1. Gravitational wave detection
The search for the elusive wave
Nergis Mavalvala
(the LIGO and Virgo Scientific Collaborations)

2. Outline What are gravitational waves (GWs)? What sources emit them?
How strong is the emission?
How can we detect them?
What are we searching for?
The GW world in the future
Bold claims!!!
Measure distance changes of m (1/1000 of an atomic nucleus!) over kilometers
Detect a mm bump on 10 km object hundreds of l.y. away
Help unravel the mysteries of Gamma Ray Bursts

3. Understanding gravity
Newton (16th century)
Universal law of gravitation
Worried about action at a distance
Einstein (20th century)
Gravity is a warpage of space-time
Matter tells spacetime how to curve  spacetime tells matter how to move

4. Gravitational wave (GW) basics
Gravitational Waves are a prediction of general relativity “Ripples in spacetime fabric” traveling at speed of light
Stretch and squeeze the space transverse to direction of propagation
Strain
Emitted by aspherical accelerating masses
Like tides  for objects that are free to move, GWs change lengths by fractional amounts
Like tides  GWs change lengths by fractional amounts

5. Astrophysics with GWs vs. Light
Very different information, mostly mutually exclusive
Difficult to predict GW sources based on EM observations
Light GW
Accelerating charge
Accelerating mass
Images
Waveforms
Absorbed, scattered, dispersed by matter
Very small interaction; matter is transparent
100 MHz and up
10 kHz and down

6. Astrophysical sources of GWs
Ingredients
Lots of mass (neutron stars, black holes)
Rapid acceleration (orbits, explosions, collisions)
Colliding compact stars
Merging binaries
Supernovae
The big bang
Earliest moments
The unexpected
GWs
neutrinos
photons
now

7. Pulsar born from a supernova
PULSAR IS BORN: A supernova is associated with the death of a star about eight times as massive as the Sun or more. When such stars deplete their nuclear fuel, they no longer have the energy (in the form of radiation pressure outward) to support their mass. Their cores implode, forming either a neutron star (pulsar) or if there is enough mass, a black hole. The surface layers of the star blast outward, forming the colorful patterns typical of supernova remnants.
ACCRETION SPINS UP THE PULSAR: When a pulsar is created in a supernova explosion, it is born spinning, but slows down over millions of years. Yet if the pulsar -- a dense star with strong gravitational attraction -- is in a binary system, then it can pull in, or accrete, material from its companion star. This influx of material can eventually spin up the pulsar to the millisecond range, rotating hundreds of revolutions per second.
GWs LIMIT ACCRETION INDUCED SPIN UP: As the pulsar picks up speed through accretion, it becomes distorted from a perfect sphere due to subtle changes in the crust, depicted here by an equatorial bulge. Such slight distortion is enough to produce gravitational waves. Material flowing onto the pulsar surface from its companion star tends to quicken the spin, but loss of energy released as gravitational radiation tends to slow the spin due to the principle of conservation of energy. This competition may reach an equilibrium, setting a natural speed limit for millisecond pulsars beyond which they cannot be spun up.
Courtesy of NASA (D. Berry)

8. Millisecond pulsar accretion
As the pulsar picks up speed through accretion, it becomes distorted from a perfect sphere due to subtle changes in the crust, depicted here by an equatorial bulge. Such slight distortion is enough to produce gravitational waves. Material flowing onto the pulsar surface from its companion star tends to quicken the spin, but loss of energy released as gravitational radiation tends to slow the spin due to the principle of conservation of energy. This competition may reach an equilibrium, setting a natural speed limit for millisecond pulsars beyond which they cannot be spun up.
Courtesy of NASA (D. Berry)

9. Black hole mergers Contours of GWs in x polarization
Yellow contours represent tidal forces
As we zoom out we see red contours of GW waves
Notice that x-pol has no emission on equatorial plane.
Contours of GWs in x polarization
Courtesy of J. Centrella, GSFC

