The universe experienced a short but extraordinary growth spurt about 10 billion-trillionths seconds after the big bang. Inflation was a cataclysmic episode that saw the entire fabric of time and space jigger with gravitational waves. GWs detected six years ago to great fanfare were smaller-scale events caused by black holes colliding. Scientists at the European Space Agency (Esa), however, are now aiming for bigger targets. They hope to soon detect faint echoes from the universe's inflationary birth throes almost 14 years after the event using the biggest instrument ever made. The Esa's gravitational wave detector, which is hundreds of times larger than the Earth, will be floating in space to search for wobbles in spacetime due to all manner of enormous astrophysical convulsions.Laser Interferometer Gravitational-Wave Observatory, (Ligo) was the international project that identified the first GW in 2015. Its success earned the 2017 Nobel Prize in Physics for three of its key supporters. Two massive detectors make up Ligo, located in Louisiana and Washington. Each tunnel is 2.5 miles (4km) in length and intersects at an angle. A laser beam travels along the tunnels to a mirror at one end, bounces back, and then it goes down another. Where the arms intersect, the returning light waves interact with each other. Spacetime is slightly stretched or contracted as a GW passes. This effect is different for each arm. It alters the synchrony between the light waves and thus alters the interference.Ligo is not the only one. In collaboration with the European detector Virgo (based in Italy), a second GW was detected on Christmas Day 2015. Kagra, a detector located in Japan, started operating in the middle of last year. Other detectors are in India and China.The collision of two black hole seem to be responsible for most of the GWs we have seen. These GWs are made from stars that are much larger than our sun and have collapsed under their own gravity. Albert Einstein's theory of general relativity states that gravity is the warping spacetime caused by mass. The collapse can continue until there is nothing left but an almost infinitely dense singularity. This creates a gravitational field so strong that no light can escape it.This still is a computer simulation of the collision of two black hole, which was detected by the Laser Interferometer Gravitational-Wave Observatory for the first ever time. Photograph by SXSprojectTwo black holes might collide with each other's gravity and spiral inward until they become one. General relativity predicted that such events would cause GWs to rippling through the cosmos more than 100 years ago, but there was no direct evidence until the Ligo detection. They can also be caused by other extreme astrophysical phenomena, like the merging neutron stars. These are burnt-out black holes with smaller masses than the stars they collided with. Their collapse has been stopped at the point when their mass is so dense that a thimbleful could weigh as many as 50m elephants.You can also produce GWs by much larger objects. The centre of our galaxy and many other galaxies is a supermassive dark hole, which has a mass several million times that of our sun. It was formed from collapsing stars and clouds cosmic gas and dust. Objects that spiral into supermassive blackholes generate GWs with lower frequencies and longer wavelengths. These GWs are more powerful than the smaller black-hole merger wave seen by Virgo and Ligo.These are not visible by ground-based detectors. It would be like trying catch a whale in an oyster pot. An interferometric detector will require longer arms to see them. This is tricky because each arm must be a straight, long channel that is free of vibrations. Instead, researchers plan to create low-frequency GW sensors in space. The Laser Interferometer Space Antenna, also known as the Lisa, is currently being developed for Esa.Lisa will send laser beams to one spacecraft in order to bounce off a free floating mirror inside another craft. You can create an L-shaped structure with two arms like Ligo by using three spacecraft. The arms don't have to be straight. Lisa will place its three spacecraft at the corners of triangles, so each corner becomes one detector. The entire array will orbit around the Earth, following our planet for approximately 30 miles.In 2015, Esa launched the pilot project Lisa Pathfinder to test the feasibility and scale up laser interferometry from space. Esas Paul McNamara was the mission scientist. The mission was completed in 2017. It was perfect from the beginning, meeting all our requirements. The spacecraft employs tiny thrusters to resist the force of the sunlight's light.McNamara says that our spacecraft was much more stable than the coronavirus. This is a good thing, as Lisa will have to detect a change of arm length due to a GW of approximately a tenth of an atom over a mile.This will make GW astronomy more interesting than going beyond visible light made a big difference for ordinary astronomy Emanuele BeriHowever, Lisa's launch will not occur for at least a decade. McNamara says that we have three satellites to construct, each with many components. It takes time. That's one of the sad facts of a complex mission. Official