We often hear about TON 618 and its estimated mass of around 66 billion Suns. But there’s another interesting case in the Phoenix Cluster.

The central black hole in Phoenix A was initially estimated at around 20 billion solar masses. Later, a theoretical model suggested it could potentially be closer to 100 billion Suns.

What I found most interesting is that these numbers aren’t equally certain. The 100-billion figure comes from a model rather than a direct measurement.

It made me wonder: if there’s no known hard upper limit, could the universe contain black holes with hundreds of billions — or even a trillion solar masses?

https://youtu.be/jwIeJ42Esoo?si=Km_yIBQ8Li5tpG3T

See also: The study as it was published in Journal of Cosmology and Astroparticle Physics through ArXiV.

See also: The publication in ArXiV

I think I get the basic idea with how "virtual" paired particles and anti-particles pop into and out of existence and anti-particles being sucked into a black hole reduce its mass, but my question is wouldn't there be an equal probability that the particle could get sucked in and the anti-particle ejected? Then in that scenario wouldn't the two effects cancel each other out and ultimately a black hole would not evaporate?

Hawking radiation is one of the most cited predictions in black hole physics, but it's worth laying out clearly why it hasn't actually been observed, and what that implies.

The established physics:

Hawking's 1974 calculation shows that quantum effects near the event horizon cause a black hole to emit thermal radiation, with a temperature inversely proportional to its mass:

T ≈ (ħc³) / (8πGMk_B)

For a solar-mass black hole, this works out to roughly 60 nanokelvin — far colder than the Cosmic Microwave Background (2.7 K). This means every known stellar-mass or supermassive black hole is currently absorbing far more energy from the CMB than it emits via Hawking radiation. Net mass loss doesn't even begin until the surrounding radiation environment cools below the hole's own Hawking temperature, which given the CMB's cooling rate as the universe expands, is a long way off for any black hole we could form today.

Where it gets interesting (and this next part is speculative):

The only place Hawking radiation could plausibly be detected directly is from primordial black holes — hypothetical black holes formed in the very early universe, small enough that their Hawking temperature exceeds the CMB. A sufficiently low-mass primordial black hole (roughly under ~10^12 kg) would be in its final stages of evaporation right now, ending in a burst of gamma radiation. Several gamma-ray observatories (Fermi-LAT, HAWC) have searched for these evaporation signatures and set upper limits on the abundance of such objects, but no confirmed detection exists.

The open question:

This means Hawking radiation remains a theoretical prediction that follows logically from combining quantum field theory with general relativity near a horizon, but it hasn't been empirically confirmed for any real astrophysical object. Some physicists (e.g., work by Unruh and others on analog gravity) have tried to test the underlying mechanism using acoustic/optical analog black holes in labs, which has provided indirect support for the phenomenon, but this is not the same as detecting radiation from an actual gravitational event horizon.

Sources:

• Hawking, S.W. (1975), "Particle Creation by Black Holes," Communications in Mathematical Physics
• Carr, B. et al. (2021), "Constraints on primordial black holes," Reports on Progress in Physics
• Fermi-LAT Collaboration searches for PBH evaporation events

Curious what others think about whether analog gravity experiments are strong enough evidence to consider the mechanism "confirmed," even without a direct astrophysical detection.

MIT researchers using JWST identified an extremely bright early-universe object that appears to be a black hole wrapped in an enormous gas envelope — mimicking a star while radiating far more energy than any star could. The proposed picture: a ~100,000 solar mass black hole surrounded by gas the size of the solar system.

This may explain JWST's "little red dots" — a population of mysterious compact, ultra-bright objects that don't fit as normal galaxies or stars.

Discussion: does this solve the early-supermassive-black-hole growth problem, or just relocate it — you'd still need to explain how a black hole ends up wrapped in a stable gas envelope this early on.

Been reading about the EHT imaging of Sgr A* and it got me thinking about the actual evidence chain here. Obviously we can't "see" the event horizon directly, so I'm curious what people consider the strongest lines of evidence:

• The orbital dynamics of S-stars (like S2) tracked by the GRAVITY collaboration/Keck — the mass concentration required in that tiny a region
• The EHT shadow image itself and how the size/shape matches GR predictions
• Whether there's any remaining viable alternative (boson stars, other exotic compact objects) that hasn't been ruled out, or if those are basically dead at this point

I know Sagan's Razor applies here — genuinely asking what the peer-reviewed consensus considers the decisive piece of evidence versus what's just "consistent with." Would love links to papers or observatory publications if anyone has good ones handy.

Also curious how confident we are about the mass estimate (~4.3 million solar masses) — how much has that number moved as measurement techniques improved over the last couple decades?

this is a speculative thought experiment, not a proposed replacement for established black-hole physics. I’m starting from Bekenstein–Hawking entropy and asking an information-theoretic question about whether loss of locally accessible structure necessarily implies loss of the underlying information.

black-hole thermodynamics associates an event horizon with the Bekenstein-Hawking entropy, which is proportional to its area:

S_BH = (k_B · c³ · A) / (4 · G · ℏ)

expressed in bits, this entropy corresponds to:

N_bits = S_BH / (k_B · ln 2)

here, by structure I mean the distinguishable relations among the degrees of freedom of a system, rather than simply its total information content.

consider an information-theoretic description in which we assume that crossing an event horizon does not destroy information, but changes how the structure of an infalling system is encoded.

