We Have Unlocked a New Layer of the Universe

On the 14th of September 2015, scientists at the Laser Interferometer Gravitational Wave Observatory detected gravitational waves directly for the first time. A stunning achievement that led to the 2017 Nobel Prize in physics. Why was this significant? Well, here's an analogy.

Let's imagine that human beings evolved without the ability to see light. For thousands of years, we'd fumble in the dark, relying on our other senses, until one day someone invented a machine that could perceive light for us. In time, we'd see everything from the tips of our noses to the farthest flung galaxies. This analogy captures the magnificence of LIGO. It's about much more than proving a scientific prediction; LIGO enables us to perceive the physical universe and understand reality on a new level.

Like photons, gravitational waves travel at the speed of light as they ripple across spacetime. Their signals are all around us. By listening for gravitational waves with some of the most sensitive instruments ever built, scientists are recording tremors of distant violent events: the formation of black holes, supernova explosions, and potentially exotic phenomena we haven't discovered yet.

So what are gravitational waves, what causes them, and why is LIGO's ability to detect them already transforming our understanding of the universe? I'm Alex Mccoan, and you're watching Astramm. Join me today as we learn about gravitational waves, unpack the groundbreaking technology behind LIGO, and anticipate some of the stunning developments that lie around the corner.

Gravitational waves are one of the stranger implications of Albert Einstein's general theory of relativity. As we've covered previously, spacetime is a model that combines the three dimensions of space and the fourth dimension of time into a single manifold. All objects with mass create curvature in spacetime, and objects with a lot of mass create a lot of curvature, which we experience as gravity. A simple way to visualize this is to think of a pool ball resting on an elastic surface and a bowling ball resting on that same surface. The more massive bowling ball will create more curvature. As objects move across spacetime, that curvature changes position with them. One of the amazing consequences is that when objects of a certain mass accelerate, they can send ripples across spacetime as gravitational energy. While this requires a special set of conditions—namely, a very massive object undergoing acceleration—such a cataclysmic event would send ripples, or gravitational waves, outward at the speed of light. Think of them like ripples on a pond, but instead of water, they travel through the fabric of spacetime in all directions. As in the pond analogy, these disturbances become weaker as they radiate outward. To an observer, the distance between objects would appear to expand and shrink as the gravitational wave passes. Mind-boggling to imagine.

Yet although Einstein predicted the existence of gravitational waves, he was pessimistic about our chances of ever detecting them. He thought that these disturbances would be so small as to escape our ability to measure them. And who could blame him? Many of the changes in distance that LIGO seeks to measure are 1/10,000th the length of a proton. Yes, you heard that correctly: 10,000 times smaller than a single proton. And yet these signals would come encoded with all kinds of information about their origins, when they originated, how far they traveled, and what kind of event produced them.

This is where LIGO comes in. It consists of two observatories funded by the United States National Science Foundation and operated by MIT and Caltech. Among its driving forces are renowned physicists Kip Thorne, Raina Weiss, and Barry Barish, all of whom shared the 2017 Nobel Prize for their decisive contributions to the detection of gravitational waves. LIGO is essentially a large-scale and very sensitive interferometer, an invention that's been around since the 1880s. An interferometer essentially measures what happens when light waves are combined from two or more sources. For example, you could use an interferometer to test whether light travels at different speeds through different substances, such as through air or water. Even a subtle difference in speed will produce an interference pattern when the light waves combine, much like what happens when two ripples on a pond intersect. If the peak of one ripple hits the valley of a second ripple, they will subtract from each other, producing a flat surface. However, if the peaks line up exactly, it means the waves are in phase and add to each other. This is essentially what the interferometer measures with light. By seeing how in or out of phase two light waves are, an observer can infer the relative speed of the waves. And the larger and more powerful the interferometer, the more sensitive it is.

