LISA Science Survey
Table of Contents
In the following a brief survey of the key scientific measurements
that LISA will perform is given. These measurements address the
basic scientific goals of the LISA mission, which are captured
formally in the LISA Science Objectives. The scientific
background for the LISA science measurements and objectives is
discussed extensively in the Science
Case Document, from which this text is adapted.
LISA records the inspirals and mergers of binary black holes, the
most powerful transformations of energy in the Universe, allowing
precision measurements of systems composed only of pure dynamical
spacetime.
The strongest gravitational waves are generated by systems with
the largest gravitational fields GM/R, hence large masses and
small sizes. The strongest of all are generated by interactions
of black holes, dense knots of pure spacetime energy with
GM/(Rc2)≈1. At LISA frequencies the strongest
sources are massive black hole binaries with about 104
to 107 times the mass of the Sun. Two black holes
orbit each other, spiral together as they lose energy by
radiation, and finally merge. The waves from these events —
many cycles over a long inspiral, climaxing in a brief series of
powerful waves during a violent merger, and a final ringdown to a
quiescent single black hole — record dynamical general
relativity not only in its purest form but also in its most
violent, nonlinear behavior: a maximally warped vacuum spacetime
interacting with itself.
The black hole binaries start with wide orbits at low frequencies. As
they lose energy their frequency increases and their radiation
strengthens. A typical source enters the LISA band a year or more
before the final merger so many orbits are recorded, encoding details
of the system properties and behavior, position on the sky, and
absolute distance. The coherent phase and polarization information
obtained over LISA's solar-orbit baseline (and variable inclination)
can often pinpoint where a source is in the sky to better than a
degree. In the last hours or minutes the signal-to-noise ratio grows
very high, often into the hundreds to thousands depending on distance.
At its peak luminosity, around the moment of merger, a black hole
binary is the most extreme transformation of mass-energy of any kind
in the Universe, radiating a power 10-3c5/G (or ~1049 watts), in a few
wave cycles, for a time of about 100 GM/c3. This peak radiated power
is about 1000 times more than all the stars in the visible Universe.
The merger throes of a million solar mass binary black hole merger
last about 500 seconds. Massive black hole binary inspiral and merger
events are such powerful radiators that LISA can detect them anywhere,
out to the largest redshifts where galaxies might exist.
The detailed study of waveforms from black hole binaries
provides a rich testbed for general relativity. Recent
breakthroughs now allow numerical computation of Einstein's field
equations throughout the entire inspiral and merger event,
yielding a detailed map of the predicted gravitational waveform
that will be the first detailed test of dynamical, strong-field
general relativity. Waveforms coherently correlated over many
orbits (from approximately 10 to about 1000 depending on mass and
redshift) recorded in the LISA signal stream, and detection of
events with a signal to noise of a thousand or more, allow precise
tests of the theory as well as precise measurements of system
parameters to a precision of order 10-2 to
10-3. Comparison with the computed details of the
inspiral and merger waveform will provide a powerful test of the
binary black hole model assumed for these systems.
[Top]
LISA will map isolated black holes with high precision, verifying
whether they are the stationary "no hair" spacetime configurations
described by the Kerr metric, completely specified by four numbers:
the mass and three components of spin.
In general relativity the final isolated spinning black hole is
described mathematically as a particular, precisely specified
spacetime shape called a Kerr metric, that depends only on the physics
of gravity and not at all on the history or environment of the black
hole. Comparison of the ringdown waveform with theory can verify that
the final black hole which arises from a merger is indeed described by
the Kerr solution, and satisfies the "no hair" theorem of general
relativity that states that an isolated, stationary black hole is
completely specified by its mass, charge and angular momentum. The
LISA signals during the merger phase are so strong that the
signal-to-noise ratio is often greater than 100 even in one
oscillation cycle: signal waveforms are visible on an oscilloscope
type display of raw data even to the naked eye, so even if general
relativity were to be wrong at the levels allowed by our existing
tests (e.g. the double binary pulsar J07037-3039) we would be able to
use LISA data to make sense of what is happening.
