Testing Einstein’s theory of relativity with the clearest gravitational-wave signal yet

Visualization of a binary black hole ringdown consistent with the gravitational-wave event GW250114.The gravitational waves are separated into two modes of the ringing remnant black hole, identified in the observation: the fundamental mode (green) and its first overtone (red). It also shows a predicted third tone (yellow) that the data places limits on. Visualization performed at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), based on a numerical relativity simulation of the Simulating Extreme Spacetimes (SXS) Project. Credit: H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), K. Mitman (Cornell University)

Ringing Black Hole Animation (GW250114)
Credit: H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), K. Mitman (Cornell University)

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To the point:

• Relativity put to the test: Relativity put to the test: A LIGO-Virgo-KAGRA team has conducted some of the most precise tests of Einstein’s theory of general relativity. The results were published in Physical Review Letters today.


• Einstein holds fast: In all tests, the observations match the theory’s predictions. In some cases, the tests based on this signal alone are two to three times more stringent than those obtained by combining data from dozens of other signals.


• The clearest signal: The team used data from GW250114, the strongest gravitational-wave signal ever detected from the merger of two black holes.


• Like a bell: For the first time, detailed analyses of the complete signal and the ringdown phase, which occurs shortly after the merger, have identified or constrained three gravitational-wave tones.

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An international team, with key contributions from AEI researchers, identified three gravitational-wave tones in GW250114 for the first time and conducted the most stringent tests of general relativity.

Relativity put to the test

A year ago, almost to the day, the LIGO-Virgo-KAGRA collaboration observed by far the clearest gravitational-wave signal seen to date. GW250114 came from a coalescence of black holes with masses between 30 to 40 times that of our Sun about 1.3 billion light-years away.

“This signal has already proven to be a great boon for a test of the nature of black holes and of Hawking’s area law,” says Alessandra Buonanno, director of the Astrophysical and Cosmological Relativity department at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in the Potsdam Science Park. “Now we have gone one step further and published some of the most stringent limits on deviations from Einstein’s theory of general relativity using GW250114.”

Additional analysis of the GW250114 data was published today in Physical Review Letters. The writing team included several AEI members: Alessandra Buonanno, who served as chair, and Lorenzo Pompili, Elisa Maggio, and Elise Sänger, who conducted several of the analyses reported in the publication.

Because GW250114 was observed so clearly, it can be compared in much greater detail to predictions from Einstein’s theory of relativity than other signals. This makes it possible to test whether general relativity holds true in the extreme conditions of a black hole coalescence, where strong gravitational fields meet rapidly changing dynamics. Any deviations from the predictions of general relativity could hint at new physics beyond Einstein’s theory.

Like a struck bell

The international research team obtained some of the key results using a method known as black hole spectroscopy. For this, the team focused on the ringdown of the GW250114 signal – the phase when the black hole settles into its final state right after the merger – and the characteristic spectrum of gravitational-wave modes, or tones, emitted during this phase. These tones resemble the sounds a bell makes when struck: Each tone is described by two numbers: its frequency and the rate at which it is fading. Measuring the spectrum of the tones and their fading times is called black hole spectroscopy.

For the first time, a triad of gravitational-wave tones

For the first time, researchers at the AEI in Potsdam found a third tone in the signal’s ringdown phase using a new data analysis tool they developed.

“Our analysis tool, originally proposed in 2018, takes into account the complete black-hole coalescence and makes no prior assumptions about the tones emitted during the ringdown phase,” explains Elisa Maggio, a former Marie Curie Fellow in the Astrophysical and Cosmological Relativity department and now an INFN Researcher in Rome, Italy. Maggio and Pompili collaborated on developing the most recent version of the tool and conducting the analysis. “By incorporating information from the entire signal, we constrained a higher-pitched tone at approximately twice the fundamental frequency for the first time, once again matching theoretical predictions.”

Together, the two tests – one looking at the ringdown alone and the other considering the full signal – complement each other. Once again, they empirically vindicate the rotating black hole solution discovered in 1963 by Roy Kerr.

