A black hole is the final form a massive star collapses to. The light (and spacetime itself) is warped around the black hole's event horizon due to extreme gravitational effects. This is as accurate as we can be to visualizing an actual black hole as it was generated with a code that implemented General Relativity accurately. Credit and Copyright: Paramount Pictures/Warner Bros.
Mathematical Model used to create the image developed by Dr. Kip Thorne
It’s no secret that black holes are objects to be avoided, were you to plot yourself a trip across the galaxy. Get too close to one and you’d find your ship hopelessly caught sliding down a gravitational slippery slope toward an inky black event horizon, beyond which there’s no escape. The closer you got the more gravity would yank at your vessel, increasingly more on the end closest to the black hole than on the farther side until eventually the extreme tidal forces would shear both you and your ship apart. Whatever remained would continue to fall, accelerating and stretching into “spaghettified” strands of ship and crew toward—and across—the event horizon. It’d be the end of the cosmic road, with nothing left of you except perhaps some slowly-dissipating “information” leaking back out into the Universe over the course of millennia in the form of Hawking radiation. Nice knowin’ ya.
That is, of course, if you were foolish enough to approach a non-spinning black hole.* Were it to have a healthy rotation to it there’s a possibility, based on new research, that you and your ship could survive the trip intact.
A team of researchers from Georgia Gwinnett College, UMass Dartmouth, and the University of Maryland have designed new supercomputer models to study the exotic physics of quickly-rotating black holes, a.k.a. Kerr black holes, and what might be found in the mysterious realm beyond the event horizon. What they found was the dynamics of their rapid rotation create a scenario in which a hypothetical spacecraft and crew might avoid gravitational disintegration during approach.
“We developed a first-of-its-kind computer simulation of how physical fields evolve on the approach to the center of a rotating black hole,” said Dr. Lior Burko, associate professor of physics at Georgia Gwinnett College and lead researcher on the study. “It has often been assumed that objects approaching a black hole are crushed by the increasing gravity. However, we found that while gravitational forces increase and become infinite, they do so fast enough that their interaction allows physical objects to stay intact as they move toward the center of the black hole.”
Because the environment around black holes is so intense (and physics inside them doesn’t play by the rules) creating accurate models requires the latest high-tech computing power.
“This has never been done before, although there has been lots of speculation for decades on what actually happens inside a black hole,” said Gaurav Khanna, Associate Physics Professor at UMass Dartmouth, whose Center for Scientific Computing & Visualization Research developed the precision computer modeling necessary for the project.
Artist’s representation of a black hole. Credit: XMM-Newton, ESA, NASA
Like science fiction movies have imagined for decades—from Disney’s The Black Hole to Nolan’s Interstellar—it just might be possible to survive a trip into a black hole, if conditions are right (i.e., you probably still don’t want to find yourself anywhere near one of these.)
Of course, what happens once you’re inside is still anyone’s guess…
The team’s paper “Cauchy-horizon singularity inside perturbed Kerr black holes” was published in the Feb. 9, 2016 edition of Rapid Communication in Physical Review D. You can find the full text here. The research was supported by the National Science Foundation.
*A true non-rotating “Schwarzschild” black hole would not, due to angular momentum etc., be readily found in the real world, thus making this research on rotating black holes all the more essential.
An illustration of Markarian 231, a binary black hole 1.3 billion light years from Earth. Their collision generated the first gravitational waves we've ever detected. Image: NASA
“Ladies and Gentlemen, we have detected gravitational waves. We did it.”
With those words, Dave Reitze, executive director of the U.S.-based Laser Interferometry Gravitational-Wave Observatory (LIGO), has opened a new window into the universe, and ushered in a new era in space science.
Predicted over 100 years ago by Albert Einstein, gravitational waves are ripples in space-time. They travel in waves, like light does, but they aren’t radiation. They are actual perturbations in the fabric of space-time itself. The ones detected by LIGO, after over ten years of “listening”, came from a binary system of black holes over 1.3 billion light years away, called Markarian 231.
The two black holes, each 30 times as massive as the Sun, orbited each other, then spiralled together, ultimately colliding and merging together. The collision sent gravitational waves rippling through space time.