LISA also uses a second type of source to explore the spacetime
near a massive black hole. Driven by chance encounters, a much
smaller mass compact objects — such as a degenerate
dwarf, neutron star or stellar-mass black hole —
sometimes finds itself captured by the massive black hole, after
which it orbits many times until it finally plunges into the
horizon and disappears. The gravitational waves from these
extreme mass ratio inspirals (EMRIs) encode a detailed map of a
relatively unperturbed massive black hole, predicted to be a
pure Kerr knot of highly curved, spinning spacetime. About
105 wave cycles are measured for each source, emitted
from orbital paths exploring deep into different parts of the
relativistic region near the massive black hole. The specific
mass quadrupole and higher moments predicted by the Kerr
solution are measured with a precision of about 10-4,
and precision tests of small variations about the equilibrium
Kerr solution — the small amounts of "hair" added by the
perturbing object — are measured at the one percent
level. Gravitational waves from these events map in exquisite
detail the cleanest and most accurately predicted structures in
all of astrophysics, whose mathematical elegance Chandrasekhar
once likened to that of atoms.
[Top]
LISA directly observes how massive black
holes form, grow, and interact over the entire history of galaxy
formation.
Optical, radio and x-ray astronomy have produced abundant
evidence that nearly all galaxies have massive black holes in
their central nuclei (and indeed that some recently merged
galaxies even have two black holes). These nuclear black holes
have a profound effect on galaxy formation; the influence of
black hole powered jets on the intergalactic gas out of which
galaxies form is in some cases directly observed. There is a
circumstantial case, but no direct evidence, that the formation of
this population of black holes was associated with a multistage
process of binary inspiral and merger, together with accretion.
LISA will obtain direct and conclusive evidence and study details
of this process via gravitational radiation.
In standard concordance cosmology, the first massive black
holes naturally arise from the very first, supermassive stars. In
this scenario, black hole binaries begin to form from a high
redshift, z ≈ 20, when galaxies start to assemble by a series
of (hundreds to thousands of) hierarchical mergers of smaller
protogalaxies. When two galaxies merge into one, their central
black holes sink to the center of the new galaxy, find each other,
inspiral and merge. There are so many galaxies forming in the
Universe observed by LISA that mergers happen quite frequently:
estimates based on standard galaxy formation theory suggest that
if black holes indeed grew by hierarchical merging, LISA detects a
merger event about once or twice every week on average, from a
wide range of redshifts extending back to massive binaries in
early protogalaxies at z ≈ 15. At any given time, in addition
to the actual mergers, these models predict that LISA observes
inspiral signals from hundreds of binaries in the final years
before their merger. LISA digs directly and intimately into the
detailed evolution of galactic nuclei: the large sample of
binaries provides a direct record of the whole history of galaxy
formation in the observable Universe, and of the processes that
grew their central black holes and shaped their nuclei.
In addition to mergers of massive black holes, LISA will also
observe the inspiral of stellar mass black holes into the massive
black holes in the centers of normal galaxies. These are the
extreme mass ratio inspiral events (EMRIs) mentioned earlier. The
parameters measured from extreme mass ratio events yield a census
of isolated massive black hole spins and masses in many galaxies
today, a revealing relic of black hole history. The local
universe also produces observable inspirals of less compact stars
and stellar remnants that probe the rich astrophysics near the
massive central black holes as they consume piecemeal the various
stellar populations in their vicinity. [Top]
LISA measures precise,
gravitationally-calibrated absolute luminosity distances to high
redshift, with the potential of contributing uniquely to
measurement of the Hubble constant and dark energy.
Because the inspiral leading up to the merger is a clean, pure
vacuum spacetime system of two black holes, properties of the
radiation can be computed exactly in general relativity, so that
the black hole masses, spins, orientations and even the exact
distance can be reconstructed from LISA data. (Roughly speaking,
the final wave cycle period tells the final absolute Schwarzschild
radius, and the ratio of that length to the distance is the metric
strain, h.) These inspiral distances are both individually
precise and absolutely calibrated, using only pure gravitational
physics, and they cover a wide range of redshift. In the absence
of lensing effects the absolute physical luminosity distance to a
single LISA inspiral event is typically estimated from the
waveform alone with on the order of one percent precision, and in
some cases with as good as 0.1% precision. If identification of
the host galaxy1 allows an independent redshift determination,
the redshift-distance relation is also measured with high
precision. Black hole binaries thus represent a unique and
independent new capability for precision cosmology that
complements other techniques. Even a small number of sources at
moderate redshift calibrates the distance scale and Hubble
constant an order of magnitude better than any current method — a
powerful constraint on dark energy models in combination with
microwave background data. The expected large sample of high
redshift inspiral events may lead to measurements of dark energy
parameters comparable in precision to other methods, but with
independent calibration and completely different systematic
errors. The main source of error, especially at high redshift, is
the noise induced by cosmic weak gravitational lensing along the
line of sight, but in a statistical sample this is controllable,
and indeed provides unique new information about the nature and
clustering of dark matter over time.
