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Black Holes Wandering the Galaxy?

Do rogue black holes wander through the distant outskirts of the Milky Way? A new theory suggests one way to find out: Look for small clusters of stars that should accompany such objects. The idea is that low-mass proto-galaxies with black holes at their center would have merged, creating a gravitational kick that would send the now larger black hole outward fast enough to escape the host dwarf galaxy, but not fast enough to leave the overall galactic halo.


Image: This artist’s conception shows a rogue black hole floating near a globular star cluster on the outskirts of the Milky Way. New calculations by Ryan O’Leary and Avi Loeb suggest that hundreds of massive black holes, left over from the galaxy-building days of the early universe, may wander the Milky Way. Fortunately, the closest rogue black hole should reside thousands of light-years from Earth. Credit: David A. Aguilar (CfA).

The ‘kick’ comes from the emission of gravitational waves as the black holes merge, carrying away linear momentum. We can sometimes find black holes because of their accretion of matter, which makes them visible. But to find these rogue objects, each massing from a thousand to hundred thousand suns, we should search for remarkably compact clusters of stars whose individual components are moving rapidly. Says Ryan O’Leary (Harvard-Smithsonian Center for Astrophysics), who developed this work along with Avi Loeb:

“The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef. Without the shining stars to guide our way, the black holes would be all but impossible to find.”

A window into galactic history? Evidently so, as the paper argues (internal references omitted for brevity):

During the hierarchical build-up of the Milky-Way (MW) galaxy, mergers of low-mass galaxies were common and should have resulted in a population of freely floating BHs [black holes]. Since the gravitational potential of an overdense region does not evolve dramatically during the growth of cosmological structure the
region that eventually collapsed to make the MW was able to trap those BHs with the most common kick velocities… even before the MW had formed. Each of the ejected BHs carries with it a star cluster that used to populate the core of its parent galaxy. In this paper we examine the observational signatures of the ejected star clusters which are expected to be floating in the MW halo. The discovery of a relic population of the star clusters attached to recoiled BHs would provide a new window for cosmology, allowing one to constrain the merger history of the MW as well as the formation history of the first population of massive BHs.

The galactic halo, then, may contain evidence of the Milky Way’s history, the number of black holes there depending upon the number of small proto-galaxies that went into its creation. It’s a theory with a useful observational test, one that can be examined by poring through sky surveys looking for these compact star clusters with their highly individualistic signature.

The paper is O’Leary and Loeb, “Star clusters around recoiled black holes in the Milky Way halo,” accepted by Monthly Notices of the Royal Astronomical Society and available online. A Harvard-Smithsonian Center for Astrophysics news release is also available.

Comments on this entry are closed.

  • James M. Essig May 1, 2009, 20:05

    Hi Folks;

    1,000 to 100,000 solar mass blackholes roaming the halo regions of the Milky Way! The first thing that came to mind is the use of such black holes as an energy source. Atleast 10 % of mass falling into a blackhole can be converted into and radiated away as energy. This bests the most efficient hydrogen to Helium fusion by a factor of about 14.

    Such blackholes may be a fantastic energy source for a future galactic human civilization and also a good way to store mass over truely cosmic epochs given the extreme lifetimes of massive blackholes.

    One thing that just occured to me is the use of such blackholes for powered and unpowered gravity assists for near C manned space craft wherein the path of a space craft would be incrementally bent by gravity assists while at the same time picking up significant kinetic energy each time the craft encountered a blackhole under powered gravity assists.

    On the scary side, I would hate to have one of these blackholes somehow get tossed back into the outer disk of the Milky Way with Earth’s name on it. I think we do not have to worry about that at present, however.

    Also, could these rouge very massive blackholes be an significant overlooked component of Cold Dark Matter? I have heard of the microlensing or transit study results and I wonder if some how massive halo blackholes could be a strongly overlooked component of CDM.

