I Is there a plan for a search for nearby black holes?

[Buzz Bloom]

The thread

https://www.physicsforums.com/threads/how-close-is-earth-to-the-closest-black-hole.939912/​

reached a conclusion that there is a 65% confidence level that there is one black hole within 50 ly of Earth.
The link

http://www.solstation.com/stars/s20ly.htm​

says there are 25 stars bigger than red dwarfs within 20 ly.

Extrapolating to a sphere centered on Earth with a 100 ly radius, this sphere should contain about 625 stars bigger than red dwarfs and 8 black holes, of which about 4 would be in binary systems. Systematically observing the Doppler shifts for these 625 stars over a period of time should detect any invisible massive companions.

Is such a search currently happening or planned?

 

[Buzz Bloom]

Extrapolating to a sphere centered on Earth with a 100 ly radius, this sphere should contain about 625 stars bigger than red dwarfs and 8 black holes, of which about 4 would be in binary systems.

CORRECTION
Extrapolating to a sphere centered on Earth with a 100 ly radius, this sphere should contain about 3125 stars bigger than red dwarfs and 125 black holes, of which about 62 would be in binary systems.

 

[davenn]

Is such a search currently happening or planned?

for what purpose ?

they are more interested in looking for exo-planets

 

[Vanadium 50]

for what purpose ?

For that matter, "how'?

 

[Chronos]

The GAIA mission is the biggest project currently underway. The first data release milestone was achieved in 2016 with; https://arxiv.org/abs/1609.04303,Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes This monumental endeavor will catalog over a billion stars in the MW, along with a number of extragalactic sources - including over 500,000 quasars. Its limiting magnitude is 20.5 and will reach stars in the MW core. The odds of it revealing hitherto undiscovered black holes in the solar neighborhood are considered very favorable. IRAC ans SWIFT have, and will continue to contribute to the observational databese as well. Our knowledge of compact stars will be significantly expanded by these missions over the next decade as researchers sift through this mountain of new data pouring in. Given current knowledge of the abundance of high mass [8+ solar] stars in the solar neighborhood, the odds of finding 125 black hole candidates within 100 light years do not appear promising.

 

[Buzz Bloom]

for what purpose ?

For that matter, "how'?

Hi davenn and Vanadium:

Since I am not one, I have no clear idea about what motivates professional astronomers to do what they do. I also have no idea about whether what motivates my curiosity would also motivate a professional astronomer. However, If I were to undertake a search for nearby black holes, my purpose would be to learn more about what can be observed about black holes. For example, if a black hole were found close enough to Earth, might it become possible to verify by observation evidence of the Hawking radiation phenomenon? If not, perhaps a probe might be sent to gather relevant information.

I am not qualified to answer "How?" However, here is a thought.
1. Take a Doppler measurement for each star that could be in a binary system with an invisible BH. Make appropriate corrections for the known movements of the Earth and Sun.
2. Repeat taking these measurements periodically.
3. For each of these stars that show the behavior of being in a binary system with an invisible large enough mass companion, look for further evidence that this invisible companion might be a black hole. For example, look for gravitational lensing. (I understand that a relatively massive invisible companion with gravitational lensing might be a neutron star. However, there is a good chance the orientation might be favorable for detecting its pulsar behavior.)

Regards,
Buzz

 

[Buzz Bloom]

The GAIA mission is the biggest project currently underway.

Hi Chronos:

The link you posted failed to open for me, but this one worked OK.

https://arxiv.org/abs/1609.04303​

Here is a quote:

For about two million of the brighter stars (down to magnitude ~11.5) we obtain positions, parallaxes, and proper motions to Hipparcos-type precision or better.​

I may not be correctly interpreting the abstract, but it seems to not say that any periodic Doppler measurements were taken, as I described in my previous post to answer the "how" question. If this is correct, then I would guess (possibly incorrectly) that the Gaia mission does not include as a goal a search for black holes.

Regards,
Buzz

 

[phinds]

For example, if a black hole were found close enough to Earth, might it become possible to verify by observation evidence of the Hawking radiation phenomenon? If not, perhaps a probe might be sent to gather relevant information.

I can't see how this would be possible since (1) Hawking Radiation is for all practical purposes so weak as to be non-existent until a BH gets to be quite small and (2) until it gets to be REALLY small, the HR is swamped by CMB (plus any other incidental radiation) so that the BH is a net absorber, not a net radiator.