10. Spinning black holes
As the animation begins, a wide-angle view shows the black hole and a nearby blue giant star in a binary (double) system. Celestial objects in binary systems orbit closely around their common center of mass. At this point, the black hole is located to the left of a blue giant star. The powerful gravity of the black hole pulls gas from the blue giant, which forms a tail-like structure as it streams toward the black hole.As the animation zooms in the gas can be seen forming a disk-shaped structure as it whirls around the black hole, like soap suds spiraling down a bathtub drain. Lines from the poles of the black hole represent jets of gas being ejected from the vicinity of the black hole at nearly the speed of light (about 186,000 miles per second). Although nothing can escape a black hole once it passes its point of no return, called the event horizon, black holes are "sloppy eaters," often expelling matter that approaches but does not cross the event horizon. The poorly understood jets are frequently seen near black holes that are swallowing copious quantities of gas.
Moving in further we reach the immediate vicinity of the black hole, with the event horizon depicted as a black sphere. The surrounding disk of gas, represented by white and blue rings, whirls around the black hole at different speeds, with the material closest to the black hole approaching the speed of light. Because it moves at different speeds, atoms that comprise the gas rub against each other and become intensely hot, causing them to emit high-energy radiation, like X-rays. These X-rays reveal an otherwise invisible black hole.
The gap between the gas disk and the event horizon represents the innermost stable orbit matter can have before plunging into the black hole. A spinning black hole modifies the fabric of space-time near it. The spinning allows matter to orbit at a closer distance than if the black hole were not spinning, and the closer matter can get the faster it can orbit.
As if black holes weren't menacing enough, astronomers now have observational evidence that at least some of them spin about like whirlpools, wrapping up the fabric of space with them.
Dr. Tod Strohmayer of NASA's Goddard Space Flight Center, Greenbelt, MD, has studied one such black hole system with NASA's Rossi X-ray Timing Explorer and found unique patterns in the X-ray radiation that have previously only been seen in spinning neutron stars. With these new parameters, he could verify that a black hole, like a neutron star, can spin.
The black hole that Strohmayer observed is the stellar variety, which is formed from a collapsed star. When stars at least 10 times more massive than our Sun exhaust their fuel supply, they no longer have the energy to support their tremendous bulk. These stars explode their outer shell of gas in an event called a supernova.
Strohmayer's target was GRO J , a microquasar 10,000 light years from Earth. A microquasar is a specific type of black hole with jets of high-speed particles shooting perpendicularly from the plane of matter that orbits it. Strohmayer observed two QPOs, a previously detected one at about 300 Hertz (Hz) and a newly detected one at 450 Hz. (A hertz is a unit of frequency equal to one cycle per second.)
The black hole mass has been established at seven times the mass of our Sun from earlier optical observations of GRO J "A spinning black hole modifies the fabric of space near it," said Strohmayer. "The spinning allows matter to orbit at a closer distance than if it were not spinning, and the closer matter can get the faster it can orbit. For GRO J we can now say that the only way for it to produce the 450 Hz oscillations is if it is spinning."

11. The sounds of the Universe
Gravitational waves can be encoded into sound
The sounds can give us a very accurate picture of how the source behaves
Change frequencies (like false color)
Binary black holes with almost equal mass (3:1 ratio)
Schwartzschild (no spin)
Kerr (spin like whirlpools)
Sounds courtesy Scott Hughes, MIT

12. Sight and sound
Supermassive black hole (106 Msun) captures small black hole (10 Msun) (frequencies shifted by factor of >1000)
Initially circular orbit rapidly spinning
Initially very elliptical orbit (like a comet) also rapidly spinning
NASA
Image: The two bright sources at the center of this composite x-ray (blue)/ radio (pink) image are co-orbiting supermassive black holes powering the giant radio source 3C 75. Surrounded by multimillion degree x-ray emitting gas, and blasting out jets of relativistic particles the supermassive black holes are separated by 25,000 light-years. At the cores of two merging galaxies in the Abell 400 galaxy cluster they are some 300 million light-years away. Astronomers conclude that these two supermassive black holes are bound together by gravity in a binary system in part because the jets' consistent swept back appearance is most likely due to their common motion as they speed through the hot cluster gas at 1200 kilometers per second. Such spectacular cosmic mergers are thought to be common in crowded galaxy cluster environments in the distant universe. In their final stages the mergers are expected to be intense sources of gravitational waves.
Sounds: These sounds encode waves generated by the spiral-in of stellar mass compact bodies captured by massive black holes - for example, a 10 solar mass black hole spiraling into a million solar mass black hole. Black holes in this mass range are found in the nuclei of almost every galaxy; sources of this type are one of the key science targets for the NASA/ESA LISA mission.
The frequencies of these sources (and of LISA's best sensitivity) are far lower than that of the human ear! (The peak sensitivity of LIGO, by contrast, corresponds almost exactly to human audio.) Accordingly, Scott had to fudge things a bit: All frequencies are shifted by a factor of a few thousand from the way that nature would actually present them. Think of it as the audio equivalent of a "false color" image.
The sound is modulated - the "buzz" you should hear in the first sound - due to the large black hole's very rapid spin. If the large black hole is one million solar masses, then the orbital radius is initial around million kilometers; it decreases to a radius of a few million kilometers before the small body plunges into the massive black hole. (A million solar mass black hole would itself have a radius of about 1 ½ - 3 million kilometers.)
The second sound illustrates a very different case. The initial orbit in this case is extremely eccentric - think of a comet's orbit around the sun. Each of the pops you should hear corresponds to the smaller body passing close to the black hole and moving very rapidly. The sequence of pops gets closer together as the eccentricity shrinks. In this calculation, the eccentricity drops all the way to zero, and the final inspiral is perfectly circular. (Note, we now know that this behavior is not quite right; its manifestation here is because Sound 3 was generated using an approximation to the real laws of GW emission.
Two co-orbiting supermassive BHs at the centers of two galaxies that are
colliding (in Abell 400 galaxy cluster)
Composite Xray and radio images
Sounds courtesy Scott Hughes, MIT

