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How scientists will keep searching for gravitational waves

There are more ways to find them than just LIGO

NASA/Miriam Nielsen

Now that the first gravitational wave signals have been detected, scientists are eager to find more. Gravitational waves offer a new way of learning about distant objects like black holes and neutron stars; we can measure the gravitational waves that they produce, similar to how we use electromagnetic radiation to observe distant stars and galaxies.

"Gravitational waves have a whole spectrum of wavelengths."

The scientific collaboration LIGO, which made yesterday's discovery, plans to upgrade their observatories to make their instruments more sensitive to wave detection. But LIGO is only suited to detect the waves with extremely high frequencies, and gravitational waves can have frequencies that span days, months, or even years. To pick those types up, scientists will need different tools.

Scientists have been exploring alternative detection methods for many years now. One idea is to build bigger space-based wave detectors, with laser beams spanning hundreds of thousands of miles. Another technique is to observe mysterious stars known as pulsars over the course of many years, to see if they are altered by gravitational waves. Those efforts have been ongoing for many years now, but haven't turned up any concrete signals yet.

However, all three types of detection methods are necessary to study the different types of gravitational waves that exist throughout the Universe. "Gravitational waves have a whole spectrum of wavelengths," says Ira Thorpe, an astrophysicist at NASA. "These methods are complementary to each other."

Extra-sensitive LIGO

A LIGO researcher examining one of the observatory mirrors. (LIGO)

The LIGO collaboration — which stands for the Laser Interferometer Gravitational-Wave Observatory — has actually been looking for waves since 2002. But for the first eight years of its operation, the collaboration didn't pick up any signals. "That initial LIGO phase, we knew probably wasn’t going to be sensitive enough to detect waves," says Shane Larson, a theoretical physicist at Northwestern and a LIGO collaborator.

LIGO still has room to improve

Then in 2010, the collaboration shut down for five years to upgrade the two LIGO observatories in Louisiana and Washington state. This included adding an extra mirror and increasing the power of the lasers. Once the changes were made, LIGO resumed operations in 2015, leading to the collaboration's revolutionary detection.

But LIGO still has room to improve — especially in regards to its lasers. LIGO uses laser beams to see if gravitational waves have warped space-time. The scientists time how long it takes the lasers to bounce off two mirrors; if a gravitational wave is passing through, the beams will take different amounts of time to come back from each mirror. Right now, the lasers are incredibly sensitive, capable of detecting changes to the mirrors' positions that are just one ten-thousandth the size of a proton. But the lasers aren't running at full power yet. "Sometime between now and next year, we’re going to be as sensitive as we can be," says Larson.

The LIGO optics lab. (Embry-Riddle Aeronautics University)

Increasing the power of these lasers means LIGO will be able to detect gravitational waves originating from even farther away in the Universe. And that's pretty far. The waves that LIGO detected came from black holes merging 1.3 billion light-years away. Larson says they'll be able to detect black hole mergers even more distant than that.

LIGO will also be able to see another type of mysterious object with full laser power: neutron stars. These super faint stellar objects form after a star dies, just as some black holes do. But neutron stars are not as heavy as black holes are, so they don’t emit gravitational waves that are quite as strong. With full laser power, though, LIGO will be able to pick up waves from neutron stars that are merging around 600 to 700 million light-years away.

eLISA

An artist rendering of the eLISA spacecraft. (ESA)

LIGO's instruments are extremely precise, but the collaboration has to constantly overcome one big problem: Earth. Our planet is a noisy place, and the movement of nearby cars can potentially throw off LIGO's instruments. But many of these issues can be eliminated by moving to space.

eLISA scales up gravitational wave detection

That's the idea behind eLISA, or the Evolved Laser Interferometer Space Antenna; it's the concept for a space-based gravitational wave detector proposed by the European Space Agency. With a planned launch date of 2034, eLISA would send three different spacecraft into orbit around the Sun, where they would form an equilateral triangle. The system would work similarly to LIGO, as the vehicles would be connected via laser beams. The lasers could then indicate if a gravitational wave has altered the position of objects inside spacecraft.

Apart from eliminating Earth's movements, eLISA also scales up gravitational wave detection. LIGO's lasers span just 2.5 miles, but eLISA's laser beams will span more than 600,000 miles. This allows the spacecraft to detect waves with much lower frequencies. LIGO can only detect high-frequency signals coming from rapidly moving objects — such as black holes spinning around each other several times per second. But eLISA could potentially detect waves coming from much slower moving objects in space, like some binary neutron star systems or supermassive black holes merging far away.

Pulsar timing

An artist rendering of a planet orbiting a pulsar. (NASA)

Another way to scale up gravitational wave detection is to observe strange dead stars, known as pulsars, for many years. Like distant cosmic lighthouses, pulsars are rotating neutron stars that emit a long beam of electromagnetic radiation. These beams can be measured from Earth when they're pointing in our planet's direction. And the timing of these "pulses" can tell us if gravitational waves are present.

A pulsar rotates in extremely precise periods, making it very easy to predict when the star's beam will swing by Earth. So if there is any deviation in a pulsar's pulse, scientists can measure it. In this way, the beams from the pulsars act a bit like the lasers that LIGO uses. Their travel time indicates if they are being manipulated by gravitational waves. "Instead of having artificial lasers travel through vacuum tubes, we actually have radio pulses from pulsars that are traveling through the vacuum of space," says Maura McLaughlin, an astrophysicist with NANOGrav — the North American Nanohertz Observatory for Gravitational Waves.

If there is any deviation in a pulsar's pulse, scientists can measure it

Using this type of detection method, scientists can pick up waves with incredibly low frequencies that last years. McLaughlin says these signals will likely come from black holes orbiting around each other very slowly or distant galaxy mergers. There's also the chance that pulsar timing could pick up more exotic wave sources known as cosmic strings. These are theoretical bumps or wiggles in space-time that vibrate and produce gravitational waves. And it's even possible that pulsars will reveal gravitational waves left over from the Big Bang.

However, observing pulsars hasn't turned up any evidence of waves just yet. The NANOGrav collaboration has been observing around 50 pulsars in our galaxy for about a decade with no definitive results yet. But McLaughlin says the sensitivity of their data only increases with time, since they are searching for waves with much longer wavelengths. And the success of LIGO will only spur the NANOGrav project forward. "It's great that LIGO made this detection, because it shows our methods should work."