 

[Chronos]

Buzz, apologies for the broken link. Some papers that may be of interest: https://arxiv.org/abs/1710.09839, Detecting Black Hole Binaries by Gaia., and https://arxiv.org/abs/1704.03455; Hunting Black Holes with Gaia.

 

[Buzz Bloom]

I can't see how this would be possible since (1) Hawking Radiation is for all practical purposes so weak as to be non-existent until a BH gets to be quite small and (2) until it gets to be REALLY small, the HR is swamped by CMB (plus any other incidental radiation) so that the BH is a net absorber, not a net radiator.

Hi phinds:

Thanks for your post.

I would appreciate seeing your thoughts about my probe idea. Re (1) Are you saying that even a probe visiting the neighborhood of a black hole would be unable to detect the Hawking radiation?

Also, re (2), are you saying that Hawking radiation is limited to photons which would therefore be swamped by CMB photons.

Regards,
Buzz

 

[Drakkith]

I would appreciate seeing your thoughts about my probe idea. Re (1) Are you saying that even a probe visiting the neighborhood of a black hole would be unable to detect the Hawking radiation?

Also, re (2), are you saying that Hawking radiation is limited to photons which would therefore be swamped by CMB photons.

Yes and yes. A solar mass black hole radiates as if it were a blackbody at 60 nanokelvins, emitting far less radiation than it absorbs from the CMB and other sources. The emitted radiation is essentially all photons until the black hole is very small.

 

[snorkack]

How significant or insignificant is an accretion disc of an isolated black hole zooming quietly through interstellar gas?

 

[Buzz Bloom]

A solar mass black hole radiates as if it were a blackbody at 60 nanokelvins, emitting far less radiation than it absorbs from the CMB and other sources.

Hi Drakkith:

Thanks for your answers. Am I correct in interpreting that this means that a black body will not begin to loose mass through Hawking radiation until the temperature of the CMB is reduced by the expansion of the universe to a smaller temperature than the temperature of the black body's Hawking radiation? If so, I would like to know the age of the universe when this would happen. My guess is it that it is orders of magnitude greater than the current age. If you have an estimate for this value, I would appreciate seeing it. If not, I will attempt to learn what I need and calculate it myself.

If this guessed age is correct, then would it be reasonable to say that Hawking radiation can not possibly be confirmed by observation within any practical future time-frame?

Regards,
Buzz

 

[Drakkith]

Thanks for your answers. Am I correct in interpreting that this means that a black body will not begin to loose mass through Hawking radiation until the temperature of the CMB is reduced by the expansion of the universe to a smaller temperature than the temperature of the black body's Hawking radiation?

I believe so.

If so, I would like to know the age of the universe when this would happen.

That depends on the mass of the black hole. Supermassive black holes will take far longer to reach that point than stellar mass black holes. Either way, it's many billions of years into the future.

 

[Buzz Bloom]

Either way, it's many billions of years into the future.

Hi Drakkith:

I make a simplifying assumption that the current ratio of matter mass-energy density to dark energy mass-energy density of about 1/5 is sufficiently small to ignore for the following calculation.

(1) a = e^H₀t​

where

(2) 1/H₀ = 13.6 Gy.​

Solving (1) for t gives

(3) t = (ln a) /H₀.
​

The value of a when a BH of 1 solar mass has a Hawking radiation temperature equivalent, T_HRsunmass, equal to the then CMB temperature, T_CMB, is given by

(4) a = T_CMB / T_HRsunmass = 4.43 x 10⁷.​

Putting (4) into (3) gives

(5) t = 17.6 × 13.6 Gy = 241 Gy.​

This is the age of the universe when a BH having a mass at the present time of 1 solar mass begins to lose mass due to Hawking radiation.

This a very long time to wait for observational data about Hawking radiation to start to become observable. I have been thinking about this problem now for a few days, and I have come up with an idea for what seems to me to be a plausible method for a probe to detect an extremely weak Hawking radiation in an environment of much stronger background of CMB radiation. I have decided to start another thread to seek informed opinions regarding the plausibility of this idea.

Regards,
Buzz

 

[Drakkith]

It's not that Hawking radiation is entirely unobservable, it's that it's buried in the "noise" of all that other radiation. I've done a bit of astrophotography, where I take pictures of stuff in the night sky with a telescope and camera. My targets almost always have extremely weak signals that can be buried in the noise of the ambient background light. However, I can get around this by increasing my exposure time to minutes or hours until the signal to noise ratio is high enough for a good picture. Various image processing techniques have also been developed to get around this problem of low SNR. A method similar to the ones used in astrophotography could potentially be used to detect hawking radiation.