13. Gravitational waves -- the Evidence
Hulse & Taylor’s Binary Neutron Star System
(discovered in 1974, Nobel prize in 1993)
PSR
Two neutron stars orbiting each other at c
Compact, dense, fast  relativistic system
Emit GWs and lose energy
Used time of arrival of radio pulses to measure change in orbital period due to GW emission
Change in orbital period
NS rotates on its axis 17 times/sec. Reaches periastron (minimum separation of binary pair) every 7.75 hours.
Systematic variation in arrival time of pulses. Variation in arrival time had a 7.75 hour period  binary orbit with another star.
Pulsar clock slowed when traveling fastest and in strongest part of grav field (periastron).
Figure shows decrease in orbital period of 76 usec/year. Shift in periastron due to decay of orbit.
Y-axis = change in orbital period relative to 1975 measurement
Define periastron as measure of orbital period
Exactly as predicted by GR for GW emission
Years

14. Strength of GWs: e.g. Binary Neutron Star
Gravitational wave amplitude (strain) for a binary neutron star pair M r R
h ~10-21
Quadrupole formalism is accurate to order of magnitude for most sources.
Involves computing wave generation and radiation reaction from Einstein eqn.
1 light year = 9.5e12 km
1 pc = 30.8e12 km
1 Mpc = 3.26e6 l.y.
Weak internal gravity and stresses  nearly Newtonian source
Kepler’s third law of planetary motion: period^2 = 4*pi^2*radius^3/(G*Msun)
Distances  1 parsec = 3.26 l.y. = 3e18 cm
r ~ 10^23 m ~ 10 Mpc (center of Virgo cluster)
Distance of earth to center of galaxy ~ l.y. ~ 10 kpc
Virgo cluster 15 Mpc or 50 million l.y.

15. In our galaxy (21 thousand light years away)
Strength of GWs
Hulse-Taylor binary pulsar at the end of its lifetime (100 million years from now)
In our galaxy (21 thousand light years away)
h ~ 10-18
In the Virgo cluster of galaxies (50 million light years away)
h ~ 10-21

16. Detecting GWs
GW from space

17. Interferometric detectors
Laser
Photodetector
GW from space
Laser
Photodetector
Very small!
1000 times smaller than the nucleus of an atom

18. Measurement and the real world
How to measure the gravitational-wave?
Measure the displacements of the mirrors of the interferometer by measuring the phase shifts of the light
What makes it hard?
GW amplitude is small
External forces also push the mirrors around
Laser light has fluctuations in its phase and amplitude

19. LIGO: Laser Interferometer Gravitational-wave Observatory3 k m ( 1 s )
MIT
4 km
2 km
NSF
Caltech LA
4 km

21. Initial LIGO during S5 Initial LIGO Seismic noise Suspension thermal
Viscously damped pendulum
Initial LIGO
Shot noise
Photon counting statistics
SQL:
h(f) = sqrt(8*hbar/M)/Omega/L

22. Global network of detectors
GEO
VIRGO
LIGO
TAMA
AIGO
LIGO
Detection confidence
Source polarization
Sky location
LISA

23. Gravitational wave searches
What can we do with the data?

24. Search for GWs from the BIG BANG

25. Cosmological GW Background
385,600
10-22 sec
10+12 sec
Waves now in the LIGO band were produced sec after the Big Bang
WMAP 2003

26. Stochastic GW background
What’s our Universe made of?
Elements in the early Universe
10-5
10-6
Dark matter 23%
Initial LIGO (1 year data)
Atoms 4%
Speculative structures (cosmic strings)
10-8
Energy density in GWs
GWs ??
10-9
Advanced LIGO (1 year data)
Sensitivity scales at sqrt(BW*T_int) for Omega, or fourth-root(BW*T_int) for strain.
Dark energy 73%
10-13
Inflation

