It has only been a couple of years since the first historic detection of gravitational waves, but now physicists are already dissecting a handful of signals that emanated hundreds of millions of light-years away to elucidate how some of the most violent events in our universe work.
Most of the gravitational wave signals detected so far involve the merger of black holes, but one signal, detected on Aug. 17, 2017, was special—it was caused by the smashup of two neutron stars. This merger also generated a powerful gamma-ray burst (GRB) that was detected at nearly the same time, linking GRBs with neutron star mergers and highlighting where heavy elements in our universe are forged. A new era of “multimessenger astronomy” had begun.
Now, the signal (designated GW170817) has been reanalyzed to understand what happened after the merger. Analysis that came before suggested that the collision of the two neutron stars would have tipped the mass balance to create a black hole. According to a new study, published in the journal Monthly Notices of the Royal Astronomical Society: Letters, two physicists suggest a contradictory scenario: GW170817 didn’t create a black hole, it produced a hypermassive neutron star, instead.
“We’re still very much in the pioneering era of gravitational wave astronomy. So it pays to look at data in detail,” said Maurice van Putten of Sejong University in South Korea. “For us this really paid off, and we’ve been able to confirm that two neutron stars merged to form a larger one.”
The secret behind this finding focuses on the datasets recorded by the US-based Laser Interferometer Gravitational-wave Observatory (LIGO) and Italian Virgo observatory. When gravitational waves are recorded during a black hole or neutron star merger event, their frequency rapidly increases (as the objects orbit one another faster and faster as they get closer and closer) and then abruptly cuts off (when they collide). When turned into an audio file, mergers sound like “chirps.” Apart from sounding like an eerie bird call coming from deep space, physicists have been able to extract surprisingly detailed information from the conditions of the merging objects, such as their mass and rates of spin.
And this is where van Putten’s work comes in.
Working with Massimo della Valle of the Osservatorio Astronomico de Capodimonte in Italy, the duo applied a new analysis technique to these data and detected a 5-second descending “chirp” (as shown by the downward arrow in the graph above). This descending chirp happened immediately after the GRB was detected coming from the same location as the gravitational wave signal’s origin. According to their analysis, the spin-down—from 1 KHz to 49 Hz—was most likely representative of a very massive neutron star and not a black hole.
If corroborated, this discovery could have profound implications for astrophysics. How hypermassive neutron stars (like the one that was created by GW170817) can exist without collapsing into a black hole will likely keep theorists busy for some time and physicists will be hopeful for another gravitational wave event like GW170817.
One hundred and thirty million years ago in a galaxy 130 million light-years away, two neutron stars met their fate, merging as one. Trapped in a gravitational embrace, these two stellar husks spiraled closer and closer until they violently ripped into one another, causing a detonation that reverberated throughout the cosmos.
On August 17, the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and Italian Virgo gravitational wave detector felt the faint ripples in spacetime from that ancient neutron star collision washing through our planet. Until now, LIGO and Virgo have only confirmed the collisions and mergers of black holes, so the fact that a nearby (a relative term in this case) neutron star merger had been detected was already historic.
But the implications for this particular neutron star signal, which is comparatively weak in comparison with the black hole mergers that have come before it, are so profound that I’ve been finding it hard to put this grand discovery into words (though I have tried).
Why It Matters
With regards to gravitational waves, I feel I’ve described each gravitational wave discovery as “historic” and “a new era for astronomy” since their first detection on Sept. 15, 2015, but the detection of GW170817 may well trump all that have come before it, even though the signal was generated by neutron stars and not black hole heavyweights.
The thing with black holes is that when they collide and merge, they don’t necessarily produce electromagnetic radiation (i.e. visible light, X-rays or infrared radiation). They can go “bump” in the cosmic night and no intelligent being with a conventional telescope would see it happen. But in the the gravitational domain, black hole mergers echo throughout the universe; their gravitational waves travel at the speed of light, warping spacetime as they propagate. To detect these “invisible” waves, we must build instruments that can “see” the infinitesimal wobbles in the fabric of spacetime itself, and this is where laser interferometry comes in.
Very precise lasers are fired down miles-long tunnels in “L” shaped buildings in the two LIGO detectors (in Washington and Louisiana) and the Virgo detector near Pisa. When gravitational waves travel through us, these laser interferometers can measure the tiny spacetime warps. The more detectors measuring the same signal means a more precise observation and scientists can then work out where (and when) the black hole merger occurred.