LIGO, which is actually two separate facilities separated by over 3,000 km, is a finely tuned system of lasers and sensors that can detect these tiny ripples in space-time. LIGO is so sensitive that it can detect ripples 10,000 times smaller than a proton, in laser beams 4 kilometres long.
The Laser Interferometer Gravitational-Wave Observatory (LIGO)facility in Livingston, Louisiana. The other facility is located in Hanford, Washington. Image: LIGO
Light is—or has been up until now—the only way to study objects in the universe. This includes everything from the Moon, all the way out to the most distant objects ever observed. Astronomers and astrophysicists use observatories that can see in not only visible light, but in all other parts of the electromagnetic spectrum, to study objects in the universe. And we’ve learned an awful lot. But things will change with this announcement.
“I think we’re opening a window on the universe,” Dave Reitze said.
Another member of the team that made this discovery, astrophysicist Szabolcs Marka from Columbia University, said, “Until this moment we had our eyes on the sky and we couldn’t hear the music.”
Gravitational waves are a new way to study notoriously difficult things to observe like black holes and neutron stars. Black holes emit no light at all, and their characteristics and properties are inferred from cause and effect relationships with objects near them. But the detection of gravitational waves holds the promise of answering questions about black holes, neutron stars, and even the early days of our universe, including the Big Bang.
It’s almost impossible to overstate the magnitude of this discovery. Once we understand how to better detect and observe gravitational waves, we may come to a whole new understanding of the universe, and we may look back on this day as truly ground-breaking and revolutionary.
And it all started 100 years ago with Albert Einstein’s prediction.
For a better understanding of Gravitational Waves, their sources, and their detection, check out Markus Possel’s excellent series of articles:
We’ve featured the videos from Kurzgesagt many times before here on Universe Today and they’re always wonderful. Sometimes a terrific animation with adorable birds is worth a million words. In their latest video, they cover black holes, from birth to death, giving you an overview of these bizarre objects.
But this episode is very special for me because: I wrote it. Well, I contributed an initial script and Matt, a PhD astrophysicist from Quarks and Coffee, added his own flare and tightened up the science.
Activity within the jet from NGC 3852 imaged by Hubble. Credit: NASA, ESA, and E. Meyer (STScI).
Even the Empire’s planet-blasting battle station has nothing compared to the immense energy being fired from the heart of NGC 3862, a supermassive black hole-harboring elliptical galaxy located 300 million light-years away.
And while jets of high-energy plasma coming from active galactic nuclei have been imaged before, for the first time activity within a jet has been observed in optical wavelengths, revealing a quite “forceful” collision of ejected material at near light speeds.
Using archived image data acquired by Hubble in 1994, 1996, and 2002 combined with new high-resolution images acquired in 2014, Eileen Meyer at the Space Telescope Science Institute (STScI) in Baltimore, Maryland identified movement in visible clumps of plasma within the jet emitted from the nucleus of NGC 3862 (aka 3C 264). One of the outwardly-moving larger clumps could be seen gaining on a slower, smaller one in front of it and the two eventually collide, creating a shockwave that brightens the resulting merged mass dramatically.
Such a collision has never been witnessed before, and certainly not thousands of light-years out from the central supermassive black hole.
Close-up image of the jet as seen in 2014. Credit: NASA, ESA, and E. Meyer (STScI).
“Something like this has never been seen before in an extragalactic jet,” Meyer said. “This will allow us a very rare opportunity to see how the kinetic energy of the collision is dissipated into radiation.”
Jets like this are created when infalling material around an active (that is, “feeding”) supermassive black hole gets caught up in its powerful spinning and twisting magnetic fields. This accelerates the material even further and, rather than permitting it to descend down past the black hole’s event horizon, results in it getting shot out into space at velocities close to the speed of light.
When material approaches the black hole in even amounts the jets are fairly consistent. But if the inflow is uneven, the jets can consist of clumps or knots traveling outward at different speeds.
Because of the motion of the galaxy itself related to our own, the speed of the clumps can appear to actually move faster than the speed of light, especially when – as seen in NGC 3862 – a large clump has already paved the way within the jet. In reality the light speed limit has not been broken, but the apparent superluminal motion so far from the SMBH indicates that the material was ejected extremely energetically.
