1. Laser interferometric gravitational wave detectors
The search for the elusive wave
Nergis Mavalvala
(LIGO Scientific Collaboration)
Danish Physical Society, June 2010

2. Outline Gravitational waves (GWs) Sources Detectors
Sampling of astrophysical searches
Next generation detectors
Detectors in space

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. Spacetime curvature The mass of an object curves the spacetime fabric
When the massive object vibrates, “ripples” of the spacetime propagate outward from it 
Image courtesy plus.math.org
GRAVITATIONAL WAVE
Image courtesy lisa.nasa.gov

5. 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

6. 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

7. Astrophysical sources of GWs
Ingredients
Lots of mass (neutron stars, black holes)
Rapid acceleration (orbits, explosions, collisions)
Colliding compact stars
Merging Black Holes
Supernovae
The big bang
Earliest moments
The unknown
Now 13 billion years
GWs 0 years
CMB 400 thousand years
Looking back in time

8. 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

9. Black hole mergers Contours of GWs in x polarization
Yellow contours  Tidal forces
Red contours  GWs
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. 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.
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."
GRO J produces 450 Hz oscillations because it is spinning
NASA

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. In our galaxy (21 thousand light years away, 8 kpc)
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, 8 kpc)
h ~ 10-18
In the Virgo cluster of galaxies (50 million light years away, 15 Mpc)
h ~ 10-21 M r R

13. Interferometric detectors
Laser
Photodetector
GW from space
Laser
Photodetector
1000 times smaller than the nucleus of an atom

14. 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

15. GW detector at a glance Mirrors hang as pendulums Quasi-free particles
Respond to passing GW
Filter external force noise
20 kW
Optical cavities
Mirrors facing each other
Builds up light power
Lots of laser power P
Signal  P
Noise 
10 W

16. 10 kg Fused Silica 25 cm diameter 10 cm thick < lambda/5000
over beam diameter

17. Initial LIGO (2005 to 2007) Initial LIGO Seismic noise Thermal noise
Ground vibrations
Initial LIGO
Thermal noise
Damped pendulum
Shot noise
Photon counting
SQL:
h(f) = sqrt(8*hbar/M)/Omega/L
Sounds – 200 Hz, 440 Hz, 1 kHz, 10 kHz

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

19. Gravitational-wave searches
Instrument and data

20. Science runs and sensitivity
1st Science Run
Sept 02
(17 days) S2
2nd Science Run
Feb – Apr 03
(59 days) S3
3rd Science Run
Nov 03 – Jan 04
(70 days)
Strain (sqrt[Hz]-1)
LIGO Target Sensitivity S5
5th Science Run
Nov 05 onward
(1 year integrated) S4
4th Science Run
Feb – Mar 05
(30 days)
Frequency (Hz)

21. Science runs and Sensitivity

22. Astrophysical searches
Coalescence of binary compact objects (neutron stars, black holes, primordial BH)
Core collapse supernovae
Black hole normal mode oscillations
Neutron star rotational instabilities
Gamma ray bursts
Cosmic string cusps
Periodic emission from pulsars (esp. accretion driven)
Stochastic background (incoherent sum of many sources or very early universe)
Expect the unexpected!
Transient
Campanelli et al., Lazarus Project
GWs
neutrinos
photons
now
High duty cycle

23. Sampling of current GW searches
Stochastic Background

24. 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

25. Stochastic GW background
What’s our Universe made of?
Elements in the early Universe
10-5
10-6
Dark matter 23%
Initial LIGO (Nature 2009)
Atoms 4%
Speculative structures (cosmic strings)
10-8
Energy density in GWs
GWs ??
10-9
Advanced LIGO (1 year data)
Sensitivity scales as sqrt(BW*T_int) for Omega, or fourth-root(BW*T_int) for strain.
S5 Nature result = 6.9e-6 (Nature 460, , 2009)
Dark energy 73%
10-13
Inflation
f ~ 100 Hz