There are many more details that can be gleaned from the gravitational wave signal from black hole mergers, of course — including the progenitor black holes’ masses, the merged mass, black hole spin etc. — but for the most part, black hole mergers are purely a gravitational affair.
Neutron stars, however, are a different beast and, on Aug. 17, it wasn’t only gravitational wave detectors that measured a signal from 130 million light-years away; space telescopes on the lookout for gamma-ray bursts (GRBs) also detected a powerful burst of electromagnetic radiation in the galaxy of NGC 4993, thereby pinpointing the single event that generated the gravitational waves and the GRB.
And this is the “holy shit” moment.
As Caltech’s David H. Reitze puts it: “This detection opens the window of a long-awaited ‘multi-messenger’ astronomy.”
What Reitze is referring to is that, for the first time, both gravitational waves and electromagnetic waves (across the EM spectrum) have been observed coming from the same astrophysical event. The gravitational waves arrived at Earth slightly before the GRB was detected by NASA’s Fermi and ESA’s INTEGRAL space telescopes. Both space observatories recorded a short gamma-ray burst, a type of high-energy burst that was theorized (before Aug. 17) to be produced by colliding neutron stars.
Now scientists have observational evidence that these types of GRBs are produced by colliding neutron stars as the gravitational wave fingerprint unquestionably demonstrates the in-spiraling and merger of two neutron stars. This is a perfect demonstration of multi-messenger astronomy; where an energetic event can be observed simultaneously in EM and gravitational waves to reveal untold mysteries of the universe’s most energetic events.
Another Nod to Einstein
The fact that the gravitational waves and gamma-rays arrived at approximately the same time is yet another nod to Einstein’s general relativity. The century-old theory predicts that gravitational waves should travel at the speed of light and, via this brand spanking new way of doing multi-messenger astronomy, physicists and astronomers have again bolstered relativity with observational evidence.
But why did the gravitational waves arrive slightly before the GRB? Well, NASA’s Fermi team explains: “Fermi’s [Gamma-ray Burst Monitor instrument] saw the gamma-ray burst after the [gravitational wave] detection because the merger happened before the explosion,” they said in a tweet.
In other words, when the two neutron stars collided and merged, the event immediately dissipated energy as gravitational waves that were launched through spacetime at the speed of light — that’s the source of GW170817 — but the GRB was generated shortly after.
Enter the Kilonova
As the neutron stars smashed together, huge quantities of neutron star matter were inevitably blasted into space, creating a superheated, dense volume of free neutrons. Neutrons are subatomic particles that form the building blocks of atoms and if the conditions are right, the neutron star debris will undergo rapid neutron capture process (known as “r-process”) where neutrons combine with one another faster than the newly-formed radioactive particles can decay. This mechanism is responsible for synthesizing elements heavier than iron (elements lighter than iron are formed through stellar nucleosynthesis in the cores of stars).
For decades astronomers have been searching for observational evidence of the r-process in action and now they have it. Soon after the merger, massive amounts of debris erupted in a frenzy of heavy element creation, triggering an energetic eruption known as a “kilonova” that was seen as a short GRB. The GRB was cataloged as “SSS17a.”
The Golden Ticket
Follow-up observations by the Hubble Space Telescope, Gemini Observatory and the ESO’s Very Large Telescope have all detected spectroscopic signatures in the afterglow consistent with the r-process taking place at the site of the kilonova, meaning heavy elements are being formed and, yes, it’s a goldmine. As in: there’s newly-synthesized gold there. And platinum. And all the other elements heavier than iron that aren’t quite so sexy.
And there’s lots of it. Researchers estimate that that single neutron star collision produced hundreds of Earth-masses of gold and platinum and they think that neutron star mergers could be the energetic process that seed the galaxies with heavy elements (with supernovas coming second).
So, yeah, it’s a big, big, BIG discovery that will reverberate for the decades to come.
The best thing is that we now know that our current generation of advanced gravitational wave detectors are sensitive enough to not only detect black holes merging billions of light-years away, but also detect the nearby neutron stars that are busy merging and producing gold. As more detectors are added and as the technology and techniques mature, we’ll be inundated with merging events big and small, each one teaching us something new about our universe.
From crashing black holes to wobbling neutron stars, these cosmic phenomena generate ripples in spacetime and not necessarily emissions in the electromagnetic spectrum. So when the Laser Interferometer Gravitational-wave Observatory (LIGO) made its first gravitational wave detection in September 2015, the science world became very excited about the reality of “gravitational wave astronomy” and the prospect of detecting some of the most massive collisions that happen in the dark, billions of light-years away.