One signal beats dozens of others

The research team also examined an earlier phase of the clearly observed black hole coalescence when the two black holes were orbiting each other more slowly.

“We used a flexible, theory-independent method developed earlier at the AEI to determine how much the gravitational-wave signal deviated from the predictions of general relativity early in the coalescence,” says Elise Sänger, a PhD student in the Astrophysical and Cosmological Relativity department who conducted the analysis. “Remarkably, using data from this one clearly observed signal alone allows us to set some of the most stringent constraints on possible deviations from general relativity.”

The constraints derived using the AEI-developed model are two to three times more stringent than those obtained by combining data from dozens of signals in the latest fourth Gravitational-Wave Transient Catalogue (GWTC-4.0).

Only the beginning

“These results demonstrate the great scientific value of accurate waveform models and ophisticated data analysis techniques,” says Alessandra Buonanno. “But this is only the beginning. Future observing runs will allow us to detect signals like GW250114 more frequently and more clearly. Each one will open new avenues for testing Einstein’s theory and searching for new physics.”

Source: Max Planck Institute for Gravitational Physics/News

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Media contact:

Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
Tel: +49 331 567-7303
elke.mueller@aei.mpg.de

Science contacts:

Prof. Dr. Alessandra Buonanno
Director | LSC
Principal Investigator
Tel: +49 331 567-7220
Fax: +49 331 567-7298
alessandra.buonanno@aei.mpg.de
Homepage of Alessandra Buonanno

Dr. Elisa Maggio
INFN Researcher
elisa.maggio@aei.mpg.de
Istituto Nazionale di Fisica Nucleare, Rome

Dr. Lorenzo Pompili
Research Fellow
Lorenzo.Pompili@nottingham.ac.uk
University of Nottingham, School of Mathematical Sciences

Elise Sänger
PhD Student
elise.saenger@aei.mpg.de

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Publication

The LIGO Scientific Collaboration; the Virgo Collaboration; the KAGRA Collaboration; Abac, A.; Abouelfettouh, I.; Acernese, F.; Ackley, K.; Adhicary, S.; Adhikari, D.; Adhikari, N. et al.

Black Hole Spectroscopy and Tests of General Relativity with GW250114. Physical Review Letters 136, 041403 (2026)

MPG.PuRe - DOI - pre-print - publisher-version

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Further information

LIGO Science Summary
of the publication “Black hole spectroscopy and tests of general relativity with GW250114”

MPG.PuRe - DOI - pre-print - publisher-version

more

Ten years of gravitational-wave astronomy and the clearest signal yet

September 10, 2025
The LIGO-Virgo-KAGRA collaboration and the Max Planck Institute for Gravitational Physics celebrate the anniversary and present new, exciting results.

more

LIGO-Virgo-KAGRA Collaboration successfully wraps up its fourth Observing Run

November 19, 2025
Further exciting results anticipated from O4’s remaining parts

more

Doubling the gravitational-wave transient catalogue

August 26, 2025

LIGO-Virgo-KAGRA researchers at the Max Planck Institute for Gravitational Physics and at Leibniz University Hannover make significant contributions to detect and analyze new gravitational-wave candidates

more

Brito, R.; Buonanno, A.; Raymond, V.
Black-hole Spectroscopy by Making Full Use of Gravitational-Wave Modeling. Physical Review D 98 (8), 084038 (2018)

MPG.PuRe - DOI - pre-print

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Are Binary Black Hole Spins and Mass Ratios Correlated?

An artist's rendition of two black holes approaching a collision.
Credit: LIGO/Caltech/MIT/R. Hurt (IPAC)

When researchers scour the detections of merging black holes made by gravitational wave observatories, they use models and statistics to make careful inferences about the population of black holes in our universe. In a recent article, researchers explored whether an emerging trend in gravitational wave data is real or an artifact of previous analysis methods.

llustration of the first black hole merger detected by LIGO.
Credit: Aurore Simmonet (Sonoma State University)

A New Window on the Universe

The detection of gravitational waves from merging black holes in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) gave scientists a new way to investigate black holes. By analyzing the spacetime ripples from colliding black holes, researchers hope to understand how the black holes formed (through the collapse of massive stars or the successive mergers of existing black holes?) and how they came to exist in binary systems (by first belonging to a stellar binary system or by forming solo and linking up with another black hole later?).

