Page 38 - The 'X' Chronicles Newspaper - October / November 2018

38 Gravitational Waves

Gravitational Waves

Could Solve A

Cosmological Crisis

Within Five Years—Or

Shake Physics To Its Core

We’re closing in on one of the most
important numbers in the universe.

By Charlie Wood

When cosmologist Daniel Holtz took off from
Hong Kong on August 17, 2017, his head
swirled with the ideas he’d spent the last week
lecturing on, including his hope that vibrations
in space would someday settle an ongoing
debate regarding the size and age of the
universe. But he knew it would take time. Time
for two of the densest objects in existence to
smack together and shake the cosmos hard
enough for us to feel the rumble here on Earth,
time to locate the disturbance, and time to swing color, a measure of how stretched its light is. says Adam Reiss, a cosmologist at John
our telescopes toward the collision before the Researchers can also do something similar with Hopkins University who works on the
accompanying burst of light faded back into information in ancient light left over from supernova method. “There's something
darkness. shortly after the big bang, known as the cosmic interesting going on, something we don’t
Optimistically, such paired observations microwave background (CMB) radiation. Once understand about the universe.”
of both gravitational waves and light from these you know the speed of expansion you can work Holtz has bet his career on the idea that
neutron star collisions were about ten years off, backward to figure out the exact size and age of gravitational waves could serve as arbiter. The
he had told the audience in his last lecture the the universe or push forward to look at its future idea, which originates from a speculative 1986
previous day. The Laser Interferometer trajectory. paper by American physicist Bernard Shutz, is
Gravitational-Wave Observatory (LIGO) The problem is that the two current that another type of dead star could replace
collaboration had already detected black hole calculations give different results. supernovae as a cleaner yardstick for cosmic
mergers and the Virgo interferometer had just The most recent estimates for the distances. After blowing up, giant stars that
come online in Italy two weeks earlier, so the supernova method (73.5 kilometers per second don’t quite have what it takes to become a black
enterprise was developing smoothly. But when per megaparsec, as of January) and the CMB hole collapse into neutron stars—a scrum of
Holtz, who works at the University of Chicago, method (67.4, as of June) differ by about 9 particles so dense even atoms get crushed.
returned to Illinois, he learned the future had percent. The discrepancy didn’t cause much When two of these stellar corpses crash
arrived early. The gravitational shockwaves set alarm in the field initially, since the together, the impact sets off a ripple known as a
off by the collision of two nearby titans had measurements are devilishly difficult in gravitational wave.
rippled through his plane—and the entire practice. A big, far off explosion looks like a Because these waves are undulations in
planet—while he was in the air, and weak, close one, so finding the distance to space itself, nothing gets in their way.
observatories around the world were scrambling supernovae relies on the “cosmic distance Unperturbed by dust and gas clouds, they
for follow-up optical observations. ladder,” a complicated technique that involves spread out from the crash site until reaching
“We landed, and my phone exploded. I relating three types of objects at different Earth, where the scientific community operates
immediately connected and just sat there on my distances, or “rungs.” Astronomers first study three L-shaped detectors (with more on the
laptop starting to work,” Holtz recalls. “That flickering stars in our galactic backyard with way) to nab them. As a wave passes through the
was the most amazing experience of my life.” basic geometry and then transfer that planet, it gives it a light squeeze. One arm of
Twelve hours after touchdown he had a back-of- knowledge to similarly behaving stars in distant each L becomes about one-proton shorter than
the-envelope calculation for the most galaxies to get a read on the supernovae the other, and the apparatus notifies physicists
contentious number in cosmology: the speed of happening there. “They're incredibly careful in and astronomers around the world. Using the
the expanding universe. With just one data point so many different ways,” Holst says. “But exacting equations of general relativity,
he couldn’t get the decisive measurement he’s there's a lot of sausage in there.” researchers can measure the distance to the
dreamed of for thirteen years, but he finally Studying the CMB calls for less collision accurately with little calculation and
knew the project was possible. Now, after doing machinery, but more assumptions. The few assumptions—no ladder climbing or
some more math, he’s back with a new background radiation preserves a record of the particle counting required.
prediction: the LIGO collaboration may be able expanding universe in its infancy, and to Holtz refined the theory in 2005,
to settle the decades-long debate within five extrapolate to the present cosmologists have to suggesting that spotting the light from a neutron
years, according to his recent letter in Nature. draw on everything they think they know about star crash along with sensing the waves would
The conversation revolves around one what gravity, matter, dark energy, and dark provide speed information to complement the
question: How fast is the universe expanding? matter have been doing during the intervening gravitational wave distance reading, and joined
Finding the answer, known as the Hubble 13 billion years. Any number of flaws could LIGO to spearhead such an effort. Many of his
constant, is simple in theory. You start with a have thrown either method off, but even as colleagues told him it was never going to
receding object, typically a star undergoing a astrophysicists in each camp have checked and happen, he recalls, since astronomical data
particular type of death. These “type 1a re-checked their math, the two estimations have predicted that neutron star mergers should occur
supernovae” always explode in the same way, refused to converge. Now the chance that the extremely rarely, but all the pieces came
so researchers can get an idea of their distance cosmological community has just had an together on August 17 just as he was flying
based on their brightness. To calculate the incredible string of statistical bad luck is home.
Hubble Constant, you also need to know the pushing 1 in 1,000.
speed at which the explosion is moving away “It's gotten to a point now where we're (Continued on Page 39)
from you, which you can get from looking at its like ‘wow, that's probably not just a fluke,’”