Here's how it works. LIGO has two observatories located in Hanford, Washington, and Livingston, Louisiana. Why two? Well, you need at least two detection sites to triangulate where the signals are coming from. Each observatory continuously fires a powerful laser at a beam splitter positioned at a 45° angle. The laser beam has to operate at around 750 kW—powerful enough to vaporize you completely if you got in its path. The splitter then splits the laser beam perpendicularly. The light in each arm travels down a 4 km vacuum cavity with a mirror at the end of it. The beams then bounce between this mirror and a recycling mirror at the other end nearly 300 times, increasing the distance from 4 to 1,200 km. Remember what we said with interferometers: bigger is better. After completing nearly 300 trips, the laser beams combine at the beam splitter and head to a photodiode, which is a light-sensitive semiconductor. If undisturbed, the beams will be in phase, meaning their frequencies will subtract each other, and no light will arrive at the photodiode. But if there's a gravitational wave, the distance each beam travels will be slightly different, and they'll be out of phase. The photodiode will pick up a signal indicating the presence of a gravitational wave.

Now, this is how it works in a perfect world. But in reality, the interferometer is constantly picking up noise. To minimize this, LIGO uses incredibly smooth 40 kg mirrors suspended by silica threads. Any particles in the interferometer's arms are also a problem, which is why LIGO pumps the air from its vacuum chambers to 1 trillionth of atmospheric pressure. But there's another problem: at these minuscule levels, even quantum mechanics are a nuisance because they introduce randomness into photon behavior. LIGO mitigates this with an optical cavity which squeezes the light. This squeezing minimizes the light phase's noise and squeezes it into amplitude noise, which the interferometer doesn't measure. In other words, the quantum randomness will show up more in the height of the waves. Quantum randomness is a fact of life. It can't be eliminated, but it can be shifted, much as you might move clutter from your bedroom floor to your closet—the chaos isn't gone, just out of sight for the moment. Plus, the goal isn't to eliminate noise completely, but to get the best signal-to-noise ratio possible.

That's a pretty good overview of how LIGO works. So what has it discovered? As I mentioned earlier, LIGO detected its first signal in 2015, named GW150914. Scientists studied the data and learned that it was caused by the merger of two black holes about 1.66 billion light-years away. These black holes, which were 29 and 36 solar masses, became a binary and spiraled around each other until they merged and released a blast in the final 20 milliseconds that was so powerful—now get ready for this number, because this is what the scientists actually think—it contained 50 times the combined light power of every star in the observable universe. At the risk of sounding crude, that is nuts. I've read this fact many times over, and I still cannot comprehend what it means. Yet, after traveling for 1.66 billion years and finally reaching LIGO, the disturbance was so faint it moved LIGO's 4 km arm 1/1,000th of the width of a proton. To visualize this, imagine the distance between us and Proxima Centauri and changing it the width of a human hair. That is the level of precision LIGO was able to detect. If that's not one of the most astonishing feats in human history, I don't know what is. And this was just the first gravitational wave LIGO detected.

The second detection occurred 3 months later, in December 2015. That signal also came from a black hole merger, which took place 1.4 billion light-years away. Over its initial three runs, LIGO recorded more than 80 black hole mergers. And in August 2017, it detected the merger of two neutron stars, named GW170817. This signal was notable for being the first gravitational wave to be corroborated by electromagnetic observations from 70 observatories across the planet. This was a breakthrough not only in gravitational wave detection but in multi-messenger astronomy.

It turns out LIGO was just warming up during these three runs. As of May 2023, LIGO has begun its fourth run with better sensitivity than ever. After its latest round of upgrades, which kept LIGO offline for 3 years, the observatories now have more reflective mirrors, better mirror suspension, and improved light squeezing with lower quantum uncertainty. And this time LIGO also has the support of KAGRA, a new interferometer observatory in Hida, Japan. KAGRA is located underground, making it the world's first subterranean gravitational wave observatory and also the first to use cryogenic mirrors. During an engineering run on the 18th of May, LIGO scientists say they already received a signal that was possibly caused by a neutron star being swallowed by a black hole. We'll have to wait a while for confirmation, but if these early results are any indication, LIGO is about to blow the doors off our understanding of gravitational wave-generating phenomena.

