Here’s a Glimpse of the Jaw-Dropping Physics Surrounding Our Supermassive Black Hole

Simulation of Material Orbiting close to a Black Hole
Simulation of material orbiting close to a black hole (ESO/Gravity Consortium/L. Calçada)

Full disclosure: I wrote the press release for the University of Waterloo, whose researcher, Avery Broderick, developed the theory behind the accretion disk hotspots that have now been observed immediately surrounding our galaxy’s supermassive black hole. Read the full release on the UW website. Below is a long-form version of my article, including quotes from my interview with Broderick.

New observations of the center of our galaxy have, for the first time, revealed hotspots in the disk of chaotic gas orbiting our Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*).

Using the tremendous resolving power of the ESO’s Very Large Telescope array in Chile, astronomers used the new GRAVITY instrument to detect the “wobble” of bright patches embedded inside the accretion disk that spins with the black hole. These bright features are clocking speeds of 30 percent the speed of light.

This is the first time any feature so close to a black hole’s event horizon has been seen and, using thirteen-year-old predictions by astrophysicists, we have a good idea about what’s causing the fireworks.

“It’s mind-boggling to actually witness material orbiting a massive black hole at 30 percent of the speed of light,” said scientist Oliver Pfuhl, of the Max Planck Institute for Extraterrestrial Physics and co-investigator of the study published in the journal Astronomy & Astrophysics. “GRAVITY’s tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented detail.”

It is thought that the accretion disk surrounding a black hole is threaded with a powerful magnetic field that frequently becomes unstable and “reconnects.” Similar to the physics that drives the explosive flares in the Sun’s lower corona, these reconnection events rapidly accelerate the plasma in the disk, discharging vast quantities of radiation. These flaring events inside Sgr A*’s accretion disk create hotspots that get pulled in the direction of the material’s spin as it slowly gets digested by the black hole. The GRAVITY instrument was able to deduce that the accretion disk material is orbiting the black hole in a clockwise direction from our perspective and the accretion disk is almost face-on.

Artist’s impression of a hot accretion disk surrounding a black hole [NASA]
The original theory behind these hotspots was derived by Avery Broderick (University of Waterloo) and Avi Loeb (Harvard University) when they were both working at Harvard-Smithsonian Center for Astrophysics in the mid-2000s. In 2005 and 2006, the pair published papers that described theoretical computer models that simulated reconnection events in a black hole’s accretion disk, which caused intense heating and bright flares. The resulting hotspot would then continue to orbit with the speeding accretion disk material, cooling down and spreading out, before another instability and reconnection event would be triggered.

Their work was inspired by the detection of enigmatic bright flares erupting in the vicinity of Sgr A*. These flares were powerful and regular, occurring almost daily. At the time, a few theories were being explored—from supernovas detonating near the supermassive black hole, to asteroids straying too close to the black hole’s gravitational maw—but Broderick and Loeb decided to focus on the extreme region immediately surrounding the black hole’s event horizon.

“Avi and I thought: ‘well, if the flare timescales are close to orbital timescales around the black hole, wouldn’t it be interesting if they are actually bright features embedded in the accretion flow orbiting close to it?’,” Broderick told me.

Black holes are gravitational masters of their domain; anything that drifts too close will be blended into a superheated disk of plasma surrounding them. The matter trapped in the accretion disk then flows toward the event horizon—the point at which nothing, not even light, can escape—and consumed by the black hole via mechanisms that aren’t yet fully understood. The researchers knew that if their model was an accurate depiction of what is going on in the core of our galaxy, these hotspots could be used as visual probes to trace out structures in the accretion disk and in space-time itself.

