Physics
Today: An investigation by scientists of the
Laser Interferometer
Gravitational-Wave Observatory (LIGO) Scientific
Collaboration and the
Virgo Collaboration
(see below right image) has put new constraints on on the
amount of gravitational waves that could have come from the
Big
Bang—the initial creation of the universe—by
not finding anything.
Most of the information gathered on the Big Bang comes from
measuring the existing ratios of elements in the universe, and
from the
cosmic
microwave background, an electromagnetic radiation "echo"
of the Big Bang found at radio wavelengths.A similar "echo" of
gravitational waves is still believed to exist in the universe
as the "
stochastic
background," analogous to a superposition of many waves of
different sizes and directions on the surface of a pond. The
amplitude of this background is directly related to the
parameters that govern the behavior of the universe during the
first minute after the Big Bang.Earlier measurements of the
cosmic microwave background have placed the most stringent
upper limits of the stochastic gravitational wave background at
very large distance scales and low frequencies. The new
measurements—taken over a two-year period between 2005
and 2007—by LIGO and Virgo directly probe the
gravitational wave background in the first minute of its
existence, at time scales much shorter than accessible by the
cosmic microwave background.The research, which appears in
Nature
, also constrains models of cosmic strings, objects that
are proposed to have been left over from the beginning of the
universe and subsequently stretched to enormous lengths by the
universe's expansion; the strings, some cosmologists say, can
form
loops
that produce gravitational waves as they oscillate, decay,
and eventually disappear.Gravitational waves carry with them
information about their violent origins and about the nature of
gravity that cannot be obtained by conventional astronomical
tools. The existence of the waves was predicted by
Albert
Einstein in 1916 in his
general
theory of relativity. The LIGO and GEO instruments have
been actively searching for the waves since 2002; the Virgo
interferometer joined the search in 2007."Combining
simultaneous data from the LIGO and Virgo interferometers gives
information on gravitational-wave sources not accessible by
other means. It is very encouraging that the first result of
this alliance makes use of the unique feature of gravitational
waves being able to probe the very early universe. This is very
promising for the future," says Francesco Fidecaro, a professor
of physics with the University of Pisa and the Istituto
Nazionale di Fisica Nucleare, and spokesperson for the Virgo
Collaboration.
The analysis used data collected from the LIGO interferometers,
a 2-km and a 4-km
detector in Hanford, Washington (left), and a
4-km instrument in
Livingston, Louisiana (below right). Each of the L-shaped
interferometers uses a laser split into two beams that travel
back and forth down long interferometer arms. The two beams are
used to monitor the difference between the two interferometer
arm lengths.According to the general theory of relativity, one
interferometer arm is slightly stretched while the other is
slightly compressed when a gravitational wave passes by.
The interferometer is constructed in such a way that it can
detect a change of less than a thousandth the diameter of an
atomic nucleus in the lengths of the arms relative to each
other.Because of this extraordinary sensitivity, the
instruments can now test some models of the evolution of the
early universe that are expected to produce the stochastic
background."Since we have not observed the stochastic
background, some of these early-universe models that predict a
relatively large stochastic background have been ruled out,"
says Vuk Mandic, assistant professor at the University of
Minnesota."We now know a bit more about parameters that
describe the evolution of the universe when it was less than
one minute old," Mandic adds. "We also know that if cosmic
strings or superstrings exist, their properties must conform
with the measurements we made—that is, their properties,
such as string tension, are more constrained than before."This
could be interesting, he says, "because such strings could also
be so-called fundamental strings, appearing in string-theory
models. So our measurement also offers a way of probing
string-theory models, which is very rare today.""This result
was one of the long-lasting milestones that LIGO was designed
to achieve," Mandic says. Once it goes online in 2014,
Advanced
LIGO, which will utilize the infrastructure of the LIGO
observatories and be 10 times more sensitive than the current
instrument, will allow scientists to detect cataclysmic events
such as black-hole and neutron-star collisions at
10-times-greater distances."Advanced LIGO will go a long way in
probing early universe models, cosmic-string models, and other
models of the stochastic background. We can think of the
current result as a hint of what is to come," he adds."With
Advanced LIGO, a major upgrade to our instruments, we will be
sensitive to sources of extragalactic gravitational waves in a
volume of the universe 1,000 times larger than we can see at
the present time. This will mean that our sensitivity to
gravitational waves from the Big Bang will be improved by
orders of magnitude," says Jay Marx of the California Institute
of Technology, LIGO's executive director."Gravitational waves
are the only way to directly probe the universe at the moment
of its birth; they're absolutely unique in that regard. We
simply can't get this information from any other type of
astronomy. This is what makes this result in particular, and
gravitational-wave astronomy in general, so exciting," says
David Reitze, a professor of physics at the University of
Florida and spokesperson for the LIGO Scientific
Collaboration.Related Links
Gravity
waves 'around the corner'
The
LIGO Scientific Collaboration & The Virgo
Collaboration.
