One of the most pressing questions in astronomy concerns black holes. We know that massive stars that explode as supernovae can leave stellar mass black holes as remnants. And astrophysicists understand that process. But what about the supermassive black holes (SMBHs) like Sagittarius A-star (Sgr A*,) at the heart of the Milky Way?
SMBHs can have a billion solar masses. How do they get so big?
Exotic dark matter theories. Gravitational waves. Observatories in space. Giant black holes. Colliding galaxies. Lasers. If you’re a fan of all the awesomest stuff in the universe, then this article is for you.
The discovery of gravitational waves by the LIGO experiment in 2015 sent ripples through the scientific community. Originally predicted by Einstein’s Theory of General Relativity, the confirmation of these waves (and two subsequent detections) solved a long-standing cosmological mystery. In addition to bending the fabric of space-time, it is now known that gravity can also create perturbations that can be detected billions of light-years away.
Seeking to capitalize on these discoveries and conduct new and exciting research into gravitational waves, the European Space Agency (ESA) recently green-lighted the Laser Interferometer Space Antenna (LISA) mission. Consisting of three satellites that will measure gravitational waves directly through laser interferometry, this mission will be the first space-based gravitational wave detector.
This decision was announced yesterday (Tuesday, June 20th) during a meeting of ESA’s Science Program Committee (SPC). It’s implementation is part of the ESA’s Cosmic Vision plan – the current cycle of the agency’s long-term planning for space science missions – which began in 2015 and will be running until 2025. It is also in keeping with the ESA’s desire to study the “invisible universe“, a policy that was adopted in 2013.
To accomplish this, the three satellites that make up the LISA constellation will be deployed into orbit around Earth. Once there, they will assume a triangular formation – spaced 2.5 million km (1.55 million mi) apart – and follow Earth’s orbit around the Sun. Here, isolated from all external influences but Earth’s gravity, they will then connect to each other by laser and begin looking for minute perturbations in the fabric of space-time.
Much like how the LIGO experiment and other gravitational wave detectors work, the LISA mission will rely on laser interferometry. This process consists of a beam of electromagnetic energy (in this case, a laser) being split in two and then recombined to look for patterns of interference. In LISA’s case, two satellites play the role of reflectors while the remaining one is the both source of the lasers and the observer of the laser beam.
When a gravitational wave passes through the triangle established by the three satellites, the lengths of the two laser beams will vary due to the space-time distortions caused by the wave. By comparing the laser beam frequency in the return beam to the frequency of the sent beam, LISA will be able to measure the level of distortion.
These measurements will have to be extremely precise, since the distortions they are looking for affect the fabric of space-time on the most minuscule of levels – a few millionths of a millionth of a meter over a distance of a million kilometers. Luckily, the technology to detect these waves has already been tested by the LISA Pathfinder mission, which deployed in 2015 and will conclude its mission at the end of the month.
In the coming weeks and months, the ESA will be looking over the design of the LISA mission and completing a cost assessment. If all goes as planned, the mission will be proposed for “adoption” before construction begins and it is expected to be launched by 2034. In the same meeting, the ESA also adopted another important mission that will be searching for exoplanets in the coming years.
This mission is known as the PLAnetary Transits and Oscillations of stars, or PLATO, mission. Like Kepler, this mission will monitor stars within a large sections of the sky to look for small dips in their brightness, which are caused by planets passing between the star and the observer (i.e. the transit method). Originally selected in February of 2014, this mission is now moving from the blueprint phase into construction and will launch in 2026.
It’s an exciting time for the European Space Agency. In recent years, it has committed itself to multiple endeavors in the hope of maintaining Europe’s commitment to and continued presence in space. These include studying the “invisible universe”, mounting missions to the Moon and Mars, maintaining a commitment to the International Space Station, and even building a successor to the ISS on the Moon!
Just a couple of weeks ago, astronomers from Caltech announced their third detection of gravitational waves from the Laser Interferometer Gravitational-Wave Observatory or LIGO.
As with the previous two detections, astronomers have determined that the waves were generated when two intermediate-mass black holes slammed into each other, sending out ripples of distorted spacetime.
One black hole had 31.2 times the mass of the Sun, while the other had 19.4 solar masses. The two spiraled inward towards each other, until they merged into a single black hole with 48.7 solar masses. And if you do the math, twice the mass of the Sun was converted into gravitational waves as the black holes merged.
These gravitational waves traveled outward from the colossal collision at the speed of light, stretching and compressing spacetime like a tsunami wave crossing the ocean until they reached Earth, located about 2.9 billion light-years away.
