Technological increments made in astronomy have allowed us to shed light on parts of the universe that long been shrouded in darkness; see objects that for a long time existed only on paper; and has forever changed the way we look at space and time. In the same vein, in February 2016, when it was announced that the Laser Interferometer Gravitational-wave Observatory (LIGO) has successfully detected the presence of gravitational waves, it marked a landmark achievement in science – followed up by the June 15 announcement that a second blackhole merger had been detected.
Albert Einstein’s celebrated general theory of relativity has been validated yet again. More, it has opened an additional window and, now, reliable for astronomers to study the universe through. At the same time, the first steps to broaden the window had already been taken in the form of the European Space Agency probe named Laser Interferometer Space Antenna (LISA) Pathfinder, shortened to LPF, launched in December 2015.
It was a proof-of-concept mission, a precursor to a much larger and ambitious project. And its success could set in motion the wheels that would truly enable gravitational waves to be a major part of the astronomer’s toolkit, which currently enables observations across the electromagnetic spectrum. An ESA article described the ability to detect gravitational waves in this way: “It will be like the difference between going from silent movies to our modern cinema experience with surround sound.”
What if we could upgrade that experience to include 4K resolution? That’s LPF.
On June 7, 2016, the ESA declared that the LPF mission’s two-month-long experiment in space had culminated in a successful demonstration of the technology that would one day form the heart of a space-based gravitational wave observatory, the Evolved Laser Interferometer Space Antenna (‘e’ for ‘evolved’). The experiment comprised two identical cubes made of gold-platinum and suspended in free fall. The aim was to measure how much faster, in some circumstances, one object would accelerate than the other. Because the ultimate aim is to use a similar but larger experiment – to sense gravitational waves – it is crucial to isolate the cubes from any other forces that might mask the gravitational wave signal. So, they are housed in a spacecraft that protects them from any external influences.
The craft also avoids hitting the suspended cubes by continuously adjusting its position in a drag-free manner using a combination of thrusters producing forces of the order of a micronewton. The entire setup is an impressive feat of engineering, with the LISA team managing to shield the test masses and account for the most sensitive forces, some of which include those arising from accumulated charge, photon pressure and even cosmic rays, while at the same time exerting control over the positions and orientations of the cubes and the spacecraft.
The resounding success of the experiment is reflected in that the relative acceleration between the cubes was measured to be less than ten millionths of a billionth of the gravitational acceleration on Earth. This meant that LPF was able to measure displacements at the femto- level, 100-times better than the expected precision. Ground-based detectors like LIGO can only detect waves of frequency 100 Hz or higher (although the lowest has been pegged at around 35 Hz). eLISA will be more sensitive, attempting to record frequencies as low as 0.1 mHz.
Why is capturing low frequency gravitational waves so important? The lower the frequency of gravitational waves a system can sense, the less pixelated the universe will seem to be, revealing itself in detail that existing astronomical methods cannot capture.
Gravitational wave astronomy relies on the ability to sense ‘ripples’ in spacetime. These ripples are created when there are sudden changes in the gravitational fields of massive objects, and are simply gravitational energy waves that travel across spacetime, compressing and stretching it in the process. The catch is that unless an extremely energetic event produces the ripple, the disturbance in spacetime is so minute that even the most sensitive detectors cannot sense it. It took the merger of two massive black holes, each almost 30 times the mass of the Sun, to produce a signal that LIGO could detect. With the frequency range that LIGO is sensitive to, astronomers can only detect compact and fast stellar objects like black holes and neutron stars, along with events that have a specific kind of high-frequency signal.
Events like galactic collisions and the merger of supermassive or heavily mismatched black holes produce gravitational waves of low frequencies. To detect such exotic events, a detector capable of capturing waves in the range 0.0001 Hz-1 Hz is necessary. Low-frequency captures can allow astronomers to make observations of events over an extended period of time.
For example, LIGO captured around two-tenths of a second of a blackhole-merger. On the other hand, with a low-frequency detector, astrophysicists can tune in to signals from when the two objects are rapidly orbiting around each other, getting closer, through to the ultimate union. An answer posted as a part of an ‘Ask Me Anything’ (AMA) session on Reddit by a team of scientists from the LPF collaboration summarises the potential advantages of low frequency band sensitivity:
“The frequency band that LISA will observe is rich with signals from different kinds of astrophysical and cosmological sources. We will observe the signals from merging black holes at the centres of galaxies, the complex waveforms produced by small, stellar mass black holes falling into the supermassive black holes in the centres of galaxies, the almost sinusoidal signals from compact binary systems in our galaxies, and possibly the stochastic ripples from the Big Bang itself. And of course, all those systems we have not yet predicted!”
Low-frequency captures, the coup de grace of the LPF results, are simply not feasible to set up on an Earth-bound system. For starters, the experiment would be required to detect fluctuations in distance between objects kept millions of kilometres apart, something that is possible only in a space mission. Also, noise arising from a variety of sources on Earth would simply drown the faint signal from a low-frequency gravitational wave. Even LIGO had systems set up to account for minute seismic tremors and vibrations caused by vehicles passing by in the vicinity. What the success of the LPF mission highlights is that the principle used to detect gravitational waves at LIGO can be adapted for a space mission, free from terrestrial interference and size bounds. And while LPF hasn’t detected gravitational waves, the news of its success is heartening as it paves the way for ESA’s ambition to set up a full scale gravitational-wave observatory in space.