We’ve Been Wrong About Black Holes

Scientific concepts, discoveries, and nature/astrophysical phenomena

Black holes and what we can/can’t know

• A black hole is defined observationally/theoretically by its event horizon: a boundary beyond which light and information cannot escape.
• Despite the “hole” misconception, black holes are described as extremely dense objects formed from collapsing stars.
• The interior beyond the event horizon is not directly observable; formation details are lost (“information is lost,” in the way the speaker frames it).

Supermassive black holes (SMBHs) and galaxy co-evolution

• Most/essentially every galaxy is said to host a supermassive black hole at its center.
• Reported observational correlations:
  • SMBH mass correlates with galaxy stellar mass.
  • It also correlates with central stellar properties and stellar kinematics.
  • More ordered rotation in central stars aligns with less massive SMBHs.
  • More scrambled/chaotic motions align with more massive SMBHs.
• Co-evolution idea: SMBHs and their host galaxies grow “hand in hand.”

Galaxy mergers and how they feed black holes

• Galaxy mergers scramble angular momentum:
  • Ordered, stable orbits become turbulent/chaotic.
  • Material is driven inward (“sent tumbling towards the middle”) and can:
    • Grow the SMBH.
    • Potentially merge SMBHs when two galaxies’ central black holes come together.
• Mergers can reshape galaxies from spiral discs toward elliptical/“bulge/sphere” morphologies.

Rethinking the merger-only picture: SMBHs in “isolated” spiral/disc galaxies

• Method/approach (as described):
  • Select galaxies with pristine spiral disc morphology (interpreted as evidence of no recent major merger).
  • Measure the SMBH masses in these galaxies.
  • Compare them against the expected SMBH–galaxy correlations.
• Key findings:
  • SMBHs in these apparently non-merged systems can be up to ~1 billion solar masses.
  • These SMBHs are often larger than expected from the “merger-driven” correlation picture.
• Simulation-derived feeding estimate:
  • In ~35% of simulated cases, SMBH fuel traces back to merger events (elsewhere mentioned, sometimes ~15%).
  • Therefore, at least ~65% of SMBH growth in these scenarios appears to come from non-merger processes.

Gas inflow mechanisms (feeding SMBHs without major mergers)

Proposed internal/large-scale inflow channels discussed include:

• Spiral arms acting as funnels for gas toward the center.
• A central bar (an elongated stellar feature) channeling gas inward.
• Gas accretion from the cosmic web (filamentary large-scale structure connecting galaxies).

Accretion discs, quasars, and black hole “burps” (feedback)

• SMBH growth via accretion discs:
  • Infalling matter forms a disc spiraling inward.
  • Strong gravity heats the gas, making discs bright and observable.
• Quasars:
  • Extremely bright active galactic nuclei, historically identified as “quasi-stellar objects.”
  • Their brightness can outshine entire galaxies, linked to accretion around SMBHs.
• “Burps” (outflows and jets):
  • If too much material accumulates near the BH, pressure and energy release can drive energetic ejections.
  • Outflows can become:
    • Winds pushing gas outward.
    • Jets launched along the black hole spin axis, associated with strong magnetic fields (per the speaker).
• Galaxy-scale consequence:
  • Feedback can heat or remove galactic gas, potentially shutting down star formation (“killing the galaxy”).

Star-forming vs “dead” galaxies; blue vs red

• Galaxy types emphasized:
  • Spiral, star-forming galaxies (often blue): still forming stars.
  • Elliptical, non-star-forming galaxies (often red): dominated by older stellar populations.
• Percentages quoted (approximate, as stated):
  • ~70% of spirals: star forming; ~30%: not.
  • ~90% of ellipticals: dead; ~10%: forming stars.
• Quenching physics idea:
  • Star formation needs cold gas that can clump under gravity.
  • Heating gas raises its energy so gravity can’t compress it into dense star-forming regions.

Two-stage evolution with a transitional “middle”

• Spiral-to-elliptical transformation discussed:
  • Spirals may quench slowly.
  • Ellipticals may transform quickly, including morphological change.
• Rejuvenation possibilities:
  • Star formation might restart if cool gas is accreted (external infall or minor interactions).

