Current Research into Black Holes

Walker O'Brien

Black holes as a phenomena have been theorized for many decades, however our knowledge of them has only started to become concrete.  The research into black holes truly began around the 1960s when consistent evidence to show that they were dark objects with enormous gravitational wells and horizons where objects seem to vanish  (Rees and Volonteri, 1).  Since they were first confirmed, our understanding of them has been growing at breakneck speeds.  These objects, as we have observed, can vary greatly in size and mass and can even have characteristics that are not present in other similarly-sized black holes.  Recent research has been focused on the phenomena such as electromagnetic radiation and the Kerr metric as they pertain to active galactic nuclei and isolated black holes.  The Kerr metric deals with the objects that are launched away from black holes with considerably more energy than they started with and if harnessed could pose a viable energy source for future generations (Rees, 4-4.2).  With so many black holes in the universe, the easiest to study are the isolated holes because they very closely follow mathematical models as a result of having little outside influence.  These isolated black holes follow models so closely it makes them ideal for the study of phenomena like pulsars and quasars which are some of the most energetic objects in the universe and because of this they are becoming more heavily researched  (Coleman 1).  With the current research being done, some scientists even think that we could harness the energy expelled by black holes as a nearly limitless energy source (Rees, 4).

In the mid-1960s, physicists began to widely acknowledge the existence and importance of active galactic nuclei, AGN and from then up to the mid-1980s the consensus on the power source of the AGN was thought to be derived from the mass of the black hole and thus gravitational in nature.  The models of dense stellar systems and massive stars also have in their models that gravitational energy is a primary source of energy in the system.  What has been found is that the AGN are supermassive black holes that have bound immense amounts of material around them.  Supermassive black holes, SMBHs, are black holes that can have masses that are up to millions of times that of our sun and are regularly located at the centers of galaxies.  These black holes are thought to be caused by a runaway process where a stellar mass black hole engulfs more and more matter, increasing its gravity well, attracting more and more matter for it to engulf through the process of accretion, or the slow accumulation of mass in a central area.  This ability to so increase their gravitational energy level makes black holes more efficient at releasing energy from matter than the star, stellar system or gas cloud that collapsed to create them.  This being said, the most likely source for the black holes in our universe are massive star collapses and not stellar clusters.  This is because the single star collapse allows the created black hole to gradually take in the surrounding materials instead of in large chunks as would happen in a stellar system which would destabilize the hole and cause it to send off large amounts of the matter surrounding it.  This idea shifted the theoretical research from trying to fit the black holes to a stellar cluster model and instead to a single, massive star model allowing for more unified research and faster development of theories.  The research that has been done from the mid-1960s to the mid-1980s has also lead to deeper investigations into quasars, radio galaxies, pulsars and other phenomena related to the AGN which could lead to other areas including energy harvesting from black holes (Rees, 1).  If the human race were able to harness the energy coming off of a black hole, energy would no longer be a limiting factor in human development enabling more rapid progression in science, math and the arts.

Another important facet of the ongoing black hole research is in the varying masses that they can have and also how they are formed.  Black holes have a large range to which their mass can vary, from about the same mass as our sun, to as much as some dwarf galaxies; however, with this immense difference in the masses that black holes can have, they all can be described using three things, the mass, charge and spin.  These three properties describe every aspect of black holes even the relativistic outflows that many black holes have. The principles currently outlined are able to define the AGN at the centers of galaxies and also the small black holes elsewhere in space even though they may be created through completely different processes  (Rees and Volonteri, 1).  The bulk of the knowledge that we have about black holes come from the application of Einstein’s theory of general relativity to the mass of the black hole, describing the gravitational energy of the black holes (Rees, 2.1-2.3).