So what other developments lie ahead? India is preparing a collaborative project called LIGO-India, or Indigo, which will help LIGO triangulate better location data. In 2027-2028, LIGO will implement its LIGO Voyager upgrade, which will achieve higher sensitivity with four times heavier mirrors and higher-frequency lasers. And in the more distant future, a third-generation facility has been proposed, called Cosmic Explorer. This facility would feature two new observatories with arms spanning 40 km and 20 km, respectively. Remember, with interferometers, bigger is better. But the proposal that really excites me is the Laser Interferometer Space Antenna, or LISA. This would be the first space-based gravitational wave observatory, which would utilize three spacecraft in a 2.5 million km long configuration. This interferometer would be so big and so precise, scientists hope it would be adept at uncovering exotic and theoretical sources of gravitational waves, such as cosmic strings and other speculative phenomena. In theory, it could help us stare directly into the fabric of reality. With a planned launch date of 2037, we're still over a decade away, but it's never too early to start counting the years.

So there you have it: an overview of LIGO and how scientists are using gravitational waves to better understand the universe. They give us evidence of extremely remote and ancient phenomena that cannot be measured by other means. And they can be a secondary way to measure observations made by other instruments, like the Webb telescope or Hubble. In time, this revolutionary field should allow us to understand the nature of our universe, its history, and even its future. I hope you found this episode as fascinating as I have. Tune into the universe with the right equipment, and you will hear it humming, no matter where you look in the sky. Of course, it's not possible for sound to travel in a vacuum of space. Instead, we are talking about the hum of gravitational waves.

You might already be familiar with a similar signal discovered decades ago. It's called the cosmic microwave background, and it is the afterglow of the Big Bang. This glow of microwave light was a groundbreaking discovery; it was the smoking gun that solidified the Big Bang's place as the leading theory of our universe's beginning. Yet this light signal had a glaring weakness: despite it being the earliest light of our universe, it was emitted almost 400,000 years after the Big Bang. If we want to go back earlier, we will need a background signal made not of light but of the ripples of the fabric of spacetime.

In 2023, astronomers at the NANOGrav Collaboration have announced that, by using a clever trick, they may have detected just that. Using exotic stars as a galactic-size gravitational wave detector, they measured the faint hum of spacetime itself. This hum of gravitational waves might just be the reverberations of the very creation of our universe.

I'm Alex Mccoan, and you're watching Astramm. Join me today as we learn about the secrets of gargantuan gravitational waves and the ingenious new method astronomers have used to detect them.

In a previous episode, we covered the LIGO detector and how it uses lasers to precisely measure the minuscule changes in the length of a tunnel caused by gravitational waves passing through. These gravitational waves stretch and squish spacetime, causing changes in the time it takes for light to travel from one end of the tunnel to the other. But detectors like LIGO are only sensitive to a specific kind of gravitational wave. Like light waves and sound waves, gravitational waves can come in different frequencies, and LIGO is only able to listen in to one part of this spectrum: the high frequencies. That's when these binaries are spiraling in on each other rapidly, completing hundreds of rotations per second before colliding in a great bang. But that is also why LIGO cannot listen in on the much earlier stages of these binaries when the objects are orbiting each other at a much greater distance. Because these orbits are much slower, the gravitational waves they produce are of a lower frequency and energy. So you could say, rather than a great bang, they give off a faint hum, combining into one collective signal. Bigger black holes give greater contributions to this signal. The loudest contributors would consist of pairs of supermassive black holes, the kind you'd find at the center of galaxies. These binaries, which consist of black holes billions of times the mass of our sun, are expected to be rare because they only form when something truly spectacular happens: the merger of two galaxies. When galaxies merge, because of the vast amounts of empty interstellar space in them, they mostly phase through each other. Very few, if any, actual collisions of stars or planets happen. But the stars gravitationally attract one another and merge into one collective galaxy. And the nuclei of these galaxies, the supermassive black holes, form binaries that continue orbiting one another long after the galaxy merger appears to be complete. It's these remnants of galaxy mergers that we would hear most strongly in the ultra-low-frequency range, specifically around the nanohertz range. Yet, just like how the human ear cannot hear sounds with ultra-low frequencies, LIGO is deaf to this cosmic cacophony. If we could listen in, we would gain new insight into galaxy mergers and black holes.