[Top]
LISA studies in detail thousands of compact
binary stars in the Galaxy, providing a new window into matter at
the extreme endpoints of stellar evolution.
In addition to mergers and meals of distant black holes, LISA
detects many lower mass binary systems in our galaxy, mostly
very compact remnants of normal stars, called white dwarfs. Very
soon after turning on, LISA will quickly detect a handful of
nearby binary compact stars already studied and named. These
"verification binaries" provide sources with known positions and
periods ensuring particular, predictable LISA signals. Signals
are also certain to appear from populations in our galaxy of
numerous and various remnants, including white dwarfs and neutron
stars, which are known to exist from some that emit electro-
magnetically. Simple extrapolation of known nearby samples to the
whole Galaxy predicts that LISA will detect thousands of binaries.
The most compact binaries (those at high frequency) will be
measured in detail as individual sources from across the Galaxy,
while at lower frequencies only the nearby ones will be indi-
vidually distinguished; millions of others from across the Galaxy
will blend together into a confusion background. LISA provides
distances and detailed orbital and mass parameters for hundreds of
the most compact binaries, a rich trove of information for
detailed mapping and reconstruction of the history of stars in our
galaxy, and a source of information about tidal and other
non-gravitational influences on orbits associated with the
internal physics of the compact remnants themselves. LISA may
also detect at high frequencies the background signal from compact
binaries in all the other galaxies.
[Top]
LISA may find entirely new phenomena of nature
not detected using light or other particles.
Given that all forms of mass and energy couple to gravity,
other sources of gravitational waves may exist that are not known
from extrapolating current electromagnetic observations. LISA's
frequency band can indeed be extrapolated to very high redshift
where we do not yet have any direct observations, and to a regime
where LISA itself will be our first information of any kind about
the nonlinear behavior and motion of matter. For example, the
LISA frequency band in the relativistic early Universe corresponds
to horizon scales at the Terascale frontier, where phase
transitions of new forces of nature or extra dimensions of space
may have caused catastrophic, explosive bubble growth and
efficient gravitational wave production. LISA is capable of
detecting a stochastic background from such events from about
100 GeV to about 1000 TeV, if gravitational waves in the
LISA band were produced with an overall efficiency more than about
10-7, a typical estimate from a moderately strong
relativistic first-order phase transition. This corresponds to
times about 3×10-18 to 3×10-10 seconds after
the start of the Big Bang, a period not directly accessible with
any other technique. Reaching much further still beyond the range
of any particle accelerator, LISA also deeply probes possible new
forms of energy such as cosmic superstrings, relics of the early
Universe predicted in some versions of string theory, that are
invisible in all ways except by the gravitational waves they emit.
In principle, their signature could provide direct evidence for
new ideas unifying all forms of mass and energy, and possibly even
spacetime itself.
Footnotes
1
LISA's waveform fitting can often
pinpoint the direction of a source to much better than a degree,
and the distance estimate also narrows the redshift range
considerably; nevertheless there may be many thousands of galaxies
in the LISA "error box" for a given source. Models suggest that
the host may be identified from a telltale nuclear starburst
associated with the merger, or from variability associated with
the disrupted disks around the merging holes, but galaxy nuclei
are too little understood to make a firm prediction. Study of LISA
electromagnetic counterparts may provide an exploratory bonanza
for wide field synoptic imaging and spectroscopy across the
electromagnetic spectrum, but it is also possible that
identification of hosts will prove elusive.
Back
Oliver Jennrich,
06 Feb 2007
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