Two Black Holes Merging Generate Gravity Waves
On September 14, 2015, 4:50 a.m. in Louisiana and 2:50 a.m. in Washington state, the two gravity wave detection systems (LIGO) had barely finished being calibrated and were in engineering runs when a loud signal came through at the Livingston, Louisiana site. “Data was streaming, and then - bam,” recalled Dr. David Reitze, the Caltech professor who is the current director of the LIGO Laboratories, the group that built and runs the detectors.
Seven milliseconds later, the signal, now labeled GW150914 (Gravitational Wave year 15, month 9, day 14), hit the Hanford, Washington site. LIGO scientists later determined that the likelihood of such identical signals landing almost simultaneously by pure chance was incredibly small. Nobody was awake, but computers recorded the events.
Comparisons with computer simulations reveals that the waves came from two black holes, 29 and 36 times as massive as the sun, starting from about 200 kilometers of each other before merging. As they approached the end at half the speed of light, they were circling each other at an amazing 250 times a second. This LIGO event observation provides the first evidence for black holes themselves that does not depend on hot gas or stars swirling around the holes.
Modeling shows that the final black hole totaled 62 solar masses, 3 solar masses less than the sum of the individual black holes. The missing 3 solar masses vanished in gravitational waves according to Einstein's formula E=mc^2, which is the gravitational radiation that LIGO observed. The signal exceeds the “five-sigma” standard of statistical significance that physicists require to claim a discovery. LIGO researchers reported this event in a paper published in the Physical Review Letters in February, 2016. For a more complete LIGO description of the event click here. Top
How Does NASA Find Black Holes?
How do we go about "finding" a black hole if they are incredibly compact and emit no visible light? We can only see the "shadow" of a black hole as light can not escape. See the black hole simulation to the left of what the Event Horizon Telescope hopes to see at the heart of the Milky Way. (EHT Project - previous page.) The light (in yellow) from the accretion disk is bent inwards towards the center due to the gravitational pull of the black hole.
There are a couple of tricks that can be used says NASA. Stellar black holes (from burnt out stars) are often part of a binary star system (two stars revolving around each other). What we "see" from earth is a visible star orbiting around what appears to be nothing. In reality, it is orbiting around an invisible black hole. We can infer the mass of the black hole by measuring the way the visible star is orbiting around it. The larger the black hole, the greater the gravitational pull, and the greater the effect on the visible star. Eventually the black hole will consume the star. See an artist's sketch to the left below of a black hole eating a star.
Another way we can find a black hole is by observing x-rays generated around it. As the gas in the accretion disk gets closer to the black hole, it heats up from the friction of ever faster moving gas molecules.
Just outside the black hole's event horizon, the gas heats up to temperatures in the millions of degrees. Gas heated to these temperatures releases tremendous amounts of energy in the form of x-rays that have a unique hump pattern which identifies a black hole.
"Supermassive" black holes also have an accretion disk that emits x-rays that form a characteristic hump curve. This is formed not by a single star as in a binary system, but by the great amount of gas present in the humongous supermassive accretion disk.
In about 10 percent of supermassive black holes, jets of energized matter thousands of light-years in length shoot out in opposite directions. This can be "seen" in radio, visible, x-ray and gamma-ray wavelengths. These jets accelerate matter to nearly the speed of light through a mechanism not well understood.
From small to large-scale black holes, NASA is valiantly trying to answer many questions such as: How is material fed directly into the black hole? How do jets form? Why do some black holes have jets, while many more do not? What keeps the jets powered for millions of years? Why were AGNs (Active Galactic Nucleus') more common in the past than today? How do supermassive black holes participate in the formation and evolution of galaxies? Top
Kip Thorne - Inside A Black Hole
Kip Thorne, theoretical physicist, was Professor of Theoretical Physics at the California Institute of Technology, and is one of the world's leading experts on Einstein's general theory of relativity (also a personal friend of Steven Hawking). He was the scientific consultant and executive producer for the movie Interstellar. See the black hole Interstellar illustration to the left. (The intense black hole gravity bends the accretion disk glow from the back side to the top and bottom of the black hole shadow.) In the following paragraphs, Kip Thorne theorizes what the internals of a black hole most likely are.
"A big misconception is that a black hole is made of matter that has just been compacted to a very small size. That is not true. A black hole is made from warped space and time. It may have been created by an imploding star. But the star's matter has been destroyed at the hole's center, where space-time is infinitely warped. There is nothing left anywhere but warped space-time."
