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The science of LISA

 

Using LISA, scientists hope to learn more about a number of exciting facets of modern astrophysics and cosmology.

Supermassive black holes

Black hole dust torus

Supermassive black holes, with masses between 100,000 and a few billion times the mass of our sun, can be found in the core region of most galaxies. The NGC 4261image shows a dust ring around the central black hole of galaxy which astronomers have designated NGC 4261.

 

LISA will be able to detect the gravitational waves that are produced when two such supermassive black holes merge with each other.  Using this data, astrophysicists hope to answer such questions as: How and when did the massive black holes in galactic nuclei form? Did they grow mostly by sucking in gas matter from their surroundings, or were they formed through the merger of smaller black holes when our universe was young?  Do the black holes in galactic nuclei rotate, and if so, what determines their rate of rotation and how did they come to rotate in the first place? What are the objects in the direct neighbourhood of a galaxy's central black hole, and how do they interact with each other?

Probing spacetime around black holes

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In the solar system, gravity is very weak. But the most interesting predictions that can be derived from Einstein's theory of general relativity concern phenomena that occur when gravity is very strong! With LISA, we will be able to probe Einstein's theory in regions in which gravity is very strong, namely in the vicinity of black holes. Notably, LISA can detect gravitational waves produced when a compact object (such as a neutron star, a white dwarf or a small black hole) comes close to a supermassive black hole, first tracing out an orbit around the central object and then, as more and more energy is lost in the form of gravitational waves, falling in. Such an infalling object would act like a probe, and the gravitational waves produced would allow us to create a precise map of the spacetime around the massive black hole.  Such a fall is likely to take months or even years, and the information carried by the gravitational wave will give us our most direct look yet at the spacetime around a black hole. The graph above represents a sideways view of a compact body's orbit around a supermassive black hole - as the object moves to the left and to the right as we view its orbit side-on, the graph goes up and down. The key feature is the difference between the blue and the red curves. For the blue curve, the supermassive black hole rotates at 50% the maximal rate allowed for black holes, for the red dotted curve, it rotates at 55% of that rate.  As the graph shows, the small compact body's orbit clearly reflects that difference, showing how such bodies can be used as sensitive probes to determine the central black hole's rate of rotation.

A census of binaries

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Whenever two astronomical bodies orbit each other, gravitational waves are produced.  LISA will be able to detect binary systems made up of white dwarfs  (the burnt-out remnants of stars like our sun) and of neutron stars (super-dense stellar corpses produced in supernova explosions) in which the two component stars orbit each other with periods between a few seconds and a few hours. The image above is an artist's impression of one of the binaries LISA will be looking for, a star system designated RXJ0806.3+1527.

LISA is expected to see thousands of individual sources of this type, enabling researchers to take a census of this kind of object in our galaxy, documenting overall numbers, system types (e.g. how many neutron-star binaries as opposed to black hole binaries?), rates of rotation, information about the masses of the components and similar data.  At lower frequencies, corresponding to white-dwarf binaries orbiting more slowly, LISA cannot distinguish between individual sources; still, the combined signal from these binaries will give us important information about the prevalence of such slower binaries in our galaxy.

Mapping the distant universe

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Another exciting area of LISA science is the precise absolute measurement of distances on cosmological scales. Where LISA measurements and conventional electromagnetic observations are concerned, the whole is greater than the sum of its parts: Combining the two, astronomers can gain a deeper understanding of our universe that would have been impossible to achieve using either type of signal alone.

More concretely, when two black holes orbit each other and then merge, the properties of the gravitational waves emitted by such a system and measured with LISA carry information about the total energy released in the process, the system's "gravitational wave brightness". Just as one and the same light bulb looks brighter when it is closer to us, and more dim when it is further away, comparing the system's intrinsic brightness with its perceived brightness gives a measure of how far away the system is. In conventional astronomy, systems for which the intrinsic brightness is known are called "standard candles"; since the detection of gravitational waves has in some respects more in common with listening to sound than with observing light, such merging black holes have been nicknamed "standard sirens".

 LISA can only locate standard sirens to about one degree in the sky, to a region containing around 100,000 galaxies. However, astronomers observing that same region using conventional telescope may  be able to identify the galaxy producing the LISA signal and measure its redshift - the way that the universe's expansion influences the frequency of light and other radiation that reaches us from a distant galaxy. The combined information would provide an unprecedented way to measure the scale of the universe, its properties such as the overall space curvature and the properties of the mysterious "dark energy" which causes the universe's expansion to accelerate.

Looking back in time

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Using electromagnetic radiation, the earliest image of our cosmos that astronomers can obtain is the so-called cosmic background radiation, produced roughly 400,000 years after the big bang (see image above). Using gravitational waves, we can "see" significantly earlier epochs of our cosmic evolution, fractions of a second after the big bang.


By observing gravitational waves emitted at the earliest moments in the life of our Universe, LISA may be able to shed light on questions that touch upon some of the most fundamental issues of modern physics, linking the largest scale of all, cosmology, with the smallest scales imaginable in elementray particle physics. Is the world really made of “strings” rather than particles? Did the early universe produce relics such as cosmic superstrings that decay predominantly into gravitational waves? There are a number of  interesting processes in the early universe which could release substantial energy in the form of macroscopic motions, resulting in the production of gravitational waves, for instance an "electroweak phase transition" (in which the electroweak force is split up into the electromagnetic force and the weak nuclear force we can measure today) or a late end to cosmic inflation (the latter being a period of rapid accelerated expansion in the early universe).  The corresponding signals, which LISA would be able to detect, carry unique information about the earliest history of our cosmos.  In the same way, LISA could also test predictions derived from string theory, including some that would give us insight in the fundamental nature of spacetime: How many space dimensions are there? If LISA detects gravitational waves created as additional dimensions curl themselves up in the early universe, we might obtain an answer to that question.

Continue with the final part of "LISA in a nutshell": A high-tech mission