This plot shows a comparison of the data with the hotspot model including various effects of General and Special Relativity. The continuous blue curve denotes a hot spot on a circular orbit with 1.17 times the innermost stable circular orbit, i.e. just outside the event horizon, of a 4 million solar mass black hole. The axis give the offset from the center in micro-arcseconds [MPE/GRAVITY collaboration]
It’s Sgr A*’s gravity of 4 million Suns that gives the flares a super-boost, however. “In our orbiting hotspot model, a key component of the brightening is actually caused by gravitational lensing,” added Broderick, referring to a consequence of Einstein’s general relativity, when the gravity of black holes warp space-time so much as to form lenses that can magnify the light from distant astronomical sources. “It’s like a black hole analog of a lighthouse.”

Now that GRAVITY has confirmed the existence of these hotspots, Broderick is overjoyed.

“I’m still absorbing it; it’s extremely exciting,” he said. “I’m bouncing around a little bit! The fact you can track these flares is completely new, but we predicted that you could do this.”

The GRAVITY study is led by Roberto Abuter of the European Southern Observatory (ESO), in Garching, Germany, and it describes the detection of three flares emanating from Sgr A* earlier this year. Although the hotspots cannot be fully resolved by the VLT, with the help of Broderick and Loeb’s predictions, Abuter’s team recognized the “wobble” of emissions from the flares as their associated hotspots orbited the supermassive black hole.

This discovery opens a brand-new understanding of the environment immediately surrounding Sgr A* and will complement observations made by the Event Horizon Telescope (EHT), an international collaboration of radio telescopes that are currently taking data to acquire the first image of a black hole, which is expected early next year.

Broderick hopes that these advances will help us to understand how black holes grow and consume matter, and if the predictions of general relativity break down at one of the most gravitationally extreme environments in the universe. But he’s most excited about how the first EHT image of a black hole will impact society as a whole: “It’s going to be a wonderful event, I think it will be an iconic image and it will make black holes real to a lot of people, including a lot of scientists,” he said.

Aside: In 2016, I had the incredible good fortune to visit the VLT at the ESO’s Paranal Observatory as part of the #MeetESO event. I interviewed several VLT and ALMA scientists, including Oliver Pfuhl, and helped produce the mini-documentary below:

Gravity and the Dark Side of the Cosmos: LIVE Perimeter Institute Lecture

Streaming LIVE here, today, at 4 p.m. PDT/7 p.m. EDT/11 p.m. GMT

The Perimeter Institute’s public lecture series is back! At 7 p.m. EDT (4 p.m. PDT) today, Erik Verlinde of the University of Amsterdam will ask: Are we standing on the brink of a new scientific revolution that will radically change our views on space, time, and gravity? Specifically, Verlinde will discuss the possibility that gravity may be an emergent phenomena and not a fundamental force of nature. Ohh, interesting.

The Perimeter Institute for Theoretical Physics (in Ontario, Canada) always puts on a superb production and you can watch Dr Verlinde’s talk via the live feed above. You can also participate via social media using the hashtag #piLIVE and follow @perimeter and @erikverlinde on Twitter.

Watch the preview:

Heavy Stellar Traffic Sends Dangerous Comets Our Way

New image of comet ISON
Comet C/2012 S1 (ISON) as imaged by TRAPPIST–South national telescope at ESO’s La Silla Observatory in 2013 (TRAPPIST/E. Jehin/ESO)

Sixty-six million years ago Earth underwent a cataclysmic change. Back then, our planet was dominated by dinosaurs, but a mass extinction event hastened the demise of these huge reptiles and paved the way for the mammalian takeover. Though there is some debate as to whether the extinction of the dinosaurs was triggered by an isolated disaster or a series of disasters, one event is clear — Earth was hit by a massive comet or asteroid and its impact had global ramifications.

The leading theory is that a massive comet slammed into our planet, creating the vast Chicxulub Crater buried under the Yucatán Peninsula in Mexico, enshrouding the atmosphere in fine debris, blotting out the sun for years.

Although there is strong evidence of comet impacts on Earth, these deep space vagabonds are notoriously hard to track, let alone predict when or how often they may appear. All we know is that they are out there, there are more than we thought, they are known to hit planets in the solar system and they can wreak damage of apocalyptic proportions.