But that's not all. One of the most exciting things about the gravitational wave background is that it is a window to the very first moments of our universe's existence. Let's quickly recap what we know about the cosmic microwave background. This microwave light is the earliest light that we can detect because, for the first 400,000 years, the universe was opaque to light. What does it mean for the universe to be opaque? Initially, the universe was far too hot for neutral matter to exist. Electrons and protons were unable to bind together and form neutral atoms simply because they had too much energy. This state of matter is called plasma—the same stuff that stars are made of—and the universe was filled with it. Light gets trapped in plasma, unable to move through space without bumping into the charged electrons and protons, being continuously absorbed and emitted. But once the universe expanded and cooled down enough for the protons and electrons to coalesce into neutral hydrogen and helium atoms, it suddenly became transparent. The light was finally freed. This light was then stretched by the expansion of the universe until it redshifted into microwave light. This is why the cosmic microwave background is the earliest light of the universe. But gravitational waves are not made of light; they are made of spacetime itself. Plasma is no obstacle to them. To gravitational waves, the universe has been transparent from almost the very instant of the Big Bang itself. They can penetrate past this plasma barrier and offer us a glimpse of the very first moments of the universe's creation. Because these primordial gravitational waves have been traveling for so long, even if they were originally produced with a high frequency, they would be greatly stretched by the expansion of spacetime. They would be redshifted into ultra-low-frequency gravitational waves, the same kind that would come from black hole binaries. Together, these multiple sources will give us a so-called stochastic gravitational wave background existing over a range of low frequencies.

How can astronomers retrieve this treasure trove of information about black holes, the Big Bang, and more? Can we build a better LIGO and detect how the Earth is bobbing up and down in these gargantuan gravitational waves? Well, LIGO works by sending two beams of light being sent along perpendicular tunnel arms. The beams are reflected by mirrors at the ends of each of those tunnels, and they return to the starting point and are compared with each other. This is so that if spacetime gets distorted along one direction, the information from the other direction can be used to get a sense of how much it has changed. Unfortunately, if we were to design a new LIGO to detect these gravitational waves, we would need to have tunnel arms that are much larger than anything we could build on Earth. This is because the frequency of a wave is inversely proportional to its wavelength. Ultra-low-frequency waves have an ultra-long wavelength and would require us to look at changes over a much greater distance to notice the effects of these gravitational waves. For nanohertz waves—that is, one full fluctuation taking a billion seconds—a quick calculation (wavelength equals speed of wave over frequency) tells us that the wavelength would be around 10^17 m. For a sense of scale, the distance between the Earth and the sun, or an astronomical unit, is 10^11 m. A light-year is around 10^16 m. The wavelengths we are discussing are tens of light-years long. Clearly, we cannot build anything suitable for this task on Earth or even in the solar system. We need to go beyond our stellar neighborhood.

But rather than wait for humanity to ascend beyond the confines of our solar system and become a space-faring civilization, astronomers at NANOGrav have used a clever trick. You may recall our recent episode on pulsars and how we discussed that these ultra-dense neutron stars are able to spin with remarkable stability. They emit light along their poles, and we can detect the pulses that we receive as those vast beams of light swoop around and hit the Earth again and again. These pulses come with such precise certainty that they are used as clocks of the cosmos, able to keep time across vast distances. Therefore, we can use already identified pulsars to supply us with reliable signals from thousands of light-years away. The pulsars are like the mirrors at the end of the LIGO arms, but rather than sending us back a signal we send across, they are generating their own. NANOGrav have cleverly concocted an imaginary interferometer that stretches for thousands of light-years. And all we need to use it are the good old radio telescopes that we've been using for decades. But our calculations will only be as good as our observations. When choosing pulsars to observe, we need to make sure that they are suitable for the task. The best candidates are handpicked and added to a group called a pulsar timing array. For NANOGrav, they have made use of 68 pulsars, which were chosen for the following special properties: they had to be spread all across the sky, so we can figure out how these massive gravitational waves are affecting spacetime in all directions. Also, they had to be millisecond pulsars, which means they complete millions of rotations per second. In our recent pulsar video, we talked about how these fast-spinning pulsars are expected to be the most stable and dependable when it comes to the regularity of their pulses. In fact, their stability even rivals that of some atomic clocks. We could not ask for better timekeepers to be dotted across space and time. This regularity means that we can take measurements of the time their pulses arrive and continue taking these measurements over multiple years to see if there are any changes happening. Of course, the times of arrival can vary due to all sorts of factors, such as the change in distance between the Earth and the pulsar as both objects continue moving around space. But we can take this into account quite easily. However, there are less predictable factors, such as random fluctuations in the interstellar gas the light is traveling through, causing delays that aren't coming from the stretching of spacetime.