It’s expected that the combined clusters of material will continue to brighten over the next several decades.
You can see a video of the observations below, and watch a Google+ Hangout with Hubble team members about these observations here.
An illustration that shows the powerful winds driven by a supermassive black hole at the centre of a galaxy. The schematic figure in the inset depicts the innermost regions of the galaxy where a black hole accretes, that is, consumes, at a very high rate the surrounding matter (light grey) in the form of a disc (darker grey). At the same time, part of that matter is cast away through powerful winds. (Credits: XMM-Newton and NuSTAR Missions; NASA/JPL-Caltech;Insert:ESA)
The combined observations from two generations of X-Ray space telescopes have now revealed a more complete picture of the nature of high-speed winds expelled from super-massive black holes. Scientist analyzing the observations discovered that the winds linked to these black holes can travel in all directions and not just a narrow beam as previously thought. The black holes reside at the center of active galaxies and quasars and are surrounded by accretion discs of matter. Such broad expansive winds have the potential to effect star formation throughout the host galaxy or quasar. The discovery will lead to revisions in the theories and models that more accurately explain the evolution of quasars and galaxies.
This plot of data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency’s (ESA’s) XMM-Newton determines for the first time the shape of ultra-fast winds from supermassive black holes, or quasars. The winds blow in every direction, in a nearly spherical fashion, coming from both sides of a galaxy (Credit: NASA/JPL-Caltech/Keele Univ.;XMM-Newton and NuSTAR Missions, [Ref])The observations were by the XMM-Newton and NuSTAR x-ray space telescopes of the quasar PDS 456. The observations were combined into the graphic, above. PDS 456 is a bright quasar residing in the constellation Serpens Cauda (near Ophiuchus). The data graph shows both a peak and a trough in the otherwise nominal x-ray emission profile as shown by the NuSTAR data (pink). The peak represents X-Ray emissions directed towards us (i.e.our telescopes) while the trough is X-Ray absorption that indicates that the expulsion of winds from the super-massive black hole is in many directions – effectively a spherical shell. The absorption feature caused by iron in the high speed wind is the new discovery.
X-Rays are the signature of the most energetic events in the Cosmos but also are produced from some of the most docile bodies – comets. The leading edge of a comet such as Rosetta’s P67 generates X-Ray emissions from the interaction of energetic solar ions capturing electrons from neutral particles in the comet’s coma (gas cloud). The observations of a super-massive black hole in a quasar billions of light years away involve the generation of x-rays on a far greater scale, by winds that evidently has influence on a galactic scale.
A diagram of the ESA XMM-Newton X-Ray Telescope. Delivered to orbit by a Ariane 5 launch vehicle in 1999. (Illustration Credit: ESA/XMM-Newton)
The study of star forming regions and the evolution of galaxies has focused on the effects of shock waves from supernova events that occur throughout the lifetime of a galaxy. Such shock waves trigger the collapse of gas clouds and formation of new stars. This new discovery by the combined efforts of two space telescope teams provides astrophysicists new insight into how star and galaxy formation takes place. Super-massive blackholes, at least early in the formation of a galaxy, can influence star formation everywhere.
The NuStar Space Telescope launched into Earth orbit by a Orbital Science Corp. Pegasus rocket, 2012. The Wolter telescope design – optics in the foreground, 10 meter truss and detectors at back – images throughout a spectral range from 5 to 80 KeV. (Credit: NASA/Caltech-JPL)
Both the ESA built XMM-Newton and the NuSTAR X-Ray space telescope, a SMEX class NASA mission, use grazing incidence optics, not glass (refraction) or mirrors (reflection) as in conventional visible light telescopes. The incidence angle of the X-rays must be very shallow and consequently the optics are extended out on a 10 meter (33 foot) truss in the case of NuSTAR and over a rigid frame on the XMM-Newton.