"A black hole really is an object with a very rich structure, just like earth has a rich structure of mountains, valleys, oceans, and so forth. Its warped internal space-time whirls around the central singularity like air in a tornado. It has time slowing as you approach the hole's edge, the so-called horizon. Then, inside the horizon time flows toward and into the singularity, dragging everything that's inside the horizon forward in time to its destruction." Top
Direct-Collapse Black Holes
Astronomers believe they know how supermassive black holes weighing in at millions of suns grow in the heart of most galaxies. They get started from a "seed" black hole, created when an extremely massive star (about 100 suns) collapses. It pulls in gas from its surroundings, becoming much more massive, and eventually may merge with other black holes. This process is called black hole accretion.
However, accretion theory does not explain supermassive black holes in extremely distant, and therefore "very young quasars". A quasar's incredible brightness comes from matter spiralling into a supermassive black hole, heated to millions of degrees, creating jets that shine as beacons across the universe.
These early galaxies may have contained the first generation of stars created after the Big Bang. And, these stars can collapse to form black holes, but they do not work out as early "quasar" seeds. There is no surrounding gas for the black hole to feed on. All the gas has been blown away by winds from the hot, newly formed stars. For decades, astronomers have called this the "quasar seed problem." There is just not enough time for supermassive black holes to grow by accretion.
The direct-collapse theory was proposed in 2003 to solve this conundrum. The direct-collapse process begins with a primordial cloud of hydrogen and helium in a sea of ultraviolet radiation. Normally, the cloud would be able to cool and fragment to form stars. However, the radiation photons keep the gas hot, thus suppressing any star formation. As the gas gets more and more compact, eventually you have the conditions for a collapse into a massive black hole.
Instead of making many normal stars, direct-collapse theory suggests that these early galaxies formed a single supermassive star at their center that ended up collapsing into a huge black hole. The gas in this environment was used to feed the black hole rather than make many normal stars. This process was unique to the early universe, it does not happen in galaxies today.
A galaxy called CR7 (pictured above) was identified in a Hubble Space Telescope survey called COSMOS by scientists from the University of Texas at Austin in 2015. Hubble found CR7 one billion years after the Big Bang. CR7 has a certain hydrogen line in its spectrum, the "Lyman-alpha line," several times brighter than expected. Also, the spectrum showed an unusually bright helium line. (It neeeds to be over 100,000 degrees Celsius to ionize helium.) Another unusual feature in the spectrum was the absence of any lines from elements heavier than helium (metals). The above, together with the source's distance, means that CR7 probably is a supermassive black hole formed by direct-collapse.
In addition, NASA recently announced the discovery of two more direct-collapse black hole candidates based on observations from the Chandra X-ray Observatory. Top
ESA And NASA - Black Holes Wobble
In a study published in July 2016, astronomers used data from the European Space Agency's (ESA's) XMM-Newton X-ray Observatory and NASA's NuSTAR telescope to measure the "wobble" in x-ray emissions from black hole H1743-322. ESA's scientists have proved the existence of a "gravitational vortex" around a black hole. (A vortex is a region within a body of fluid in which the fluid elements behave like a whirlpool.)
In the 1980s, pioneering astronomers using early x-ray telescopes discovered that x-rays coming from stellar-mass (dead star) black holes in our galaxy "flicker". The flickering followed a set pattern. When it begins, the dimming and re-brightening can take 10 seconds to complete. As the days, weeks and then months progress, the period shortens until the oscillation takes place 10 times every second. Then, the flickering suddenly stops altogether.
Here is the general picture: hot puffed-up plasma very near the black hole radiates x-rays. Some of these x-rays hit the surrounding gas accretion disk, knocking electrons off of "iron atoms" in the swirling gas. As those iron atoms snatch back their electrons, they "fluoresce", emitting x-rays at a specific energy.
See the three part illustration to the left. The black hole drags spacetime with it as it spins. The whole system - the black hole, the hot inner plasma (white section), and the surrounding accretion disk (yellow), - is spinning like a top. And if the disk is tilted relative to the black hole, then the top will wobble, or "precess".
The x-ray-emitting plasma near the black hole precesses (creating a vortex). The x-rays strike matter in the surrounding disk, making it glow like a fluorescent bulb. The glow rotates around the accretion disc to the right (top), to the front (middle), and to the left (bottom). The time for the orbit to return to its initial condition is known as the precession cycle. The precession cycle takes just a matter of seconds or less to complete. When one sees a part of the disk spinning around towards the earth, one will see its iron emission blueshifted; emissions from a part of the disk spinning away from the earth will be red shifted.
