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
The String Section
Although they sound exotic, all of the sources just discussed, even
the huge binary black-hole mergers, are actually expected to happen
in the normal course of events according to our current
understanding of the universe. But what about really weird stuff?
What new, unexpected things might really knock our socks off?
Physics now extends its reach back to the early moments of the Big
Bang, to incredibly high temperatures, even to the inflation epoch
when the cosmic expansion got the kick that made it as big as it is
today. If you go back far enough, even space and time were not like
they are today. A still-untested quantum version of Einstein's
theory, string theory, suggests that space has 10 dimensions, many
of which are highly curved or compact, and that all the particles of
matter, and maybe even spacetime and gravitational waves, are
ultimately composed of tiny quantum strings. The problem with string
theory is that despite its seemingly miraculous ability to tie ideas
from different parts of physics and mathematics together, nobody has
yet found any real-world evidence for it. Might LISA hear any
whispers from that new physics?
There is at least one kind of new, truly "stringy" object
that, if it exists at all, fills the universe with gravitational
radiation that LISA might hear. The tiny quantum strings might also
form into cosmic superstrings, which are microscopically
thin but astronomically long.
In the very early universe, a dense network of them forms by rapid
quenching as the universe expands. This formation process resembles
the cracking of cold ice cubes when you drop them suddenly into
water, the mottled patterning of alloy domains in a finely forged
Samurai sword or the trapped vortex lines that sometimes form in
sudden cooling of superconductors, superfluids or liquid crystals.
As the universe expands further, the strings unravel and rush around
at almost the speed of light; when they cross they can exchange
partners, spawning closed loops of string. A large population of
these loops accumulates and doesn't easily disappear. The loops
thrash around but are almost stable and remain around for a long
time, shrinking only slowly. Indeed the main way the loops lose
energy is by gravitational waves! If we estimate the strength of
gravitational waves, they turn out to be easily detectable by LISA
for some scenarios suggested by string-theory inflation.
The most interesting stringy events from these loops are rather rare
occasions when an unusually nearby loop beams gravitational waves in
our direction by a sort of whipping action, or cusp catastrophe. The
motion of the string for an instant, in one place, formally
approaches the speed of light, and as this moment approaches, the
gravitational waves are beamed and amplified. Such bursts, if
detected, would be a rich source of data and a completely new window
on how string theory works in the real world.
It's also possible that we might see gravitational waves directly
from the early universe, possibly from the end of inflation when the
fields driving the Big Bang converted their energy into normal
light, matter and antimatter, or at a later phase transition when
light and matter spawned the excess of matter over antimatter that
became our atoms. Gravitational waves are so penetrating that they
reach us from the entire history of the universe, right back to the
start of the Big Bang.
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