Einstein’s theory of general relativity helps to simplify the physics of dense star clusters and supermassive objects such as black holes.  The general relativity models for black holes show that no matter how disorganized the collapse of the system was, the black holes that are produced are still characterized almost completely by their mass and spin.  Using this knowledge, the Kerr Metric helps to explain the behavior of a few specific black holes who are able to split particles and eject massive amounts of energy into the surrounding space.  Kerr holes must have the same spin as the particles within the ergosphere, the area between the static limit, the maximum range that the Kerr metric can be applied which is unique to each hole, and the event horizon.  The particles that are located in the ergosphere can be split into two or more parts where one part is consumed by the hole and the other part is ejected from the hole with more energy than it started with.  In order for the Kerr metric to be applied, everything in the ergosphere must rotate with the black hole including light itself.  The particles that are ejected from this situation have extra energy given to them from the hole and the extra energy that the ejected part obtains is acquired from the gravitational energy of the hole.  The Kerr metric describes holes who are significantly denser than objects like the earth or our sun and the study of these holes has yielded the understanding that objects around black holes must have a minimum angular momentum or they will plunge directly into the hole.  The spin of the black holes determines both how the orbiting objects will be oriented and where the outflow will be directed.  The orbits of objects around a black hole will be primarily in the same plane as the rotation of the hole and the outflow will be directed parallel to the spin axis.  The Kerr metric also helps to show how the outflows from a black hole could be harnessed and used as an energy source (Rees, 4-4.2).

Using the Kerr metric, the outflow of a black hole can be more closely studied and through this scientists have found that the primary component of the outflow from a black hole is in the form of electromagnetic radiation and, in theory, can be tapped as an energy source through the application of magnetic fields.  The electromagnetic outflow from a black hole can be described easily with a select set of mathematical formulas and in pulsars, located at the centers of small galaxies or elsewhere, the electric properties can enable us to harvest an almost infinite amount of energy from a black hole much like the active galactic nuclei at the center of the milky way  (Rees 4-4.2).   Astronomers are not yet able to confirm whether some or all of the black holes observed in the universe can be described definitively using the Kerr metric; however, the observations that we have made on the flow patterns close to the holes allow us to gain a rudimentary idea as to the metric that describes these holes.  To confirm the Kerr metric’s viability X-ray interferometry as a scientific method of making measurements must be developed further.  The use of X-ray interferometry, the process of studying the wave property of X-rays emitted by an object to show with extreme precision the shape and size of astronomical objects, would allow astronomers to image the inner disks that surround the black holes showing with greater detail what the black holes are feeding on and what they expel.  With a better understanding of the structure of the surrounding matter in a black hole, the accretion process can also be more completely understood  (Rees and Volonteri, 2).  The black holes that we could tap for energy are mostly supermassive black holes that exist at the centers of galaxies and other intermediate mass black holes, however not much is known about how these black holes are formed (Rees 4-4.2).  The current understanding to the accretion of a black hole is that the faster the spin of a black hole, the slower the accretion.  For the supermassive black holes at the centers of galaxies, it has been theorized that they were produced through the collisions of galaxies where the central black holes would coalesce into a larger black hole and pull the other black holes strewn about the galaxy towards the center, growing larger and larger with every consumed hole (Rees and Volonteri, 2).  This model for the formation of SMBHs helps to alleviate some of the doubt as to the true source of these cosmic phenomena but doesn’t answer all questions that we currently have, especially about the relativistic outflows.

In the past, scientists thought that black holes came in two mass ranges, stellar mass black holes and super massive black holes however an intermediate class of black holes was theorized in the early 2000s and, if true, could show the origin of SMBHs.  An intermediate class of black hole could also change our understanding on the dynamics of star clusters and small stellar systems along with explaining the origin of SMBHs and AGN. If an intermediate size of black hole was discovered, the idea that SMBHs are the result of the large scale growth of a smaller black hole would have another piece of evidence supporting it.  To find this intermediate class of black holes astronomers are focusing their search on the isolated black holes that liter space because these isolated black holes are less likely to be super massive and are also extremely easy to study.  For Intermediate mass black holes, the process of formation is still shaky because these black holes are above the average mass range of most extremely large stars so an idea for their formation is that these holes could be forming in binary systems or in stellar clusters instead of single stars.  The possibility of the intermediate mass black holes coming from large stars was ruled out because these large stars lose significant amounts of mass to solar winds and have life spans of only a few million years and are created near the centers of galaxies, which already have SMBHs in them.  This goes against what was the general consensus for scientists just 20 years prior, showing that the science still has a way to go before a complete knowledge of the subject is achieved.  The idea that black holes formed in the early universe just after the big bang has also been explored, however the mass of the black holes created in this time could not have exceeded the mass of our sun because of the process of their formation using only dispersed quarks instead of the more tightly packed matter that is present now.  The most likely source for intermediate mass black holes is that they started as stellar mass black holes that grew gradually through accretion (Coleman, 1-5).

