A new tool to study neutron stars

Even the faintest gravitational ripple heading for Earth could soon be observable. 

A team of US and Australian researchers have developed new hardware that could boost the sensitivity of gravitational wave detectors to pick up waves weaker than what can be currently observed.

Four months after we heard news of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detecting gravitational waves, scientists are anything but idle.

With a new way to observe the universe, they’re working to push the limits of what can be seen.

“We want to use the Advanced LIGO detectors to sense the farthest gravitational wave or weakest gravitational wave possible,” explained Massachusetts Institute of Technology’s Nergis Mavalvala, author of the current study.

At the heart of the Advanced LIGO detectors is laser light in a vacuum, travelling back and forth down the interferometer’s two arms. This light is used to measure the distance between the mirrors at each end – which gravitational waves will stretch and squeeze ever so slightly.

Problem is, detecting a gravitational wave signal is limited by the quantum fluctuations in the laser light – the sporadic appearance of energetic particles from empty space.

These fluctuations create noise and a gravitational wave that’s weaker than the level of noise will go unnoticed.

It’s “like trying to measure the length of a piece of paper while the ruler’s tick marks keep wiggling and moving about,” explains Mavalvala. “Because this noise causes the tick marks on our meter stick to jitter, we want to reduce that.”

Her team’s solution for reducing the noise is tweaking a “squeezed vacuum source” – and no, that’s not some kind of distressed household appliance.

A new squeezed vacuum source could make gravitational wave detectors sensitive enough to study neutron stars. Credit: Eric Oelker, Massachusetts Institute of Technology

In a squeezed vacuum, the vacuum is modified so that the phase of the light fluctuates less. (The amplitude of the light fluctuates more because of this, but the phase noise is what really matters for LIGO.)

In their new system, published in the journal Optica, the scientists reduced the phase fluctuation further by reducing vibrations in the instrument, and by adding a feedback system that corrects any remaining noise.

“The best approach is to try to reduce the amount of intrinsic phase noise, but if you can’t do that, you can measure how much it’s jittering and then use feedback to correct it,” said paper author Eric Oelker.

This correction system allowed the team to cover a larger bandwidth of light frequencies, “suppressing the phase noise in a completely new way,” he explained.

The new vacuum source, the scientists discovered, has 10 times less noise than other sources. Adding this technology to Advanced LIGO could double the detector’s sensitivity, allowing them to detect weaker gravity waves, the researchers say.

So what are they keen to observe?

More sensitive detectors could help reveal the composition of neutron stars, said Mavalvala. These stars are so extremely dense they could hold the mass of the Sun within a 10-kilometre radius.

“Nobody knows exactly how the neutrons in these stars behave when you crush them into such a dense package,” she said. But the nuclear matter spat out when two neutron stars collide and rip apart can be studied using gravitational waves, she added.

With their squeezed vacuum source ready to deploy at Advanced LIGO within the next year, we’re staying tuned.