinning cases.
  • Another excerpted line notes that “stupendously large” black holes might exceed 100 billion or even 1 trillion solar masses, suggesting current models are uncertain or evolving.
  • Some argue that, in principle, there may be no hard upper bound: mass could keep being added or merged, with the only practical limit being available matter and cosmic expansion.

Growth mechanisms and the Eddington limit

  • The Eddington limit is described as capping how fast a black hole can grow via luminous accretion: radiation pressure from infalling matter can blow material away when luminosity is too high.
  • Crucially, this limit does not restrict growth by mergers, so in theory arbitrarily rapid growth is possible if enough smaller black holes are supplied.
  • There is mention of the “final parsec problem”: we know supermassive black holes merge, but the detailed mechanism for removing orbital energy so they can actually coalesce is not fully understood.

Universe-as-black-hole speculation

  • Some participants mention ideas that our observable universe might be inside a giant black hole, or be a “post-evaporated BH-like thing” from a previous universe.
  • Others strongly dismiss this as “nonsense,” noting: black holes have an exterior while our universe does not; black hole interiors collapse toward a singularity, whereas our universe appears to expand away from an initial singularity and more closely resembles a white hole.
  • Counterpoints stress that “interior/exterior” may be unobservable and that coordinate choices (expanding vs contracting) can be ambiguous; debate remains unresolved in the thread, with consensus that such models are at best highly speculative.

Black hole collisions and gravitational waves

  • Commenters ask what happens if two maximally massive black holes (near the theoretical 270B solar-mass limit) collide.
  • Others clarify that black hole mergers are well-established: LIGO detects gravitational waves from such events, including some very massive, rapidly spinning black holes.
  • The idea of colliding supermassive black holes at relativistic speeds is floated as a way to probe physics at unification energies, though framed as pure thought experiment.

Possibility of seeing our own past via lensing

  • A question is raised: could gravitational lenses like the Cosmic Horseshoe let us see Sun/Earth light from billions of years ago, e.g., early Earth–Moon history.
  • One response: in principle, a sufficiently precise lensing path could redirect our own light back to us, but practical issues (dust, intervening matter, required alignment, extreme faintness) would likely make it unobservable even with hypothetical “orbital hypertelescopes.”
  • Another commenter explains that the Horseshoe lens is ~5.6 billion light-years away; a round-trip path via that lens would show light older than the Sun itself, so it couldn’t show our solar system.
  • The required deflection angles (e.g., ~180°) and narrow “sweet spots” between the photon sphere and the shadow make such paths astronomically unlikely, especially for stellar-mass black holes.

Size, density, and interior physics

  • For a 36-billion-solar-mass black hole, estimates in the thread give:
    • Schwarzschild radius ≈ 7–8 light-days.
    • Event horizon radius ≈ 1,000 times the Earth–Sun distance.
  • Using the standard “mass / horizon volume” trick, one commenter notes the average density would be comparable to Mars’s thin atmosphere; others stress this is a formal calculation, since we don’t actually know the internal matter distribution.
  • Multiple people emphasize that very massive black holes have low average density (because radius ∝ mass, so density ∝ 1/mass²).
  • There is extended debate about what happens inside the event horizon:
    • One side uses the common heuristic that inside, “space becomes timelike,” all future-directed paths lead to the singularity, and you cannot move “outward” in any meaningful sense.
    • Others respond that this depends on coordinate choices; locally, a freely falling observer near the horizon of a huge black hole experiences little curvature (“no drama”), can still raise a hand or throw a ball in their local frame, and tidal forces at the horizon can be small.
  • On time dilation:
    • From far away, an infalling object appears to freeze near the event horizon and never cross it within finite external time.
    • From the infaller’s perspective, they cross the horizon and hit the singularity in finite proper time (minutes to hours for very large black holes).
    • This leads to confusion and speculative musings about matter “never really” entering, or mass being stuck near the horizon; other comments push back, saying the external view doesn’t halt interior physics.

“Teaspoon of black hole” vs neutron star matter

  • Someone asks how the Mars-atmosphere density squares with popular lines like “a teaspoon of black hole weighs more than a mountain.”
  • Several replies:
    • That colorful analogy fits neutron star matter better; black hole density, defined via horizon volume, decreases with size.
    • You can’t literally scoop a teaspoon of black hole; for a point-like singularity, any “scoop” is either all or nothing.
    • A “teaspoon-sized black hole” is possible but is conceptually different from “a teaspoon of black hole.”

Cosmic Horseshoe geometry and evolution

  • A link to the Cosmic Horseshoe’s images is shared.
  • One commenter notes the Horseshoe results from near-perfect alignment of a background galaxy (19 Gly away) and a foreground lens (6 Gly away).
  • A question is raised about how motions of those galaxies (and ours) will change the lensing configuration over time and how quickly the arc shape would evolve; no quantitative answer is given in the thread.

Galaxy dynamics and central masses

  • An anecdote describes early PC-era simulations where approximating each galaxy’s gravity as coming from its center of mass produced realistic-looking colliding galaxies, suggesting a dominant central mass.
  • Another commenter points out that even the most massive known supermassive black holes are only a tiny fraction of their host galaxy’s total mass, so not all dynamics can be reduced to stars orbiting a central black hole.
  • There is brief discussion of when the “mass at the center” approximation is justified (e.g., roughly radial symmetry, large distances) versus when detailed N-body effects matter (e.g., galaxy–galaxy encounters with overlapping extents).

Scale comparisons and reactions

  • The black hole is noted as ~9,000 times more massive than Sagittarius A* in the Milky Way.
  • Some readers find the scale “mind-boggling” and look for visualizations; links to videos and diagrams (e.g., TON 618 scale graphics) are shared to help intuition.

Humor and meta-discussion

  • The “36B” in the title triggers multiple AI/LLM jokes (quantization, pruning, fitting on a “consumer galaxy,” black hole consuming AI venture capital).
  • There’s also light language pedantry (“armchair physician” vs physicist) and a small side-thread about the usefulness vs irritation of nitpicking words and dropping context-less links.