Which cosmic objects will LISA study?
ESA’s mission LISA will be the first to observe gravitational waves in space. The mission consists of three spacecraft that fly in a triangular formation with arms of 2.5 million km. By closely monitoring the distance travelled by laser beams, the observatory will measure tens of thousands of gravitational waves coming from everywhere all at once. Scientists will have to detangle the stream of intertwined gravitational waves to learn more about our Universe.
1. Discover the Milky Way through binary systems
More than a hundred billion stars call the Milky Way their home. Among them, most are born in pairs, especially those that are much more massive than Sun. While these systems of two stars orbit each other, they create gravitational waves that are big enough for LISA to measure. Given their abundance, this group will be the most represented in LISA’s data.
Even though systems of two stars are common, their mutual evolution is not yet fully understood. At the end of their lives, stars undergo a transformation and leave behind compact stellar objects: white dwarf stars, neutron stars, or black holes. By studying the gravitational signal of these double systems, LISA will learn more about their common evolution.
Pairs of lower mass stars form binary systems of white dwarf stars at the very end of their lives. White dwarf stars are hard to spot with ordinary telescopes because they are very small, around the size of Earth. LISA will detect tens of thousands of white dwarf stars using the gravitational waves they create. By measuring where these stars are located and how they are distributed, LISA will learn more about the structure of our galaxy.
On the other hand, systems of high mass stars end their lives as a pair of neutron stars and/or black holes. Again, while they orbit one another LISA can pick up their signal. Interestingly, because of its three laser-beam arms, LISA will be able to localise them on the sky within one arc degree (an area about the size of the full Moon). This means that when the system eventually merges, an event accompanied by a flash of light, telescopes with a large field-of-view will be able to look for the source of light in the right direction. This will enable astronomers to study the event in various types of light (called multi-messenger astronomy) and learn as much as possible about the event.
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2. Merging massive black holes to study the Universe
At the centres of galaxies sit supermassive black holes. They are between a hundred thousand and a billion times more massive than the Sun. How these black holes form is not yet known. Most astronomers think that they grew from smaller 'seed' black holes that accreted material and merged with other back holes over their lifetime. One other theory is that these black holes already existed since the beginning of the Universe.
Massive black holes can be studied by measuring the light coming from objects that orbit the black holes or by light emitted when stars fall into these black holes. LISA will allow for the first gravitational study of massive black holes in binary systems at the centres of galaxies.
Most current telescopes, with the exception of the James Webb Space Telescope, cannot capture light from the Early Universe – 13 billion light-years away. But by studying gravitational waves from massive black holes in the early Universe, we can discover how they were born. LISA is sensitive to gravitational waves from black holes that form pairs. The mission will learn more about the initial formation and early evolution of these extraordinary objects.
When two massive black holes spiral toward each other, LISA can detect their gravitational signals already from up to two weeks before they eventually collide and merge. This is earlier than any ground-based detector could measure them. It also gives other telescopes time to point towards the black hole pair to look for possible light signals from the black holes colliding.
3. Compact objects falling onto single supermassive black holes
Not all supermassive black holes exist in pairs. Single black holes have been spotted before by observing light that was created by stars falling into them. You may remember the image of the supermassive black hole at the heart of our galaxy produced by the Event Horizon Telescope in radio wavelengths.
Coincidently, LISA is also sensitive to gravitational waves created by compact objects interacting with these massive black holes. The observatory is sensitive to interactions of smaller black holes, neutron stars and possibly some white dwarf stars orbiting the massive black holes.
In conclusion, during its four-year mission LISA will study millions of gravitational signals coming from binary systems and single objects with masses ranging from heavy to super heavy. The mission will shed light on the structure of our galaxy, learn more about the evolution of stars and black holes and locate them in the sky so other telescopes can perform follow-up observations.