Adam Ingram (University of Amsterdam, The Netherlands) and colleagues set out to observe this effect directly. They pointed the XMM-Newton and NuSTAR space telescopes at the black hole known as H1743-322 with a mass of about 10 suns drawing in gas from its companion star. The observations clearly show the iron line shifting back and forth in the spectrum over the course of 4 to 5 seconds. This was exactly the way that the frame-dragging effect predicts. However, an intriguing aspect of this black hole precession is that it has not been seen in the "supermassive" variety of black holes." Top
How Do Black Holes Flare?
Dan Wilkins, Saint Mary’s University, Canada, and colleagues explored what makes a flare happen by observing the active black hole at the center of the galaxy Markarian (Mrk) 335, which lies roughly 350 million light-years away in the constellation Pegasus. Mrk 335 is well known for its spurts of brilliance, its x-ray emissions rising and falling at whim. The team used the Swift and NuSTAR space telescopes to watch how Mrk 335’s x-ray emission changed during a month long flare in September 2014.
The corona is a haze of high-energy electrons that hovers over the black hole and its accretion disk. The corona is real, but not very well understood. The team thinks that before a flare, the corona spreads out across the disk’s surface. Then it gathers itself together and contracts into a vertical, jet like structure. The compact corona then launches off the disk at roughly 20% the speed of light.
When an object moves at a fraction of the speed of light, it beams a lot of its radiation in the direction it is moving, which in this case is away from the disk. The effect is called relativistic beaming. So although the corona is still effectively blazing in x-rays, most of those x-rays are not pointed at the disk and so are not reflecting off it. Since the corona is now somewhat away from the black hole, many of the photons can escape the black hole’s gravity and reach us.
The purplish glow in the artist's conception above, represents the corona, which contains highly energetic particles that generate x-ray light. X-ray observations suggest that, when a black hole appears to flare in x-rays, it's actually the corona that's changing. The corona gathers inward (left), becoming brighter (middle), before shooting away from the black hole (right). Astronomers don't know why coronas shift, but they have learned that this process leads to a brightening of x-ray light that can be observed by telescopes. The flare ends when the jet like corona collapses back to the disk.
However, why does the corona launch off the disk in the first place? Astronomers don’t know the answer. Maybe the accretion rate of the disk goes up, bumping up the energy available to make things happen. Maybe magnetic structures in the disk-corona system go crazy. Or maybe instabilities in the disk or corona itself trigger the contraction and launch. Stay tuned. Top
How Big Can A Black Hole Grow?
The limit of a black hole is about 50 billion times the mass of the sun, according to calculations by Andrew King, University of Leicester, UK, and University of Amsterdam, The Netherlands. In the February 2016 issue of the Monthly Notices Letters of the Royal Astronomical Society, King shows that once a black hole reaches the mass of 50 billion suns, the accretion disk of gas that feeds the black hole begins to crumble apart, collapsing under its own weight into stars.
“If the black hole is very massive, then the gas disc would have to be correspondingly large and massive,” explains Zoltan Haiman of Columbia University. “The main idea in King’s paper is that above a certain mass, the gas in such a disk would be gravitationally unstable - i.e., it would collapse into clumps under its own weight before the gas can funnel inward into the black hole.”
But that is not to say the black hole stops growing altogether. It just has to gobble down mass in secret, without emitting any light. A star might happen to fall straight into it, swallowed whole, or it could merge with another black hole.
Astronomers have found black holes with masses of about 20 billion suns, near King’s theoretical limit, but they have found them by looking for the accretion disk’s beacon of light. “The mass limit means that this procedure should not turn up any masses much bigger than those we know, because there would not be a luminous disk,” Andrew King said in a press release. Top
Dark Matter Density Limit
Astronomers studying the behavior of supermassive black holes, the gigantic beasts at the heart of most galaxies, have discovered that dark matter most likely has a density limit.
As a supermassive black hole sucks in material from the surrounding galaxy, part of that material is dark matter, which is invisible except for its gravitational pull. See the blue dark matter in the processed photo of Abell 1689 to the left.
Astronomers know that galaxies need the extra gravity from dark matter to keep their stars from flying off into space. But if too much dark matter got pulled into a supermassive black hole, there would not be enough left over to hold the galaxy together.
Researchers calculate in an issue of the Monthly Notices of the Royal Astronomical Society, that the maximum density for dark matter halos must be seven times the mass of the sun per cubic light-year of space. Any more than this amount, and supermassive black holes would grow so large that they would devour giant swaths of dark matter from their galaxies, possibly obliterating them entirely.