Now, using fresh observations from the European Space Agency’s Gaia mission, astronomer Coryn Bailer-Jones, who works at the Max Planck Institute for Astronomy in Munich, Germany, has added an interesting component to our understanding of cometary behavior.

Stellar Traffic

Long-period comets are the most mysterious — and troubling — class of comet. They will often appear from nowhere, after falling from their distant gravitational perches, zoom through the inner solar system and disappear once more — often to be never seen again. Or they hit something on their way through. These icy bodies are the pristine left-overs of our solar system’s formation five billion years ago, hurled far beyond the orbits of the planets and into a region called the Oort Cloud.

In the Oort Cloud these ancient masses have remained in relative calm far from the gravitational instabilities close to the sun. But over the eons, countless close approaches by other stars in our galactic neighborhood have occurred, causing very slight gravitational nudges to the Oort Cloud. Astronomers believe that such stellar encounters are responsible for knocking comets from this region, sending them on a roller-coaster ride to the inner solar system.

The Gaia mission is a space telescope tasked with precisely mapping the distribution and motion of stars in our galaxy, so Bailer-Jones has investigated the rate of stellar encounters with our solar system. Using information in Gaia’s first data release (DR1), Bailer-Jones has published the first systematic estimate of stellar encounters — in other words, he’s estimated the flow of stellar traffic in the solar system’s neighborhood. And the traffic was found to be surprisingly heavy.

In his study, to be published in the journal Astronomy & Astrophysics, Bailer-Jones estimates that, on average, between 490 and 600 stars will come within 16.3 light-years (5 parsecs) of our sun and 19-24 of them will come within 3.26 light-years (1 parsec) every million years.

According to a press release, all of these stars will have some gravitational effect on the solar system’s Oort Cloud, though the closest encounters will have a greater influence.

This first Gaia data release is valid for five million years into the past and into the future, but astronomers hope the next data release (DR2) will be able to estimate stellar traffic up to 25 million years into the past and future. To begin studying the stellar traffic that may have been responsible for destabilizing the dinosaur-killing comet that hit Earth 66 million years ago will require a better understanding of the mass distribution of our galaxy (and how it influences the motion of stars) — a long-term goal of the Gaia project.

An Early Warning?

Spinning this idea into the future, could this project be used to act as an early warning system? Or could it be used to predict when and where a long-period comet may appear in the sky?

In short: “No,” Bailer-Jones told Astroengine via email. “Some close stellar encounters will for sure shake up the Oort cloud and fling comets into the inner solar system, but which comets on which orbits get flung in we cannot observe.”

He argues that the probability of comets being gravitationally nudged can be modeled statistically, but this would require a lot of assumptions to be made about the Oort Cloud, a region of space that we know very little about.

Also, the Oort Cloud is located well beyond the sun’s heliosphere and is thought to be between 50,000 and 200,000 AU (astronomical units, where 1 AU is the average distance between the sun and the Earth) away, so it would take a long time for comets to travel from this region, creating a long lag-time between stellar close approach and the comet making an appearance.

“Typically it takes a few million years for a comet to reach the inner solar system,” he added, also pointing out that other factors can complicate calculations, such as Jupiter’s enormous gravity that can deflect the passage of comets, or even fling them back out of the solar system again.

This is a fascinating study that goes to show that gravitational perturbations in the Oort Cloud are far from being rare events. A surprisingly strong flow of stellar traffic will constantly rattle otherwise inert comets, but how many are dislodged and sent on the long journey to the solar system’s core remains a matter for statistics and probability.

Geodesy and GOCE: Astrocast.TV with Bente Lilja Bye

In the first episode of A Green Space — A Green Earth at Astrocast.TV, my friend and astrophysicist Bente Lilja Bye gives a superb overview about the Gravity field and steady-state Ocean Circulation Explorer (GOCE) that was finally launched in March. It’s a captivating show, detailing the history and science behind the study of geodesy (the gravitational field, shape and rotation of the Earth).