How do we get rid of the random noise in our data from pulsars located thousands of light-years away? With statistics! We can isolate the gravitational wave background from the random noise by considering correlations between pairs of pulsars in the pulsar timing array. Just like LIGO, one tunnel isn't sufficient. We need two pulsars on each end of our imaginary tunnels. If similar fluctuations show up with two pulsars, it is unlikely that the random noise aligned in such a way to make it appear by chance that they are acting in unison. It is far more likely that there is a common cause underlying both those fluctuations, such as the possibility that the photons from the two pulsars are riding the same gravitational wave that are washing by them both. We can then relate these correlations of pulsar pairs with the angular distance in the sky. If there is no gravitational wave background, we expect there to be no correlation and the graph to show a straight line at zero. However, if the data shows a special curve called a Hellings and Downs curve, that would indicate that a gravitational wave background is responsible for the variances. That's what NANOGrav have succeeded in doing. On the 28th of June 2023, based on over 15 years of continuous observations of their 68 pulsars, they announced strong evidence for the gravitational wave background. This is the graph we've been waiting for. This is a hallmark of gravitational waves, which stretch along one direction and squish in the perpendicular direction. This is exactly what we are seeing with this graph. When pulsars are separated at right angles, their photons are experiencing an opposing effect as they travel to Earth. The error bars in the data appear quite large, but the researchers have calculated that the data shows that the existence of a gravitational wave background is statistically significant at the three-sigma level. This means that there's only a 1 in 1,000 chance that this is a false alarm and that this data was the result of mere chance. In physics, the five-sigma level is considered the gold standard for a discovery, but three-sigma is still very strong evidence. As they collect and analyze more data, astronomers will be able to reduce those error bars and finally announce an official discovery.

So what secrets does this gravitational wave background reveal? While we cannot pick apart the sources of the collective noise just yet, we hope that these techniques will eventually allow us to listen in to specific supermassive black hole binaries and figure out what the signal can tell us about the creation of the universe. Until then, astronomers will continue to use their galaxy-size detector to calculate how we are bobbing up and down in the turbulent sea of spacetime. Have you heard about NANOGrav before? What secrets do you think gravitational waves could reveal? Let us know in the comments below.

Every year since 2018, astronomers have spotted a mysterious blue flash in the sky. It is one of the brightest phenomena in the universe—an explosion that makes the average supernova look like a faintly lit candle in the distance. Some have been spotted closer to us, and others billions of light-years away. They look similar enough to one another, but no one knows what they are or what causes them. Through rigorous observation and analysis, astronomers began decoding a pattern and formulating a tentative but plausible theory, until in 2023 they saw something that left them completely baffled. These explosions are called luminous fast blue optical transients, and there's simply nothing else like them out there.

I'm Alex Mccoan, and you're watching Astramm. Join me today as we dive into the mystery of space's brightest explosions, how they were discovered, and why they keep stumping scientists again and again.

