Introduction

NANOGrav stands for North American Nanohertz Observatory for Gravitational Waves. As the name implies, NANOGrav members are drawn from across the United States and Canada and our goal is to study the Universe using gravitational waves. Gravitational waves are ripples in the fabric of space and time that cause objects to shrink and stretch by very, very small amounts. NANOGrav uses the Galaxy itself to detect gravitational waves with the help of objects called pulsars — exotic, dead stars that send out pulses of radio waves with extraordinary regularity. This is known as a Pulsar Timing Array, or PTA. NANOGrav scientists make use of some of the world's best telescopes and most advanced technology, drawing on physics, computer science, signal processing, and electrical engineering. Our short term goal is to detect gravitational waves within the next decade. But detection is only the first step towards studying our Universe in a completely new and revolutionary way, and we are sure to make unexpected discoveries in the process.

NANOGrav cooperates with similar experiments in Australia (the Parkes Pulsar Timing Array) and Europe (the European Pulsar Timing Array). Together, we make up the International Pulsar Timing Array, or IPTA. By sharing our resources and knowledge, we hope to usher in the era of gravitational wave astronomy more quickly and with greater impact.

NANOGrav was founded in October 2007 and has since grown to over 100 members at over 40 institutions. The NANOGrav Physics Frontiers Center is supported through a $14.5M award which started in 2015.

Understanding Gravity

When most people think of gravity, they think of a force that keeps keeps things together: it keeps people on the surface of the Earth, it keeps the Earth in orbit around the Sun, and it even keeps entire galaxies together. This way of thinking about gravity — as a long range force of attraction — was firmly established in the 17th century by Isaac Newton. Newton's law of gravity is a spectacular example of how some simple mathematical rules can accurately explain what we observe in nature, but it isn't the end of the story. By the end of the 19th century, people had found several situations in which the classical physical laws, such as Newton's law of gravity, didn't quite work. Newton's theory isn't totally wrong, but it is incomplete. Few people realized just how profoundly a more complete law of gravity would change our view of the Universe, but that is exactly what happened after Albert Einstein weighed in.

In 1916, Einstein published his general theory of relativity, a completely new way of thinking about gravity. In general relativity, or GR, we think of gravity as a distortion, or curvature, of the fabric of space and time itself (called space-time). In this context, space means the distance between two objects, or the shortest path you could take to get between point A and point B. This is not the same thing as "outer space" — every thing in the Universe exists in space-time, including the Earth and everything on (and in) it. In the concept of space-time, time refers to that which is measured by clocks.

What does this all mean? You can think of space-time as a sheet of fabric that is pulled tight (it isn't a perfect analogy, but it is the simplest one). Now imagine placing a heavy object like a bowling ball on this sheet. The sheet curves around the bowling ball, and according to GR, this curvature is gravity. A smaller object like a golf ball placed on the sheet will naturally fall towards the larger object, and if you give the golf ball a little push, it will circle the bowling ball just like a planet orbiting a star (friction eventually causes the golf ball to hit the bowling ball, but in space this doesn't happen). Anything and everything with mass, from stars to actual golf balls, will cause space-time to curve, and hence create a gravitational field. This may all sound pretty wild, but GR is the most elegant and complete description of gravity ever, and it has passed every test that scientists have ever put it through. In other words, it works, and it works well.

Understanding Gravitational Waves

One of the predictions of GR is the existence of gravitational waves. Gravitational waves are ripples in space-time that are caused when massive objects move in a certain way. These ripples actually cause objects to shrink and stretch as the wave passes through them, but the effect is tiny — even a very strong gravitational wave will cause an object to shrink and stretch by one part in a quadrillion! (1 quadrillion = 1,000,000,000,000,000)

This animation demonstrates the effect that a passing gravitational wave has on a circular ring of particles. The particles are stretched and pulled in different directions as the wave passes through them. Gravitational waves have two polarizations—"plus" (top) and "cross" (bottom).

