In the past few years, ground-based experiments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo observatory have shed light on one of the darkest and most elusive phenomena in the Universe: gravitational waves.
These waves are ripples in the very fabric of spacetime, and cannot be explored using telescopes that observe electromagnetic light – they require more complex forms of investigation, involving extremely precise lasers that measure minuscule variations over huge distances. While a number of incredible recent discoveries have opened a whole new window onto the cosmos, ground-based observatories are limited by their size and sensitivity to observe high-frequency waves between 10 and 1000 Hz (10 to 1000 oscillations per second). These waves enable us to explore objects with masses ranging between roughly 1 and 100 times the Sun's mass, and thus cannot probe the waves created by some of the most massive, extreme, and distant objects.
LISA, on the other hand, will be able to scour the entire Universe in its hunt for these elusive waves, from the very tiniest to the very largest of scales. To detect gravitational waves with lower frequencies down to 0.1 mHz, such as those from the merging of supermassive black holes at the centre of massive galaxies, an observatory must span millions of kilometres – something that can only be achieved in space.
|Artist's impression of merging black holes. Credit: ESA|
The invisible Universe
Gravitational waves are, and will always be, invisible to observers using electromagnetic light. Their effects are incredibly subtle, as they travel, virtually undisturbed, over immense cosmological distances. Much of the cosmos remains electromagnetically 'dark', requiring missions such as LISA to 'listen' to gravity: the force that dominates and organises the Universe at the very largest scales.
LISA will reveal much about high-energy phenomena in the distant, early, large-scale Universe at far higher precisions than ever before, by looking for elusive waves that could only be detectable by such a large, precise, space-based interferometer. The mission will also test and measure gravity in its strong regime, map spacetime itself (especially around black holes), and enable further tests of Einstein's general theory of relativity.
Gravitational waves are created through the acceleration of mass – but the gravitational waves produced in everyday life are minuscule in amplitude and impossible to measure. However, the Universe is filled with accelerating bodies: planets orbiting the Sun, binary star systems, and even pairs of black holes caught in each other's intense gravitational field, causing them to accelerate around each other at speeds approaching the speed of light. These black hole systems emit gravitational waves, removing energy from the binary system and eventually leading the black holes to collide, forming a new, bigger, black hole. During the final stages, as they inspiral towards each other and finally merge, the energy released in gravitational waves from one single event is greater than the rest of the quiescent electromagnetic universe put together.
To explore these dramatic events, LISA will probe the cosmos back to greater and greater distances from us – and thus to earlier and earlier epochs in its history – to find the seeds from which the first black holes formed (corresponding to redshifts up to z=20, back to the cosmic dark ages), and to observe dramatic black hole mergers. The observatory will detect and explore massive black holes of between 105 and 107 solar masses at earlier cosmic epochs than ever observed before, and shed light on the unexplored population of black holes at the low-mass end of this range. It will be possible for the first time to detect gravitational waves from the collision of supermassive black holes, and to receive the signal from the inspiral of smaller, stellar-mass black holes years before ground-based observatories like LIGO and Virgo could detect their final merger (and the corresponding higher frequency gravitational waves).
LISA will study how black holes form, evolve, and merge, and characterise their masses, spins, and redshifts. It will answer questions about how black holes form in galactic nuclei, how they feed on the surrounding matter and how this affects their evolution, and create the widest black hole census yet compiled, covering all stages of their lifetimes.
The early Universe
The onset of cosmic structure formation and the very building blocks of our Universe remain poorly explored. The earliest days of our Universe's formation, corresponding to redshifts between 20 and 6 – when the Universe was between a few to several hundred million years old – saw the formation of much of the large-scale structure of the cosmos, with the growth of a 'cosmic web' of invisible dark matter that would provide the scaffolding for ordinary matter to assemble and form the seeds of stars and galaxies. The following epoch, corresponding to redshifts between 6 and 2 and cosmic ages up to a few billion years, saw galaxies emerge, evolve, and merge together, as massive amounts of stars burst into existence and black holes grew ever more massive.
The most recent epoch, corresponding to redshifts from 1 to the present day, is more easily accessible to traditional observations that gather light across the electromagnetic spectrum, but still holds many secrets that could be unlocked with gravitational waves. LISA will explore the Universe from the cosmic dark ages up to the present epoch, mapping the history of our cosmos via gravity back to the epochs preceding the formation of stars and galaxies.
Extreme Mass Ratio Inspirals
Building on this timeline of the Universe, LISA will explore quiescent, massive black holes lurking at the centres of galaxies, and provide the deepest ever view of galactic nuclei – including regions that are inaccessible and invisible to electromagnetic observations. It will do so by using Extreme Mass Ratio Inspirals, or EMRIs for short: these are compact stellar remnants, such as black holes or neutron stars, that have become trapped in a chaotic orbit around a supermassive black hole, and whirl around until they are consumed.
These observations will provide information on how stellar remnants are distributed near the centres of galaxies, and the masses and characteristics of both the compact objects and the supermassive black holes that consume them. The gravitational waves emitted as a compact object spirals towards its inevitable doom will also map the spacetime in the near vicinity of a supermassive black hole, providing a test of general relativity in the strongest regime that could ever be probed.
Compact binary star systems
Most of the stars in galaxies across the Universe, as well as in our Milky Way, exist in multiple star systems. LISA will explore compact binary systems in which stellar remnants exist close to and interact with one another; these systems often include white dwarfs, neutron stars, or possibly small, stellar-mass black holes.
We now know of less than 50 such systems that whirl around so fast that they produce gravitational waves – LISA will be able to detect several of these systems already in the first weeks of operations, and discover many thousands of new ones. In fact, the gravitational waves constantly released by these relatively nearby ultra-compact binaries represent a local source of noise on top of the signal from more distant sources (such as the mergers of supermassive black holes). One of LISA's key objectives is to characterise the population of these binary systems, surveying their prevalence and distribution throughout our Galaxy – all-important information for probing the history, structure, and composition of the Milky Way.