October 2019, ScientificAmerican.com 21GETTY IMAGESCOSMOLOGYTabletop
Detector
A mini gravitational-wave
detector could probe dark matterWithin one second of the big bang,
the first newborn black holes may have
announced their formation with gravita-
tional waves that stretched and squeezed
the fabric of existence as they rippled out-
ward into the expanding universe. Now
researchers at Northwestern University
have begun planning a tabletop-size sen-
sor that could detect these primordial
howls for the first time.
The gigantic $1-billion Laser Interfer-
ometer Gravitational-Wave Observatory
(LIGO) first measured the spacetime rip-
ples known as gravitational waves in 2016;
these phenomena came from the collision
and merging of distant supermassive black
holes. Since then, massive detectors have
also recorded gravitational waves from
merging neutron stars. Northwestern’s
proposed mini detector, which received
an influx of funding in July, could measure
higher-frequency waves from objects that
have never been measured before—such
as black holes in the earliest universe.
Current gravitational-wave detectors
such as U.S.-based LIGO and Europe’s Vir-
go use a sprawling system of mirrors and
laser “arms” that stretch for kilometers to
measure tiny changes in distance caused
by passing gravitational waves. North-
western’s Levitated Sensor Detector
would use lasers to suspend a glass bead
inside a vacuum chamber, creating anextremely force-sensitive sensor with arms
just a meter long. It would listen for echoes
from the formation of primordial black
holes and the activity of theoretical parti-
cles called axions, both of which are candi-
dates for mysterious dark matter—hidden
materials that may constitute much of the
universe’s mass and are invisible except for
their gravitational presence.
“I think there is more interest in ex -
pand ing the frequency range in the search
for gravitational waves, particularly after
the recent exciting LIGO discoveries,” says
Andrew Geraci, a physicist at Northwest-
ern and principal investigator on the new
detector project. “These sources that are
dark matter–related are a bit more specu-
lative—the sources that LIGO found were
pretty much expected to exist.”
To try detecting waves from such
sources, the Northwestern project will use
$1 million from the W. M. Keck Foundation,
a U.S. charitable foundation based in Los
Angeles, and additional support from the
university. After two years of develop-
ment, the meter-long prototype would
operate for a preliminary year and poten-
tially pave the way for a larger detector
that could reach 10 meters in length.
Many researchers question whether
anything has the energy to be a strong
gravitational-wave source at such high fre-
quencies—above 10 kilohertz—says Rana
Adhikari, an experimental physicist at the
California Institute of Technology, who is not
involved in the levitated sensor project. But
he adds that the hypothetical sources linked
to dark matter could prove the exception:
“We may be surprised by all of the exotica
the universe produces in the ultrasonic grav-
itational-wave regime.” — Jeremy Hsu© 2019 Scientific AmericanOctober 2019, ScientificAmerican.com 21GETTY IMAGESCOSMOLOGYTabletop
Detector
A mini gravitational-wave
detector could probe dark matterWithin one second of the big bang,
the first newborn black holes may have
announced their formation with gravita-
tional waves that stretched and squeezed
the fabric of existence as they rippled out-
ward into the expanding universe. Now
researchers at Northwestern University
have begun planning a tabletop-size sen-
sor that could detect these primordial
howls for the first time.
The gigantic $1-billion Laser Interfer-
ometer Gravitational-Wave Observatory
(LIGO) first measured the spacetime rip-
ples known as gravitational waves in 2016;
these phenomena came from the collision
and merging of distant supermassive black
holes. Since then, massive detectors have
also recorded gravitational waves from
merging neutron stars. Northwestern’s
proposed mini detector, which received
an influx of funding in July, could measure
higher-frequency waves from objects that
have never been measured before—such
as black holes in the earliest universe.
Current gravitational-wave detectors
such as U.S.-based LIGO and Europe’s Vir-
go use a sprawling system of mirrors and
laser “arms” that stretch for kilometers to
measure tiny changes in distance caused
by passing gravitational waves. North-
western’s Levitated Sensor Detector
would use lasers to suspend a glass bead
inside a vacuum chamber, creating anextremely force-sensitive sensor with arms
just a meter long. It would listen for echoes
from the formation of primordial black
holes and the activity of theoretical parti-
cles called axions, both of which are candi-
dates for mysterious dark matter—hidden
materials that may constitute much of the
universe’s mass and are invisible except for
their gravitational presence.
“I think there is more interest in ex -
pand ing the frequency range in the search
for gravitational waves, particularly after
the recent exciting LIGO discoveries,” says
Andrew Geraci, a physicist at Northwest-
ern and principal investigator on the new
detector project. “These sources that are
dark matter–related are a bit more specu-
lative—the sources that LIGO found were
pretty much expected to exist.”
To try detecting waves from such
sources, the Northwestern project will use
$1 million from the W. M. Keck Foundation,
a U.S. charitable foundation based in Los
Angeles, and additional support from the
university. After two years of develop-
ment, the meter-long prototype would
operate for a preliminary year and poten-
tially pave the way for a larger detector
that could reach 10 meters in length.
Many researchers question whether
anything has the energy to be a strong
gravitational-wave source at such high fre-
quencies—above 10 kilohertz—says Rana
Adhikari, an experimental physicist at the
California Institute of Technology, who is not
involved in the levitated sensor project. But
he adds that the hypothetical sources linked
to dark matter could prove the exception:
“We may be surprised by all of the exotica
the universe produces in the ultrasonic grav-
itational-wave regime.” — Jeremy Hsusad1019Adva3p.indd 21 8/21/19 4:17 PM
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