The study of black holes that has been done in the last fifty years has been enabled largely by the advances in technology that have occurred since scientists first began theorizing about their existence.  The availability of facilities suitable for long term research on supermassive black holes has been a limiting factor in the progression of the field of study.  With the new facilities, studies have been done on the radiation emitted by AGN and other aspects of them that has helped to firm our understanding of all black holes and their behavior.  One of the main ways that scientists are attempting to study AGN was through emission-line variability which shows the changes that can occur in black holes over both days, weeks and even years.  This method of observing the AGN is used because black hole brightness can change wildly over the course of only a few days, enabling researchers to extrapolate the width and mass of most black holes.  The ability to detect changes in the brightness of black holes has led to a better understanding of many aspects of black holes and when applied to quasars shows us that these AGN expel up to the same energy as a trillion stars over the same period of time.  Through this method, scientists now have a more concrete understanding about the AGN that reside at the centers of both active and inactive galaxies.  Another way that scientists are now studying black holes is through X-ray interferometry which, although currently unproven as accurate, shows great promise for future studies of black hole size and behavior.  These new methods are a far cry from the naked eye observations that were being used before any of these methods became possible  (Peterson, 1-3).

The current research that has been completed on AGN has allowed for a unified model that is used to describe the vast majority of SMBHs.  The research completed is also in agreement over the breadth of length and timescales for SMBHs, establishing a model for how they were formed.  This agreement comes in light of the variety of different galaxy shapes and AGN activity and brightness.  With the variety of galaxy luminosities and AGN configurations, the models that are being created are still holding true.  This fact is interesting because of the fact that there are such differences in both the AGN and the galaxies that surround them.  This was observed through the X-ray emissions from the galaxies who’s AGN were being studied and the relationship that was observed has opened up more doors for more research into the field of SMBHs and AGN (Corton et al., 1-5).  What scientist are also working towards is the ability to harness black holes as an energy source instead of fossil fuels or the alternative energy sources currently being contemplated.  The research into black holes has come a long way since the first days in the field, but there is still a ways to go before these astronomical phenomena are truly understood.

Works Cited

Miller, M Coleman, and E J M Colbert. “INTERMEDIATE-MASS BLACK HOLES.”World Scientific 13.1 (2004): 1-64. World Scientific. World Scientific Publishing Company, 3 Sept. 2003. Web. 20 Jan. 2014. <>.

Rees, M. J. “Black Hole Models for Active Galactic Nuclei.” Annual Review of Astronomy and Astrophysics 22.1 (1984): 471-506. Print.

Peterson, Bradley M. “Variability of Active Galactic Nuclei.” The Starburst-AGN Connection (2001): n. pag. Print.

Rees, Martin J., and Marta Volonteri. “Massive Black Holes: Formation and Evolution.”International Astronomical Union 238 (2006): 1-8. Print.

Croton, Darren J., Volker Springel, Simon D. M. White, G. De Lucia, C. S. Frenk, L. Gao, A. Jenkins, G. Kauffmann, J. F. Navarro, and N. Yoshida. “The Many Lives of AGN: Cooling Flows, Black Holes, and the Luminosities and Colours of Galaxies.” Monthly Notices of the Royal Astronomical Society Febuary (2005): 1-21. Print.