One potential result that has emerged from several analyses of gravitational wave signals is that the effective spins and the ratios of the masses of merging black hole binaries appear to be anticorrelated. But as with all results that are extracted delicately, statistically from complex data sets, it’s important to ask if this is a real feature of the data, with real implications for how black hole binary systems are assembled, or if it’s a result of our models or statistical analyses.

Top: The black hole spins are aligned with the system’s orbital angular momentum (positive effective spin). Bottom: The black hole spins are misaligned with the system’s orbital angular momentum (negative effective spin). Credit: Kerry Hensley

Statistical Investigation

Christian Adamcewicz (Monash University and OzGrav) and collaborators approached this question by applying a new statistical treatment to detections of black hole mergers. This new treatment features a new model for effective spin and allows for a subpopulation of black hole binaries with zero effective spin, which hasn’t yet been ruled out and might have an impact that hasn’t been accounted for.

The team applied their population model to the third catalog of gravitational wave signals from the LIGO and Virgo detectors and used Bayesian statistical methods to extract the properties of the overarching population of black holes. They found that the previously reported anticorrelation between effective spin and mass ratio is likely real, ruling out the possibility of there being no correlation at 99.7% probability.

More Work, a Paradox, and Astrophysical Possibilities

Adamcewicz and collaborators acknowledge that this work doesn’t provide a final verdict on this question (as they put it, “a modeler’s job is never done”), and that other statistical effects need to be rooted out. One lingering possibility is that this result is due to the amalgamation paradox, which arises when trends present in different factors disappear or flip when the factors are considered together.

If the observed anticorrelation holds up to further statistical scrutiny, a number of astrophysical phenomena could be responsible for this effect. Extensive mass transfer between black hole progenitor stars, stars evolving within a common envelope and accreting matter at a high rate, or even black hole binary systems assembled within the accretion disks of active galactic nuclei should all be investigated with future black hole population models.

By Kerry Hensley

Citation

“Evidence for a Correlation Between Binary Black Hole Mass Ratio and Black Hole Spins,” Christian Adamcewicz et al 2023 ApJ 958 13. doi:10.3847/1538-4357/acf763

Source: American Astronomical Society/AAS-Nova

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Binary Black Hole Simulations Provide Blueprint for Future Observations

Gas glows brightly in this computer simulation of supermassive black holes only 40 orbits from merging. Models like this may eventually help scientists pinpoint real examples of these powerful binary systems. Credits: NASA’s Goddard Space Flight Center.  Download this video in HD formats from NASA Goddard's Scientific Visualization Studio

The first image of a black hole shows the core of galaxy Messier 87 as resolved by radio waves by the Event Horizon Telescope in 2019. Credits: National Science Foundation/Event Horizon Telescope Consortium

Astronomers continue to develop computer simulations to help future observatories better home in on black holes, the most elusive inhabitants of the universe.

Though black holes likely exist abundantly in the universe, they are notoriously hard to see. Scientists did not capture the first radio image of a black hole until 2019, and only about four dozen black hole mergers have been detected through their signature gravitational ripples since the first detection in 2015.

That is not a lot of data to work with. So scientists look to black hole simulations to gain crucial insight that will help find more mergers with future missions. Some of these simulations, created by scientists like astrophysicist Scott Noble, track supermassive black hole binary systems. That is where two monster black holes like those found in the centers of galaxies orbit closely around each other until they eventually merge.

The simulations, created by computers working through sets of equations too complicated to solve by hand, illustrate how matter interacts in merger environments. Scientists can use what they learn about black hole mergers to identify some telltale characteristics that let them distinguish black hole mergers from stellar events. Astronomers can then look for these telltale signs and spot real-life black hole mergers.

Noble, who works at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said these binary systems emit gravitational waves and influence surrounding gases, leading to unique light shows detectable with conventional telescopes. This allows scientists to learn about different aspects of the same system. Multimessenger observations that combine different forms of light or gravitational waves could allow scientists to refine their models of black hole binary systems.