The waves swept past each of the two LIGO facilities, located in different parts of the United States, stretching the length of carefully calibrated laser measurements. And from this, researchers were able to detect the direction, distance and strength of the original merger.
Seriously, if this isn’t one of the coolest things you’ve ever heard, I’m clearly easily impressed.
Now that the third detection has been made, I think it’s safe to say we’re entering a brand new field of gravitational astronomy. In the coming decades, astronomers will use gravitational waves to peer into regions they could never see before.
Being able to perceive gravitational waves is like getting a whole new sense. It’s like having eyes and then suddenly getting the ability to perceive sound.
This whole new science will take decades to unlock, and we’re just getting started.
As Einstein predicted, any mass moving through space generates ripples in spacetime. When you’re just walking along, you’re actually generating tiny ripples. If you can detect these ripples, you can work backwards to figure out what size of mass made the ripples, what direction it was moving, etc.
Even in places that you couldn’t see in any other way. Let me give you a couple of examples.
Black holes, obviously, are the low hanging fruit. When they’re not actively feeding, they’re completely invisible, only detectable by how they gravitational attract objects or bend light from objects passing behind them.
But seen in gravitational waves, they’re like ships moving across the ocean, leaving ripples of distorted spacetime behind them.
With our current capabilities through LIGO, astronomers can only detect the most massive objects moving at a significant portion of the speed of light. A regular black hole merger doesn’t do the trick – there’s not enough mass. Even a supermassive black hole merger isn’t detectable yet because these mergers seem to happen too slowly.
This is why all the detections so far have been intermediate-mass black holes with dozens of times the mass of our Sun. And we can only detect them at the moment that they’re merging together, when they’re generating the most intense gravitational waves.
If we can boost the sensitivity of our gravitational wave detectors, we should be able to spot mergers of less and more massive black holes.
But merging isn’t the only thing they do. Black holes are born when stars with many more times the mass of our Sun collapse in on themselves and explode as supernovae. Some stars, we’ve now learned just implode as black holes, never generating the supernovae, so this process happens entirely hidden from us.
Is there a singularity at the center of a black hole event horizon, or is there something there, some kind of object smaller than a neutron star, but bigger than an infinitely small point? As black holes merge together, we could see beyond the event horizon with gravitational waves, mapping out the invisible region within to get a sense of what’s going on down there.
We want to know about even less massive objects like neutron stars, which can also form from a supernova explosion. These neutron stars can orbit one another and merge generating some of the most powerful explosions in the Universe: gamma ray bursts. But do neutron stars have surface features? Different densities? Could we detect a wobble in the gravitational waves in the last moments before a merger?
And not everything needs to merge. Sensitive gravitational wave detectors could sense binary objects with a large imbalance, like a black hole or neutron star orbiting around a main sequence star. We could detect future mergers by their gravitational waves.
Are gravitational waves a momentary distortion of spacetime, or do they leave some kind of permanent dent on the Universe that we could trace back? Will we see echoes of gravity from gravitational waves reflecting and refracting through the fabric of the cosmos?
Perhaps the greatest challenge will be using gravitational waves to see beyond the Cosmic Microwave Background Radiation. This region shows us the Universe 380,000 years after the Big Bang, when everything was cool enough for light to move freely through the Universe.
But there was mass there, before that moment. Moving, merging mass that would have generated gravitational waves. As we explained in a previous article, astronomers are working to find the imprint of these gravitational waves on the Cosmic Microwave Background, like an echo, or a shadow. Perhaps there’s a deeper Cosmic Gravitational Background Radiation out there, one which will let us see right to the beginning of time, just moments after the Big Bang.
And as always, there will be the surprises. The discoveries in this new field that nobody ever saw coming. The “that’s funny” moments that take researchers down into whole new fields of discovery, and new insights into how the Universe works.
The LIGO project was begun back in 1994, and the first iteration operated from 2002 to 2012 without a single gravitational wave detection. It was clear that the facility wasn’t sensitive enough, so researchers went back and made massive improvements.
In 2008, they started improving the facility, and in 2015, Advanced LIGO came online with much more sensitivity. With the increased capabilities, Advanced LIGO made its first discovery in 2016, and now two more discoveries have been added.
LIGO can currently only detect the general hemisphere of the sky where a gravitational wave was emitted. And so, LIGO’s next improvement will be to add another facility in India, called INDIGO. In addition to improving the sensitivity of LIGO, this will give astronomers three observations of each event, to precisely detect the origin of the gravitational waves. Then visual astronomers could do follow up observations, to map the event to anything in other wavelengths.