Diagram of one of three x-ray telescopes of the XMM-Newton design. Only a few of the grazing angle concentric mirrors are shown. Inset: a simplified illustration of how a Wolter telescope works. (Credits: Wikimedia, ESA) [click to enlarge]The spectral ranges of the XMM-Newton and NuSTAR Telescopes. (Credits: NASA, ESA)
The ESA built XMM-Newton was launched in 1999, an older generation design that used a rigid frame and structure. All the fairing volume and lift capability of the Ariane 5 launch vehicle was needed to put the Newton in orbit. The latest X-Ray telescope – NuSTAR – benefits from tens years of technological advances. The detectors are more efficient and faster and the rigid frame was replaced with a compact truss which required all of 30 minutes to deploy. Consequently, NuSTAR was launched on a Pegasus rocket piggybacked on a L-1011, a significantly smaller and less expensive launch system.
So now these observations are effectively delivered to the theorists and modelers. The data is like a new ingredient in the batter from which a galaxy and stars are formed. The models of galaxy and star formation will improve and will more accurately describe how quasars, with their active super-massive black-holes, transition into more quiescent galaxies such as our own Milky Way.
This artist’s impression shows the surroundings of the supermassive black hole at the heart of the active galaxy NGC 3783 in the southern constellation of Centaurus (The Centaur). Credit: ESO/M. Kornmesser
We’ve come a long way in 13.8 billion years; but despite our impressively extensive understanding of the Universe, there are still a few strings left untied. For one, there is the oft-cited disconnect between general relativity, the physics of the very large, and quantum mechanics, the physics of the very small. Then there is problematic fate of a particle’s intrinsic information after it falls into a black hole. Now, a new interpretation of fundamental physics attempts to solve both of these conundrums by making a daring claim: at certain scales, space and time simply do not exist.
Let’s start with something that is not in question. Thanks to Einstein’s theory of special relativity, we can all agree that the speed of light is constant for all observers. We can also agree that, if you’re not a photon, approaching light speed comes with some pretty funky rules – namely, anyone watching you will see your length compress and your watch slow down.
But the slowing of time also occurs near gravitationally potent objects, which are described by general relativity. So if you happen to be sight-seeing in the center of the Milky Way and you make the regrettable decision to get too close to our supermassive black hole’s event horizon (more sinisterly known as its point-of-no-return), anyone observing you will also see your watch slow down. In fact, he or she will witness your motion toward the event horizon slow dramatically over an infinite amount of time; that is, from your now-traumatized friend’s perspective, you never actually cross the event horizon. You, however, will feel no difference in the progression of time as you fall past this invisible barrier, soon to be spaghettified by the black hole’s immense gravity.
So, who is “correct”? Relativity dictates that each observer’s point of view is equally valid; but in this situation, you can’t both be right. Do you face your demise in the heart of a black hole, or don’t you? (Note: This isn’t strictly a paradox, but intuitively, it feels a little sticky.)
And there is an additional, bigger problem. A black hole’s event horizon is thought to give rise to Hawking radiation, a kind of escaping energy that will eventually lead to both the evaporation of the black hole and the destruction of all of the matter and energy that was once held inside of it. This concept has black hole physicists scratching their heads. Because according to the laws of physics, all of the intrinsic information about a particle or system (namely, the quantum wavefunction) must be conserved. It cannot just disappear.
Dr. Stephen Hawking of Cambridge University alongside illustrations of a black hole and an event horizon with Hawking Radiation. He continues to engage his grey matter to uncover the secrets of the Universe while others attempt to confirm his existing theories. (Photo: BBC, Illus.: T.Reyes)
Why all of these bizarre paradoxes? Because black holes exist in the nebulous space where a singularity meets general relativity – fertile, yet untapped ground for the elusive theory of everything.
Enter two interesting, yet controversial concepts: doubly special relativity and gravity’s rainbow.
Just as the speed of light is a universally agreed-upon constant in special relativity, so is the Planck energy in doubly special relativity (DSR). In DSR, this value (1.22 x 1019 GeV) is the maximum energy (and thus, the maximum mass) that a particle can have in our Universe.
Two important consequences of DSR’s maximum energy value are minimum units of time and space. That is, regardless of whether you are moving or stationary, in empty space or near a black hole, you will agree that classical space breaks down at distances shorter than the Planck length (1.6 x 10-35 m) and classical time breaks down at moments briefer than the Planck time (5.4 x 10-44 sec).