We have very good circumstantial evidence that gravitational waves exist as predicted by GR. Observations of neutron stars (massive, dead stars) that are in binary star systems with another neutron star or white dwarf star show that the stars are slowly getting closer. This happens because these binary systems emit gravitational waves that carry away orbital energy. However, we have never seen the actual shrinking and stretching of space-time caused by a passing gravitational wave because the effect is so tiny and difficult to measure. The goal of NANOGrav is to make just such a detection and to use gravitational waves as a tool for studying the Universe.

Not all gravitational waves are identical. Like light waves, we can characterize a gravitational wave by its frequency (the number of waves that pass by us in one second). NANOGrav is sensitive to very, very low frequency gravitational waves, hence the term "nanohertz" in our name. Even though gravitational waves travel at the speed of light, the waves NANOGrav can detect take a billion seconds to go from one peak to the next (in other words, they have very long wavelengths). NANOGrav and other similar experiments are the only way of studying these types of gravitational waves.

A New Frontier

For the whole of human history, almost everything we have learned about the distant Universe (everything outside our Solar System) has come from studying light. In this sense, light includes all parts of the electromagnetic spectrum: radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma-ray. Some very important contributions have also been made by studying sub-atomic particles, but for the most part light is the main messenger that carries information about the Universe to us. Of course, not everything emits light, or at least light that we can easily detect. Some examples are black holes, white dwarfs, and neutron stars, as well as hypothetical objects called cosmic strings. It is also impossible for us to detect light from when the Universe was younger than about 379,000 years old. But all of these things are predicted to emit gravitational waves.

The direct detection of gravitational waves by the LIGO collaboration has opened an entirely new window on our Universe. Detecting gravitational waves at the much lower frequencies that NANOGrav is sensitive to will allow us to gain unique and complementary knowledge about our universe. NANOGrav will be able to answer questions about how the most massive black holes in the Universe form, how galaxies merge and grow throughout cosmic history, and how gravity behaves at the limit of our understanding. These are just a few of the discoveries we expect to make, but the unexpected and unpredictable may be even more exciting. In the past, whenever we have opened up new frontiers in astronomy, we have discovered things we never even imagined, and the same will almost certainly be true as we enter the new era of gravitational wave astronomy.

Pulsar Timing Arrays

Neutron stars are the remains of a dead star that was more massive than our Sun, but not massive enough to form a black hole. Pulsars, which form a special class of neutron stars, are extreme, fascinating, and just plain cool objects in and of themselves — a teaspoon of neutron star material on Earth would weigh as much as the entire human race! And pulsars have magnetic fields so strong that they would erase every credit card and computer hard drive on Earth...even if the pulsar was as far away as the Moon! But from the point of view of NANOGrav, what makes a pulsar so interesting is that it can be used as a very precise clock. This is because pulsars send out a beam of radio waves and because they rapidly spin. Each time the beam of a pulsar points towards the Earth, we see a pulse of radio waves, hence the name pulsar. These pulses can be used just like the tick of a clock. The most precise pulsars, known as millisecond pulsars, spin hundreds of times a second and approach the stability of the best atomic clocks on Earth.

NANOGrav uses pulsars to form a type of cosmic global positioning system that is capable of detecting the minute effects of passing gravitational waves. The GPS that you use in your car or on your phone works by communicating with satellites in orbit around the Earth, and can determine your position very accurately. NANOGrav is using millisecond pulsars to do the same basic thing: to look for tiny changes in the position of the Earth that are due to the shrinking and stretching effect of passing gravitational waves. Just like GPS uses several satellites spread throughout the sky, NANOGrav uses an array of pulsars, forming a pulsar timing array. And since gravitational waves have such a small effect, only the most stable millisecond pulsars that make the best clocks will work.

Since pulsars are distributed throughout our Milky Way galaxy, in a very real sense we are using the Galaxy itself as a gravitational wave detector.