A European experiment known as Virgo has been operating for a few years as well, agreeing to collaborate with the LIGO team if any detections are made. So far, the Virgo experiment hasn’t found anything, but it’s being upgraded with 10 times the sensitivity, which should be fully operational by 2018.
A Japanese experiment called the Kamioka Gravitational Wave Detector, or KAGRA, will come online in 2018 as well, and be able to contribute to the observations. It should be capable of detecting binary neutron star mergers out to nearly a billion light-years away.
Just with visual astronomy, there are a set of next generation supergravitational wave telescopes in the works, which should come online in the next few decades.
The Europeans are building the Einstein Telescope, which will have detection arms 10 km long, compared to 4 km for LIGO. That’s like, 6 more km.
There’s the European Space Agency’s space-based Laser Interferometer Space Antenna, or LISA, which could launch in 2030. This will consist of a fleet of 3 spacecraft which will maintain a precise distance of 2.5 million km from each other. Compare that to the Earth-based detection distances, and you can see why the future of observations will come from space.
And that last idea, looking right back to the beginning of time could be a possibility with the Big Bang Observer mission, which will have a fleet of 12 spacecraft flying in formation. This is still all in the proposal stage, so no concrete date for if or when they’ll actually fly.
Gravitational wave astronomy is one of the most exciting fields of astronomy. This entirely new sense is pushing out our understanding of the cosmos in entirely new directions, allowing us to see regions we could never even imagine exploring before. I can’t wait to see what happens next.
NASA Astrobiologist Dr. Chris McKay organized an August 2014 workshop to discuss the future of a permanent moon base, and the ultimate goal of establishing a human settlement on Mars. The resultant nine papers have been recently published in a special issue of the journal New Space.
We’ve had an abundance of news stories for the past few months, and not enough time to get to them all. So we are now using a tool called Trello to submit and vote on stories we would like to see covered each week, and then Fraser will be selecting the stories from there. Here is the link to the Trello WSH page (http://bit.ly/WSHVote), which you can see without logging in. If you’d like to vote, just create a login and help us decide what to cover!
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Chalk another one up for Citizen Science. Earlier this month, researchers announced the discovery of 24 new pulsars. To date, thousands of pulsars have been discovered, but what’s truly fascinating about this month’s discovery is that came from culling through old data using a new method.
A pulsar is a dense, highly magnetized, swiftly rotating remnant of a supernova explosion. Pulsars where first discovered by Jocelyn Bell Burnell and Antony Hewish in 1967. The discovery of a precisely timed radio beacon initially suggested to some that they were the product of an artificial intelligence. In fact, for a very brief time, pulsars were known as LGM’s, for “Little Green Men.” Today, we know that pulsars are the product of the natural death of massive stars.
The data set used for the discovery comes from the Parkes 64-metre radio observatory based out of New South Wales, Australia. The installation was the first to receive telemetry from the Apollo 11 astronauts on the Moon and was made famous in the movie The Dish. The Parkes Multi-Beam Pulsar Survey (PMPS) was conducted in the late 1990’s, making thousands of 35-minute recordings across the plane of the Milky Way galaxy. This survey turned up over 800 pulsars and generated 4 terabytes of data. (Just think of how large 4 terabytes was in the 90’s!)
The nature of these discoveries presented theoretical astrophysicists with a dilemma. Namely, the number of short period and binary pulsars was lower than expected. Clearly, there were more pulsars in the data waiting to be found.
Enter Citizen Science. Using a program known as [email protected], researchers were able to sift though the recordings using innovative modeling techniques to tease out 24 new pulsars from the data.
“The method… is only possible with the computing resources provided by [email protected]” Benjamin Knispel of the Max Planck Institute for Gravitational Physics told the MIT Technology Review in a recent interview. The study utilized over 17,000 CPU core years to complete.
[email protected] is a program uniquely adapted to accomplish this feat. Begun in 2005, [email protected] is a distributed computing project which utilizes computing power while machines are idling to search through downloaded data packets. Similar to the original distributed computing program [email protected] which searches for extraterrestrial signals, [email protected] culls through data from the LIGO (Laser Interferometer Gravitational Wave Observatory) looking for gravity waves. In 2009, the [email protected] survey was expanded to include radio astronomy data from the Arecibo radio telescope and later the Parkes observatory.