In other words, spacetime is discrete. It exists in indivisible (albeit vanishingly small) units. Quantum below, classical above. Add general relativity into the picture, and you get the theory of gravity’s rainbow.
Physicists Ahmed Farag Ali, Mir Faizal, and Barun Majumder believe that these theories can be used to explain away the aforementioned black hole conundrums – both your controversial spaghettification and the information paradox. How? According to DSR and gravity’s rainbow, in regions smaller than 1.6 x 10-35 m and at times shorter than 5.4 x 10-44 sec… the Universe as we know it simply does not exist.
“Say what??” -Albert Einstein
“In gravity’s rainbow, space does not exist below a certain minimum length, and time does not exist below a certain minimum time interval,” explained Ali, who, along with Faizal and Majumder, authored a paper on this topic that was published last month. “So, all objects existing in space and occurring at a time do not exist below that length and time interval [which are associated with the Planck scale].”
Luckily for us, every particle we know of, and thus every particle we are made of, is much larger than the Planck length and endures for much longer than the Planck time. So – phew! – you and I and everything we see and know can go on existing. (Just don’t probe too deeply.)
The event horizon of a black hole, however, is a different story. After all, the event horizon isn’t made of particles. It is pure spacetime. And according to Ali and his colleagues, if you could observe it on extremely short time or distance scales, it would cease to have meaning. It wouldn’t be a point-of-no-return at all. In their view, the paradox only arises when you treat spacetime as continuous – without minimum units of length and time.
“As the information paradox depends on the existence of the event horizon, and an event horizon like all objects does not exist below a certain length and time interval, then there is no absolute information paradox in gravity’s rainbow. The absence of an effective horizon means that there is nothing absolutely stopping information from going out of the black hole,” concluded Ali.
No absolute event horizon, no information paradox.
And what of your spaghettification within the black hole? Again, it depends on the scale at which you choose to analyze your situation. In gravity’s rainbow, spacetime is discrete; therefore, the mathematics reveal that both you (the doomed in-faller) and your observer will witness your demise within a finite length of time. But in the current formulation of general relativity, where spacetime is described as continuous, the paradox arises. The in-faller, well, falls in; meanwhile, the observer never sees the in-faller pass the event horizon.
“The most important lesson from this paper is that space and time exist only beyond a certain scale,” said Ali. “There is no space and time below that scale. Hence, it is meaningless to define particles, matter, or any object, including black holes, that exist in space and time below that scale. Thus, as long as we keep ourselves confined to the scales at which both space and time exist, we get sensible physical answers. However, when we try to ask questions at length and time intervals that are below the scales at which space and time exist, we end up getting paradoxes and problems.”
To recap: if spacetime continues on arbitrarily small scales, the paradoxes remain. If, however, gravity’s rainbow is correct and the Planck length and the Planck time are the smallest unit of space and time that fundamentally exist, we’re in the clear… at least, mathematically speaking. Unfortunately, the Planck scales are far too tiny for our measly modern particle colliders to probe. So, at least for now, this work provides yet another purely theoretical result.
The paper was published in the January 23 issue of Europhysics Letters. A pre-print of the paper is available here.
When a star encounters a black hole, tidal forces stretch the star into an elongated blob before tearing it apart, as seen in these images from a computer simulation by James Guillochon of Harvard University.
A telescope peers into the blackness of deep space. Suddenly – a brilliant flash of light appears that wasn’t there before. What could it be? A supernova? Two massively dense stars fusing together? Perhaps a gamma ray burst?
Five years ago, researchers using the ROTSE IIIb telescope at McDonald Observatory noticed just such an event. But far from being your run-of-the-mill stellar explosion or neutron star merger, the astronomers believe that this tiny flare was, in fact, evidence of a supermassive black hole at the center of a distant galaxy, tearing a star to shreds.
