A new era of gravitational wave astronomy is on its way as the ambitious upcoming LISA space mission joins a host of huge detectors on Earth.
The Laser Interferometer Space Antenna will be a space-based constellation of 3 satellites that will fly in formation to study ripples in spacetime.
Cast your mind back to September 2015, when some of the most sensitive instruments ever built made a remarkable discovery: the first-ever detection of tiny ripples in space and time, known as gravitational waves.
Created by a pair of black holes spiralling towards each other and crashing together, the observed wave travelled through space at the speed of light until it was detected by two separate observatories here on Earth.
Now scientists are setting their sights on grander goals, hoping to observe the entire Universe, looking back in time to its very origin, with gravitational waves.
In January 2024, the European Space Agency (ESA) gave the green light for an international team of scientists to begin building the largest gravitational wave detector ever built – only this time it will be in space.
Its name is LISA, the Laser Interferometer Space Antenna, and it will revolutionise our understanding of the Universe.
Gravitational waves simply explained
Gravitational waves are ripples in space and time, similar to those formed on the surface of water when a pebble is dropped from a height.
Gravitational waves, however, are caused by some of the most violent astrophysical events in the Universe, such as black holes smashing together.
They were predicted by Albert Einstein in his general theory of relativity more than a century ago.
According to theory, gravitational waves expand and contract spacetime itself.
Everything, including you and me, will stretch and squeeze as a gravitational wave passes by.
Thankfully, although gravitational waves are thought to be like tsunamis at the source, by the time they reach us here on Earth their effects are minuscule.
They are so small, in fact, that gravitational waves produced by some of the most energetic events in the Universe are thought to only stretch and squeeze the entire Earth by a fraction of the width of an atom.
Detecting gravitational waves
Detecting such small changes might appear an impossible task, but the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) managed it.
The first observation of gravitational waves, dubbed GW150914, was caused by the cataclysmic collision of two black holes, each with a mass around 30 times that of our Sun.
At the precise moment the two merged, the amount of energy emitted in gravitational waves was larger than the luminosity of all stars in the visible Universe added together.
Since this Nobel Prize-winning discovery, additional gravitational wave detectors joined the observational campaign.
Close to 100 signals have been observed by the LIGO–Virgo–KAGRA collaborations (a partnership encompassing four separate detectors located around the world) across three distinct observing runs.
The treasure trove of knowledge gleaned from these observations has already transformed our understanding of the cosmos.
They have revealed that black holes collide far more frequently than expected, uncovered the origin of exotic elements such as platinum and gold, and constrained fundamental properties of black holes such as their mass and spin.
In the 2020s, the existing gravitational wave detectors entered an ambitious commissioning period where numerous upgrades were applied to the instruments.
On 24 May 2023, the LIGO–Virgo–KAGRA collaboration entered its latest 18-month observing run, otherwise known as the fourth gravitational wave observing run, O4 (though the run was briefly paused between 16 January and 10 April for maintenance and additional improvements).
O4 will be 30% more sensitive than previous iterations, making it the most sensitive search for gravitational wave signals to date.
This increased sensitivity will result in a gravitational wave detection every two or three days, compared to every week as seen previously.
The increased number of gravitational wave signals will improve our ability to infer the true population of black holes in the local Universe.
However, if we want to set our sights on grander targets and detect gravitational waves from the Universe’s birth, we must venture to a new location – space.
Enter LISA, the Laser Interferometer Space Antenna
LISA, the Laser Interferometer Space Antenna, will be a gravitational wave detector in space, and the largest scientific instrument ever built.
It will be comprised of three individual and identical satellites flying in a triangle formation separated by 2.5 million km (1.5 million miles) – more than six times larger than the orbit of the Moon around Earth.
Similar to existing gravitational wave detectors on Earth, the Laser Interferometer Space Antenna will use lasers to precisely measure the distance between each satellite and monitor changes in the light’s arrival time.
It will be able to detect gravitational waves in the 0.1mHz to 1Hz window, a low-frequency region that can’t be detected by ground-based observatories.
Though similar to current observatories in principle, LISA poses many additional technical challenges that need to be solved before it can launch in the mid-2030s.
Are we even able to place objects within a spacecraft in a near-perfect gravitational freefall, while controlling their motion with unprecedented accuracy?
In order to test proposed solutions, ESA led a test mission called LISA Pathfinder, which launched from the European spaceport in French Guiana in 2015.
Within the first two months of operations, LISA Pathfinder successfully demonstrated that the technology required for the Laser Interferometer Space Antenna is possible.
The final results, published in 2018, far exceeded expectations and in January 2024 ESA formally adopted the LISA space mission, recognising that the mission’s concept, design and technology are advanced enough to start building the instrument.
LISA is now firmly established as one of the major missions in ESA’s upcoming programme.
What might the Laser Interferometer Space Antenna discover?
The Laser Interferometer Space Antenna will address many scientific goals, including understanding how galaxies form.
Although there is no direct evidence, galaxies are thought to be formed from the mergers of hundreds to thousands of smaller protogalaxies (a cloud of gas undergoing active star formation).
Nearly all galaxies have massive black holes at their centres with masses ranging from 1,000 to 10 million times the mass of our Sun.
When two galaxies merge, the two massive black holes at their centre will eventually find each other, merge and release a huge amount of energy through gravitational waves.
Unfortunately, the gravitational waves from merging massive black holes are at a much lower frequency than the sensitivity window of existing ground-based detectors.
Owing to LISA’s monumental size, it will be uniquely able to observe the collision of massive black holes, from the current-day Universe all the way back in time to when the Universe was 0.18 billion years old (its estimated age is 13.7 billion years).
Similarly, the Laser Interferometer Space Antenna will observe stellar-mass compact objects falling into massive black holes, otherwise known as extreme mass ratio inspirals, creating gravitational waves without the need for some cataclysmic event.
By measuring the properties of the waves produced, LISA will yield a unique census of isolated and relatively unperturbed massive black holes, a relic of black hole history.
Einstein’s gravity
The Laser Interferometer Space Antenna will even be able to test Einstein’s theory of gravity in the most extreme regime that can ever be probed.
The strongest gravitational waves are produced by systems with the largest gravitational fields – for example, the merger of massive black holes.
Recent breakthroughs have allowed us to solve Einstein’s equations of gravity on a computer and thus make high-precision predictions of how such gravitational waves should appear.
By identifying loud gravitational waves from the mergers of such massive black holes, astronomers will be able to compare with predictions, creating the most stringent test of general relativity to date.
Similarly, the final phase of a gravitational wave signal should allow astronomers to verify something called the no-hair theorem, which states that black holes can be completely described by three properties: their mass, charge and angular momentum.
The last stages of a gravitational wave signal describe the properties of the final black hole, allowing for the no-hair theorem to be tested.
LISA will be a revolutionary instrument, both in terms of its design and what it promises to achieve for gravitational wave research.
The adoption of LISA, the Laser Interferometer Space Antenna, by ESA represents a tremendous milestone in the mission.
An international team of scientists are now hard at work building the largest astronomical observatory ever built, opening an unprecedented new window into the Universe, revolutionising our understanding of cosmology and gravitation.
No doubt, there will be some surprises in there too.
This article appeared in the June 2024 issue of BBC Sky at Night Magazine