 

[Chronos]

As they say in real estate - location, location, location. A black hole located in a matter rich region of space can be a copious source of EM emissions [e.g., quasars]. In a matter poor environment, a BH can become anonymous to the point of invisibility. A microquasar, the low mass relative of quasars, is the most promising black hole harbinger. The nearest of which is probably GRO J1655-40, at about 5500 light years. Less flamboyant representatives of the black hole candidate family are known as x-ray binaries. These come in two basic flavors - 1] high [10+ solar] mass and 2] low [<10 solar] mass systems. Many hundreds of these systems have been catalogued by missions including INTEGRAL and CHANDRA, to name a couple. High mass x-ray binaries along with microquasars are deemed likely to host black holes. The best known and nearest high mass x-ray binary is probably Cygnus X-1 at about 7000 light years. Unsurprisingly, Cygnus X-1 is located in one of the most active star formation regions of the MW Low mass x-ray binaries are generally suspected to host neutron stars. GAIA should add considerably to the list of BH suspects. Isolated BH's will remain resistant to detection and their abundance is subject to considerable speculation. I would hazard to guess isolated BH's are probably little more common than their isolated progenitors: spectral class O and B stars, which constitute a tiny fraction of the known stellar population. The widely quoted estimate of 100 milion black holes populating the MW appears overly generous.

 

[snorkack]

Isolated BH's will remain resistant to detection and their abundance is subject to considerable speculation. I would hazard to guess isolated BH's are probably little more common than their isolated progenitors: spectral class O and B stars, which constitute a tiny fraction of the known stellar population. The widely quoted estimate of 100 milion black holes populating the MW appears overly generous.

Isolated O stars are short lived. Isolated black holes can only be removed by merger or escape. Milky Way should contain thousands of generations of black holes.

 

[Vanadium 50]

A method similar to the ones used in astrophotography could potentially be used to detect hawking radiation.

I don't think so. For the simpler problem of spotting a close black hole because it's black (and Hawking radiation is a tiny, tiny perturbation from this), you need about a billion year exposure, if you know where the BH is on the sky. (It blocks the CMBR) But over a billion years not only moves across the sky, so you have to find it as it moves, but it also moves in distance.

 

[Drakkith]

you need about a billion year exposure

Well, scratch that idea then.

 

[Chronos]

Agreed. But I suspect the production of black hole is severely depressed by evolving metallicity sufficient to preclude them from amounting to more than a microscopic fraction of the current MWr population. While it is true the lifespan of a BH,'once formed, is virtually eternal compared to that of a star, the same is also true of any degenerate star.

Per BlackCAT [re: https://arxiv.org/abs/1510.08869] fewer than 60 stellar mass black hole candidates have thus far been identified. The ATNF pulsar catalog includes overr 2600 entries [http://www.atnf.csiro.au/research/pulsar/psrcat/] According to NASA [re::https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html] "Astronomers have found less than 2,000 pulsars, yet there should be about a billion neutron stars in our Milky Way Galaxy." This yielda a suspiciously high ratio of easily more than 30 neutron stars for each black hole candidate detected. NASA attempt to reconcile the apparent disparity between BH and NS detection rates by noting [re:https://science.nasa.gov/astrophysics/focus-areas/black-holes] "Most stellar black holes, however, lead isolated lives and are impossible to detect. Judging from the number of stars large enough to produce such black holes, however, scientists estimate that there are as many as ten million to a billion such black holes in the Milky Way alone." This assertion appears weak given it suggests black holes could be as plentiful as neutron stars.

 

[phinds]

Hi Drakkith:

Thanks for your answers. Am I correct in interpreting that this means that a black body will not begin to loose mass through Hawking radiation until the temperature of the CMB is reduced by the expansion of the universe to a smaller temperature than the temperature of the black body's Hawking radiation? If so, I would like to know the age of the universe when this would happen. My guess is it that it is orders of magnitude greater than the current age. If you have an estimate for this value, I would appreciate seeing it. If not, I will attempt to learn what I need and calculate it myself.

If this guessed age is correct, then would it be reasonable to say that Hawking radiation can not possibly be confirmed by observation within any practical future time-frame?

