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
Chirps, Rings, Black Holes and Binaries
The most spectacular LISA events will be huge, roaring events when
two very big black holes, somewhere in the universe, spiral together
and merge into a single bigger black hole. The final hole weighs a
lot less than did the original two, and the difference in mass is
radiated as gravitational waves. As I mentioned above, in terms of
radiated power, just one of these mergers far outshines everything
else in the universe combined.
These events will be fun to listen to. They sing a song called a
"chirp." For a long while the orbiting black holes emit a
nearly constant set of tones, like a single note on a violin that
gets higher only very slowly. Then, just before the holes merge, the
note quickly gets higher and louder at the same time, like a
virtuoso's flourish. Finally, after the merger, there is a
"ringdown" when the sound rapidly goes away, like the
reverberations in a vast concert hall.
We think mergers happen pretty frequently somewhere in the universe.
Most galaxies have a massive black hole right in the middle, and
every galaxy has swallowed or merged with another galaxy more than
once in the past; that is how galaxies grow. When two galaxies
merge, their two massive black holes sink to the middle of the new
galaxy because they lose energy to stars and gas by gravitational
interactions. Finally the holes find each other and merge together.
There are roughly ten billion galaxies to listen to, and if each of
them does this just once during the ten billion years of active
galaxy assembly, that's about one event every year, on average.
But most massive black holes don't have to wait so long to swallow
something; they are snacking all the time on the smaller occupants
of the galaxies around them. The big holes live in dense swarms of
stars in the centers of galaxies, and every now and then one of the
stars gets too close to its neighbor for its own good.
Sometimes a very compact stellar remnant—a neutron star or a
stellar-mass black hole—finds itself in a death dance, where
it whirls in and out and around a massive black hole many times
until it finally plunges into the oblivion of the event horizon and
disappears from view. All the time it is doing that dance, it emits
gravitational radiation. The gravitational radiation records a
history of the orbit and makes a detailed map of the spacetime
around the massive black hole. Remember that the black hole is made
of gravity alone, and Einstein's theory tells us what the structure
of black holes ought to be. This kind of event will tell us a lot
about the structure of black holes themselves—how spacetime
ties itself into the stable spinning knots we call black holes.
LISA also has some sure targets. Our galaxy is full of stars. Stars
have a life cycle—they only last as normal stars until their
hydrogen fuel runs out—and many of them have burned out and
died. Most of the time the remnant is a very small and dense ember
such as a white dwarf or a neutron star, and much of the time,
because stars tend to form in binaries, the remnant is in a binary
system with a similar companion. Those remnants that orbit each
other once every few minutes to an hour radiate at frequencies that
LISA can hear.
In fact we already know of a few nearby binaries, discovered by
astronomers using normal telescopes, that LISA will be able to hear.
We call these "calibration binaries" because we already
have a pretty good idea of many of their properties, such as their
frequency and distance. After LISA, we will know a lot
more—the gravitational waveform will tell us their inclination
and much about their detailed masses and other properties. The
nearby binaries will also reassure us that LISA is actually working
and detecting gravitational waves. Thousands of more distant
binaries blend into a noisy backup chorus that will also be heard as
soon as LISA turns on.
» Post Comment