Astronomers at McDonald had been using the telescope to scan the skies for such nascent flashes for years, as part of the ROTSE Supernova Verification Project (SNVP). And at first blush, the event seen in early 2009, which the researches nicknamed “Dougie,” looked just like many of the other supernovae they had discovered over the course of the project. With a blazing – 22.5-magnitude absolute brightness, the event fit squarely within the class of superluminous supernovae that the researchers were already familiar with.
But as time went on and more data on Dougie rolled in, the astronomers began to change their minds. X-ray observations made by the orbiting Swift satellite and optical spectra taken by McDonald’s Hobby-Eberly Telescope revealed an evolving light curve and chemical makeup that didn’t fit with computer simulations of superluminous supernovae. Likewise, Dougie didn’t appear to be a neutron star merger, which would have reached peak luminosity far more quickly than was observed, or a gamma ray burst, which, even at an angle, would have appeared far brighter in x-ray light.
That left only one option: a so-called “tidal disruption event,” or the carnage and spaghettification that occurs when a star wanders too close to a black hole’s horizon. J. Craig Wheeler, head of the supernova group at The University of Texas at Austin and a member of the team that discovered Dougie, explained that at short distances, a black hole’s gravity exerts a much stronger pull on the side of the star nearest to it than it does on the star’s opposite side. He explained, “These especially large tides can be strong enough that you pull the star out into a noodle.”
The team refined their models of the event and came to a surprising conclusion: having drawn in Dougie’s stellar material a bit faster than it could handle, the black hole was now “choking” on its latest meal. This is due to an astrophysical principle called the Eddington Limit, which states that a black hole of a given size can only handle so much infalling material. After this limit has been reached, any additional intake of matter exerts more outward pressure than the black hole’s gravity can compensate for. This pressure increase has a kind of rebound effect, throwing off material from the black hole’s accretion disk along with heat and light. Such a burst of energy accounts for at least part of Dougie’s brightness, but also indicates that the original dying star – a star not unlike our own Sun – wasn’t going down without a fight.
Combining these observations with the mathematics of the Eddington Limit, the researchers estimated the black hole’s size to be about 1 million solar masses – a rather small black hole, at the center of a rather small galaxy, three billion light years away. Discoveries like these not only allow astronomers to better understand the physics of black holes, but also properties of their often unassuming home galaxies. After all, mused Wheeler, “Who knew this little guy had a black hole?”
To get a simulated glimpse of Dougie for yourself, check out the amazing animation below, courtesy of team member James Guillochon:
The research is published in this month’s issue of The Astrophysical Journal. A pre-print of the paper is available here.
This artist's rending shows "before" and "after" images of a changing look quasar. Credit: Yale University.
Last week, astronomers at Yale University reported seeing something unusual: a seemingly stedfast beacon from the far reaches of the Universe went quiet. This relic light source, a quasar located in the region of our sky known as the celestial equator, unexpectedly became 6-7 times dimmer over the first decade of the 21st century. Thanks to this dramatic change in luminosity, astronomers now have an unprecedented opportunity to study both the life cycle of quasars and the galaxies that they once called home.
A quasar arises from a distant (and therefore, very old) galaxy that once contained a central, rotating supermassive black hole – what astronomers call an active galactic nucleus. This spinning beast ravenously swallowed up large amounts of ambient gas and dust, kicking up surrounding material and sending it streaming out of the galaxy at blistering speeds. Quasars shine because these ancient jets achieved tremendous energies, thereby giving rise to a torrent of light so powerful that astronomers are still able to detect it here on Earth, billions of years later.
In their hey-day, some active galactic nuclei were also energetic enough to excite electrons farther away from the central black hole. But even in the very early Universe, electrons couldn’t withstand that kind of excitement forever; the laws of physics don’t allow it. Eventually, each electron would drop back down to its rest state, releasing a photon of corresponding energy. This cycle of excitation happened over and over and over again, in regular and predictable patterns. Modern astronomers can visualize those transitions – and the energies that caused them – by examining a quasar’s optical spectrum for characteristic emission lines at certain wavelengths.