Regards,
Buzz

I see you are practicing understatement today

 

[Chronos]

What I was trying to say in 21, which appears a bit unclear, is we need to explain why we see so many neutron strars and so few black hole candidates. Neither of the two are marketing geniuses. Most known NS's are pulsars which need to be oriented favorably to be detected. On the other hand nearly all BH candidates are members of multiple star systems, which require a visible companion to rat them out. The biggest exception is microquasars, of which only a handful have been found. Since they can be detected at great distances [some even in other galaxies] their scarcity is even more a mystery. Assuming the MW is teeming with black holes, where are all the microquasars? I dare to dub this the PF paradox.

 

[Buzz Bloom]

On the other hand nearly all BH candidates are members of multiple star systems

Hi Chronos:

I have seen statements that: (1) about 1/2 of BHs are in multiple systems, more than 1/2 are ..., (3) much more than 1/2 are ..., and now your (4) almost all are ..

Can you post a reference to a source for (4)?

Regards,
Buzz

 

[Buzz Bloom]

I see you are practicing understatement today

Hi phinds:

I confess I was attempting to do exactly that. However in my post #43, I gave the result of my calculation: 241 Gy. So, I guess I was actually over estimating when I said, "My guess is it that it is orders of magnitude greater than the current age."

Regards,
Buzz

 

[snorkack]

Hi Chronos:

I have seen statements that: (1) about 1/2 of BHs are in multiple systems, more than 1/2 are ..., (3) much more than 1/2 are ..., and now your (4) almost all are ..

Can you post a reference to a source for (4)?

Regards,
Buzz

It´s not that most black holes are in multiple systems. It is that almost all black hole candidates are.
Single black holes may be common, probably are, but they are hard to detect.

 

[Chronos]

Buzz, regarding (4): "Stellar evolution models, chemical enrichment by supernovae within the Milky Way, and gravitational microlensing events all indicate that a population of about ∼108 -109 stellar-mass black holes resides in our Galaxy (Shapiro & Teukolsky 1983; van den Heuvel 1992; Brown & Bethe 1994; Samland 1998; Agol et al. 2002). And yet, despite such high estimates, fewer than fifty stellar black hole candidates have been studied and confirmed, all of which are in X-ray binary systems." re;;https://arxiv.org/pdf/1704.03455.pdf ,Hunting Black Holes with Gaia]

Regarding the other flip floppy sounding mass fractions, I defer to; https://arxiv.org/pdf/1704.03455.pdf, Measuring the binary fraction of massive stars using young star clusters: the resolved cluster RSGC1, "It has long since been known that a substantial fraction (∼ >50%) of massive stars are in binaries, and that the companion mass distribution is skewed to higher masses than would be expected if it were randomly sampled from the initial mass function (IMF) (e.g. Abt & Levy 1978; Garmany et al. 1980; Gies 1987; Mason et al. 1998; Garc´ıa & Mermilliod 2001; Kobulnicky & Fryer 2007). In terms of the impact of binarity on stellar evolution, the important quantity is not what fraction of massive stars are in binaries, but what fraction will interact during their lifetime. The most recent measurements indicate that the interacting binary fraction for massive stars may be very high, 50 − 70% (Sana et al. 2012, 2013). These results imply that effects of binarity on the evolution of massive stars cannot be neglected, and therefore call into question all predictions for massive stellar evolution which have their foundation in single-star models." The paper further asserts "Formally, the derived binary fraction for the stars of the same mass as the RSGs, in this case ∼18M, is fbin=37+27 −23. ...This result is somewhat at odds with other recent estimates of binary fractions of massive stars as measured from radial velocity surveys in massive your clusters, which have yielded values of fbin in the region of 50-80%, somewhat lower than other recent estimates of >50%." Further confounding matters is this paper; https://arxiv.org/abs/1306.1811, The Multiplicity of High-Mass Stars, "We report about an ongoing photometric and spectroscopic monitoring survey of about 250 O- and 540 B-type stars in the southern Milky Way with the aim to determine the fraction of close binary systems as a function of mass and to determine the physical parameters of the individual components in the multiple systems. Preliminary results suggest that the multiplicity rate drops from 80% for the highest masses to 20% for stars of 3 solar masses. Our analysis indicates that the binary systems often contain close pairs with components of similar mass. This coincidence cannot originate from a random tidal capture in a dense cluster but is likely due to a particular formation process for high-mass stars. "

 

[Vanadium 50]

A solar mass black hole radiates as if it were a blackbody at 60 nanokelvins, emitting far less radiation than it absorbs from the CMB and other sources.