mission adoption is expected in 2024. We will then know all the details about the mission, including who from the Esa member countries and what amount the US is contributing, according to Emanuele Berti, an astrophysicist at Johns Hopkins University in Baltimore.Japan and China are also planning space-based GW detections. McNamara doesn't see these as competition but rather as a positive thing. With more than one detector it will be possible for triangulation to determine where the waves are coming.Lisa will make GW astronomy more interesting in the same way that going beyond visible light (to radio waves, Xrays, etc.) was a game changer for ordinary astronomy, according to Berti. The project will examine different types of GW sources. We hope to learn a lot more about the formation and evolution of structure in the universe and gravity. If Lisa sees primordial GWs caused by inflation in the early part of the big bang it could help to test theories about how everything started.Another way to observe low-frequency GWs might not require any special detectors. The North American Nanohertz Observatory for Gravitational waves (NanoGrav), a collaboration, uses observations from a global network radio telescopes to examine the effects GWs have upon the timing of cosmic clocks known as pulsars.Pulsars are fast-spinning neutron stars which emit intense radio waves from their poles. These beams sweep across the sky in lighthouse beams. Pulsar signals can be predicted and are very predictable. According to Stephen Taylor, a NanoGrav member from Vanderbilt University, Tennessee, a GW can pass between the pulsar, and the Earth. This deforms spacetime and causes the pulse to arrive earlier or later than expected.The Green Bank Telescope (GBT), part of NanoGrav, is located at the National Radio Astronomy Observatory (Virginia), as part of the NanoGrav Project. Jon Arnold Images Ltd/AlamyThe detectors are in effect the pulsars. According to Julie Comerford, a NanoGrav team member from the University of Colorado at Boulder, the detector arms are as long as the distance between Earth and the Pulsars. This could be thousands of light-years. NanoGrav detects signals with very long wavelengths at very low frequencies. This is beyond the reach of Lisa. They are made by supermassive blackholes billions of times larger than the sun and merge when entire galaxies collide. Taylor says that no other detector can detect these signals. These mergers, though unimaginably catastrophic, are quite common and NanoGrav would be able to detect a type of hubbub caused by many of them. Comerford says that there are many pairs of supermassive, black holes in the universe. They orbit around each other, producing GWs. These ripples create a sea of GWs, which we are now bobbing in.A NanoGrav team, led by Joseph Simon, a Colorado postdoctoral researcher, reported a January discovery of GWs. Comerford says that although more research is needed to confirm the signal is caused by GWs but that this is the most exciting result in astrophysics he has seen over the past few years.NanoGrav is a GW detector that measures light years in length. Sougato Bose, a physicist at University College London, believes we can make one small enough for it to fit in a cupboard. His idea is based on one of the strangest effects in quantum theory. This generally describes extremely small objects such as atoms. Superpositions are a way for quantum objects to be placed, which means that their properties can be defined in more than one way.Although quantum scientists are able to routinely place atoms into a quantum superposition, such unusual behaviour is not possible for large objects like footballs. They are always there regardless of whether you look. It is not impossible to maintain a superposition for something so large, it is just difficult to detect.Sougato Bose is a University College London physicist who leads a group of researchers that plans to experimentally access quantum gravitation. Photograph by Sougato BoseBose and colleagues propose that if we can make a quantum superposition from an object that is intermediate in size between an atom or a football, a tiny crystal measuring about a hundred nanometres across and around the size of a large virus particles, the superposition would be so fragile that it could be sensitive to a passing GW. The two states that make up a quantum superposition could be interacted like two light waves, and distortions of spacetime due to a GW would appear as a change in the interference.Bose believes that diamond nanocrystals kept in a vacuum more empty than outerspace and cooled to within a whisker or absolute zero could be kept for long enough to perform the trick. Although it would be difficult, he believes that the technical difficulties have been shown individually and that it is just a matter of joining them all. He says that there is no barrier to doing this over the next 10 or so years, provided enough funding is available.What will we see if these and other developments lead to a boom in GW astronomy? McNamara states that you often see unexpected things when you look into the universe. We might also see more of the events that cause GWs than we know. McNamara says that's when the real fun begins.
