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As far as I understood the biggest black holes we know are at the centre of very distant quasars and they are enormous. All the big black holes are very far away. At the centre of the closer galaxies there are big black holes, but not as big as the distant ones.

If we take into account that distance is inversely proportional to age[*] could we interpret it as a sign of their evolution? Could this be a hint that somehow they lose mass at a rate that is faster than the Hawking radiation?

[*] I am not taking into account recycled matter here.

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    $\begingroup$ Note that none of the known black holes lose mass at all -- they are colder than the background radiation, let alone radiation from stars. Any BH heavier than the moon gains weight from background radiation alone. (This has cosmological implications because maximum entropy, "heat death", will only occur after the background radiation has cooled down to the temperature of the super massive black holes that remain (e.g. 1E-18K for TON 618 with 6e10 M☉)!), which only then start to evaporate, extremely slowly (~1E107 s). Together, that's basically "never" even cosmologically.) $\endgroup$
    – Peter - Reinstate Monica
    Commented May 14 at 7:52
  • $\begingroup$ (I understand you were asking for a different mechanism, I just wanted to point this out though.) $\endgroup$
    – Peter - Reinstate Monica
    Commented May 14 at 7:53
  • $\begingroup$ It's not what you're asking about, but black holes can lose mass faster than by Hawking radiation in specific circumstances. For example, mergers of binary black holes can result in multiple stellar masses being converted to gravitational radiation as the resulting black hole rings down (hence why these events were targeted for the first gravitational wave detections). However, this takes a very short period of time and only removes a small part of the total mass. $\endgroup$
    – Christopher James Huff
    Commented May 14 at 22:13
  • $\begingroup$ @Peter-ReinstateMonica, Re, "~1E107 s...that's basically 'never' even cosmologically." Depends on whether or not you believe Roger Penrose. en.wikipedia.org/wiki/Conformal_cyclic_cosmology His mass-market book refers to "the long boring phase" of the evolution of the Universe. (I.E., when it consists of nothing but black holes.) And, when he says, "long," he mentions a number of years which is larger than any number I have ever seen in any other scientific literature,* ever. [*Not counting literature about pure mathematics as "scientific."] $\endgroup$
    – Solomon Slow
    Commented May 14 at 23:25
  • $\begingroup$ @Peter-ReinstateMonica Nope. I was not asking for any mechanism and I am well aware that wiih our current knowledge we could not make any hypothesis about how it would happen. My question is if the current distribution hints whether something unknown might have happened in the past several billion years. The current answer is actually in topic, I am just waiting to see if there are other answers before closing the question. $\endgroup$
    – FluidCode
    Commented May 15 at 6:57

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The nearby supermassive black holes are (on average) bigger than the distant ones. Of course there are more distant galaxies than nearby ones, so the largest black holes tend to be in fairly distant galaxies, (but not the very distant ones). The largest black holes are found in extremely large elliptical galaxies, such as Abell 1201, and these are a relatively late development in the universe.

So your basic observation is incorrect. Black holes tend to grow with age. Older supermassive black holes, in nearby galaxies tend to be larger.

More distant black holes tend to be more active, they are accreting more matter, and this makes them brighter. Most of the brightest quasars are distant.

Statistical aside: If I generate 10 normally distributed random values with a mean of 100, and then 1000 random values with a mean of 80 (both with a standard deviation of 10), you would not be surprised that the largest value is in the set of .1000, even though it has a lower average. This is why the largest black hole is fairly distant.

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