Being such a recently discovered event, the data we have on luminous fast blue optical transients, or LFBots for short, is minimal. The first LFBOT ever detected was identified just 6 years ago, in 2018, and we've only witnessed a handful of them since. As so little is known about them, it makes agreeing on a universal definition a little tricky. For now, what scientists do all agree on are some common characteristics these LFBots seem to share. So far, they all display a predominantly blue emission, very high optical luminosity, and being bright in X-rays, ultraviolets, and radio waves. They are also very fast, as their name indicates: LFBots go off like a cosmic camera flash. They reach peak brightness and then dim very rapidly, usually in the space of hours or days. As you might know, a supernova follows the same pattern of brightening and then dimming, but this dimming takes weeks or months. This short-lived nature of LFBots makes them difficult to spot and study. So how did we manage to capture such an elusive event? NASA's ATLAS-HKO telescope in Hawaii is part of…

An early asteroid impact warning system that scans the entire sky several times a night for moving objects. On the 16th of June 2018, Atlas HKO was performing its routine scan when it captured something very unusual: a flash 100 times brighter than a regular supernova that disappeared within days. The scramble was on. Scientists immediately started analyzing the data to understand what they'd seen. They pinpointed the explosion as coming from the Hercules constellation, some 180 million lightyears away. Officially designated as 2018 cow, the event was affectionately nicknamed the "cow" after the last three letters in its name. Once located, it was quickly classified as a type 1b supernova. This kind of supernova, also known as a core collapse explosion, is formed when massive stars collapse under their own gravity. But something about this particular explosion didn't quite fit.

When taking a closer look at the cow's emission spectrum, it didn't look very typical for a type 1B supernova. It had unusually broad emission lines and very weak helium lines. Scientists thought it appeared more reminiscent of a type 1C BL supernova and so was quickly reclassified. However, the more scientists poured over the data, the more surprised they were by what they found. Whatever this explosion was, it started to look less and less like a supernova, at least not the kind of supernova we'd expect. For starters, it appeared out of nowhere. This blast went from inactive to peak luminosity in just a few days. Like we mentioned earlier, supernovae usually take a few months to reach their brightest and dim again at a similar pace. Not only that, it was registered to be 10 to 100 times brighter than an ordinary supernova. Also, everything about the way it exploded was wrong.

When supernovae explode, they tend to release their energy in a spherical shape. We know now that the shock wave sent out after a supernova can be aspherical due to the presence of strong magnetic fields, which can distort the shape of the initial blast. However, upon closer inspection, this mystery explosion did not even explode spherically. The researchers themselves called it the most aspherical explosion ever seen. Soon enough, other theories were being put forward. Some thought the cow could be a monster black hole shedding a passing star. Others suggested it was a supernova that gave birth to a black hole or a neutron star. But let's step back for a moment. It's really hard to draw any conclusions when you only have one of something. As the saying goes, once is an anomaly, twice is a coincidence, and three times is a pattern. What researchers really needed was a bigger sample size, more instances of this kind of explosion so they could compare observations and deduce any patterns that might arise.

Luckily, it didn't take long for scientists to get their eyes on a second similar explosion. A few months later, in September 2018, the cosmic camera flashed again, and again for a third time in 2020. Keeping in the tradition of pulling animal nicknames from the last three letters of their official names, we had the beginnings of an LFBot zodiac. The cow was joined by the koala and the camel. And just like that, they had data on three separate LFBot events. They could start hunting for patterns. When analyzing the cow, koala, and camel, one of the first things that stood out to researchers was the location of these events. Even though they all happened in different parts of the universe, each blast was registered as coming from inside the spiral arm of a galaxy. At first, this information emboldened the initial theory that LFBots were just a type of core collapse supernova. Let me explain why. The kind of star that causes a core collapse supernova is a massive star, one of the biggest types of stars you can get. And as you know, the bigger the star, the shorter its lifespan. This means that these stars don't get the chance to travel very far before they die. It would make sense then that their supernovae occur very close to the star cluster where they were born. And what part of the galaxy is known for having such clusters? Yep, the spiral arms—the exact place we saw all three LFbots.