Among the discoveries were some rare finds. For example, PSR J1748-3009 Has the highest known dispersion measure of any millisecond pulsar (The dispersion measure is the density of free electrons observed moving towards the viewer). Another find, J1750-2531 is thought to belong to a class of intermediate-mass binary pulsars. 6 of the 24 pulsars discovered were part of binary systems.
These discoveries also have implications for the ongoing hunt for gravity waves by such projects as LIGO. Specifically, a through census of binary pulsars in the galaxy will give scientists a model for the predicted rate of binary pulsar mergers. Unlike radio surveys, LIGO seeks to detect these events via the copious amount of gravity waves such mergers should generate. Begun in 2002, LIGO consists of two gravity wave observatories, one in Hanford Washington and one in Livingston Louisiana just outside of Baton Rouge. Each LIGO detector consists of two 2 kilometre Fabry-Pérot arms in an “L” configuration which allow for ultra-precise measurements of a 200 watt laser beam shot through them. Two detectors are required to pin-point the direction of an incoming gravity wave on the celestial sphere. You can see the orientation of the “L’s” on the display on the [email protected] screensaver. Two geographically separate detectors are also required to rule out local interference. A gravity wave from a galactic source would ripple straight through the Earth.
Such a movement would be tiny, on the order of 1/1,000th the diameter of a proton, unnoticed by all except the LIGO detectors. To date, LIGO has yet to detect gravity waves, although there have been some false alarms. Scientists regularly interject test signals into the data to see if system catches them. The lack of detection of gravity waves by LIGO has put some constraints on certain events. For example, LIGO reported a non-detection of gravity waves during the February 2007 short gamma-ray burst event GRB 070201. The event arrived from the direction of the Andromeda Galaxy, and thus was thought to have been relatively nearby in the universe. Such bursts are thought to be caused by neutron star and/or black holes mergers. The lack of detection by LIGO suggests a more distant event. LIGO should be able to detect a gravitational wave event out to 70 million light years, and Advanced LIGO (AdLIGO) is set to go online in 2014 and will increase its sensitivity tenfold.
Knowledge of where these potential pulsar mergers are by such discoveries as the Parkes radio survey will also give LIGO researchers clues of targets to focus on. “The search for pulsars isn’t easy, especially for these “quiet” ones that aren’t doing the equivalent of “screaming” for our attention,” Says LIGO Livingston Data Analysis and EPO Scientist Amber Stuver. The LIGO consortium developed the data analysis technique used by [email protected] The direct detection of gravitational waves by LIGO or AdLIGO would be an announcement perhaps on par with CERN’s discovery of the Higgs Boson last year. This would also open up a whole new field of gravitational wave astronomy and perhaps give new stimulus to the European Space Agencies’ proposed Laser Interferometer Space Antenna (LISA) space-based gravity wave detector. Congrats to the team at Parkes on their discovery… perhaps we’ll have the first gravity wave detection announcement out of LIGO as well in years to come!
-Read the original paper on the discovery of 24 new pulsars here.
Two white dwarfs similar to those in the system SDSS J065133.338+284423.37 spiral together in this illustration from NASA. Credit: D. Berry/NASA GSFC
Locked in a spiraling orbital embrace, the super-dense remains of two dead stars are giving astronomers the evidence needed to confirm one of Einstein’s predictions about the Universe.
A binary system located about 3,000 light-years away, SDSS J065133.338+284423.37 (J0651 for short) contains two white dwarfs orbiting each other rapidly — once every 12.75 minutes. The system was discovered in April 2011, and since then astronomers have had their eyes — and four separate telescopes in locations around the world — on it to see if gravitational effects first predicted by Einstein could be seen.
According to Einstein, space-time is a structure in itself, in which all cosmic objects — planets, stars, galaxies — reside. Every object with mass puts a “dent” in this structure in all dimensions; the more massive an object, the “deeper” the dent. Light energy travels in a straight line, but when it encounters these dents it can dip in and veer off-course, an effect we see from Earth as gravitational lensing.
Einstein also predicted that exceptionally massive, rapidly rotating objects — such as a white dwarf binary pair — would create outwardly-expanding ripples in space-time that would ultimately “steal” kinetic energy from the objects themselves. These gravitational waves would be very subtle, yet in theory, observable.