Like waves rippling over the surface of the ocean, gravitational waves travel through spacetime, a prediction that was made by Albert Einstein over a century ago. And like those ocean waves, gravitational waves might reveal something about the nature of spacetime.
We’re talking extra-dimensions and a new study suggests that gravitational waves may carry an awful lot more information with them beyond the characteristics of what generated them in the first place.
Our 4-D Playing Field
First things first, remember that we interact only with four-dimensional spacetime: three dimensions of space and one dimension of time. This is our playing field; we couldn’t care less whether there are more dimensions out there.
Unless you’re a physicist, that is.
And physicists are having a hard job describing gravity, to put it mildly. This might seem weird considering how essential gravity is for, well, everything. Without gravity, no stars would form, planets wouldn’t coalesce and the cosmos would be an exceedingly boring place. But gravity doesn’t seem to “fit” with the Standard Model of physics. The “recipe” for the universe is perfect, except it’s missing one vital ingredient: Gravity. (It’s as if a perfect cake recipe is missing one crucial ingredient, like flour.)
There’s another weird thing about gravity: Although it’s very important in our universe (yes, there might be more than one universe, but I’ll get to that later), it is actually the weakest of all forces.
String theory (and, by extension, superstring theory) predicts that the universe is composed of strings that vibrate at different frequencies. These strings form something like a vast, superfine noodle soup and these strings thread through many dimensions (many more than our four-dimensions) creating all the particles and forces that we know and love.
Now, the possible reason why gravity is so weak when compared with the other fundamental forces could be that gravity is interacting with many more dimensions that are invisible to us 4-D beings. Although string theory is a wonderful mathematical tool to describe this possibility, there is little physical evidence to back up this superfine noodly mess, however.
But as already mentioned, if string theory holds true, it would mean that our universe contains many more dimensions than we regularly experience. (The unifying superstring theory, called “M-theory”, predicts a total of 11 dimensions and may provide the framework that unifies the fundamental forces and could be the diving board that launches us into the vast ocean that is the multiverse… but I’ll stop there, I’ve said too much.)
Groovy. But what the heck has this got to do with gravitational waves? As gravitational waves travel through spacetime, they might be imprinted with information about these extra dimensions. Like our wave analogy, as the sea washes over a beach, the frequency of the waves increase as the water becomes shallower — the ocean waves are imprinted with information about how deep the water is. Could gravitational waves washing over (or, more accurately, through) spacetime also create some kind of signature that would reveal the presence of very, very tiny extra-dimensions as predicted by superstring theory?
Possibly, say researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Potsdam, Germany.
“Physicists have been looking for extra dimensions at the Large Hadron Collider at CERN but up to now this search has yielded no results,” says Gustavo Lucena Gómez, second author of a new study published in the Journal of Cosmology and Astroparticle Physics. “But gravitational wave detectors might be able to provide experimental evidence.”
The researchers suggest that these extra-dimensions might modify the signal of gravitational waves received by detectors like LIGO and leave a very high-frequency “fingerprint.” But as this frequency would be exceedingly high — of the order of 1000 Hz — it’s not conceivable that the current (and near-future) ground-based gravitational wave detectors will be sensitive enough to even hope to detect these frequencies.
However, extra-dimensions might modify the gravitational waves in a different way. As gravitational waves propagate, they stretch and shrink the spacetime they travel through, like this:
The amount of spacetime warping might therefore be detected as more gravitational wave detectors are added to the global network. Currently, LIGO has two operating observing stations (one in Washington and one in Louisiana) and next year, the European Virgo detector will start taking data.
More detectors are planned elsewhere, so it’s possible that we may, one day, use gravitational waves to not only “see” black holes go bump in the night, we might also “see” the extra-dimensions that form the minuscule tapestry of the fabric beyond spacetime. And if we can do this, perhaps we’ll finally understand why gravity is so weak and how it really fits in with the Standard Model of physics.
Want to know more about gravitational waves? Well, here’s an Astroengine YouTube video on the topic:
Imagine speeding down the highway and plowing into an unfortunate swarm of mosquitoes. Now imagine that you had the ability to precisely measure the mass of each mosquito, the speed at which it was traveling and the direction it was going before it exploded over your windscreen.
