Using Stars to See Gravitational Waves (Patreon)

Now that gravitational waves are definitely a thing, it’s time to think about some of the crazy things we can figure out with them. In some cases we’re going to need a gravitational wave observatory - in fact, we've already built one. 

We are at the cusp of a golden age of gravitational wave astronomy. We’ve already talked about the Laser Interferometer Gravitational Wave Observatory–LIGO, and the first discovery of gravitational waves here. Those videos came out almost two years ago. A lot has happened since then. First, an update on LIGO. In its two and a half years of operation, LIGO has observed five certain black hole–black hole mergers. These events have been consistently surprising. The black holes in question were enormous. In three cases, both members of the black hole binary pair were well over twenty times the mass of the sun, and in the other cases they were 30 plus solar masses. But if these black holes formed in the deaths of massive stars, which we think they must, then they should weigh in at between five and fifteen solar masses-20 at most. It’s hard to imagine black holes forming bigger than this, or both of a binary pair growing this large after formation.

Researchers are spending lots of brainpower trying to come up with new and creative solutions to explain this anomaly. Some are trying to adjust stellar evolution models to allow for the formation of more massive black holes. Others are calculating whether these black holes may have grown in globular clusters, where the stellar density is so high that we expect lots of black hole mergers. There’s also some more creative speculation: perhaps we’ve observed the merging of primordial black holes, formed in the instant after the big bang, or perhaps they grew after being swept up in a quasar accretion disk – I actually did some work on that last one. Yeah, everyone wants in on the gravitational wave game.

A very recent paper has a very different explanation for the apparent large black hole masses. Perhaps these gravitational wave signals were amplified by another phenomenon predicted by Einstein’s general relativity: gravitational lensing. The paths of gravitational waves should also be warped by intervening gravitational fields, which can amplify the signal and stretch out the wavelengths. That could make a binary merger in the 5-15 solar mass range look like a much higher mass merger.

Of course the really big recent news was the observation of a neutron star-neutron star merger. We covered that in detail here, when the detection was still a rumor. Yeah, that one turned out to be true. For the first time the event behind a gravitational wave signal was also seen as light–first as a gamma ray burst, but then also across the electromagnetic spectrum. Observatories across the planet – and in orbit around the planet – swiveled to watch the afterglow of this collision. Besides what they tell us about neutron stars, more observations like this should allow us to figure out whether gravitational wave signals are often also gravitationally lensed.

Virgo, an Italian based gravitational wave observatory, was online for the neutron star merger and was extremely important in narrowing down its origin. More are coming. LIGO-INDIA or INDIGO, will join the LIGO network, aiming to begin operations in 2024.

So LIGO is great and all, but with its 4km arm length it’s only sensitive to a frequency range from around one Hertz to 10 kHz. That roughly corresponds to the frequency of the binary orbits just before merger, which for black holes and neutron stars clock in at a few to maybe a thousand orbits per second in that last instant. The more massive the binary the lower the frequency. To catch the merger of the million-to-billion-solar mass black holes – supermassive black holes – that live in the centers of galaxies we’d need to observe gravitational waves in the .1 mHz to .1 Hz range. That would require an interferometer with arms millions of kilometers in length.

Sure, why not. That’s LISA, the laser interferometer space antenna. This spacecraft by the European Space Agency is scheduled for a 2034 launch. The three components of this craft will trail behind the Earth in an orbit around the Sun. Its arms will be two point five million kilometers in length. That’s roughly ten times the distance from the Earth to the Moon. It has to be that big to listen in the .1 mHz to .1 Hz range. It’ll see those merging Supermassive Black Holes, as well as the faint hum of thousands of binary pairs of white dwarfs, neutron stars and black holes long before their merger.

LISA pathfinder, launched at the end of 2015, was a mission designed as a proof of concept for the technology involved in the eLISA mission. And it proved to be a rousing success demonstrating the feasibility of the mission.

The universe is flooded with spacetime ripples. We expect a faint gravitational wave background buzz from an earlier epoch of the universe in which binary supermassive black holes were common, or from cosmic strings, if they exist, or even from the instant after the big bang when the speculative graviton decoupled from the other fundamental forces. LISA may detect some of this gravitational wave background, but much of it may have wavelengths as long as many  light years. That's beyond any gravitational wave interferometer we could ever physically construct. We’d need a network of perfect timing devices scattered across the galaxy.

Oh, wait, we already have one. We have pulsars. Remember those? Neutron stars whose jets sweep past the Earth as the star processes, resulting in a sequence of flashes more regular that an atomic clock. We’re already using these to study the gravitational wave background, at the 1 to 100 nHz range. The International Pulsar Timing Array is a massive international effort spanning many universities and radio observatories around the world. It monitors dozens of the fastest-rotation pulsars – millisecond pulsars - spanning many thousands of light years. 

It looks for shifts in the time of arrival of their signals. Such shifts could indicate tiny fluctuations in the fabric of space within the pulsar array volume due to the passage of impossibly vast gravitational waves. This galaxy-scale observatory is already in operation, and has placed valuable limits on the amplitude of the gravitational wave background. More time and more data will hopefully yield a detection. This may be the only way we can look directly at the instant after the big bang.

Some scientists are even trying to see how gravitational waves would interact with stars. Gravitational radiation should have physical effects as shown by the sticky bead argument first presented by Richard Feynman. He came up with a thought experiment of a simple gravitational wave detector of a rod with two sliding beads. As a gravitational wave passes by, the beads are free to follow expansion and contraction of space, but the rod resists due to the atomic forces between its atoms, but. As they move along the rod, they generate friction and thus heat. That heat energy comes from the gravitational wave. This thought experiment isn’t practical, but it demonstrates that in the right circumstances gravitational waves should be able to dump some of their energy into matter.

For example, into stars. Stars oscillate at particular frequencies. We spoke about this in our episode on asteroseismology. If a gravitational wave frequency matches a natural resonant frequency of a star, oscillations can be set up inside the star. This generates an internal friction, heating up the stellar interior. The star should then brighten. It may be possible to observe this effect in the dense star fields of galactic cores if those galaxies also contain a binary supermassive black hole that is generating gravitational waves. A similar effect may cause white dwarf stars in binary orbits to explode, as they absorb the gravitational radiation from their own orbits.

Links to all of the programs and papers I mentioned are in the description. We couldn’t talk about everything – there’s a lot happening. Gravitational wave astronomy is currently in a gold-rush. Questions long unanswered are now being resolved, and new mysteries open up. There are going to be some extremely important, and also entirely unexpected discoveries as we learn to decode the incredible wealth of information carried to us in the rippling fabric of spacetime.