But just as researchers seemed to be making progress, something happened that caught them completely off guard again. Like the Atlas HKO in Hawaii, the Tuiki transient facility is a very wide-angle ground-based camera that scans the whole northern sky every 2 days. In 2022, it detected another similar explosion, except after the initial blinding flash, this optical transient started behaving rather strangely. Instead of exploding once and fading away in a few days like the other LFbots before it, the so-called Tasmanian Devil continued to produce short-duration bursts far longer than expected. What's more, each of these bursts seemed to be just as bright as the original explosion, which was very strange indeed. The Tasmanian Devil emitted more energy than hundreds of billions of stars like our sun combined. Researchers were deeply puzzled yet again. It was unlike anything they'd ever seen before in astronomy. Now I know the skeptics among you may be thinking, "Alex, this has got to be due to some kind of technical error, miscalibrated equipment, a fluke in quality control tests, or a simple mathematical mistake." Indeed, the history of science is littered with scientists shouting "Eureka," supposed breakthroughs or discoveries only to be deflated once someone checked their workings. But this is not one of those moments. The data was corroborated by 14 other telescopes around the world. They confirmed that the Tasmanian Devil did, in fact, pulsate a minimum of 14 times. The total number was likely much higher. Equally surprising, these mega-powerful pulses were only minutes apart. It looked to scientists like a star that kept dying and being revived again and again. If that's what it is, the strange phenomenon could provide brand new insight into the life of stars. So far, stellar life cycles have only been studied as snapshots of different stages, never as a continuous process.

In short, even though the multiple bursts of light from the Tasmanian Devil were unexpected, there was still reason to suspect these explosions could have been a strange type of supernova, seeing as none of the observations directly violated the core collapse supernova theory. But what came next did. On April 10th, 2023, astronomers picked up yet another big blue explosion with all the same characteristics as their LFbot zodiac relatives, except this one was not even close to where it was supposed to be. Unlike all the other LFbots before it, the Finch was not in the spiral arm of a distant galaxy. Three billion lightyears away from us and 15,000 lightyears from the closest galaxy, it stood solitary in space. It was in the middle of nowhere and exploded between two galaxies. This was a huge blow to the leading theory. If these explosions really were the result of massive stars core collapsing, there's no way it would be happening in between galaxies. So it was back to the drawing board for researchers. They came up with two new theories to explain what could have caused the finch to explode where it did. The first suggests that the finch could be the result of stars being torn apart by an intermediate-mass black hole, a black hole that has 100 to 10,000 times more mass than our sun. Except the existence of these black holes has never been proven. Their smaller and larger relatives, stellar remnants and supermassive black holes, have both been confirmed to actually exist, but this middle child remains purely theoretical. However, should they exist, could they be responsible for this elusive light show? Astronomers have shown in simulations that stars can orbit intermediate-mass black holes as many as five times before being ejected. With each orbit around the black hole, the star is effectively being ripped apart by losing more and more mass. Finally, the remaining stellar matter would be flung back out into the galaxy at speeds as high as 10% the speed of light. This would be consistent with the speed and brightness observed in LFbots, lending weight to this theory. One of the most likely places researchers would expect to find intermediate-mass black holes is in globular star clusters in a galaxy's outer halo. Galaxies have halos that extend far beyond the main disc and bulge. They are most visible in spiral galaxies like our Milky Way. In our own home galaxy, the outer halo stretches some impressive 1 million lightyears from its galactic center. Some scientists think that it is possible that the finch could have been located inside a globular star cluster such as this. If this is the case, and intermediate-mass black holes do exist there, it could be plausible that the finch was caused by these unusually sized black holes ripping up large stars.

