Press Release:
The fields of astronomy and astrophysics are about to undergo a revolution and the University of Alabama in Huntsville is helping blaze the trail.
Albert Einstein’s last great prediction, the existence of gravitational waves, is finally on the cusp of being confirmed. Beyond just validating a 100-year-old idea, the direct measurement of gravitational waves will forever change our understanding of the universe and open up new unfathomable opportunities to learn more of its secrets.
Research done at UAH is paramount to the quest for making gravitational wave observations a routine tool in astronomy and astrophysics, and the university will be able to proudly claim an ownership stake in the pending discoveries.
Throughout human history we have looked to the stars. Either through boredom or curiosity, we began to recognize patterns in the night sky and attributed meaning to the constant, as well as changing, cosmos. The field of astronomy was born when Galileo trained his telescope to the heavens, confirming that the Earth was not at the center of the universe. This was a very unpopular idea in Galileo’s time and he paid dearly for it. However, passing on knowledge is a close facsimile for immortality, and his ideas have persisted in glory.
For hundreds of years after Galileo lived out his days under house arrest, the arc of astronomy followed human kind’s ability to construct bigger and better telescopes. By utilizing better astronomical tools, detailed observations of planetary motion ultimately contributed to Isaac Newton’s groundbreaking description of mechanics — why and how bodies move around — arguably the dawn of “physics” as a discipline. Observations made by Edwin Hubble showed that our galaxy was but one island in a sea of others and, paradoxically, distant galaxies all appear to be receding into the distance. Playing the movie backwards we are led to the conclusion that the universe — once thought to be infinite and static — started out as an impossibly small, impossibly dense bundle of energy before bursting into existence at the Big Bang.
Little did we know these great strides were merely setting the stage for our true intellectual voyage through the cosmos. With the dawn of the Space Age — the heritage of which is still plainly visible here in Huntsville — we were able to place telescopes beyond the confines of the Earth and receive signals otherwise blocked by our nurturing atmosphere. A bizarre, expansive, violent universe was revealed to us, redefining our perceived place in the universe and spurring human insight and ingenuity to continually push the limits of our abilities to peer deep into the cosmos.
As of today, it is fair to say that nearly everything we know about the universe — how big it is, how old it is, how it will end, how stars are born, how stars die, how planets form, which planets are habitable — comes to us from light. We have learned a lot from light, and it has not finished revealing its secrets. However, the story we are able to piece together is incomplete. It is as if we have been watching news broadcasts for hundreds of years on mute. We can make educated guesses about what the story has to tell, and often the images are enough. However, to truly understand the details of the story, we need to hear the reporter. Before the end of this decade, we will — for the first time — finally turn on the “sound” of the universe and with it gain deeper understanding of how the cosmos works. To do so we must add a new sense to our toolbox for learning about the universe. Enter Albert Einstein.
Einstein’s greatest contribution to science was the General Theory of Relativity, or “GR” for short. Completed in 1915 (100 years prior to this writing), GR replaced hundreds of years of Newtonian physics with a revolutionary idea: Space and time are not absolute, but form a sort of fabric in the cosmos, space-time, that is, in a sense, malleable. The “force” of gravity is not a force at all, but a consequence of objects moving through warped space-time. These ideas are so far from our conscious day-to-day experience that they can be off-putting, however they have become integral to our daily lives. Without accounting for the fact that the rate at which time passes depends on one’s location in the universe (time passes faster where there is less gravity), the Global Positioning System (GPS) would fail to be of any use within a matter of minutes.
Like all theories of science that have stood the test of time, at the time of its genesis GR was able to explain provided insight into unexplained truths (in GR’s case, the orbital motion of the planet Mercury) as well as make predictions for phenomena not yet observed. The much-celebrated observation by Sir Arthur Eddington that massive objects (like the Sun) bend the trajectory light takes through space-time was the first confirmation of GR, and cemented Einstein’s place among the pantheon of humanity’s greatest minds. All other great predictions from General Relativity — time passing slower near massive objects, the existence of black holes, the twisting of space-time near massive spinning objects — have been experimentally or observational confirmed save one: Gravitational waves.