A simple example of an atomic spectrum, showing emission lines at particular wavelengths. Broad humps correspond to brighter emission lines, while lines that arise from narrow, lower-intensity emissions appear dimmer. Credit: NASA
Not all quasars are created equal, however. While the spectra of some quasars reveal many bright, broad emission lines at different energies, other quasars’ spectra consist of only the dim, narrow variety. Until now, some astronomers thought that these variations in emission lines among quasars were simply due to differences in their orientation as seen from Earth; that is, the more face-on a quasar was relative to us, the broader the emission lines astronomers would be able to see.
But all of that has now been thrown into question, thanks to our friend J015957.64+003310.5, the quasar revealed by the team of astronomers at Yale. Indeed, it is now plausible that a quasar’s pattern of emission lines simply changes over its lifetime. After gathering ten years of spectral observations from the quasar, the researchers observed its original change in brightness in 2010. In July 2014, they confirmed that it was still just as dim, disproving hypotheses that suggested the effect was simply due to intervening gas or dust. “We’ve looked at hundreds of thousands of quasars at this point, and now we’ve found one that has switched off,” explained C. Megan Urry, the study’s co-author.
How would that happen, you ask? After observing the comparable dearth of broad emission lines in its spectrum, Urry and her colleagues believe that long ago, the black hole at the heart of the quasar simply went on a diet. After all, an active galactic nucleus that consumed less material would generate less energy, giving rise to fainter particle jets and fewer excited atoms. “The power source just went dim,” said Stephanie LaMassa, the study’s principal investigator.
LaMassa continued, “Because the life cycle of a quasar is one of the big unknowns, catching one as it changes, within a human lifetime, is amazing.” And since the life cycle of quasars is dependent on the life cycle of supermassive black holes, this discovery may help astronomers to explain how those that lie at the center of most galaxies evolve over time – including Sagittarius A*, the supermassive black hole at the center of our own Milky Way.
“Even though astronomers have been studying quasars for more than 50 years, it’s exciting that someone like me, who has studied black holes for almost a decade, can find something completely new,” added LaMassa.
The team’s research will be published in an upcoming issue of The Astrophysical Journal. A pre-print of the paper is available here.
An artist's conception of a black hole binary in a heart of a quasar, with the data showing the periodic variability superposed.
Credit: Santiago Lombeyda/Caltech Center for Data-Driven Discovery
Most large galaxies harbor central supermassive black holes with masses equivalent to millions, or even billions, of Suns. Some, like the one in the center of the Milky Way Galaxy, lie quiet. Others, known as quasars, chow down on so much gas they outshine their host galaxies and are even visible across the Universe.
Although their brilliant light varies across all wavelengths, it does so randomly — there’s no regularity in the peaks and dips of brightness. Now Matthew Graham from Caltech and his colleagues have found an exception to the rule.
Quasar PG 1302-102 shows an unusual repeating light signature that looks like a sinusoidal curve. Astronomers think hidden behind the light are two supermassive black holes in the final phases of a merger — something theoretically predicted but never before seen. If the theory holds, astronomers might be able to witness two black holes en route to a collision of incredible scale.
The light curve combines data from two CRTS telescopes (CSS and MLS) with historical data from the LINEAR and ASAS surveys. Image Credit: Graham et al.
Graham and his colleagues discovered the unusual quasar on a whim. They were aiming to study quasar variability using the Catalina Real-Time Transient Survey (CRTS), which uses three ground-based telescopes to monitor some 500 million objects strewn across 80 percent of the sky, when 20 or so periodic sources popped up.
Of those 20 periodic quasars, PG 1302-102 was the most promising. It had a strong signal that appeared to repeat every five years or so. But what causes the repeating signal?
The black holes that power quasars do not emit light. Instead the light originates from the hot accretion disk that feeds the black hole. Orbiting clouds of gas, which are heated and ionized by the disk, also contribute in the form of visible emission lines.
“When you look at the emission lines in a spectrum from an object, what you’re really seeing is information about speed — whether something is moving toward you or away from you and how fast. It’s the Doppler effect,” said study coauthor Eilat Glikman from Middlebury College in Vermont, in a news release. “With quasars, you typically have one emission line, and that line is a symmetric curve. But with this quasar, it was necessary to add a second emission line with a slightly different speed than the first one in order to fit the data. That suggests something else, such as a second black hole, is perturbing this system.”