The second theory researchers put forward suggested that the finch could have been the result of two neutron stars moving towards each other in increasingly tight spirals until they collided. This kind of event causes a kilonova, which is known to be one of the biggest stellar blasts in the universe. One telltale sign of a kilonova is the presence of gravitational waves, which come through as a hallmark chirp caused by a rapid increase in frequency as two massive objects spin around each other, eventually colliding and merging. If we could find something like this when analyzing data from the finch, it would lend strong credence to the neutron stars theory. However, as bad luck would have it, our chirp detector, the Laser Interferometer Gravitational-Wave Observatory or LIGO, was down for maintenance at the time the finch occurred. That means we don't know if such gravitational waves were emitted as we have no data on them. One thing we do know is that no gamma-ray burst was detected, which is something you might expect to see with a kilonova of this size. But remember, this explosion happened 3 billion lightyears away. That's really far, perhaps too far for whatever gamma-ray data there might have been to be detected. All in all, none of this data can conclusively confirm or reject the neutron star theory. It's frustrating to bear witness to such an incredibly rare, new, and powerful phenomenon and not have the data to conclusively understand what is causing it. I guess patience is an underrated virtue for an astronomer to have. But they are not sitting on their hands either. Scientists are already planning to use the optics of the James Webb Space Telescope to carry out a search for any faint globular clusters in the same location as the Finch. This would hopefully clarify if they are on the right path with the intermediate-mass black hole theory.

In the meantime, other researchers are focused on broadening the sample size. The more LFbots we can detect, the more we will learn about them. The six we have seen so far have taught us some things, but we have a lot left to learn. The only way we're going to get a larger sample is to keep sweeping the sky with wide-field surveys like the Atlas HKO and this Vicki transient facility. Scientists' hopes are pinned on the Vera C. Rubin Observatory, currently under construction in Chile, a telescope which will scan the entire southern sky every few nights, which I've done a video about here. It is expected to be operational by 2025 and will be able to capture 10 times more light than all previous facilities. Follow-up observations with Hubble and ground-based telescopes will help analyze more information, which could lead to more breakthroughs. LFbots are a super new phenomenon that we still don't understand well. Scientists themselves admit that the discovery of the finch raises more questions than it answers. The more we learn about these bright blue explosions, the more they keep surprising us. All there is left to do is what hundreds of generations of astronomers have done for centuries before us: point our telescopes up to the sky, watch, and wait.

Back in 1916, Albert Einstein predicted that black holes emanate gravitational waves. Imagine throwing a rock into a pond. The water ripples. You can think of gravitational waves like those ripples, except instead of acting on water, they act on the fabric of spacetime, ever so slightly stretching and contracting it as it passes. These disturbances were predicted to be so small that even Einstein himself didn't think humans would ever be able to detect them. Fast forward to today. In just over a century, we've managed to build a global network of observatories that have picked up on gravitational waves from about 100 cosmic events. For example, there's the Laser Interferometer Gravitational-Wave Observatory or LIGO in the US, the Virgo in Europe, and the Kagra in Japan, which all work together to cooperate data and pinpoint the waves' direction and source location. If you aren't familiar with their setup, the way these instruments work will surprise you because they are nothing like traditional telescopes. They don't use lenses. Instead, they rely on something much more fun: lasers. Scientists send lasers down two perpendicular tunnels several kilometers long. When gravitational waves from space arrive at Earth, they cause tiny disturbances in the fabric of spacetime, which is picked up by these lasers. And I mean tiny—something like 10^-8 m or 1/10,000th of a proton. As the laser's paths are stretched or contracted by the gravitational waves, they emit different frequencies, kind of like the strings of a guitar. A higher frequency is a higher pitch. Converting this data into sound is how we get the signature chirp of a black hole collision. Each collision has its distinct personality. Long, low buildups like this one indicate a slower collision between pretty lightweight black holes. Faster, heavier black holes sound more like this. You just heard the sound of two black holes combining to form an even bigger black hole 5 billion years ago. It's the most massive and distant gravitational wave source ever observed, emitting about the energy equivalent of five solar masses in gravitational radiation. Pretty epic, if you ask [Music] me. Thanks for watching. I really want to give a huge thank you to our astronauts on Patreon. It's really becoming a thriving community, and I've loved reading all your messages and comments over there. If you'd like to join in, then you can visit the link in the description to become an astronaut and bring the channel more stability than the algorithm. When you join, you'll be able to watch the whole video ad-free, see your name in the credits, and submit questions to our team. Meanwhile, click the link to this playlist for more Astramm content. I'll see you next time.