This week I'll be "live-posting" (or more accurately, "time-delayed-
-posting") some highlights from the 2024 LISA Symposium,
https://www.lisasymposium2024.ie/programme/which was held in Dublin, Ireland, and streamed by zoom.
BACKGROUND
LISA is the Laser Interferometer Space Antenna, a European Space Agency
(ESA) mission (with also an important NASA contribution) from to fly a
space-based gravitational-wave (GW) detector sensitive to GWs in the
millihertz frequency band. (In contrast, current ground-based GW
detectors like LIGO, Virgo, and KAGRA, are sensitive to GWs with
frequncies between (roughly) 20 and 3000 hertz.)
LISA was formally adopted by ESA earlier this year, and is currently
planned to be launched around 2035. The nominal science mission is
scheduled to be 4.5 years long, with possible extension to 10 years.
Basically, LISA will be a constellation of 3 spacecraft, orbiting the
Sun in orbits chosen such that the constellation is approximately a
(rotating) equilateral triangle with each side 2.5 million km long.
Each LISA spacecraft sends and receives laser beams to/from each of
the other two LISA spacecraft (so there are a total of 6 propagating
laser beams). By using very clever interferometry techniques, the
inter-spacecraft separations can be measured to picometer (!) accuracy.
GWs passing through the solar system will change those separations;
LISA will measure those GWs between (roughly) 0.1 and 100 millhertz in,
with a best sensitivity on the order of 10^{-20}/sqrt(Hz) at frequencies
of a few millihertz.
SOURCES & SCIENCE
LISA should be sensitive to a number of types of astrophysical GW sources:
* Compact white-dwarf binary stars in the Milky Way with orbital periods
on the order of 10 minutes or less -- these are strong enough sources
that LISA should detect essentially every such system in the Milky Way.
This, in turn, should tell us a lot about galactic structure. There
will also be many millions of similar systems with longer orbital periods,
which LISA won't invidually resolve.
* Orbiting binary systems of neutron stars and/or "stellar-mass"
black holes (BHs) ("stellar-mass" means BHs with masses below ~100
solar masses). Any given such sytstem may cross through the LISA
frequency band 5-10 years before the same system finally merges (and
the merger is visible to ground-based GW detectors such as LIGO, Virgo,
Kagra, and the planned Cosmic Explorer and/or Einstein Telescope).
* The decay and coalescence of supermassive BH binaries (masses ~10^6
solar masses). These are very strong sources lasting for hours to a
day or so, which LISA should be able to detect at reasonable signal/noise
ratios up to redshift ~5, and at very high signal/noise ratios (plural
thousands) for nearby sources. These observations should give a lot
of information about the formation and merger histories of the BH's
host galaxies. As well, each BH-merger observation yields the luminosity
distance to the BHs in meters, with no need for any "cosmic distance
ladder" calibration, so there's a lot of cosmology information available
too.
* Intermediate- or extreme-mass-ratio inspirals (usually abbreviated
IMRIs and EMRIs, respectively): these are stellar-mass BHs orbiting
supermassive BHs. These have relatively weak and complicated, but
very long-lasting signals (they spend years to decades within the LISA
frequency band), so clever data analysis can extract them from the
noise at cumulative signal/noise ratios of up to 100. Because the
signals last so long (millions of radians of GW phase), these are
ideal for precision tests of general relativity.
There's lots more information at
https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antennaincluding a nice animation of the LISA orbits.
-- -- "Jonathan Thornburg [remove -color to reply]" <dr.j.thornburg@gmail-red.com> currently on the west coast of Canada "There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors." -- anon
LISA: A space-based gravitational-wave detector