Evidence and tools: spectroscopy, “red dots,” and JWST

• Spectral method:
  • Galaxy light is split into a spectrum.
  • Element-specific emission lines are expected at known wavelengths.
• How SMBHs broaden lines:
  • Gas near BHs moves rapidly; emitted light is Doppler shifted (blue/red depending on motion), producing broadened emission lines.
• JWST “little red dots”:
  • A population of compact sources found in early Universe surveys.
  • Interpretation issues:
    • They show some spectral signatures consistent with growing SMBHs, but not always the expected broadened lines.
    • The spectrum shape sometimes shows a V-like feature (speaker’s description).
  • Suggested explanation:
    • Dust and dense gas cocoon reprocess light from the SMBH accretion disc, altering observed spectral features.

Limits on SMBH growth; possible “when discs stop” crossover

• Conceptual boundaries/regions discussed:
  • Event horizon (point of no return).
  • Innermost stable circular orbit (ISCO): closest stable orbit for matter; inside it, matter spirals in.
  • Self-gravitational radius: where a gas cloud’s self-gravity dominates over the BH’s tidal influence, allowing the gas to form stars rather than feeding the disc.
• Growth-limit idea:
  • As BH mass increases, ISCO shifts, while the self-gravitational radius may not expand as fast, potentially creating a regime where accretion discs can’t form efficiently.
  • Speaker mentions a possible upper scale around ~100 billion solar masses, citing an object around ~60 billion solar masses as possibly near the limit.
• If discs can’t form, SMBHs might “wink out” observationally because the bright accretion signature disappears.

The “black hole mass gap” and other black hole categories

• Stellar-mass BHs:
  • Form from supernovae, on the order of ~10 solar masses (speaker: “around 10 times heavier than the sun”).
• Intermediate-mass BHs:
  • Roughly ~10,000–100,000 solar masses, described as a poorly populated range.
  • Often discussed as a mass gap (speaker notes difficulty finding them and gaps from ~100 to a million solar masses).
• Primordial black holes (PBHs):
  • Hypothetical BHs formed in the early Universe from high-density fluctuations.
  • Could range from extremely small masses to Earth-mass-scale in some scenarios.
  • PBHs are proposed as a potential dark matter candidate, with both supportive and conflicting evidence mentioned.
• Hawking radiation and evaporation:
  • PBHs can lose mass via quantum effects near the event horizon.
  • The effect is extremely weak for large BHs, making detection difficult.
  • Speaker references Stephen Hawking and possible gamma-ray detection ideas.

Dark matter evidence and relation to gravity

• What dark matter is (as described):
  • Matter that does not interact with light (no electromagnetic interaction), making it invisible.
• Key evidence highlighted:
  • Gravitational lensing: galaxies/clusters curve spacetime and warp background light; lensing implies more mass than visible matter accounts for (quoted as ~5–10× more mass than visible, after visible components are accounted for).
  • Cosmological simulations: without cold dark matter, galaxy formation struggles to reproduce observed structures.
• Alternative gravity caveat:
  • The argument assumes Einstein’s general relativity is correct at these scales.
  • Modified-gravity alternatives have been explored, but (per the speaker) they have not produced a better comprehensive explanation.

Black holes as “extreme test beds” for physics

• Gravity is maximized near BHs, making them useful for testing:
  • General relativity predictions (speaker references Event Horizon Telescope imaging consistent with GR).
  • How to connect quantum mechanics with gravity (the broader “quantum gravity” challenge).

Observatories and upcoming survey/data prospects

• Reuben Observatory / time-domain surveys:
  • Expected to produce frequent sky imaging to capture variability from accretion discs (speaker analogy: film → video).
• Euclid space telescope:
  • Wide sky survey aimed at resolving galaxy morphologies (spirals vs ellipticals; bulges).
• Data explosion theme:
  • Future radio surveys like SKA will generate data too fast for naive transfer, driving advances in data access, bandwidth, and processing.

Researchers / sources featured (mentioned explicitly)

• Dr. Becky Smith (University of Oxford; interview subject)
• Stephen Hawking (Hawking radiation; mentioned)
• Albert Einstein (general relativity; mentioned)
• Brady Haran (producer; mentioned as running channels including Numberphile/60 Symbols/Computerphile)
• JWST (James Webb Space Telescope; mentioned)
• Hubble Space Telescope (mentioned)
• Event Horizon Telescope (mentioned)
• SKA (Square Kilometre Array; mentioned)
• Euclid (mentioned)
• Reuben Observatory (named as “Reuben”; mentioned)
• Messier 87 / M87 (M87 black hole; mentioned)
• Sagittarius A* (Sgr A*; mentioned)
• Milky Way / Andromeda (M31) (galaxies mentioned)
• JC / “JC just kept being like …” (not clearly identifiable as a specific person; appears to refer to JWST/James Webb, but no definite researcher name is provided)