FEATURE ARTICLE
The Sounds of Spacetime
In the biggest events in the universe, massive black holes collide with a chirp and a ring. Physicists are finding ways to listen in
Craig Hogan
Mapping the Distant Universe
Using the known physics of gravitational-wave emission, LISA will
let us use gravitational waves as a tool to map distances to
galaxies in the distant universe. By measuring the chirp of a
distant binary black-hole merger—how long it lasts until its
tone changes—we can tell how massive the merging black holes
are. By measuring the loudness, we can then tell how far away the
holes are. This is a completely new way to map the cosmic expansion
that can be more precise and direct than other techniques we have,
insofar as the physics of black-hole mergers is completely understood.
A tricky aspect of this project is that astronomers need to actually
identify the host galaxy with visible light (because we need an
independent measure of the redshift, the amount by which wavelengths
have been stretched by the expanding universe). We don't know for
sure that this will be possible. For loud binaries, LISA will
sometimes let us measure the direction the sound is coming from, by
combining the data from different times of year to act as a stereo
microphone. The best precision is about one degree of arc, narrowing
it down to a patch of sky which includes tens of thousands of galaxy
images. It is reasonable to hope that the very special galaxy among
these with a merging pair of black holes will look different enough
for us to identify it—perhaps by time-varying nuclear activity
in optical light, perhaps by changes in shape in response to the
disturbance created by a recent galaxy merger.
Accurate distance mapping with supernovae led to the discovery of
the cosmic dark energy that is accelerating the expansion of the
universe; measuring distances better with gravitational waves will
be a way to learn more details about that new force of nature.
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