  • Erik Anderson May 3, 2009, 13:20

    The idea of massive black holes significantly comprising the Dark Matter of the Galactic Halo has indeed been kicked around before, some time ago. This was the theory bannered by the acronym MACHOs (MAssive Compact Halo Objects). One of the main arguments that torpedoed the MACHO scenario is that if the Halo was so chock-full of the black holes, then globular clusters (which also populate the Halo) would be so often tidally disrupted by black hole encounters that hardly any globular clusters would remain in existence at the present day. This realization helped clear the way for Dark Matter theorists to nearly unanimously embrace CDM – also known as WIMPs (Weakly Interacting Massive Particles). MACHOs vs WIMPs – get it? ;-)

    I would presume that O’Leary & Loeb’s scenario does not even nearly approach the black hole density prescribed by the MACHO scenario, or else it would run aground against the same objection. Thus it has no potential to account for the galactic rotation problem. However, there is an ideological connection between CDM and O’Leary & Loeb’s hypothesis. In Cosmology, the massiveness of alleged CDM halos is considered to be a catalyst for galactic mergers, so it’s fair to say that what the CDM paradigm portends for Galactic history (an abundance of mergers) is a principal motivator for O’Leary & Loeb’s hypothesis (an abundance of leftover black holes). I myself do not subscribe to this ideology, so I do not expect their hypothesis to be confirmed by observation.

  • Adam May 3, 2009, 18:28

    Personally I like the idea that Dark Matter is actually shadow matter. Since it’s more abundant than regular matter it would’ve clumped together easier and catalysed galaxy formation. A black hole would be same regardless of what it was made of, but would form quicker from the shadow matter.

    Of course the tidal disruption problem is also the same for both options, so it’s not a perfect idea…

  • Adam May 3, 2009, 21:30

    Hi Erik

    I looked at your website and it looks really interesting – I’m enjoying the physics tutorial in particular. I have a question about the cosmology you describe: will the Universe recollapse and how does the radius evolve with time?

  • Erik Anderson May 4, 2009, 13:31

    Hi Adam,

    Other than my bio page (linked to my name above), the verbal content featured at RQGravity.net is the creation of my colleague and mentor, Charles Francis. But I am pleased to answer your question…

    Prior to 1997, there were three broadly accepted scenarios of cosmic expansion, aka the “Friedmann models,” having mathematical origins dating back to 1922. Whether or not the universe recollapses depends on a “critical” value of total mass in the universe, which is determinable by observing the spectrographic redshifts of galaxies at known distances. Plotting this correlation should profile the historic deceleration of cosmic expansion. But the famous Supernova Ia observational campaigns in the late 1990’s produced an unexpected surprise: cosmic ACceleration. Since then, the possibility of cosmic recollapse has fallen into disfavor.

    However, the scenario of cosmic acceleration depends upon how one interprets spectrographic redshift. Doing so requires the joint-application the principles of quantum mechanics and general relativity. But how to apply the principles jointly is actually one of the outstanding unsolved problems in physics today. Charles Francis proposes a solution which just so happens to predict an unmodeled component of spectrographic redshifts that exaggerates line-of-sight velocities. Once that factor is corrected for, velocities are reduced, and acceleration vanishes. Cosmology reverts to the situation 1997, the need for “Dark Energy” is obviated, and the possibility of cosmic recollapse is back on the table.

  • ljk May 14, 2009, 12:55

    the physics arXiv blog

    Could all particles could be mini-black holes?

    Posted: 13 May 2009 09:10 PM PDT

    The idea that all particles are mini black holes has major implications both for particle physics and astrophysics, say scientists

    Could it really be possible that all particles are mini-black holes? That’s the tantalising suggestion from Donald Coyne from UC Santa Cruz (now deceased) and D C Cheng from the Almaden Research Center near San Jose.

    Black holes are regions of space in which grsavity is so strong that nothing, not evenlight, can escape.

    trouble with gravity is that on anything other than an astrophysical scale, it is so weak that it can safely be ignored. However, many physicists have assumed that on the tiniest scale, the Planck scale, gravity regains its strength.

    In recent years some evidence to support this contention has emerged from string theory where gravity plays a stronger role in higher dimensional space. It’s only in our four dimensional space that gravity appears so weak.

    Since these dimensions become important only on the Planck scale, it’s at that level that gravity re-asserts itself. And if that’s the case, then mini-black holes become a possibility.

    Coyne and Cheng ask what properties black holes might have on that scale and it turns out that they may be far more varied than anyone imagined. The quantisation of space on this level means that mini-black holes could turn up at all kinds of energy levels. The predict the existence of huge numbers of black hole particles at different energy level. So common are these black holes that the authors suggest that:

    “All particles may be varying forms of stabilized black holes”

    That’s an ambitious claim that’ll need plenty of experimental backing. The authors say this may come from the LHC, which could begin to probe the energies at which these kinds of black holes will be produced.

    The authors end with the caution that it would be wrong to think of the LHC as a “black hole factory”; not because it won’t produce black holes (it almost certainly will), but because, if they are right, every other particle accelerator in history would have been producing black holes as well.