“We’ve been relying on light to see everything out there,” Noble said. “But not everything emits light, so the only way to directly ‘see’ two black holes is through the gravitational waves they generate. Gravitational waves and the light from surrounding gas are independent ways of learning about the system, and the hope is that they will meet up at the same point.”

Binary black hole simulations can also help the Laser Interferometer Space Antenna (LISA) mission. This space-based gravitational wave observatory, led by the European Space Agency with significant contributions from NASA, is expected to launch in 2034. If simulations determine what electromagnetic characteristics distinguish a binary black hole system from other events, scientists could detect these systems before LISA flies, Noble said. These observations could then be confirmed through additional detections once LISA launches.

That would allow scientists to verify that LISA is working, observe systems for a longer period before they merge, predict what is going to happen, and test those predictions.

“We’ve never been able to do that before,” Noble said. “That’s really exciting.”

Noble is working with Goddard and university partners, including Bernard Kelly at the University of Maryland, Manuela Campanelli leading a team of researchers at the Rochester Institute of Technology, and Julian Krolik leading a Johns Hopkins University research team.

Kelly creates simulations using a special approach called a moving puncture simulation.

These simulations allow scientists to avoid representing a singularity inside the event horizon — the part of the black hole from which nothing can escape, Kelly said. Everything outside of that event horizon evolves, while the objects inside remain frozen from earlier in the simulation. This allows scientists to overlook the fact that they do not know what happens within an event horizon.

To mimic real-life situations, where black holes accumulate accretion disks of gas, dust, and diffuse matter, scientists have to incorporate additional code to track how the ionized material interacts with magnetic fields.

“We’re trying to seamlessly and correctly glue together different codes and simulation methods to produce one coherent picture,” Kelly said.

In 2018, the team published an analysis of a new simulation in The Astrophysical Journal that fully incorporated the physical effects of Einstein’s general theory of relativity to show a merger’s effects on the environment around it. The simulation established that the gas in binary black hole systems will glow predominantly in ultraviolet and X-ray light. 

This visualization of supercomputer data shows the X-ray glow of the inner accretion disc of a black hole. Credits: NASA Goddard/Jeremy Schnittman/Scott Noble. View additional images and video at our Scientific Visualization Studio.

Simulations also showed that accretion disks in these systems are not completely smooth. A dense clump forms orbiting the binary, and every time a black hole sweeps close, it pulls off matter from the clump. That collision heats up the matter, producing a bright signal and creating an observable fluctuation of light.

In addition to improving their confidence in the accuracy of the simulations, Goddard astrophysicist Jeremy Schnittman said they also need to be able to apply the same simulation code to a single black hole or a binary and show the similarities and also the differences between the two systems.

“The simulation are going to tell us what the systems should look like,” Schnittman said. “LISA works more like a radio antenna as opposed to an optical telescope. We’re going to hear something in the universe and get its basic direction, but nothing very precise. What we have to do is take other telescopes and look in that part of the sky, and the simulations are going to tell us what to look for to find a merging black hole.”

Kelly said LISA will be more sensitive to lower gravitational wave frequencies than the current ground-based gravitational wave observer, the Laser Interferometer Gravitational-Wave Observatory (LIGO). That means LISA will be able to sense smaller-mass binary systems much earlier and will likely detect merging systems in time to alert electromagnetic telescopes.

For Schnittman, these simulations are key to understanding the real-life data LISA and other spacecraft collect. The case for models may be even stronger for binary black holes, Schnittman said, because the scientific community has little data.

“We probably will never find a binary black hole with a telescope until we simulate them to the point we know exactly what we’re looking for, because they’re so far away, they’re so tiny, you’re going to see just one speck of light,” Schnittman said. “We need to be able to look for that smoking gun.”

Source: NASA/Hi-Tech Computing

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By Emma Edmund

NASA’s Goddard Space Flight Center, Greenbelt, Md.

