Ripples in the Galaxy

The Green Bank Telescope, one of the NANOGRAV instruments.

The big news this week was the announcement from the NANOGRAV consortium of the detection of low-frequency gravitational waves:

On Wednesday evening, an international consortium of research collaborations revealed compelling evidence for the existence of a low-pitch hum of gravitational waves reverberating across the universe.

The scientists strongly suspect that these gravitational waves are the collective echo of pairs of supermassive black holes — thousands of them, some as massive as a billion suns, sitting at the hearts of ancient galaxies up to 10 billion light-years away — as they slowly merge and generate ripples in space-time.

“I like to think of it as a choir, or an orchestra,” said Xavier Siemens, a physicist at Oregon State University who is part of the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, collaboration, which led the effort. Each pair of supermassive black holes is generating a different note, Dr. Siemens said, “and what we’re receiving is the sum of all those signals at once.”

The findings were highly anticipated, coming more than 15 years after NANOGrav began taking data. Scientists said that, so far, the results were consistent with Albert Einstein’s theory of general relativity, which describes how matter and energy warp space-time to create what we call gravity. As more data is gathered, this cosmic hum could help researchers understand how the universe achieved its current structure and perhaps reveal exotic types of matter that may have existed shortly after the Big Bang 13.7 billion years ago.

So what’s the story here? Let’s slowly assemble the pieces of this remarkable discovery, starting with the husks of dead stars.

Pulsars

When a massive stars dies, it leaves behind a compact remnant of its core, crushed under its own gravity. If the core is more than three times the mass of the Sun, this remnant will be a black hole. But if it’s less than that, it will be what we call a neutron star. Small (less than 10 km across), unthinkably dense and compressed so tightly that it is composed entirely of neutrons.

If this neutron stars has an intense magnetic field, that will create emissions of light at its north and south magnetic polls. And as the neutron star spins, those beams of light become like a lighthouse, sending a flash of light to any observer in the beam every time the neutron star spins. This is what we call a pulsar.

Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones.

The thing about pulsars is that because they are so compact and because they are relatively simple systems, their spin is extremely regular, to a tiny fraction of a second. Theis status as the most precise astronomical clocks known can make them very useful.

For example, in 1974, Taylor and Hulce discovered a pulsar whose signal was varying in a regular pattern, with the pulses arriving early or late in a regular cycle. It turned out that this pulsar was in a binary system with another neutron star, orbiting each other every 8 hours. The pull of the other star’s gravity was causing those variations as it moved toward or away from us depending on where it was in its orbit.

But that wasn’t the cool part. The cool part is that, for the last 49 years, we’ve been monitoring this system. And over that time, the orbit of the two neutrons stars has shortened by 40 seconds. Why is this? Because the Theory of Relativity predicts that any two bodies orbiting each other will lose some of their energy in the form of gravitational waves. And work done by my undergrad advisor has shown the two neutron stars have moved together exactly in line with Einstein’s theory. This was the first proof that gravitational waves exist.

Gravitational Waves

Gravitational waves are literal ripples in the fabric os spacetime itself. If one were to pass through the room you are reading this in, the room would stretch and shrink in response to the wave. Thankfully, most of them are tiny by the time they reach Earth, smaller than a proton. But this also unfortunate as it makes them extremely difficult to detect.

But not impossible. In Louisiana, Washington, Japan and France, there are large interferometers that look for gravitational waves. They shine a laser down kilometers-long corridors at right angles to each other. The lasers reflect off a mirror, come back and create an interference pattern. If a gravitational wave passes through the Earth, one or both of those corridors will get just a little bit longer or shorter and make the interference pattern change.

In 2016, the LIGO arrays made the first direct detection of gravitational waves — ripples in the universe created by two black holes smashing together a billion light years away. Since then, over a hundred such events have been detected, opening a new era in astronomy.

NANOGRAV

LIGO and its brethren are excellent tools to identify high-frequency gravitational waves produced by merging compact objects. But gravitational waves are emitted in any context. Our own sun emits gravitational waves, albeit a very very tiny amount, too tiny to detect (the total energy wouldn’t be enough to power your house). However, theoretically, our universe is awash in low-frequency gravitational waves — ripples that are light-years in size. These could hold information critical to understanding our universe. But they are too tiny in amplitude and too massive in wavelength to be detected on Earth.

But imagine if instead of being a few km long, LIGO’s laser corridors were light years in length. Then you could detect even the tiniest ripple in the fabric of spacetime. You could detect the low frequency gravitational waves, ripples in the universe that are many light years in size. These are the kind produced by merging supermassive black holes in the early universe. Or perhaps by the universe itself during its inflationary phase. Or maybe even from cosmic superstrings.

That’s where this all comes together. You can’t shine a laser a hundred light years and have it return. But you can use the exquisitely precise timing of pulsars as your laser. If the fabric of space time stretches, the pulses should come in a tiny bit later. If it contracts, they should come in a tiny bit slower. And by studying many pulsars all over the sky, you can see those ripples propagate across the galaxy, like measuring a tsunami wave from orbit.

This is not easy. Pulsars glitch. Material gets in the way. Pulsars move. The precision you need is something like one part in a quadrillion. But over 15 years and with 68 pulsars, the NANOGRAV team was able to remove the interference and measure the first low-frequency gravitational waves.

The Green Bank Telescope, one of the NANOGRAV instruments.

The collaboration involved institutions all over the world pouring over data from telescopes in West Virginia, New Mexico and Puerto Rico on 68 pulsars spread over the sky to measure variations to one part in a quadrillion. But over 15 years, they have seen a clear ripple across the cosmos, most likely produced by two gigantic black holes — millions or billions of times the mass of the Sun — colliding in a distant galaxy. There are other possible explanations, but that seems the most likely.

The best thing is that we’re only to get better at doing this. The capabilities of the NANOGRAV experiment are growing, the time basline is lengthening and our ability to measure with precision is increasing. So stand by for more results. A new window on the universe just opened.