With power on the order of 10^-29 watts. Not easy to see light years away. Furthermore, it's spectrum will be peaked at extremely low frequencies - to the point where it looks like quasi-static fields instead of radiation. Even harder to see from light years away.

 

[Chronos]

NASA has placed a bet here: https://science.nasa.gov/astrophysics/focus-areas/black-holes, of 10 mlllion to a billion BH denizans in the MW. I'm partial to the low end figure because stars maasive enough to become black holes [>10 solar] are pretty darn scarce and problebly always have been since the earlest generations of pop III stars succumbed to their own overindulgence and polluted the IIGM with metals. I don't see anything to suggest their cumulative corpse numbers can approach the upper limit of the NASA prediction. LIGO has been collecting data since 2002 and did not get their first hit until 2015 and has only managed to record a couple more hits to date. That is not the kind of success that inspires confidence our quaint little backwater hosts 100+ million black holes after 20+ years of scanning the observable universe for BH events. Sure they are hard to detect but, we are talking about a vast ocean and billions of light years scanned, making 3 look pretty frail as the backbone for this case. Here's to hoping my defense attorney can pull off a Perry Mason I can hear it now "Please share with the court, detective, just how long did you pan the accused to extract these 3 nuggets admitted to evidence?"

 

[snorkack]

LIGO has been collecting data since 2002 and did not get their first hit until 2015 and has only managed to record a couple more hits to date. That is not the kind of success that inspires confidence our quaint little backwater hosts 100+ million black holes after 20+ years of scanning the observable universe for BH events. Sure they are hard to detect but, we are talking about a vast ocean and billions of light years scanned, making 3 look pretty frail as the backbone for this case. Here's to hoping my defense attorney can pull off a Perry Mason I can hear it now "Please share with the court, detective, just how long did you pan the accused to extract these 3 nuggets admitted to evidence?"

Compare with neutron star mergers. Gravity wave detectors detect both black hole and neutron star mergers, yet there have been 2 black hole mergers and only 1 neutron star merger. Neutron stars are common, so what it suggests is not that neutron stars are rare but that they rarely merge. Ditto about black holes.

 

[nikkkom]

It looks obvious to me: small BHs usually do not radiate to a detectable extent - they can only radiate if they have an accretion disk.

The other way to detect them in multiple star systems is to detect other star's movements (easy-ish) and prove that another body is heavy, but does not radiate anything (relatively hard). Maybe it's just the case that no one yet embarked on a systematic combing through binary star catalogs for these cases.

 

[Chronos]

GAIA is in the hunt for things like transient luminosity changes due to microlensing caused by a black hole [or other massive body] passing in front of a background star GAIA is particularly well suited for this task since it will catalog the entire sky down to a limiting magnitude of 20.5 - which is sufficient to cover a very large number of stars. The first such events have, in fact, already been detected, as noted here; http://sci.esa.int/gaia/58546-gaia-spies-two-temporarily-magnified-stars/.If the MW really is 'teeming' with black holes', as suggested by some, these are just a hint of what can be expected over the life of the mission - considering objects like planets and black holes tend to have relatively high proper motions. As usual, the real challenge is in making sense out of all the data. Fortunately, there is a large pool of slave labor [grad students] available to draw upon.

 

[snorkack]

In case of a microlens, you also have to search for nature of the microlensing body.

Does microlensing event disclose anything about the distance to lens?

 

[Buzz Bloom]

Hi @Chronos:

Thank you for your link to the ESA GAIA site. From there I found

http://gsaweb.ast.cam.ac.uk/alerts/alertsindex .​

I am hoping you can explain a detail to me. What is the class "CV"?
On page 3, items 7 and 10 have class CV. Below are the details for item 7.

Name Gaia18alh
TNS: AT2018tl
Observed: 2018-02-12 17:56:06
RA (deg.): 100.63864
Dec. (deg.): 40.04256
Mag.: 16.31
Historic mag.: N/A
Historic scatter: N/A
Class: CV
Published: 2018-02-18 15:17:37
Comment: Confirmed CV also reported as ASASSN-18ci and 18cn (see vsnet-alert 21883)​

At

https://en.wikipedia.org/wiki/List_of_astronomy_acronyms#C​

I found

• CV – (celestial object) Cataclysmic Variable, a type of variable binary star system that contains a white dwarf and a companion star that changes

Is this what the index "CV" means?