Nature
460, 990-994 (2009)
Most of the information gathered on the Big Bang comes from
measuring the existing ratios of elements in the universe, and
from the
cosmic
microwave background, an electromagnetic radiation "echo"
of the Big Bang found at radio wavelengths.A similar "echo" of
gravitational waves is still believed to exist in the universe
as the "
stochastic
background," analogous to a superposition of many waves of
different sizes and directions on the surface of a pond. The
amplitude of this background is directly related to the
parameters that govern the behavior of the universe during the
first minute after the Big Bang.Earlier measurements of the
cosmic microwave background have placed the most stringent
upper limits of the stochastic gravitational wave background at
very large distance scales and low frequencies. The new
measurements—taken over a two-year period between 2005
and 2007—by LIGO and Virgo directly probe the
gravitational wave background in the first minute of its
existence, at time scales much shorter than accessible by the
cosmic microwave background.The research, which appears in
Nature
, also constrains models of cosmic strings, objects that
are proposed to have been left over from the beginning of the
universe and subsequently stretched to enormous lengths by the
universe's expansion; the strings, some cosmologists say, can
form
loops
that produce gravitational waves as they oscillate, decay,
and eventually disappear.Gravitational waves carry with them
information about their violent origins and about the nature of
gravity that cannot be obtained by conventional astronomical
tools. The existence of the waves was predicted by
Albert
Einstein in 1916 in his
general
theory of relativity. The LIGO and GEO instruments have
been actively searching for the waves since 2002; the Virgo
interferometer joined the search in 2007."Combining
simultaneous data from the LIGO and Virgo interferometers gives
information on gravitational-wave sources not accessible by
other means. It is very encouraging that the first result of
this alliance makes use of the unique feature of gravitational
waves being able to probe the very early universe. This is very
promising for the future," says Francesco Fidecaro, a professor
of physics with the University of Pisa and the Istituto
Nazionale di Fisica Nucleare, and spokesperson for the Virgo
Collaboration.
The analysis used data collected from the LIGO interferometers,
a 2-km and a 4-km
detector in Hanford, Washington (left), and a
4-km instrument in
Livingston, Louisiana (below right). Each of the L-shaped
interferometers uses a laser split into two beams that travel
back and forth down long interferometer arms. The two beams are
used to monitor the difference between the two interferometer
arm lengths.According to the general theory of relativity, one
interferometer arm is slightly stretched while the other is
slightly compressed when a gravitational wave passes by.
The interferometer is constructed in such a way that it can
detect a change of less than a thousandth the diameter of an
atomic nucleus in the lengths of the arms relative to each
other.Because of this extraordinary sensitivity, the
instruments can now test some models of the evolution of the
early universe that are expected to produce the stochastic
background."Since we have not observed the stochastic
background, some of these early-universe models that predict a
relatively large stochastic background have been ruled out,"
says Vuk Mandic, assistant professor at the University of
Minnesota."We now know a bit more about parameters that
describe the evolution of the universe when it was less than
one minute old," Mandic adds. "We also know that if cosmic
strings or superstrings exist, their properties must conform
with the measurements we made—that is, their properties,
such as string tension, are more constrained than before."This
could be interesting, he says, "because such strings could also
be so-called fundamental strings, appearing in string-theory
models. So our measurement also offers a way of probing
string-theory models, which is very rare today.""This result
was one of the long-lasting milestones that LIGO was designed
to achieve," Mandic says. Once it goes online in 2014,
Advanced
LIGO, which will utilize the infrastructure of the LIGO
observatories and be 10 times more sensitive than the current
instrument, will allow scientists to detect cataclysmic events
such as black-hole and neutron-star collisions at
10-times-greater distances."Advanced LIGO will go a long way in
probing early universe models, cosmic-string models, and other
models of the stochastic background. We can think of the
current result as a hint of what is to come," he adds."With
Advanced LIGO, a major upgrade to our instruments, we will be
sensitive to sources of extragalactic gravitational waves in a
volume of the universe 1,000 times larger than we can see at
the present time. This will mean that our sensitivity to
gravitational waves from the Big Bang will be improved by
orders of magnitude," says Jay Marx of the California Institute
of Technology, LIGO's executive director."Gravitational waves
are the only way to directly probe the universe at the moment
of its birth; they're absolutely unique in that regard. We
simply can't get this information from any other type of
astronomy. This is what makes this result in particular, and
gravitational-wave astronomy in general, so exciting," says
David Reitze, a professor of physics at the University of
Florida and spokesperson for the LIGO Scientific
Collaboration.Related Links
Gravity
waves 'around the corner'
The
LIGO Scientific Collaboration & The Virgo
Collaboration.
Nature
460, 990-994 (2009)
