UNSW Canberra astrophysicist, Ashley Ruiter tells us about the Laser Interferometer Space Antenna (LISA) mission to detect gravitational radiation from astrophysical objects in space, like supermassive black holes at the centres of far-away galaxies.
What is the Laser Interferometer Space Antenna (LISA)?
The Laser Interferometer Space Antenna (LISA) is a planned space mission to detect gravitational radiation from astrophysical objects in space – things like double white dwarf binary stars in our own Milky Way Galaxy and supermassive black holes at the centres of far-away galaxies.
LISA will consist of three satellites in space, all connected by laser beams, and will be the first gravitational wave detection platform that isn’t ground-based. Because LISA will have a very different design compared to current ground-based gravitational wave detectors (like LIGO, Virgo and KAGRA) it will be able to see many more different types of objects that exist in the depths of space.
The European Space Agency (ESA) is spearheading the mission along with NASA and it has a proposed launch date in the 2030s.
What are gravitational waves?
Gravitational waves (also called gravitational radiation) travel at the speed of light, and they are literally ripples in the fabric of spacetime!
Unlike electromagnetic radiation (i.e. visible light), gravitational radiation is not obstructed by things like interstellar dust, so we can detect these waves from very far away, when the Universe was much younger. Gravitational waves are created when a mass experiences a change in velocity. For example, when there are two stars orbiting each other at an ever-increasing speed the stars generate gravitational waves, which in turn pulls them closer together, shrinking their orbit until the two get close enough to merge.
As a gravitational wave passes by things on Earth it causes things to move in the most miniscule way - say it passes between two people standing next to each other, the distance between the two people will change ever so slightly. This change in distance is so small (think the size of an atom’s nucleus!) that we need very sensitive, precise instruments and intricate data analysis techniques to detect it.
Gravitational waves actually pass through us every day because so many things in the Universe are accelerating. But the effect is extremely small and we only stand a decent chance of detecting the passing waves in special circumstances, like when two heavy objects, like black holes, are orbiting very close together and a gravitational wave detector happens to be running at the required sensitivity.
What do gravitational waves tell us?
When gravitational waves are detected - which happened for the first time with the LIGO gravitational wave detector in 2015 - we can study the nature of the astrophysical event that created them. For example, researchers can figure out how heavy two black holes were before they collided to make a new, much heavier black hole. Without the gravitational wave signal, we would have no way of learning that these black holes even existed, let alone how heavy and old they were, which teaches us about the original stars that created them, and so on. A summary of compact objects (black holes and neutron stars) detected during the LIGO Scientific Collaboration’s 3rd observation run can be found here.
The most famous example of an event that was detected in gravitational waves and then searched for and rapidly found by regular telescopes was the merger of two neutron stars; an event called GW170817.
Neutron stars are formed when a massive star runs out of fuel and collapses in a ‘core-collapse supernova’. When this happens, gravity overwhelms the star’s ability to hold itself up because nuclear fusion has become unsustainable, and the core of the star is crushed into a ball of densely packed neutrons. Neutron stars are small - about the size of Canberra - and incredibly dense. When two neutron stars collide it’s called a kilonova.
Unlike the collision of two black holes, which does not emit light, colliding neutron stars create not only a gravitational wave signal, but also emit light across the electromagnetic spectrum. Without this ‘merger warning’ from the gravitational wave signal, other telescopes around the globe would not have been able to find and catch the event in the vastness of space before it rapidly faded away forever. These kinds of events - that get bright and then fade away - are called transients. The search and follow-up of transient events is a rapidly growing area of astrophysical research.
How does LISA help us understand astronomy better?
Gravitational waves have opened up a whole new era in astronomy; harnessing information in this new domain allows astrophysicists to not only see a new range of exotic space objects, but also determine their physical properties, which helps to refine astrophysical models.
While the majority of the events that the Earth-based LIGO detects are not visible in electromagnetic light, LISA will be able to detect a myriad of sources that are visible in both gravitational waves and electromagnetic light. The more observational information we can get, the better we can robustly test and constrain our theoretical models that describe astrophysical phenomena.
LISA will be able to detect many types of astrophysical objects, like 1000’s of white dwarf binary star pairs that are calmly orbiting each other (not merging for many centuries!) in the Galaxy.
Today, most of these white dwarf binaries are essentially invisible, because these stars are dim in electromagnetic light. However, astronomers think that white dwarf binaries are a major contributor to the population of exploding stars called "type Ia supernovae”, which are the most trusted ‘standard candles’ used in cosmology. Standard candles are astronomical objects that provide a kind of measuring stick to help determine the distance to galaxies and clusters of galaxies that are very far away.
Type Ia supernovae are also a main source of iron and some other heavy elements in the periodic table. LISA will help us to study white dwarf binary systems before the stars merge to assess whether they exist with the right properties, and in the right numbers, to explain the origin of type Ia supernovae and other interesting sources like the giant, dusty R Coronae Borealis stars.
In addition to learning about stars in our own Milky Way Galaxy, LISA will detect colliding supermassive black holes at the centres of distant galaxies - we’re talking black holes millions of times more massive than the Sun! These observations will allow astronomers to learn more about how galaxies form and evolve over cosmic time.