Regards,
Buzz

 

[Chronos]

I agree with your conclusion and source for the meaning of CV, Astrophysicists are notoriously fond of acronyms, so a handy reference guide is generally helpful. Some of acronyms are rather obscure, so, It is considered good practice and polite to first spell out the whole thing, followed by its acronym [in brackets], or even provide an index to avoid alienating the audience. The IAU ia the authority for all things astronomical, and even has a system for recognizing additions to its official list - re:http://cdsarc.u-strasbg.fr/viz-bin/DicForm

 

[Chronos]

You need to know something about the distance to the lensee [background object] and lens to estimate the lens mass as noted here; https://oneminuteastronomer.com/9237/gravitational-lens/,

"Astronomers can do more than take pretty pictures of gravitational lenses. Using the ideas developed by Einstein and others, if the distance to the foreground and background objects can be determined using http://oneminuteastronomer.com/5478/edwin-hubble/, and if the degree of deflection by the “lens” is measured, then astronomers can calculate the mass of the foreground object. This is, of course, simply amazing. That we can determine the mass of, for example, a cluster of hundreds of galaxies billions of light years away by measuring some light with a telescope and applying some inspired mathematics. And yet science enables such things."

Obfiously, the background star is the easy part since it can be seen and its distance is probably known. The lens - not so much. Being all black and such this gets pretty tricky. One way would be to time the duration of the lensing event. A good measurement would provide its apparent size and probable distance. Not saying this is easy, and still leaves some uncertainty.

 

[glappkaeft]

LIGO has been collecting data since 2002 and did not get their first hit until 2015 and has only managed to record a couple more hits to date. That is not the kind of success that inspires confidence our quaint little backwater hosts 100+ million black holes after 20+ years of scanning the observable universe for BH events.

First that is misrepresenting events more than just a little. The original LIGO equipment was much less sensitive than the Advanced LIGO that started gathering data for the first run September 12, 2015 and found the first confirmed merger two days later. Before the first run was ended January, 2016 it discovered one candidate and one more confirmed merger.

The second even more sensitive run started November 30, 2016 but was was suspended between May 8 and June 8 for maintenance, Virgo joined the two LIGOs on August 1 and the run was shut down August 25. This run discovered four mergers, two of them in August alone.

Secondly they detect mergers not black holes. I don't know the estimated percentage of black holes in close binary systems but it must be tiny.

 

[Chronos]

Understood, My point is, massive stars are not renowned as hermits as discussed here;https://www.e-education.psu.edu/astro801/content/l5_p7.html , by John Dutton of PSU. Furthermore; https://arxiv.org/abs/1306.1811, The Multiplicity of High-Mass Stars, states

"Preliminary results suggest that the multiplicity rate drops from 80% for the highest masses to 20% for stars of 3 solar masses. Our analysis indicates that the binary systems often contain close pairs with components of similar mass. This coincidence cannot originate from a random tidal capture in a dense cluster but is likely due to a particular formation process for high-mass stars."

Assuming black hole originate from high mass stars and high mass stars prefer company, expecting a high proportion of black holes to reside in binary systems does not seem unreasonable.

Yes, the sensitivity of LIGO has much improved since it first came on line, but, so has that of telescopes, The number of planets found in the solar system did not appreciably increase in the years following deployment of the telescope. I do not find it rash to infer this tends to oppose any claim the solar system may be 'teeming' with faint planets.

 

[Monsterboy]

What is the minimum feasible mass of a black hole which will allow it to exist without losing mass due to hawking radiation ? Given that, the only source of energy for the black hole is the CMB i.e energy lost due to hawking radiation = energy gained by absorbing the CMB radiation.

 

[Chronos]

The smallest known black holes tip the scales at around 5 solar masses. A few may even be as small as ~3 solar masses, but this is difficult to confirm. In any event, that is not nearly small enough to emit a detectable amount of Hawking radiation. For that, you need a subsolar mass black hole. Aside from PBH [primordial black holes], there is no known way [even in theory] for such tiny black holes to form.

 

[JMz]

I would appreciate seeing your thoughts about my probe idea. Re (1) Are you saying that even a probe visiting the neighborhood of a black hole would be unable to detect the Hawking radiation?