So a tight supermassive black hole binary is the most likely explanation for this oddly periodic quasar.
“Until now, the only known examples of supermassive black holes on their way to a merger have been separated by tens or hundreds of thousands of light years,” said study coauthor Daniel Stern from NASA’s Jet Propulsion Laboratory. “At such vast distances, it would take many millions, or even billions, of years for a collision and merger to occur. In contrast, the black holes in PG 1302-102 are, at most, a few hundredths of a light year apart and could merge in about a million years or less.”
But astronomers remain unsure about what physical mechanism is responsible for the quasar’s repeating light signal. It’s possible that one quasar is funneling material from its accretion disk into jets, which are rotating like beams from a lighthouse. Or perhaps a portion of the accretion disk itself is thicker than the rest, causing light to be blocked at certain spots in its orbit. Or maybe the accretion disk is dumping material onto the black hole in a regular fashion, causing periodic bursts of energy.
“Even though there are a number of viable physical mechanisms behind the periodicity we’re seeing — either the precessing jet, warped accretion disk or periodic dumping — these are all still fundamentally caused by a close binary system,” said Graham.
Astronomers still don’t have a good handle on what happens in the final few light-years of a black hole merger. And of course these two black holes still won’t collide for thousands to millions of years. Even watching for the period to shorten as they spiral inward would dwarf human timescales. But the discovery of a system so late in the game proves promising for future work.
An artistic image of the explosion of a star leading to a gamma-ray burst. (Source: FUW/Tentaris/Maciej Fro?ow)
Gamma ray bursts (GRBs) are some of the brightest, most dramatic events in the Universe. These cosmic tempests are characterized by a spectacular explosion of photons with energies 1,000,000 times greater than the most energetic light our eyes can detect. Due to their explosive power, long-lasting GRBs are predicted to have catastrophic consequences for life on any nearby planet. But could this type of event occur in our own stellar neighborhood? In a new paper published in Physical Review Letters, two astrophysicists examine the probability of a deadly GRB occurring in galaxies like the Milky Way, potentially shedding light on the risk for organisms on Earth, both now and in our distant past and future.
There are two main kinds of GRBs: short, and long. Short GRBs last less than two seconds and are thought to result from the merger of two compact stars, such as neutron stars or black holes. Conversely, long GRBs last more than two seconds and seem to occur in conjunction with certain kinds of Type I supernovae, specifically those that result when a massive star throws off all of its hydrogen and helium during collapse.
Perhaps unsurprisingly, long GRBs are much more threatening to planetary systems than short GRBs. Since dangerous long GRBs appear to be relatively rare in large, metal-rich galaxies like our own, it has long been thought that planets in the Milky Way would be immune to their fallout. But take into account the inconceivably old age of the Universe, and “relatively rare” no longer seems to cut it.
In fact, according to the authors of the new paper, there is a 90% chance that a GRB powerful enough to destroy Earth’s ozone layer occurred in our stellar neighborhood some time in the last 5 billion years, and a 50% chance that such an event occurred within the last half billion years. These odds indicate a possible trigger for the second worst mass extinction in Earth’s history: the Ordovician Extinction. This great decimation occurred 440-450 million years ago and led to the death of more than 80% of all species.
Today, however, Earth appears to be relatively safe. Galaxies that produce GRBs at a far higher rate than our own, such as the Large Magellanic Cloud, are currently too far from Earth to be any cause for alarm. Additionally, our Solar System’s home address in the sleepy outskirts of the Milky Way places us far away from our own galaxy’s more active, star-forming regions, areas that would be more likely to produce GRBs. Interestingly, the fact that such quiet outer regions exist within spiral galaxies like our own is entirely due to the precise value of the cosmological constant – the factor that describes our Universe’s expansion rate – that we observe. If the Universe had expanded any faster, such galaxies would not exist; any slower, and spirals would be far more compact and thus, far more energetically active.
In a future paper, the authors promise to look into the role long GRBs may play in Fermi’s paradox, the open question of why advanced lifeforms appear to be so rare in our Universe. A preprint of their current work can be accessed on the ArXiv.