Granted, the technology to accomplish that probably isn’t feasible in such an uncontrolled environment. Factors such as vibration from the car’s motor and tires on the road, plus wind and air turbulence will completely drown out any “splat” from a minuscule insect’s body, rendering any signal difficult to decipher from noise.
The European LISA Pathfinder spacecraft is a proof of concept mission that’s currently in space, orbiting a region of gravitational stability between the Earth and the sun — called the L1 point located a million miles away. The spacecraft was launched there in late 2015 to carry out precision tests of instruments that will eventually be used in the space-based gravitational wave detector eLISA. Inside the payload is a miniaturized laser interferometer system that measures the distance between two test masses.
When launched in 2034, eLISA (which stands for Evolved Laser Interferometer Space Antenna) will see three spacecraft, orbiting the sun at the L1 point, firing ultra-precise lasers at one another as part of a space-based gravitational wave detector. Now we actually know gravitational waves exist — after the US-based Laser Interferometer Gravitational-wave Observatory (or LIGO) detected the space-time ripples created after the collisions of black holes — excitement is building that we might, one day, be able to measure other phenomena, such as the ultra-low frequency gravitational waves that were created during the Big Bang.
But the only way we can do this is to send stunningly precise interferometers into space, away from our vibration-filled atmosphere to stand a chance of detecting some of the faintest space-time rumbles in our cosmos that would otherwise be drowned out by a passing delivery truck or windy day. And LISA Pathfinder is currently out there, testing a tiny laser interferometer in a near-perfect gravitational free-fall, making the slightest of slight adjustments with its “ultra-precise micro-propulsion system.”
Although LISA Pathfinder is a test (albeit a history-making test of incredible engineering ingenuity), NASA thinks that it could actually be used as an observatory in its own right; not for hunting gravitational waves, but for detecting comet dust.
Like our mosquito-windscreen analogy, spacecraft get hit by tiny particles all the time, and LISA Pathfinder is no exception. These micrometeoroides come from eons of evaporating comets and colliding asteroids. Although measuring less than the size of a grain of sand, these tiny particles zip around interplanetary space at astonishing speeds — well over 22,000 miles per hour (that’s 22 times faster than a hyper-velocity rifle round) — and can damage spacecraft over time, slowly eroding unprotected hardware.
Therefore, it would be nice if we could create a map of regions in the solar system that contain lots of these particles so we can be better prepared to face the risk. Although models of solar system evolution help and we can estimate the distribution of these particles, they’ve only ever been measured near Earth, so it would be advantageous to find the “ground truth” and measure them directly from another, unexplored region of the solar system.
This is where LISA Pathfinder comes in.
As the spacecraft gets hit by these minuscule particles, although they are tiny, their high speed ensures they pack a measurable punch. As scientists want the test weights inside the spacecraft to be completely shielded from any external force — whether that’s radiation pressure from the sun or marauding micro-space rocks — the spacecraft has been engineered to be an ultra-precise container that carefully adjusts its orientation an exact amount to directly counter these external forces (hence the “ultra-precise micro-propulsion system”).
This bit is pretty awesome: Whenever these tiny space particles hit the spacecraft, it compensates for the impact and that compensation is registered as a “blip” in the telemetry being beamed back to Earth. After careful analysis of the various data streams, researchers are learning a surprising amount of information about these micrometeoroides — such as their mass, speed, direction of travel and even their possible origin! — all for the ultimate goal of getting to know the tiny pieces of junk that whiz around space.
“Every time microscopic dust strikes LISA Pathfinder, its thrusters null out the small amount of momentum transferred to the spacecraft,” said Diego Janches, of NASA’s Goddard Space Flight Center in Greenbelt, Md. “We can turn that around and use the thruster firings to learn more about the impacting particles. One team’s noise becomes another team’s data.”
So, it turns out that you can precisely measure a mosquito impact on your car’s windshield — so long as that “mosquito” is a particle of space dust and your “car” is a spacecraft a million miles from Earth.
NASA put together a great video, watch it:
Aside: So it turned out that I inadvertently tested the “car-mosquito” hypothesis when driving home from Las Vegas — though some of these were a lot bigger than mosquitoes…
Before we can understand what this “noise” is, we need to understand how equipment designed to look for the space-time ripples caused by collisions between black holes and supernova explosions.