    In fact, if this thinking is correct, there’s a very real sense in which we are made from black holes. Curious!

    Ref: http://arxiv.org/abs/0905.1667: A Scenario for Strong Gravity in Particle Physics: An Alternative Mechanism for Black Holes to Appear at Accelerator Experiments

  • ljk June 15, 2009, 10:36

    A sonic black hole in a density-inverted Bose-Einstein condensate

    Authors: O. Lahav, A. Itah, A. Blumkin, C. Gordon, J. Steinhauer

    (Submitted on 7 Jun 2009)

    Abstract: We have created the analogue of a black hole in a Bose-Einstein condensate. In this sonic black hole, sound waves, rather than light waves, cannot escape the event horizon. The black hole is realized via a counterintuitive density inversion, in which an attractive potential repels the atoms. This allows for measured flow speeds which cross and exceed the speed of sound by an order of magnitude. The Landau critical velocity is therefore surpassed. The point where the flow speed equals the speed of sound is the event horizon. The effective gravity is determined from the profiles of the velocity and speed of sound.

    Subjects: Quantum Gases (cond-mat.quant-gas); General Relativity and Quantum Cosmology (gr-qc)

    Cite as: arXiv:0906.1337v1 [cond-mat.quant-gas]

    Submission history

    From: Jeff Steinhauer [view email]

    [v1] Sun, 7 Jun 2009 09:56:15 GMT (199kb,X)


  • ljk June 15, 2009, 11:10


    “Burning walls” may stop black hole formation

    A new effect may oppose the formation of black holes and explain the mysterious energy of gamma ray bursts

    Monday, June 15, 2009

    Black holes are thought to form when stars of sufficient size collapse creating a force so strong that nothing can oppose it. The result is a region of space with infinite density and a gravitational field so strong that nothing, not even light, can escape.

    The idea that no known force can oppose the collapse of a large star sits uncomfortably with many physicists. Einstein believed that black holes could not form because the angular momentum of the star would eventually become high enough to stabilise a collapse.

    Others say that our inability to find a force that opposes collapse says more about our limited understanding of physics than about the existence of black holes.

    The current thinking is that any star 3 or 4 times bigger than the Sun ought to form a black hole in a supernova at the end of its life. Anything smaller than that and the the degeneracy pressure of neutrons, which prevents neutrons being squashed too closely together, can succcessfuly oppose the collapse. Hence the formation of neutron stars.

    Now Ilya Royzen from the P.N. Lebedev Physical Institute of the Russian Academy of Sciences in Moscow has put his finger on an even more powerful force at work in supernovas.

    He says that quantum chromodynamics predicts that when a collapse overcomes the pressure of neutron degeneracy, another effect comes into play: matter undergoes a phase change.

    This change is from a hadronic form to a so-called subhadronic form, which is very different to ordinary matter. In subhadronic form, space is essentially empty. So the phase change creates a sudden reduction in pressure allowing any ordinary matter in the star to implode into this new vacuum. The result is a massive increase in temperature of this matter to 100 MeV or so, creating, what Royzen calls, a “burning wall” within the supernova.

    He says it is this”burning wall that stops the formation of a black hole during a supernovas of stars up to about 4 times the mass of the Sun, not just the degeneracy pressure of neutrons.

    Now 100 MeV is several orders of magnitude more energy than any theory of supernovas has predicted so far. And that’s interesting because it ought to produce very powerful gamma rays.

    Strangely enough, the most powerful gamma rays are several orders of magnitude more powerful than can be explained by existing theories of supernovas.

    As Royzen says, it’s hard to resist making the link. If it stands up, he may have put his finger on the mechanism that finally explains the most powerful gamma ray bursts in the universe and one of the great modern day mysteries of astrophysics.

    Ref: http://arxiv.org/abs/0906.1929: QCD Against Black Holes?

  • ljk June 15, 2009, 22:32

    A MC simulation of showers induced by microscopic black holes

    Authors: D. Gora, M. Haag, M. Roth

    (Submitted on 15 Jun 2009)

    Abstract: Large surface detectors might be sensitive not only with respect to extensive air showers induced by ultra high energy neutrinos but also to showers induced by hypothetical objects like microscopic black-holes.