Editor: Karl Hille

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Researchers find the origin and the maximum mass of massive black holes observed by gravitational wave detectors

Figure 1: Schematic diagram of the binary black hole formation path for GW170729. A star below 80 solar mass evolves and develops into a core-collapse supernova. The star does not experience pair-instability, so there is no significant mass ejection by pulsation. After the star forms a massive iron core, it collapses by its own gravity and forms a black hole with a mass below 38 solar mass. A star between 80 and 140 solar mass evolves and develops into a pulsational pair-instability supernova. After the star forms a massive carbon-oxygen core, the core experiences catastrophic electron-positron pair-creation. This excites strong pulsation and partial ejection of the stellar materials. The ejected materials form the circumstellar matter surrounding the star. After that, the star continues to evolve and forms a massive iron core, which collapses in a fashion similar to the ordinary core-collapse supernova, but with a higher final black hole mass between 38 - 52 solar mass. These two paths could explain the origin of the detected binary black hole masses of the gravitational wave event GW170729. (Credit：Shing-Chi Leung et al./Kavli IPMU)

Simulation: Pulsational pair-instability supernova evolutionary process

Credit: Shing-Chi Leung et al.

Figure 2: The red line shows the time evolution of the temperature and density at the center of the initially 120 solar mass star (PPISN: pulsational pair-instability supernova). The arrows show the direction of time. The star pulsates (i.e., contraction and expansion twice) by making bounces at #1 and #2 and finally collapses along a line similar to that of a 25 solar mass star (thin blue line: CCSN (core-collapse supernova)). The thick blue line shows the contraction and final expansion of the 200 solar mass star which is disrupted completely with no black hole left behind (PISN: pair-instability supernova). Top left area enclosed by the black solid line is the region where a star is dynamically unstable. (Credit：Shing-Chi Leung et al.)

Figure 3: The red line (that connects the red simulation points) shows the mass of the black hole left after the pulsational pair-instability supernova (PPISN) against the initial stellar mass. The red and black dashed lines show the mass of the helium core left in the binary system. The red line is lower than the dashed line because some amount of mass is lost from the core by pulsational mass loss. (Pair-instability supernova, PISN, explodes completely with no remnant left.) The peak of the red line gives the maximum mass, 52 solar mass, of the black hole to be observed by gravitational waves. (Credit：Shing-Chi Leung et al.)

Figure 4: The masses of a pair of the black holes (indicated by the same color) whose merging produced gravitational waves (GW) detected by advanced LIGO and VIRGO (merger event names GW150914 to GW170823 indicate year-month-day). The box enclosed by 38 - 52 solar mass is the remnant mass range produced by PPISNe. Black hole masses falling inside this box must have an origin of PPISN before collapse. Below 38 solar mass is the black hole formed by a massive star undergoing CCSN. In addition to GW170729, GW170823 is a candidate of a PPISN in the lower mass limit side. (Credit：Shing-Chi Leung et al.)

Through simulations of a dying star, a team of theoretical physics researchers have found the evolutionary origin and the maximum mass of black holes which are discovered by the detection of gravitational waves as shown in Figure 1.

The exciting detection of gravitational waves with LIGO (laser interferometer gravitational-wave observatory) and VIRGO (Virgo interferometric gravitational-wave antenna) have shown the presence of merging black holes in close binary systems.

The masses of the observed black holes before merging have been measured and turned out to have a much larger than previously expected mass of about 10 times the mass of the Sun (solar mass). In one of such event, GW170729, the observed mass of a black hole before merging is actually as large as about 50 solar mass. But it is not clear which star can form such a massive black hole, or what the maximum of black holes which will be observed by the gravitational wave detectors is.

To answer this question, a research team at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) consisting of Project Researcher at the time Shing-Chi Leung (currently at the California Institute of Technology), Senior Scientist Ken’ichi Nomoto, and Visiting Senior Scientist Sergei Blinnikov (professor at the Institute for Theoretical and Experimental Physics in Mosow) have investigated the final stage of the evolution of very massive stars, in particular 80 to 130 solar mass stars in close binary systems. Their finding are shown in Illustrations (a - e) and Figures (1 - 4).