Gravitational wave detectors are incredibly sensitive to the tiniest change in distance. For example, the GEO600 experiment can detect a fluctuation of an atomic radius over a distance from the Earth to the Sun. This is achieved by firing a laser down a 600 meter long tube where it is split, reflected and directed into an interferometer. The interferometer can detect the tiny phase shifts in the two beams of light predicted to occur should a gravitational wave pass through our local volume of space. This wave is theorized to slightly change the distance between physical objects. Should GEO600 detect a phase change, it could be indicative of a slight change in distance, thus the passage of a gravitational wave.
While looking out for a gravitational wave signal, scientists at GEO600 noticed something bizarre. There was inexplicable static in the results they were gathering. After canceling out all artificial sources of the noise, they called in the help of Fermilab’s Craig Hogan to see if his expertise of the quantum world help shed light on this anomalous noise. His response was as baffling as it was mind-blowing. “It looks like GEO600 is being buffeted by the microscopic quantum convulsions of space-time,” Hogan said.
The signal being detected by GEO600 isn’t a noise source that’s been overlooked, Hogan believes GEO600 is seeing quantum fluctuations in the fabric of space-time itself. This is where things start to get a little freaky.
According to Einstein’s view on the universe, space-time should be smooth and continuous. However, this view may need to be modified as space-time may be composed of quantum “points” if Hogan’s theory is correct. At its finest scale, we should be able to probe down the “Planck length” which measures 10-35 meters. But the GEO600 experiment detected noise at scales of less than 10-15 meters.
As it turns out, Hogan thinks that noise at these scales are caused by a holographic projection from the horizon of our universe. A good analogy is to think about how an image becomes more and more blurry or pixelated the more you zoom in on it. The projection starts off at Planck scale lengths at the Universe’s event horizon, but its projection becomes blurry in our local space-time. This hypothesis comes out of black hole research where the information that falls into a black hole is “encoded” in the black hole’s event horizon. For the holographic universe to hold true, information must be encoded in the outermost reaches of the Universe and it is projected into our 3 dimensional world.
But how can this hypothesis be tested? We need to boost the resolution of a gravitational wave detector-type of kit. Enter the “Holometer.”
Currently under construction in Fermilab, the Holometer (meaning holographic interferometer) will delve deep into this quantum realm at smaller scales than the GEO600 experiment. If Hogan’s idea is correct, the Holometer should detect this quantum noise in the fabric of space-time, throwing our whole perception of the Universe into a spin.
Forget building gravitational wave detectors costing hundreds of millions of dollars (I’m looking at you, LIGO), make use of the most accurate cosmic timekeepers instead and save a bundle.
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a proposal that involves closely monitoring the regular flashes of spinning neutron stars (or pulsars) to detect very slight “shimmers” in their signal. Although the physics is crazy-complex, by tracking these shimmers over a suitably distributed number of pulsars could reveal the passage of gravitational waves.
The problem is that you need to closely monitor rapidly-spinning millisecond pulsars, which are (a) tough to find (only 150 have been found over nearly three decades since pulsars were first discovered), and (b) not very plentiful in the part of the night sky of interest to scientists (northern hemisphere). They tend to clump together in globular star clusters, which makes them useless for detecting gravitational waves.
However, according to results announced by the National Radio Astronomy Observatory (NRAO) at this week’s American Astronomical Society (AAS) meeting in Washington D.C., they’ve discovered 17 new pulsars with the help of NASA’s Fermi Gamma-Ray Space Telescope.
In addition to recent Fermi telescope pulsar discoveries, it would appear that the number of potential targets for NANOGrav are increasing, making a stronger case for the 10 year, $65 million project…
Could our cosmos be a projection from the edge of the observable Universe?
Sounds like a silly question, but scientists are seriously taking on this idea. As it happens, a gravitational wave detector in Germany is turning up null results on the gravitational wave detection front (no surprises there), but it may have discovered something even more fundamental than a ripple in space-time. The spurious noise being detected at the GEO600 experiment has foxed physicists for some time. However, a particle physicist from the accelerator facility Fermilab has stepped in with his suspicion that the GEO600 “noise” may not be just annoying static, it might be the quantum structure of space-time itself… Continue reading “Is the Universe a Holographic Projection?”
Gravitational waves are a theoretical consequence of a propagating energy disturbance through space-time. They are predicted by Einstein’s general relativity equations, and astrophysicists are going to great pains to try to detect the faint signature from the passage of these waves through local space. Unfortunately, even though millions of dollars have been spent on international experiments, the gravitational wave remains in equation form; there is little (direct) evidence to support their existence.