    Microscopic black-holes might be produced in high energy particle collisions with the center of mass energies above the fundamental scale of gravity. These black-holes would decay rapidly by Hawking radiation into characteristic high multiplicity states of Standard Model particles and induce extensive air showers potentially detectable by a large surface neutrino detector.

    In this paper we study the possibility to detect microscopic black-holes exemplifying it in case of the surface detector of the Pierre Auger Observatory. The expected event rate is calculated for up-going and down-going showers induced by microscopic black-holes. Our calculations show a significant deviation of the expected rate compared to the rate expected by Standard Model predictions. The rates of up-going neutrinos are almost completely suppressed, whereas the rate of down-going neutrinos increase by a factor of about 50 with respect to standard model calculations.

    The non observation of up-going neutrinos by the Pierre Auger Observatory in conjunction with a high rate of down-going neutrino-induced showers, would be a strong indication of physics beyond the Standard Model.

    Comments: Conference Proceedings of ICRC 2009

    Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

    Cite as: arXiv:0906.2650v1 [astro-ph.HE]

    Submission history

    From: Gora Dariusz Dr. [view email]

    [v1] Mon, 15 Jun 2009 09:44:38 GMT (90kb)


  • ljk June 24, 2009, 15:36

    Are Black Holes Elementary Particles?

    Authors: Yuan K. Ha

    (Submitted on 19 Jun 2009)

    Abstract: Quantum black holes are the smallest and heaviest conceivable elementary particles. They have a microscopic size but a macroscopic mass. Several fundamental types have been constructed with some remarkable properties.

    Quantum black holes in the neighborhood of the Galaxy could resolve the paradox of ultra-high energy cosmic rays detected in Earth’s atmosphere. They may also play a role as dark matter in cosmology.

    Comments: Lecture delivered in Conference on Particle Physics, Astrophysics and Quantum Field Theory: 75 Years since Solvay, 27 -29 November 2008, Nanyang Executive Centre, Singapore. 10 pages

    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics – Phenomenology (hep-ph); High Energy Physics – Theory (hep-th)

    Cite as: arXiv:0906.3549v1 [gr-qc]

    Submission history

    From: Yuan K. Ha [view email]

    [v1] Fri, 19 Jun 2009 00:48:01 GMT (6kb)


  • ljk July 12, 2009, 10:20

    Entangled black holes as ciphers of hidden information

    Authors: Samuel L. Braunstein, Hans-Jürgen Sommers, Karol Życzkowski

    (Submitted on 4 Jul 2009)

    Abstract: The black-hole information paradox has fueled a fascinating effort to reconcile the predictions of general relativity and those of quantum mechanics. Gravitational considerations teach us that black holes must trap everything that falls into them.

    Quantum mechanically the mass of a black hole leaks away as featureless (Hawking) radiation. However, if Hawking’s analysis turned out to be accurate then the information would be irretrievably lost and a fundamental axiom of quantum mechanics, that of unitary evolution, would likewise fail.

    Here we show that the information about the matter that collapses to form a black hole becomes encoded into pure correlations within a tripartite quantum system, the quantum analog of a one-time pad until very late in the evaporation, provided we accept the view that the thermodynamic entropy of a black hole is due to entropy of entanglement.

    In this view the black hole entropy is primarily due to trans-event horizon entanglement between external modes neighboring the black hole and internal degrees of freedom of the black hole.

    Comments: 5 pages, 1 figure

    Subjects: Quantum Physics (quant-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics – Theory (hep-th)

    Cite as: arXiv:0907.0739v1 [quant-ph]

    Submission history

    From: Samuel Braunstein [view email]

    [v1] Sat, 4 Jul 2009 05:02:52 GMT (32kb)


  • ljk July 12, 2009, 12:13

    The role of black holes in galaxy formation and evolution

    Authors: A. Cattaneo, S. M. Faber, J. Binney, A. Dekel, J. Kormendy, R. Mushotzky, A. Babul, P. N. Best, M. Brueggen, A. C. Fabian, C. S. Frenk, A. Khalatyan, H. Netzer, A. Mahdavi, J. Silk, M. Steinmetz, L. Wisotzki

    (Submitted on 9 Jul 2009)

    Abstract: Virtually all massive galaxies, including our own, host central black holes ranging in mass from millions to billions of solar masses. The growth of these black holes releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. A tiny fraction of this energy, if absorbed by the host galaxy, could halt star formation by heating and ejecting ambient gas.

    A central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies, which typically have little cold gas and few young stars, unlike spiral galaxies.

