In the moments immediately following the Big Bang, the very first
gravitational waves rang out. The product of quantum fluctuations in the new
soup of primordial matter, these earliest ripples through the fabric of
space-time were quickly amplified by inflationary processes that drove the
universe to explosively expand.
Primordial gravitational waves, produced nearly 13.8 billion years ago,
still echo through the universe today. But they are drowned out by the
crackle of gravitational waves produced by more recent events, such as
colliding black holes and neutron stars.
Now a team led by an MIT graduate student has developed a method to tease
out the very faint signals of primordial ripples from gravitational-wave
data. Their results are published today in Physical Review Letters.
Gravitational waves are being detected on an almost daily basis by LIGO and
other gravitational-wave detectors, but primordial gravitational signals are
several orders of magnitude fainter than what these detectors can register.
It’s expected that the next generation of detectors will be sensitive enough
to pick up these earliest ripples.
In the next decade, as more sensitive instruments come online, the new
method could be applied to dig up hidden signals of the universe’s first
gravitational waves. The pattern and properties of these primordial waves
could then reveal clues about the early universe, such as the conditions
that drove inflation.
“If the strength of the primordial signal is within the range of what
next-generation detectors can detect, which it might be, then it would be a
matter of more or less just turning the crank on the data, using this method
we’ve developed,” says Sylvia Biscoveanu, a graduate student in MIT’s Kavli
Institute for Astrophysics and Space Research. “These primordial
gravitational waves can then tell us about processes in the early universe
that are otherwise impossible to probe.”
Biscoveanu’s co-authors are Colm Talbot of Caltech, and Eric Thrane and Rory
Smith of Monash University.
A concert hum
The hunt for primordial gravitational waves has concentrated mainly on the
cosmic microwave background, or CMB, which is thought to be radiation that
is leftover from the Big Bang. Today this radiation permeates the universe
as energy that is most visible in the microwave band of the electromagnetic
spectrum. Scientists believe that when primordial gravitational waves
rippled out, they left an imprint on the CMB, in the form of B-modes, a type
of subtle polarization pattern.
Physicists have looked for signs of B-modes, most famously with the BICEP
Array, a series of experiments including BICEP2, which in 2014 scientists
believed had detected B-modes. The signal turned out to be due to galactic
dust, however.
As scientists continue to look for primordial gravitational waves in the
CMB, others are hunting the ripples directly in gravitational-wave data. The
general idea has been to try and subtract away the “astrophysical
foreground” — any gravitational-wave signal that arises from an
astrophysical source, such as colliding black holes, neutron stars, and
exploding supernovae. Only after subtracting this astrophysical foreground
can physicists get an estimate of the quieter, nonastrophysical signals that
may contain primordial waves.
The problem with these methods, Biscoveanu says, is that the astrophysical
foreground contains weaker signals, for instance from farther-off mergers,
that are too faint to discern and difficult to estimate in the final
subtraction.
“The analogy I like to make is, if you’re at a rock concert, the primordial
background is like the hum of the lights on stage, and the astrophysical
foreground is like all the conversations of all the people around you,”
Biscoveanu explains. “You can subtract out the individual conversations up
to a certain distance, but then the ones that are really far away or really
faint are still happening, but you can’t distinguish them. When you go to
measure how loud the stagelights are humming, you’ll get this contamination
from these extra conversations that you can’t get rid of because you can’t
actually tease them out.”
A primordial injection
For their new approach, the researchers relied on a model to describe the
more obvious “conversations” of the astrophysical foreground. The model
predicts the pattern of gravitational wave signals that would be produced by
the merging of astrophysical objects of different masses and spins. The team
used this model to create simulated data of gravitational wave patterns, of
both strong and weak astrophysical sources such as merging black holes.
The team then tried to characterize every astrophysical signal lurking in
these simulated data, for instance to identify the masses and spins of
binary black holes. As is, these parameters are easier to identify for
louder signals, and only weakly constrained for the softest signals. While
previous methods only use a “best guess” for the parameters of each signal
in order to subtract it out of the data, the new method accounts for the
uncertainty in each pattern characterization, and is thus able to discern
the presence of the weakest signals, even if they are not
well-characterized. Biscoveanu says this ability to quantify uncertainty
helps the researchers to avoid any bias in their measurement of the
primordial background.
Once they identified such distinct, nonrandom patterns in gravitational-wave
data, they were left with more random primordial gravitational-wave signals
and instrumental noise specific to each detector.
Primordial gravitational waves are believed to permeate the universe as a
diffuse, persistent hum, which the researchers hypothesized should look the
same, and thus be correlated, in any two detectors.
In contrast, the rest of the random noise received in a detector should be
specific to that detector, and uncorrelated with other detectors. For
instance, noise generated from nearby traffic should be different depending
on the location of a given detector. By comparing the data in two detectors
after accounting for the model-dependent astrophysical sources, the
parameters of the primordial background could be teased out.
The researchers tested the new method by first simulating 400 seconds of
gravitational-wave data, which they scattered with wave patterns
representing astrophysical sources such as merging black holes. They also
injected a signal throughout the data, similar to the persistent hum of a
primordial gravitational wave.
They then split this data into four-second segments and applied their method
to each segment, to see if they could accurately identify any black hole
mergers as well as the pattern of the wave that they injected. After
analyzing each segment of data over many simulation runs, and under varying
initial conditions, they were successful in extracting the buried,
primordial background.
“We were able to fit both the foreground and the background at the same
time, so the background signal we get isn’t contaminated by the residual
foreground,” Biscoveanu says.
She hopes that once more sensitive, next-generation detectors come online,
the new method can be used to cross-correlate and analyze data from two
different detectors, to sift out the primordial signal. Then, scientists may
have a useful thread they can trace back to the conditions of the early
universe.
Reference:
Measuring the primordial gravitational-wave background in the presence of
astrophysical foregrounds
Phys. Rev. Lett.
Sylvia Biscoveanu, Colm Talbot, Eric Thrane, and Rory Smith DOI: Link
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Physics