27. Search for GWs from PULSARS

28. Continuous Wave Sources
Single frequency (nearly) continuous GW radiation
A neutron star with non-axisymmetric shape distortion (a “bump”)
Assume the neutron star is a rigid rotor
Get limit on ellipticity of the rotating star
0.4 mm “bump” on a 10 km radius object
Joint Bayesian parameter estimation of unknown pulsar parameters: GW amplitude h0, initial phase f0, polarisation angle y and inclination angle i, using data from all interferometers
Produce probability distribution functions for unknown parameters and marginalise over angles to set 95% upper limit on h0
PSR J frequency = Hz
distance = 0.25kpc (800 l.y.)

29. The mystery of GAMMA RAY BURSTs

30. Two kinds of bursts observed
Gamma Ray Bursts
Short-lived bursts of gamma ray photons (the most energetic photons known)
Observed from very distant sources – most are a few billion light years away
Two kinds of bursts observed
Long duration (>2 sec) bursts are associated with supernova explosions
Short duration (<2 sec) bursts were hypothesized to be due to mergers of double neutron stars or black holes (2005)

31. Supernova or merger?
Using observations from Swift, HETE-2, Hubble, Chandra, and BATSE, astronomers found that the parent stars
Were too old (>5 billion years) to be supernova explosions
Remaining candidates for progenitors: old double neutron star (DNS) or neutron star-black hole (NS-BH) mergers
SHBs in non-star-forming region or gaint ellipticals which contain large population of LMXBs that accrete and merge
Core collapse in young, star forming regions

32. GRB070201
Intense short hard GRB located in an error box that includes the spiral arms of the nearby Andromeda galaxy (M31)
About 2.5 million l.y. away
Detected by Konus-Wind, Integral, Swift and Messenger
LIGO detectors were on the air at the time

33. What can LIGO say about GRB070201?
If a signal is detected
Confirms that the progenitor is a binary star merger
Could give more accurate distance to the object
If no signal is detected
Can exclude the progenitor model for a certain mass and distance region
Put a limit on the total energy emitted by the system, assuming the source was in M31

34. What did we find? LIGO did not detect a signal
DM31
25%
50%
75%
90%
LIGO did not detect a signal
Consequence: we can say with >99% confidence that GRB was NOT caused by a binary star merger in M31

35. Coming soon… to an interferometer near you
Enhanced LIGO Advanced LIGO

36. Ultimate limits ? Seismic gravity gradient
When ambient seismic waves pass near and under an interferometric gravitational-wave detector, they induce density perturbations in the Earth, which in turn produce fluctuating gravitational forces on the interferometer’s test masses.
Human gravity gradient
The beginning and end of weight transfer from one foot to the other during walking produces the strongest human-made gravity-gradient noise in interferometric gravitational-wave detectors (e.g. LIGO). The beginning and end of weight transfer entail sharp changes (time scale τ∼20 msec) in the horizontal jerk (first time derivative of acceleration) of a person’s center of mass.

37. Initial LIGO – S5 Input laser power ~ 6 W Initial LIGO
Circulating power ~ 20 kW
Mirror mass
10 kg
Initial LIGO
SQL

38. Enhanced LIGO (Fall 2007) Input laser power ~ 30 W
Circulating power ~ 100 kW
Mirror mass
10 kg
Enhanced LIGO

39. Advanced LIGO (2011) Input laser power > 100 W
Circulating power > 0.5 MW
Mirror mass
40 kg

40. Farther in the future
Space observatory

41. Laser Interferometer Space Antenna (LISA)
Three space craft
Triangular formation
Separated by 5 million km
Formation trails earth by 20º
Approx. constant length arms
Constant solar illumination
1 AU = 1.5x108 km

42. LISA and LIGO

43. When the elusive wave is captured…
Tests of general relativity
Waves  direct evidence for time-dependent metric
Black hole signatures  test of strong field gravity
Polarization of the waves  spin of graviton
Propagation velocity  mass of graviton
Astrophysics
Predicted sources: compact binaries, SN, spinning NS
Inner dynamics of processes hidden from EM astronomy
Dynamics of neutron stars  large scale nuclear matter
The earliest moments of the Big Bang  Planck epoch
Precision measurement below the quantum noise limit

44. Ultimate success… New Instruments, New Field, the Unexpected…

45. The End