In close binary systems, initially 80 to 130 solar mass stars lose their hydrogen-rich envelope and become helium stars of 40 to 65 solar mass. When the initially 80 to 130 solar mass stars form oxygen-rich cores, the stars undergo dynamical pulsation (Illustrations a - b and Figure 2), because the temperature in the stellar interior becomes high enough for photons to be converted into electron-positron pairs. Such “pair-creation” makes the core unstable and accelerates contraction to collapse (Illustration b).

In the over-compressed star, oxygen burns explosively. This triggers a bounce of collapse and then rapid expansion of the star. A part of the stellar outer layer is ejected, while the inner part cools down and collapses again (Illustration c). The pulsation (collapse and expansion) repeats until oxygen is exhausted (Illustration d). This process is called “pulsational pair-instability”(PPI). The star forms an iron core and finally collapses into a black hole, which would trigger the supernova explosion (Illustration e), being called PPI-supernova (PPISN).

By calculating several such pulsations and associated mass ejection until the star collapses to form a black hole, the team found that the maximum mass of the black hole formed from pulsational pair-instability supernova is 52 solar mass (Figure 3).

Stars initially more massive than 130 solar mass (which form helium stars more massive than 65 solar mass) undergo “pair instability supernova” due to explosive oxygen burning, which disrupts the star completely with no black hole remnant. Stars above 300 solar mass collapse and may form a black hole more massive than about 150 solar mass.

The above results predict that there exists a “mass-gap” in the black hole mass between 52 and about 150 solar mass. The results mean that the 50 solar mass black hole in GW170729 is most likely a remnant of a pulsational pair-instability supernova as shown in Figures 3 and 4.

The result also predicts that a massive circumstellar medium is formed by the pulsational mass loss, so that the supernova explosion associated with the black hole formation will induce collision of the ejected material with the circumstellar matter to become a super-luminous supernovae. Future gravitational wave signals will provide a base upon which their theoretical prediction will be tested.

Source: Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU)

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Paper details:

Journal: The Astrophysical Journal

Title: Pulsational Pair-instability Supernovae. I. Pre-collapse Evolution and Pulsational Mass Ejection

Authors: Shing-Chi Leung (1, 2), Ken'ichi Nomoto (1) and Sergei Blinnikov (1, 3, 4)

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Author affiliations:

1. Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.
2. Walter Burke Institute for Theoretical Physics, California Institute of Technology (TAPIR at Caltech), Pasadena, CA 91125, USA.
3. National Research Center, “Kurchatov Institute,” Institute for Theoretical and Experimental Physics (ITEP), B. Cheremushkinkaya 25, 117218 Moscow, Russia.

4. Automatics Research Institute (VNIIA), Suschevskaya 22, 127055 Moscow, Russia.

DOI: https://doi.org/10.3847/1538-4357/ab4fe5 (Published 10 December, 2019)

Images All images can be downloaded from here

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Research contact

Ken'ichi Nomoto
Senior Scientist
Kavli Institute for the Physics and Mathematics for the Universe,
University of Tokyo
E-mail: nomoto@stron.s.u-tokyo.ac.jp
Phone: +81-4-7136-5940

Media Contact

John Amari
Press officer
Kavli Institute for the Physics and Mathematics of the Universe
The University of Tokyo
E-mail: press_at_ipmu.jp
Tel: 080-4056- 2767

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Funky Light Signal From Colliding Black Holes Explained

Intricate Dance of Black Holes

Credit: Columbia University 

This simulation helps explain an odd light signal thought to be coming from a close-knit pair of merging black holes, PG 1302-102, located 3.5 billion light-years away. The close-up view at right shows that the smaller of the two black holes gives off more light (left side of picture). While the black holes themselves don't emit light, they accumulate and heat up surrounding gas, which then radiates light. The reason the smaller black hole gives off more light is that it is orbiting farther from the center of mass and closer to the surrounding gas disk, allowing it to gather up most of the gas as it orbits. The result is that the more massive central black hole is starved of gas and doesn't glow as brightly.