What researchers led by a team at The University of Texas at Austin have found is optical evidence of gravitational waves slowing down the stars in J0651. Originally observed in 2011 eclipsing each other (as seen from Earth) once every six minutes, the stars now eclipse six seconds sooner. This equates to a predicted orbital period reduction of about 0.25 milliseconds each year.*
“These compact stars are orbiting each other so closely that we have been able to observe the usually negligible influence of gravitational waves using a relatively simple camera on a 75-year-old telescope in just 13 months,” said study lead author J.J. Hermes, a graduate student at The University of Texas at Austin.
Based on these measurements, by April 2013 the stars will be eclipsing each other 20 seconds sooner than first observed. Eventually they will merge together entirely.
Although this isn’t “direct” observation of gravitational waves, it is evidence inferred by their predicted effects… akin to watching a floating lantern in a dark pond at night moving up and down and deducing that there are waves present.
“It’s exciting to confirm predictions Einstein made nearly a century ago by watching two stars bobbing in the wake caused by their sheer mass,” said Hermes.
As of early last year NASA and ESA had a proposed mission called LISA (Laser Interferometer Space Antenna) that would have put a series of 3 detectors into space 5 million km apart, connected by lasers. This arrangement of precision-positioned spacecraft could have detected any passing gravitational waves in the local space-time neighborhood, making direct observation possible. Sadly this mission was canceled due to FY2012 budget cuts for NASA, but ESA is moving ahead with developments for its own gravitational wave mission, called eLISA/NGO — the first “pathfinder” portion of which is slated to launch in 2014.
The study was submitted to Astrophysical Journal Letters on August 24. Read more on the McDonald Observatory news release here.
Inset image: simulation of binary black holes causing gravitational waves – C. Reisswig, L. Rezzolla (AEI); Scientific visualization – M. Koppitz (AEI & Zuse Institute Berlin)
*The difference in the eclipse time is noted as six seconds even though the orbital period decay of the two stars is only .25 milliseconds/year because of a pile-up effect of all the eclipses observed since April 2011. The measurements made by the research team takes into consideration the phase change in the J0651 system, which experiences a piling effect — similar to an out-of-sync watch — that increases relative to time^2 and is therefore a larger and easier number to detect and work with. Once that was measured, the actual orbital period decay could be figured out.
Do you remember the LISA mission? I do! The proposed launch for this unique vision is slated for 2014 and the latest sensor technology is making its own waves… by being far more accurate than expected. Now ESA’s LISA Pathfinder mission is better than ever, and ready to tackle the vast ocean of space in search of elusive gravitational waves…
So what’s new? By employing a near complete version of LISA, the Optical Meteorology Subsystem passed its first test under space-like temperature and vacuum conditions. Not only did it make the grade, but it went far beyond. It surpassed the precision requirement needed to detect gravitational waves by 300%!
Einstein predicted them, but to physically record this phenomenon in space, the LISA Pathfinder will utilize a laser to measure the distance between two free-floating gold–platinum cubes. Here on the ground, the team in Ottobrunn, Germany, are performing the tests using mirrors instead of cubes. Not only will the distance between them be cataloged, but their angles with respect to the laser beams. Is LISA good? Darn right. She had an accuracy rating of 10 billionths of a degree!
“This is equivalent to the angle subtended by an astronaut’s footprint on the Moon!” notes Paul McNamara, Project Scientist for the LISA Pathfinder mission.
So how are gravitational waves detected? If perfect conditions do exist in space, then the free-floating cubes should mirror each other’s motions. Now, enter Einstein’s general theory of relativity. If some gravitational event should occur – such as the collision of two black holes – this should cause a minute distortion in the fabric of space. These tiny changes should be detectable. However, the accuracy needed to record such an event would need to be about one hundredth the size of an atom… a size called a “picometre”. Originally, LISA was optimized at 6 picometres measured over a timeline of 1000 seconds. But she bettered her record in 2010 and has now reached an amazing accuracy level of 2 picometres.
“The whole team has worked extremely hard to make this measurement possible,” said Dr McNamara. “When LISA Pathfinder is launched and we’re in the quiet environment of space some 1.5 million km from Earth, we expect that performance will be even better.”
The instrument team from Astrium GmbH, the Albert Einstein Institute and ESA are testing the Optical Metrology Subsystem during LISA Pathfinder thermal vacuum tests in Ottobrunn by spacecraft prime contractor Astrium (UK) Ltd. Tentatively set to launch in mid-2014, the LISA Pathfinder is well on its way to ride the gravitational waves and set the pace for ESA’s New Gravitational Wave Observatory. Perhaps within the next 10 years we’ll see even more advancements in finding the “final piece in Einstein’s cosmic puzzle.”