Archive for supermassive black holes

Black Holes, Hawking Radiation (and AI…)

Posted in Artificial Intelligence, The Universe and Stuff with tags , , , , , on December 7, 2025 by telescoper

It seems to be a common misapprehension that the energy released by the supermassive black holes in, for example, active galactic nuclei is in the form of Hawking radiation. It isn’t. Hawking radiation is only significant for black holes of very low mass. The radiation produced around supermassive black holes is due to the extremely high density and temperate of matter falling into the black hole through an accretion disk not due to the evaporation of the black hole itself. Hawking radiation has never been experimentally detected.

Hawking showed that the a black hole will produce black-body radiation with a temperature, the Hawking Temperature, given by TH in a beautiful formula below that brings together constants relating to gravity, statistical mechanics, quantum theory and relativity:

You can see that the Hawking Temperature is inversely proportional to the mass of the black hole M so is largest for very small black holes. In fact for a black hole with mass of order that of the Moon, the Hawking Temperature is just 3 Kelvin. Since the Universe is bathed in cosmic radiation with this temperature, such a black hole would not evaporate at all because it would absorb as much radiation as it emits by the Hawking mechanism as would any black hole of mass greater than this. The Hawking temperature for a supermassive black hole is many orders of magnitude lower than this, so Hawking radiation is completely irrelevant.

Notice that if a black hole does start to evaporate then its mass begins to decrease. Its Hawking temperature therefore increases so its mass decreases even more quickly. In the end the mass gets so low and the temperature so high that the black hole effectively explodes. Nobody really knows how to describe the final stage as it relies on physics we don’t understand.

Anyway, this all reminds that years ago I set an examination question that involved applying the Hawking formula above to calculate the lifetime of a black hole of mass M. It’s not too hard to show that it scales as M-3. Another part of the question asked: what is the mass of a black hole whose Hawking Temperature is room temperature (say 300 K), what would be the Schwarzschild radius of such a black hole, and what would be its lifetime?

I’ll leave it to my readers to plug the numbers into the Hawking formula above to derive the mass, etc. Please submit your answers through the comments box below. The first correct entry does not win a prize, not even a joke Peace Prize.

For a laugh I asked Google for the answer. Here is the AI summary:

Bonus marks for pointing out everything that’s wrong in this summary.

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Weekly Update at the Open Journal of Astrophysics – 22/03/2025

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , , , , , , , on March 22, 2025 by telescoper

It’s Satuday morning once again, and time for another update of papers published at the Open Journal of Astrophysics. Since the last update we have published two papers, which brings the number in Volume 8 (2025) up to 29 and the total so far published by OJAp up to 264.

The papers we have published this week are connected by the theme of black holes and their role in galaxy formation, which is a very hot topic nowadays!

The first paper to report is “Hawking Radiation from non-evaporating primordial black holes cannot enable the formation of direct collapse black holes” by Jonathan Regan, Marios Kalomenopoulos and Kelly Kosmo O’Neil of the University of Nevada, USA. This paper, which is based on an undergraduate thesis, is a study of the irradiating effects of primordial black holes and a discussion of whether these might influence the subsequent formation of supermassive black holes. It is in the section marked Astrophysics of Galaxies, and was published on Tuesday  18th March.

The overlay is here:

and you can find the final accepted version on arXiv here.

The second paper, which was published on Wednesday 19th March and is also in the folder Astrophysics of Galaxies, is “First Light and Reionization Epoch Simulations (FLARES) – XV: The physical properties of super-massive black holes and their impact on galaxies in the early universe” by Stephen Wilkins & Jussi K. Kuusisto (U. Sussex, UK), Dimitrios Irodotou (Institute of Cancer Research, UK), Shihong Liao (Beijing, China) Christopher C. Lovell (Portsmouth, UK), Sonja Soininen (Insitute of Cancer Research), Sabrina C. Berger (Melbourne, Australia), Sophie L. Newman (Portsmouth, UK), William J. Roper (Sussex), Louise T. C. Seeyave (Sussex), Peter A. Thomas (Sussex) and Aswin P. Vijayan Sussex). This paper uses cosmological hydrodynamical zoom simulations to study the formation of supermassive black holes and their impact on star formation in the early Universe.

Here is the overlay, which you can click on to make larger if you wish:

 

You can read the officially accepted version of this paper on arXiv here.

That’s all for this week. It’s been a bit frustrating for me as Managing Ediutor, because we have built up a backlog of several papers that were accepted for publication some time ago, but are still waiting for the authors to place the final version on arXiv. I hope these won’t take too long to appear, not least because I would like to clear my workflow on the Scholastica platform!

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NANOGrav Newsflash!

Posted in Astrohype, The Universe and Stuff with tags , , , , , on June 29, 2023 by telescoper

In a post earlier this week I wrote that

There is a big announcement scheduled for Thursday by the NANOGrav collaboration. I don’t know what is on the agenda, but I suspect it may be the detection of a stochastic gravitational wave background using pulsar timing measurements. I may of course be quite wrong about that, but will blog about it anyway.

The press conference is not until 1pm EDT (6pm Irish Time) but the papers have already arrived and it appears I was correct in my inference. The papers can be found here, along with a summary. The main results paper is entitled The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background. Here is the abstract (click on the image to make it bigger):

In a nutshell, this evidence differs from the direct detection of gravitational waves by interferometric experiments, such as Advanced LIGO, in that it: (a) does not detect individual sources but an integrated background produced by many sources; (b) it is sensitive to much longer gravitational waves (measured in light-years rather than kilometres).; and (c) the statistical evidence of this detection is far less clear-cut.

While Advanced LIGO can – and does – detect gravitational waves from mergers of stellar mass black holes, the NANOGrav signal would correspond to similar events involving much more massive objects – supermassive black holes (SMBHs) – with masses exceeding a million times the mass of the Sun, such as the one found in the Galactic Centre. If this is the right interpretation, the signal will provide important information about how many such mergers are happening across the Universe and hence about the formation of such objects and their host galaxies.

SMBH mergers are not the only possible source of the NANOGrav signal, however, and you can bet your bottom dollar that there will now be an avalanche of theory papers on the arXiv purporting to explain the results in terms of more exotic models.

Incidentally, for a nice explanation of the Hellings-Downs correlation, see here. The figure from the paper is

I haven’t had time to go through the papers in detail so won’t comment on the results, at least partly because I find the presentation of the statistical results in the abstract a very confusing jumble of Bayesian and frequentist language which I find hard to penetrate. Hopefully it will make more sense when I have time to read the papers and/or when I watch the announcement later.

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New Publication at the Open Journal of Astrophysics!

Posted in Maynooth, OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , on August 24, 2020 by telescoper

So another new paper has been published in the Open Journal of Astrophysics! This one is in the folder marked Astrophysics of Galaxies and is entitled Massive Star Formation in Metal-Enriched Haloes at High Redshift. I should explain that “Metal” here is the astrophysicist’s definition which basically means anything heavier than hydrogen or helium: chemists may look away now.

The authors of this paper are John Regan (of the Department of Theoretical Physics at Maynooth University), Zoltán Haiman (Columbia), John Wise (Georgia Tech), Brian O’Shea (Michigan State) and Michael Norman (UCSD). And before anyone asks, no I don’t force members of staff in my Department to submit papers to the Open Journal of Astrophysics and yes I did stand aside from the Editorial process because of the institutional conflict.

Here is a screen grab of the overlay:

You can click on the image to make it larger should you wish to do so.

You can find the arXiv version of the paper here.

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