You may not be familiar with geodesy, but it is critical to advancing our understanding of the planet we live on. For example, GOCE observations could aid prediction techniques for earthquakes, or refine GPS data; suddenly geodesy has a very real and immediate relevance to us on the ground.

Be sure to check out the video below, it’s a very slick production. Great job Bente!

GOCE is Suffering Major Delays, But Should be Dominating Space by February

No, it isn't sci-fi. It's the Porche of orbital engineering (GOCE/ESA)
No, it isn't sci-fi. It's the Porche of orbital engineering (GOCE/ESA)

The European Space Agency’s Gravity field and state-steady Ocean Circulation Explorer (GOCE) should be in space by now. In fact it should have been launched back on September 10th, but it wasn’t to be. After the spacecraft (which has a striking resemblance to something a little more sci-fi… like a star destroyer) had been sealed into the payload bay of the Rockot launch vehicle at Plesetsk cosmodrome 800 km from Moscow, I assumed that was it, we wouldn’t be seeing GOCE ever again. But there was a glitch in the guidance and navigation subsystem of the Breeze KM third stage, thus postponing GOCE’s big day. GOCE was cracked open from its rocket powered cocoon to await a Rockot oil change.

Now it seems the delays are mounting up for this amazing experiment and a launch doesn’t seem possible until February at the earliest…
Continue reading “GOCE is Suffering Major Delays, But Should be Dominating Space by February”

Last Look at GOCE Before Being Sealed Inside Rockot

The last look at GOCE before it is packed away inside the rocket two half-shells (ESA)
The last look at GOCE before it is packed away inside the rocket two half-shells (ESA)

As you probably know, I am a huge fan of the Gravity field and steady-state Ocean Circulation Explorer (GOCE) as it is the sleekest, most aesthetically pleasing spacecraft I have ever seen. Rather than looking like a generic satellite, GOCE has been constructed in the shape of an aerodynamic spaceship as its orbit is so low that atmospheric drag will be a factor. Adding to the wow! factor is the GOCE ion engine giving a small but steady thrust to make sure GOCE doesn’t lose altitude during its Sun-synchronous orbit. Combine all these factors with the incredibly advanced science it will be carrying out during its 20 month lifetime, this is about as advanced as a terrestrial satellite can get.

So, ahead of its launch on September 10th, GOCE has been packed safely inside the Breeze-KM Upper Stage at the Plesetsk Cosmodrome in northern Russia. Next time the craft sees light will be three-minutes after launch in six days time…
Continue reading “Last Look at GOCE Before Being Sealed Inside Rockot”

GOCE Will be the Coolest Satellite to Orbit Earth, Ever

Solar panels have never looked so good. GOCE is the Porsche of orbital engineering (GOCE/ESA)
Solar panels have never looked so good. GOCE is the Porsche of orbital engineering (GOCE/ESA)

The European Space Agency is set to launch the Gravity field and steady-state Ocean Circulation Explorer (GOCE) Star Destroyer satellite on September 10th. This advanced mission will be the most sophisticated piece of kit ever to orbit the Earth, investigating the Earth’s gravitational field. It will perform a highly accurate mapping campaign, producing a high resolution reference shape of the geoid (i.e. the shape of our planet). The mission will be unprecedented, but that’s not the reason why I’m drawing attention to it…

Only last week I remarked on the coolness of the 2013 Mars rover mission in the shape of the dazzling Pasteur Rover (set to drill two-metres into Mars), and today with the announcement of the launch GOCE, it looks like ESA has done it again. They’ve encased their state-of-the-art instrumentation inside something that belongs in a science fiction movie, more reminiscent of the Imperial Star Destroyer from Star Wars than a tin box satellite…
Continue reading “GOCE Will be the Coolest Satellite to Orbit Earth, Ever”