Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Feb 24, 2018 #1

    Buzz Bloom

    User Avatar
    Gold Member

    The thread
    reached a conclusion that there is a 65% confidence level that there is one black hole within 50 ly of Earth.
    The link
    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?
     
  2. jcsd
  3. Feb 24, 2018 #2

    Buzz Bloom

    User Avatar
    Gold Member

    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.
     
  4. Feb 24, 2018 #3

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    for what purpose ?

    they are more interested in looking for exo-planets :smile:
     
  5. Feb 24, 2018 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    For that matter, "how'?
     
  6. Feb 24, 2018 #5

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    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.
     
  7. Feb 24, 2018 #6

    Buzz Bloom

    User Avatar
    Gold Member

    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
     
    Last edited: Feb 24, 2018
  8. Feb 24, 2018 #7

    Buzz Bloom

    User Avatar
    Gold Member

    Hi Chronos:

    The link you posted failed to open for me, but this one worked OK.
    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
     
    Last edited: Feb 24, 2018
  9. Feb 24, 2018 #8

    phinds

    User Avatar
    Gold Member

    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.
     
  10. Feb 25, 2018 #9

    Chronos

    User Avatar
    Science Advisor
    Gold Member

  11. Feb 25, 2018 #10

    Buzz Bloom

    User Avatar
    Gold Member

    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
     
  12. Feb 25, 2018 #11

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    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.
     
  13. Feb 25, 2018 #12
    How significant or insignificant is an accretion disc of an isolated black hole zooming quietly through interstellar gas?
     
  14. Feb 25, 2018 #13

    Buzz Bloom

    User Avatar
    Gold Member

    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
     
  15. Feb 25, 2018 #14

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    I believe so.

    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.
     
  16. Feb 25, 2018 #15

    Buzz Bloom

    User Avatar
    Gold Member

    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 = eH0t
    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, THRsunmass, equal to the then CMB temperature, TCMB, is given by
    (4) a = TCMB / THRsunmass = 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
     
    Last edited: Feb 25, 2018
  17. Feb 25, 2018 #16

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    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.
     
  18. Feb 25, 2018 #17

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    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.
     
  19. Feb 26, 2018 #18
    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.
     
  20. Feb 26, 2018 #19

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    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.
     
  21. Feb 26, 2018 #20

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    Well, scratch that idea then.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Loading...
Similar Threads for plan search nearby
I Frequency of nearby Supernova Explosions