    Comments: Nature Review 7 pages, 5 figures

    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA)

    Journal reference: Nature 460, 213-219 (9 July 2009)

    Cite as: arXiv:0907.1608v1 [astro-ph.CO]

    Submission history

    From: Arman Khalatyan [view email]

    [v1] Thu, 9 Jul 2009 16:32:36 GMT (682kb,X)


  • ljk July 12, 2009, 12:17

    Looking for Intermediate-Mass Black Holes

    Authors: Paul H. Frampton

    (Submitted on 9 Jul 2009)

    Abstract: A discussion of the entropy of the universe leads to the suggestion of very many intermediate-mass black holes between thirty and three hundred thousand solar masses in the halo. It is consistent with observations on wide binaries as well as microlensing and considerations of disk stability that such IMBHs constitute all cold dark matter

    Comments: 4pp latex. Plenary talk at BSM-LHC conference, Northeastern University, June 2009

    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

    Cite as: arXiv:0907.1646v1 [astro-ph.CO]

    Submission history

    From: Paul Frampton [view email]


  • ljk September 22, 2009, 11:12


    Tuesday, September 22, 2009

    Black Holes Cannot Exist in Latest Theory of Quantum Gravity

    Nobel prize-wining physicist says black holes and space-time singularities cannot exist in his latest model of the universe

    One of the great challenges of modern science is to unite our thinking about the universe on the largest scale with our notions of how it works on the smallest; in other words to combine relativity and quantum mechanics into a single theory.

    The current best effort is a notion called string theory, an idea born of quantum thinking and in which gravity is a byproduct of complexity, a so-called emergent phenomenon.

    The trouble with with this process of emergence is that it pays mere lip service to our intuitive ideas about causality: that an affect must be preceded by its cause. At least, that’s how the Nobel prize winning physicist Gerard ‘t Hooft sees things.

    To put this right, he has constructed a different model of reality that preserves causality and has some interesting side effects. The fundamental change in his thinking is to accept a new kind of symmetry in the universe.

    A symmetry is a property of a system that is left unchanged under a certain transformation. So for example, our laws of physics are derived from the idea that they must remain constant under any change of position or direction in space. It’s a hugely powerful idea.

    Now ‘t Hooft says that to preserve the idea of causality in a theory of quantum gravity we need to accept the idea of a symmetry of scale. In other words, teh laws of physcis are the same at every scale. He also introduces an idea called “black hole complementarity” in which an observer inside a black hole sees the universe in a different way to an observer outside a black hole.

    The consequences of this thinking are profound. t’ Hooft puts it like this:

    “If we add these to our set of symmetry transformations, black holes, space-time singularities, and horizons disappear,”

    In exchange, we keep the notion of causality intact.

    You can argue the merits of such an exchange but the important question is whether ‘t Hooft’s new universe bears any relation to the one in which we live.

    In answer we can say that the existence of black holes is well accepted. Astronomers can see their gravitational effects. And while nobody has directly observed a black hole or the Hawking radiation physicists assume they emit, few doubt that the evidence in favour will mount.

    A more serious problem is the notion of scale invariance itself. Here’s a thought experiment for ‘t Hooft. Imagine you were suddenly shrunk or enlarged by some unknown factor inside a closed box, what experiment could you do to determine your new scale?

    If the laws of physics were scale invariant, it would be impossible to determine your scale with an experiment.

    But suppose you were to measure the position of a ball. Surely, in our universe, the accuracy of your measurement would be a good indication of your scale, since quantum effects would be easily distinguishable from Newtonian ones.

    ‘t Hooft seems to recognise this limitation admitting that “Newton’s constant G is not scale-invariant at all.”

    But that’s a problem of his own making. In answer to the question of how to unite the physics of the very big with the physics of the very small, ‘t Hooft says there is no difference between them.

    That may not be as crazy as it sounds. The differences we see could be the result of some other symmetry-breaking process, a kind of illusion. But how does this happen?

    He says the answer may come from a better understanding of the way information flows through his universe. “Obviously, this leaves us with the problem of defining what exactly information is, and how it links with the equations of motion, ” he says.

    ‘t Hooft is not the first to run up against information. When pushed to its limits, every fundamental theory of physics runs foul of our poor understanding of information.

    It may be no understatement to say that the biggest breakthrough in physics must come in information theory rather than quantum mechanics or relativity.

    Ref: http://arxiv.org/abs/0909.3426: Quantum Gravity without Space-time Singularities or Horizons