As these black holes orbit around each other, they are thought to send out a varying light signal. The signal was detected by astronomers using telescopes on the ground and in space.

The simulation comes from Brian Farris of Columbia University and New York University, both in New York City. Credit: Columbia University

Entangled by gravity and destined to merge, two candidate black holes in a distant galaxy appear to be locked in an intricate dance. Researchers using data from NASA's Galaxy Evolution Explorer (GALEX) and NASA's Hubble Space Telescope have come up with the most compelling confirmation yet for the existence of these merging black holes and have found new details about their odd, cyclical light signal.

The candidate black hole duo, called PG 1302-102, was first identified earlier this year using ground-based telescopes. The black holes are the tightest orbiting pair detected so far, with a separation not much bigger than the diameter of our solar system. They are expected to collide and merge in less than a million years, triggering a titanic blast with the power of 100 million supernovae.

Researchers are studying this pair to better understand how galaxies and the monstrous black holes at their cores merge -- a common occurrence in the early universe. But as common as these events were, they are hard to spot and confirm.

PG 1302-102 is one of only a handful of good binary black hole candidates. It was discovered and reported earlier this year by researchers at the California Institute of Technology in Pasadena, after they scrutinized an unusual light signal coming from the center of a galaxy. The researchers, who used telescopes in the Catalina Real-Time Transient Survey, demonstrated that the varying signal is likely generated by the motion of two black holes, which swing around each other every five years. 

While the black holes themselves don't give off light, the material surrounding them does.

In the new study, published in the Sept. 17 issue of Nature, researchers found more evidence to support and confirm the close-knit dance of these black holes. Using ultraviolet data from GALEX and Hubble, they were able to track the system's changing light patterns over the past 20 years.

"We were lucky to have GALEX data to look through," said co-author David Schiminovich of Columbia University in New York. "We went back into the GALEX archives and found that the object just happened to have been observed six times."

Hubble, which sees ultraviolet light in addition to visible and other wavelengths of light, had likewise observed the object in the past.

The ultraviolet light was important to test a prediction of how the black holes generate a cyclical light pattern. The idea is that one of the black holes in the pair is giving off more light -- it is gobbling up more matter than the other one, and this process heats up matter that emits energetic light. As this black hole orbits around its partner every five years, its light changes and appears to brighten as it heads toward us.

"It's as if a 60-Watt light bulb suddenly appears to be 100 Watts," explained Daniel D'Orazio, lead author of the study from Columbia University. "As the black hole light speeds away from us, it appears as a dimmer 20-Watt bulb."

What's causing the changes in light? One set of changes has to do with the "blue shifting" effect, in which light is squeezed to shorter wavelengths as it travels toward us in the same way that a police car's siren squeals at higher frequencies as it heads toward you. Another reason has to do with the enormous speed of the black hole.

The brighter black hole is, in fact, traveling at nearly seven percent the speed of light -- in other words, really fast. Though it takes the black hole five years to orbit its companion, it is traveling vast distances. It would be as if a black hole lapped our entire solar system from the outer fringes, where the Oort cloud of comets lies, in just five years. At speeds as high as this, which are known as relativistic, the light becomes boosted and brighter.

D'Orazio and colleagues modeled this effect based on a previous Caltech paper and predicted how it should look in ultraviolet light. They determined that, if the periodic brightening and dimming previously seen in the visible light is indeed due to the relativistic boosting effect, then the same periodic behavior should be present in ultraviolet wavelengths, but amplified 2.5 times. Sure enough, the ultraviolet light from GALEX and Hubble matched their predictions.

"We are strengthening our ideas of what's going on in this system and starting to understand it better," said Zoltan Haiman, a co-author from Columbia University who conceived the project.

The results will also help researchers understand how to find even closer-knit merging black holes in the future, what some consider the holy grail of physics and the search for gravitational waves. In the final moments before the ultimate union of two black holes, when they are tightly spinning around each other like ice skaters in a "death spiral," they are predicted to send out ripples in space and time. 

These so-called gravitational waves, whose existence follows from Albert Einstein's gravity theory published 100 years ago, hold clues about the fabric of our universe.