In 1916, one year after completing the equations of GR, Einstein realized that the theory of gravity predicted waves, which would propagate through space when massive objects undergo accelerations. These waves were predicted to travel at the speed of light with phenomenally weak amplitudes. So weak, in fact, that Einstein immediately dismissed them as being a mere mathematical curiosity, and having no practical importance to physics. Today we realize that even Albert Einstein’s celebrated imagination was prematurely limited.
Strong, dynamic, gravitational fields emit gravitational waves. In other words, we need to get a lot of matter moving around at close to the speed of light. We cannot do this in the lab and in Einstein’s day there was no evidence that any natural process could be violent enough. Advances in astronomy and astrophysics over the intervening century have revealed that the universe is a much more violent place than we had previously thought.
Stars end their lives as dense stellar remnants also referred to as compact objects. Stars like our Sun will end up burning out as white dwarfs — a dense, glowing, object near the mass of the Sun made mostly of carbon and compressed to the size of the Earth. Stars more than about 10 times as massive as the Sun collapse into neutron stars — in effect a giant atomic nucleus with the mass of the Sun packed so densely that it would fit comfortably inside Huntsville city limits. Should the mass of the stellar remnant exceed about three times the mass of the Sun, the force of gravity is too strong to support a neutron star and the object collapses into perhaps the most exotic denizen of our universe, a black hole. Black holes, neutron stars, and white dwarf stars are the universe’s most accomplished way of packing a lot of material into a tiny volume, which gets us half way to a good source of gravitational waves. The remaining step is to get these objects moving near the speed of light.
Fortunately for gravitational wave astronomers, the universe has one more trick up its sleeve. Most of the stars you see in the night sky are actually pairs of stars orbiting so closely that you cannot tell them apart. These binary stars are typically similar in age and mass, and will result at the end of their lives as binary white dwarfs, binary neutron stars, or binary black holes. As the two objects orbit one another they emit gravitational waves that relentlessly, over the course of billions of years, drives the stars closer together as they orbit one another. As the separation between objects shrinks, they move around one another faster, emitting stronger gravitational waves. This runaway process leads to a cataclysmic merger of the two objects. The mergers of compact objects are the most energetic events in the universe. If we could see gravitational waves, the mergers of two black holes would outshine the Moon from a billion light years away. Alas, this incredible output of energy is dumped into gravitational waves instead of light, and we have yet to directly observe this phenomenon. However, merging neutron stars are hypothesized to produce brilliant flashes of gamma ray light seen as gamma ray bursts. Huntsville has long been an epicenter of gamma ray burst observations, currently through NASA’s Fermi Gamma-ray Space Telescope. Now, with the advent of gravitational wave astronomy, we aim to combine the gravitational and gamma ray observations to deeply understand the violent mergers of compact stellar remnants.
Detecting gravitational waves requires highly specialized, and enormous, observatories. Gravitational waves are a disturbance traveling through space-time. As gravitational waves pass through a region of space they will alter the distance between two points. By carefully monitoring distances we can determine when gravitational waves are passing by. Accurately measuring distances between objects sounds easy enough, but the change imparted by a passing gravitational wave is mind-bogglingly small. If we wanted to use the distance to nearby stars as our ruler for gravitational wave detection, we would have to monitor changes in those distances to accuracies of less than an inch. Not only is the change we are looking for at the limits of our measurement capabilities, but there is a monumental technological challenge in protecting our measuring device from any other disturbances which might cause similarly small changes.
Currently operating or under development are three complementary approaches to gravitational wave detection. Each is specialized to detect signals in a particular wavelength regime, much like we have specialized telescopes to observe different kinds of light.
The Laser Interferometer Gravitational wave Observatory (LIGO) consists of two identical facilities in the U.S., one in Hanford, WA the other in Livingston, LA. Viewed from above, each observatory is an “L” shaped building four kilometers long on each side (or “arm”). At the corner of the “L” is a state-of-the-art laser that sends light down each arm towards meticulously polished mirrors hung from a suspension system designed to shield them from external disturbances. If no gravitational waves are passing by, laser light sent down each arm will return to the corner simultaneously. By looking for slight changes in the arrival time of light from each arm, we can measure changes in the length of one arm relative to the other. The precision of the measurement is the stuff of science fiction. A passing gravitational wave will alter the length of one arm of the detector by less than the width of a proton!
