Why search for GWs?
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New tests of general relativity
Study known sources – potential new discoveries that are inaccessible using EM
View the universe prior to recombination. Observe T=0? The ultimate naked singularity?
Because it’s fun (besides, Einstein said it’s worth doing!)
5695 papers with the words “Gravitational
Waves” in the title, 2004-2014
http://science.gsfc.nasa.gov/663/images/gravity/GWspec.jpg
http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html
GW Sources:
Relic background: A stochastic signal from the Big Bang itself, this consists of quantum
fluctuations in the initial explosion that have been amplified by the early expansion of the
Universe. While the spectral shape of this source can be predicted, its overall strength is
highly uncertain, but is constrained by the fact that gravitational wave perturbations are
one of several components contributing to the observed temperature fluctuations in the
cosmic microwave background. This limits the maximum strength of gravitational waves at
cosmological length scales. Two curves are shown: one at the upper limit of the
observational constraints, and another an order of magnitude weaker.
Binary background: Another stochastic signal, this one arising from thousands of binary
systems emitting gravitational waves continuously in overlapping frequency bands. The
individual signals are unresolveable. At long wavelengths (larger than 1014m), the binaries
in question are pairs of supermassive black holes (millions of times the mass of the Sun)
orbiting in the centers of galaxies. The hump at shorter wavelengths (1013 to 1011m) is
contributed by binary white dwarf stars within our own Galaxy.
SMBHB (Super-Massive Black Hole Binaries): Occasionally, one of the supermassive black
hole systems mentioned above will merge, producing a huge burst of gravitational waves
at millihertz frequencies. Such bursts would be detectable throughout most of the known
Universe, though the rate is highly uncertain: one per year is an optimistic estimate.
http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html
GW Sources:
WDB (White Dwarf Binaries): Above the white dwarf stochastic background are a few
thousand individually-resolveable white dwarf binary systems in our Galaxy. Some of these
systems have already been charted with conventional astronomy, and thus would be
known callibrators for future gravitational-wave detectors.
EMRI (Extreme-Mass-Ratio Inspirals): These are compact stellar remnants (white dwarfs,
neutron stars, or stellar-mass black holes only a few times more massive than our Sun) in
the process of being captured and swallowed by a supermassive black hole (millions of
times more massive than the Sun).
BHB (Black Hole Binaries): These are binary systems consisting of two stellar-mass black
holes (a few times the mass of the Sun).
NSB (Neutron Star Binaries): These are binary systems consisting of two neutron stars.
NS (Neutron Stars): This refers to the gravitational waves generated by individual neutron
stars as they spin. In order to generate gravitational waves, the neutron star must deviate
from pure axisymmetry. Several mechanisms have been proposed for generating or
sustaining such asymmetries, but their magnitudes are highly uncertain; the plot indicates
some optimistic upper limits.
http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html
GW Detectors:
Cosmic microwave background: Several thousand years after the Big Bang, when the hot
plasma of protons and electrons cooled and combined to form the first atoms,
electromagnetic radiation was released into the newly-transparent Universe. Today, this
cooled and redshifted radiation is seen as a pervasive microwave background. Density
fluctuations in the plasma resulted in small fluctuations in observed temperature across
the sky, but long-wavelength gravitational waves will also contribute their own
perturbations to the spectrum. At present these contributions are difficult to separate
out, so the total observed fluctuations place an upper limit on the size of gravitationalwave fluctuations.
http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html
GW Detectors:
Pulsar timing: Pulsars are spinning neutron stars that emit beams of electromagnetic
radiation, seen as "pulses" when they sweep over the Earth. Since the spin of a neutron
star is very stable, these pulses can be predicted and fit with high precision. A passing
gravitational wave alters the path length between the pulsar and the Earth, changing the
pulse arrival times in a fluctuating manner. The lack of such fluctuations can be
interpreted as an upper limit on gravitational waves that have wave periods shorter than
the total duration of the pulse observations (years or decades). A gravitational wave could
be detected if two or more pulsars show a correlated pattern of fluctuations in pulse
arrival times.
http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html
Detectors:
LIGO (Laser Interferometer Gravitational-wave Observatory): This consists of two
facilities in separated locations in North America. Each facility has an L-shaped vacuum
tube 4 kilometres long, with masses hanging at the corner and ends of each arm, carefully
shielded against vibrations or other outside disturbances. A passing gravitational wave
changes the relative distances between the masses in the two arms, which can be
detected by interfering laser beams traveling along each arm. Present-day sensitivity is at
a level where detection of gravitational waves is plausible, if not likely.
http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html
Gravitational waves from inflation generate twisting pattern in the polarization of the CMB,
known as a "curl" or B-mode pattern. Shown here is the actual B-mode pattern observed with
the BICEP2 telescope, with the line segments showing the polarization from different spots on
the sky. The red and blue shading shows the degree of clockwise and anti-clockwise twisting of
this B-mode pattern. (BICEP2 Collaboration)