The findings are also a doorway to understanding other merging black holes across the universe, a widespread population that is only now beginning to yield its secrets.

The California Institute of Technology in Pasadena led the Galaxy Evolution Explorer mission, which ended in 2013 after more than a decade of scanning the skies in ultraviolet light. NASA's Jet Propulsion Laboratory, also in Pasadena, managed the mission and built the science instrument. JPL is managed by Caltech for NASA.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington.

Source:  Galaxy Evolution Explorer (GALEX)

Hubble Finds That the Nearest Quasar Is Powered by a Double Black Hole Artist's View of a Binary Black Hole

Artist's View of a Binary Black Hole

Credit: NASA, ESA, and G. Bacon (STScI)

 Release Images

Optical-to-Ultraviolet Spectrum of Markarian 231

This simplified spectral plot shows the radiation emitted from the center of a nearby galaxy that hosts a quasar. Visible and infrared light coming from a disk surrounding a central black hole in the middle of the galaxy is measured. Surprisingly, ultraviolet light from the disk, as measured by the Hubble Space Telescope, shows a drop in radiation from the disk. This is evidence for a large gap in the center of the disk that is likely carved out by a second black hole orbiting the primary black hole. Credit: NASA, ESA, and P. Jeffries (STScI)

Astronomers using NASA's Hubble Space Telescope have found that Markarian 231 (Mrk 231), the nearest galaxy to Earth that hosts a quasar, is powered by two central black holes furiously whirling about each other.

The finding suggests that quasars — the brilliant cores of active galaxies — may commonly host two central supermassive black holes that fall into orbit about one another as a result of the merger between two galaxies. Like a pair of whirling skaters, the black-hole duo generates tremendous amounts of energy that makes the core of the host galaxy outshine the glow of the galaxy's population of billions of stars, which scientists then identify as quasars.

Scientists looked at Hubble archival observations of ultraviolet radiation emitted from the center of Mrk 231 to discover what they describe as "extreme and surprising properties."

If only one black hole were present in the center of the quasar, the whole accretion disk made of surrounding hot gas would glow in ultraviolet rays. Instead, the ultraviolet glow of the dusty disk abruptly drops off towards the center. This provides observational evidence that the disk has a big donut hole encircling the central black hole. The best explanation for the observational data, based on dynamical models, is that the center of the disk is carved out by the action of two black holes orbiting each other. The second, smaller black hole orbits in the inner edge of the accretion disk, and has its own mini-disk with an ultraviolet glow.

"We are extremely excited about this finding because it not only shows the existence of a close binary black hole in Mrk 231, but also paves a new way to systematically search binary black holes via the nature of their ultraviolet light emission," said Youjun Lu of the National Astronomical Observatories of China, Chinese Academy of Sciences.

"The structure of our universe, such as those giant galaxies and clusters of galaxies, grows by merging smaller systems into larger ones, and binary black holes are natural consequences of these mergers of galaxies," added co-investigator Xinyu Dai of the University of Oklahoma.

The central black hole is estimated to be 150 million times the mass of our sun, and the companion weighs in at 4 million solar masses. The dynamic duo completes an orbit around each other every 1.2 years.

The lower-mass black hole is the remnant of a smaller galaxy that merged with Mrk 231. Evidence of a recent merger comes from the host galaxy's asymmetry, and the long tidal tails of young blue stars.

The result of the merger has been to make Mrk 231 an energetic starburst galaxy with a star-formation rate 100 times greater than that of our Milky Way galaxy. The infalling gas fuels the black hole "engine," triggering outflows and gas turbulence that incites a firestorm of star birth.

The binary black holes are predicted to spiral together and collide within a few hundred thousand years.

Mrk 231 is located 581 million light-years away.

The results were published in the August 14, 2015, edition of The Astrophysical Journal.

Contact 

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

Jana Smith
University of Oklahoma, Norman, Ok.
405-325-1701
jana.smith@ou.edu

Xinyu Dai
University of Oklahoma, Norman, Ok.
405-325-3961
xdai@ou.edu

Source: HubbleSite