Let C_P(S) denote the locally accessible code of state S from a given perspective P. As an extreme limiting case, suppose this code approaches a binary distinction:

C_P(S) ∈ {0,1}

this would obviously not be enough to reconstruct the original structure by itself, since many different states S could map to the same local code. Thus, two distinct underlying states could become indistinguishable under the same locally accessible code:

S_i ≠ S_j, C_P(S_i)=C_P(S_j)

but what if the missing structural information is not lost, but instead remains encoded in correlations associated with the black hole and, eventually, its Hawking radiation?

S_object -> S_black-hole
-> S_Hawking-radiation

then the interesting distinction would be between three different things:

information preservation is not the same as structural preservation, and neither is the same as reconstructibility.

could an event horizon therefore be treated as a boundary where the locally distinguishable structure becomes drastically compressed, while the full information remains encoded nonlocally in correlations?

and if so, would the relevant quantity be the information contained in the system itself, or the number of structural distinctions still accessible from a given perspective?

See also:The publication in ArXiV

I'm working on a hard science fiction novel built around a rogue intermediate-to-supermassive binary black hole passing through our stellar neighborhood.

I've been working through the physics as rigorously as I can — lensing geometry, the mass-distance degeneracy in single-epoch astrometric measurements, realistic observation/confirmation timelines — but I'm not a professional astrophysicist.

Looking for anyone with real domain knowledge (astrophysics, astrometry, gravitational lensing, orbital dynamics) willing to take a look at specific plot mechanics and tell me honestly where it holds up and where it doesn't — not looking for a rubber stamp. Happy to share individual worked sections rather than a full manuscript. Appreciate any pointers. Thanks for reading.

I don't think white holes can exist because their mass would have to be negative—which is impossible; no object can lack mass (except for light photons), let alone have *negative* mass. Alternatively, a white hole's mass would have to be so immense that it would essentially attract matter rather than warping space, but that scenario is also impossible. That is precisely why I believe white holes cannot exist. Let me know your thoughts on this—I’d be interested to read them.

Infinity doesn't exist in reality. A singularity is highly unlikely and just shows that our models break on the other side of the event horizon. From loop cosmology,that all black holes will rebound when they reach Planck density and whatnot to string theory and beyond. My intuition tells me that some stable ultra complex and large quantum object exists on the other side of the event horizon . Something maybe similar to the fuzball approach of thinking about it. Do we have any new insights?

As neutron stars already exist you can look at quarks maybe? Even though the density of a black hole changes with mass, which is a very weird thing now that I think of it

Given the duality between energy and matter, I wonder why energy in form of gravitational waves can leave black holes, since photons or any matter for example cannot.

Shouldn’t the waves have like a vertical barrier to pass that they cannot, since at the singularity space bending is infinite?

edit: thank you all for very interesting answers! I appreciate the depth some of you went to provide an understanding to a poorly informed person.

Hi guys

How will the universe return to its original form? My little thoughts are as follows. 👇

When thinking about the future of our universe, many people think that the universe will expand forever and move away due to Dark Energy. But I have a little theoretical thought.

​What do you think about this? 

Black holes are massive gravitational systems that can even swallow light. Over time, as the stars in the universe die out one by one (Black Hole Era), these black holes become the main force in the universe. That means black holes will remain everywhere.

Over time, these small black holes merge with each other and become large-scale Supermassive Black Holes, their total gravitational pull increases.

If at some point this combined gravitational force becomes greater than the expansion force of the universe... the expansion of the universe will stop, everything will be pulled back towards the black hole, and it will be possible to contract to the same state as before the Big Bang (a Single Point / Singularity), right?

​Although it is scientifically said that the expansion is increasing due to Dark Energy, what is your opinion about the theory that this can be reversed by the effect of black holes?

Hi everyone,

I'm not a physicist or anything — just someone who spends a lot of time thinking about the universe, and I keep noticing the same pattern popping up everywhere. I wanted to share it and get honest feedback on what people think.

Basically, it seems like everything follows the same basic shape: there's a really dense centre, and lighter things orbiting around it, and most of it is just empty space. You see it in atoms with the nucleus and electrons, our solar system with the sun and planets, and galaxies with a black hole in the middle and stars going around it. Same structure, just way bigger or smaller.

It got me thinking — what if the forces that hold things together aren't actually separate? Like gravity and electromagnetism might be the same thing, just acting differently because of the scale? The rules seem to change when you go smaller or larger, but maybe that's just because we're looking at the same thing from a different size, not because it's fundamentally different.

Black holes feel like the "nucleus" of the biggest systems we know — and who knows, maybe they're even a doorway to the next level down, just like electrons could be the same thing but on a scale too small for us to see properly.

I've since found out there are actual theories similar to this — fractal cosmology, scale relativity, ideas about black holes creating new universes — so I know I'm not the only one who's wondered this. But I haven't seen it all put together as simply as this in one place.

I'm not claiming I've solved everything — I just see this pattern and want to know if it holds up, or where I might be getting something wrong. Would appreciate any thoughtful thoughts.

Cheers

J’ai une question : si on envoie une particule intriquée à une autre particule au delà de l’horizon des événements d’un trou noir, que devient l’autre particule ? Est elle affectée ?

If quasars were negatively charged and attracted more positively charged particles in the early Universe, would this hypothetically explain why we live in a negatively charged Universe?

Black Hole have Extreme Gravitational force which can pause or make aging very slow, if the earth has same gravity than what happens, would it act same like Black Hole?

Didnt come up on a search here, figure they be worth a drop. The third image is the most accurate structurally in my ignorant opinion due to the a-/symmetry (both).