As for sending a probe to the BH's neighborhood: How long are you willing to wait for it to get there? (You are obviously willing to wait ~ 100 years for it to send back data on whatever it finds when it gets there, so I imagine you are the patient sort.) For reference, the Voyagers and Pioneers travel at ~ 1 LY per 10,000 years.

 

[Buzz Bloom]

As for sending a probe to the BH's neighborhood: How long are you willing to wait for it to get there? (You are obviously willing to wait ~ 100 years for it to send back data on whatever it finds when it gets there, so I imagine you are the patient sort.) For reference, the Voyagers and Pioneers travel at ~ 1 LY per 10,000 years.

Hi JMz:

My suggestion was intended as a thought experiment perhaps involving somewhat more advanced technology for the spaceship engines than what is available today. At the time I wrote that I had an idea for a design for a telescope which would orbit the black hole, with a shield to prevent any CMB radiation for entering the telescope from all directions except that facing the BH. My thought was that the BH itself would block CMB from that direction. However, after exploring the possibilities of the design in some detail, I realized that it would not work.
See
https://www.physicsforums.com/threa...velength-em-gets-through-a-small-hole.943357/

I continued that discussion as a "INBOX Conversation" with @vsv, and I learned that my telescope design would have to be much larger than the size of the BH in order for a significant amount of the Hawking radiation to enter the telescope. Therefore the CMB coming from the direction of the BH would overwhelm the Hawking radiation entering the telescope.

My personal conclusion for all of this investigation was that I should not give any serious credence to the existence of Hawking radiation since it appears that actually ever detecting it is at the very least close to impossible.

Regards,
Buzz

 

[JMz]

My personal conclusion for all of this investigation was that I should not give any serious credence to the existence of Hawking radiation since it appears that actually ever detecting it is at the very least close to impossible.

There is, however, the interest in watching for very-low-mass primordial BHs to evaporate. But no BHs smaller than a few solar masses have ever been detected, AFAIK. And I don't know what the estimated abundance is, so the expectation value for the distance to the closest one could be much larger than 100 LY.

 

[Buzz Bloom]

There is, however, the interest in watching for very-low-mass primordial BHs to evaporate.

Hi JMz:

I read somewhere (sorry, I don't remember where) that any primordial BHs would have already evaporated a long time ago.

Regards,
Buzz

 

[glappkaeft]

Hi JMz:

I read somewhere (sorry, I don't remember where) that any primordial BHs would have already evaporated a long time ago.

Only if they are small enough. The range of sizes speculated about then it comes to primordial black holes is vast (micrograms to 1000s solar masses) and lifetimes vary even more.

Putting 13.7 Gyr lifetime into the calculator at http://xaonon.dyndns.org/hawking/ (looks resonable but I don't know enough to tell for sure) gives a mass of 1.72*10¹¹ kg which is similar to a sub 1 km stony asteroid.

 

[Buzz Bloom]

Putting 13.7 Gyr lifetime into the calculator at http://xaonon.dyndns.org/hawking/ (looks resonable but I don't know enough to tell for sure) gives a mass of 1.72*1011 kg which is similar to a sub 1 km stony asteroid.

Hi glappkaeft:

Thanks for your post. It caused me to remember that the original source I read was about primordial BHs (PBHs) as an unsatisfactory candidate to be a form of dark matter, rather than PBHs in general. I have not seen any analysis regarding models for the formation of PBHs. Do you know of any? There is also an plausible issue about small PBHs merging with other stuff (including other PBHs) into large ones, perhaps then becoming the seeds for galaxy formation.

Regards,
Buzz

 

[glappkaeft]

When it comes to most areas of physics I'm a happy self though amateur (the only pure physics course required was electromagnetism in the computer science master I studied at university). However just googling "formation of PBH" and looking at the research papers from creditable sources will give you material for months to come if you want to self-study.

 

[JMz]

Only if they are small enough. The range of sizes speculated about then it comes to primordial black holes is vast (micrograms to 1000s solar masses)

Yes. For any target date (like, 13.8 GYr), there is a batch of PBH's that are evaporating just then. But how common that size is, is unknown -- as are any issues about the distribution compared to, say, baryonic matter.

 

[glappkaeft]

Yes. For any target date (like, 13.8 GYr), there is a batch of PBH's that are evaporating just then. But how common that size is, is unknown -- as are any issues about the distribution compared to, say, baryonic matter.

Well, everything about PBH is unknown since it is unknown if they exist at all...

 

[JMz]

Quite right! :-)