LIGO is searching for high frequency (short wavelength) gravitational waves from the mergers of neutron stars and black holes with masses less than around 100 times that of the Sun. LIGO began operating in the early 2000s but, at the time of this writing, has just completed its first observations after a major upgrade to improve the instrument sensitivity. The analysis of the first data collected is well underway and enthusiasm for this new field is at an all time high. With LIGO’s new capabilities the first gravitational wave detection could come any day.
The detection of ultra-low frequency gravitational waves does not require specially made detectors on Earth. Instead we look to the stars once more. Pulsars are a special class of neutron stars, which emit a pulse of radio waves at precise intervals. The regular pulses are believed due to a small spot on the neutron star emitting a narrow beam of radio waves. As the neutron star spins the beam passes over the Earth, similar to how a lighthouse is visible in regular flashes.
The fastest spinning pulsars complete a thousand revolutions per second — a rate similar to the blades in a kitchen blender and do some with incredible stability. The radio signals from some pulsars are more reliable than atomic clocks. The North American Nanohertz Observatory for Gravitational waves (NANOGrav) monitors the arrival times of the radio signals using the world’s most capable radio telescopes. Passing gravitational waves will stretch and shrink the distance between the pulsar and the Earth, causing the signals to arrive behind or ahead of schedule. The most likely source of gravitational waves detectable by NANOGrav comes from supermassive black hole binaries at the centers of galaxies. Most, if not all, galaxies house gigantic black holes — millions of times the mass of the sun — at their center. Throughout a galaxy’s life it will merge with several neighboring galaxies and the black holes will “sink” to the center of the merged galaxy and begin orbiting one another, emitting gravitational waves all the while.
Detecting gravitational waves by timing pulsars is a game for the patient. The sensitivity of our measurement increases the longer we monitor individual pulsars. While NANOGrav has not yet made any detection, the absence of any gravitational wave signals is helping astrophysicists rule out certain hypothesis about the conditions at the centers of galaxies and the rate at which they merge throughout cosmic history.
While the field of gravitational wave astronomy will be born on the ground through experiments like LIGO and NANOGrav, it will reach maturity when we are able to build detectors in space. Detectors in space can built to hit the “sweet spot” in the gravitational wave spectrum — where the gravitational signals that go through one cycle every 1000 seconds or so. This frequency regime is rich with sources from our own galaxy (white dwarf binaries) and throughout the visible universe, most importantly the final moments of the supermassive black hole binary mergers.
Placing a gravitational wave detector in space is an unprecedented challenge and will require a large international effort to pull off. One mission concept, the Laser Interferometer Space Antenna (LISA), has been thoroughly studied and a variant is scheduled to launch in the 10 to 15 years. In December 2015, the European Space Agency and NASA launched the LISA Pathfinder mission to test many of the critical hardware components needed to fly a mission like LISA for gravitational wave observations.
The University of Alabama in Huntsville is on the front lines of gravitational wave detection. We are members of the LIGO Scientific Collaboration and NANOGrav, and are working closely with the LISA Pathfinder team to use the lessons learned during this experimental mission as a guiding force as we develop a space-based gravitational wave observatory. At the time of this writing, UAH gravitational astrophysics researchers are pouring through the new LIGO data and my find the first the first gravitational wave signal. Meanwhile our colleagues on the Fermi Gamma-ray Space Telescope are sifting through their data looking for a gamma ray signature from any gravitational wave events we may have observed.
Regardless of the outcome from the first observing runs for LIGO, gravitational waves will be detected this decade both from the detectors built on Earth and from the careful monitoring of pulsars in our galaxy. A new field of astronomy will be born and UAH will participate in the discovery one way or another.
One thing is for certain: This decade will go down in the record books as the dawn of a new field of astronomy. Once gravitational wave observations become a routine tool for astronomers and astrophysicists to learn about the universe science will never turn back.
Dr. Tyson B. Littenberg is a research scientist at the Center for Space Plasma and Aeronomic Research at The University of Alabama in Huntsville. He earned a Ph.D. in physics at Montana State University and joined UAH after research positions at NASA/Goddard Space Flight Center and Northwestern University. He is a senior member of the LIGO Scientific Collaboration and an associate member of the North American Nanohertz Observatory for Gravitational Wave. Dr. Littenberg’s research interests include gravitational physics, high-energy astrophysics and Bayesian inference.