A black hole is usually where information goes to disappear—but scientists
may have found a trick to use its last moments to tell us about the history
of the universe.
In a new study, two University of Chicago astrophysicists laid out a method
for how to use pairs of colliding black holes to measure how fast our
universe is expanding—and thus understand how the universe evolved, what it
is made out of, and where it’s going.
In particular, the scientists think the new technique, which they call a
“spectral siren,” may be able to tell us about the otherwise elusive
“teenage” years of the universe.
A cosmic ruler
A major ongoing scientific debate is exactly how fast the universe is
expanding—a number called the Hubble constant. The different methods
available so far yield slightly different answers, and scientists are eager
to find alternate ways to measure this rate. Checking the accuracy of this
number is especially important because it affects our understanding of
fundamental questions like the age, history and makeup of the universe.
The new study offers a way to make this calculation, using special detectors
that pick up the cosmic echoes of black hole collisions.
Occasionally, two black holes will slam into each other—an event so powerful
that it literally creates a ripple in space-time that travels across the
universe. Here on Earth, the U.S. Laser Interferometer Gravitational-Wave
Observatory (LIGO) and the Italian observatory Virgo can pick up those
ripples, which are called gravitational waves.
Over the past few years, LIGO and Virgo have collected the readings from
almost 100 pairs of black holes colliding.
The signal from each collision contains information about how massive the
black holes were. But the signal has been traveling across space, and during
that time the universe has expanded, which changes the properties of the
signal. “For example, if you took a black hole and put it earlier in the
universe, the signal would change and it would look like a bigger black hole
than it really is,” explained UChicago astrophysicist Daniel Holz, one of
the two authors on the paper.
The method may provide a unique window into the “teenage” years of the universe that are hard to study with other methods.
If scientists can figure out a way to measure how that signal changed, they
can calculate the expansion rate of the universe. The problem is
calibration: How do they know how much it changed from the original?
In their new paper, Holz and first author Jose María Ezquiaga suggest that
they can use our newfound knowledge about the whole population of black
holes as a calibration tool. For example, current evidence suggests that
most of the detected black holes have between five and 40 times the mass of
our sun. “So we measure the masses of the nearby black holes and understand
their features, and then we look further away and see how much those further
ones appear to have shifted,” said Ezquiaga, a NASA Einstein Postdoctoral
Fellow and Kavli Institute for Cosmological Physics Fellow working with Holz
at UChicago. “And this gives you a measure of the expansion of the
universe.”
The authors dub it the “spectral siren” method, a new approach to the
‘standard siren’ method which Holz and collaborators have been pioneering.
(The name is a reference to the ‘standard candle’ methods also used in
astronomy.)
The scientists are excited because in the future, as LIGO’s capabilities
expand, the method may provide a unique window into the “teenage” years of
the universe—about 10 billion years ago—that are hard to study with other
methods.
Researchers can use the cosmic microwave background to look at the very
earliest moments of the universe, and they can look around at galaxies near
our own galaxy to study the universe’s more recent history. But the
in-between period is harder to reach, and it’s an area of special scientific
interest.
“It’s around that time that we switched from dark matter being the
predominant force in the universe to dark energy taking over, and we are
very interested in studying this critical transition,” said Ezquiaga.
The other advantage of this method, the authors said, is that there are
fewer uncertainties created by gaps in our scientific knowledge. “By using
the entire population of black holes, the method can calibrate itself,
directly identifying and correcting for errors,” Holz said. The other
methods used to calculate the Hubble constant rely on our current
understanding of the physics of stars and galaxies, which involves a lot of
complicated physics and astrophysics. This means the measurements might be
thrown off quite a bit if there’s something we don’t yet know.
By contrast, this new black hole method relies almost purely on Einstein’s
theory of gravity, which is well-studied and has stood up against all the
ways scientists have tried to test it so far.
The more readings they have from all black holes, the more accurate this
calibration will be. “We need preferably thousands of these signals, which
we should have in a few years, and even more in the next decade or two,”
said Holz. “At that point it would be an incredibly powerful method to learn
about the universe.”
Reference:
Jose María Ezquiaga and Daniel E. Holz, Spectral Sirens: Cosmology from
the Full Mass Distribution of Compact Binaries, Phys. Rev. Lett. 129,
061102, DOI: 10.1103/PhysRevLett.129.061102
Tags:
Space & Astrophysics