26. Examples of current GW searches
Gamma-ray bursts
Pulsars
Binary Inspirals

27. Gamma-ray Bursts GRB 070201 Looked for a GW signal in LIGO
Short, hard gamma-ray burst
Consistent with being in M31
Leading model for short GRBs: binary merger involving a neutron star
Looked for a GW signal in LIGO
Searched for both inspiral and burst signals
No plausible GW signal found
Abbott et al., Ap. J 681, 1419 (2008)
Conclusion: it was most likely an SGR giant flare in M31
Mazets et al., Ap. J 680, 545 (2008)
Ofek et al., Ap. J 681, 1464 (2008)
This is the improved position localization, using Konus-Wind, INTEGRAL and MESSENGER
Leading model for short hard GRBs: binary merger involving a neutron star (found in giant elliptical galaxies, too old (> 5 Gyr) to be supernovae. 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.
SGR: Crustal cracking may excite quasinormal modes which emit GW.
Significance of extragalactic soft gamma repeater (SGR) giant flares as origin of some short hard GRBs
The LIGO result is mentioned in these papers as evidence against a merger
Ofek et al.: “Given the properties of this GRB, along with the fact that LIGO data argue against a compact
binary merger origin in M31, it is an excellent candidate to have been an extragalactic SGR giant flare…. However, we cannot rule out the possibility that it was a short-duration GRB in the background.”
Mazets et al.: Title of paper (paraphrased) is: “A giant flare from an SGR in M31”
IPN 3-sigma error region from Mazets et al., ApJ 680, 545

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
0.07 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.)
Abbott et al., Ap. J (2009)

29. Search for Binary Inspirals
Number of galaxies
Distance (~50 Mly)
Initial LIGO
Sources
Binary neutron stars (~1 – 3 Msun)
Binary black holes (< 30 Msun)
Primordial black holes (< 1 Msun)
Search method
Look for “chirps”
Limit on rate at which NS are coalescing in galaxies like our own
BBH
BNS
Here shown is inspiral range – averaged over all sky
BNS inspiral horizon distance – two 1.4 Msun NS optimally oriented, SNR =8
For binary black hole searches the effective distance is for two 5 Msun BHs optimally oriented with SNR =8.
50 Mly is about 15 Mpc. So units of the Kalogera graph is distance to Virgo cluster.
S5/VSR1 results (arXiv: )
S5 90% confidence rate corresponds to one merger every 67 years in a MW-like galaxy.
The S5 results are quoted in terms of blue solar luminosity, L_10. The MW has ~1.7*L_10.
The rate of compact binary coalescences is expected to be proportional to be the blue light luminosity of a galaxy. L_10 is 10^10 times the blue solar luminosity.
The S5 result is < 8.7e-3 per year per L10 or 8.7e-3*1.7 = per year per MWEG.
S5/VSR1  R < per year per MW-like galaxy

30. Farther away
1000x event rate
NS - NS : 15  300 Mpc

31. Next generation detectors
Advanced LIGO

32. 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.

33. Astrophysics with Advanced LIGO
Factor 10 improved amplitude sensitivity
(Range)3 = rate
Factor 4 lower frequency bound
Construction begins 2011
Keep infrastructure of initial LIGO but replace detector components with new designs
NS Binaries
Initial LIGO: ~15 Mpc
Adv LIGO: ~300 Mpc
BH Binaries
Initial LIGO: 10 Mo, 100 Mpc
Adv LIGO : 50 Mo, z=2
Stochastic background
Initial LIGO: ~3e-6
Adv LIGO ~3e-9

34. Strain sensitivity

35. Advanced LIGO improvements
Seismic noise
Active isolation system
Mirrors suspended as fourth (!!) stage of quadruple pendulums
Thermal noise
Suspension  monolithic fused quartz
Test mass  higher mechanical Q material; more massive (40 kg)
Optical noise
Laser power  increase to ~200 W
Optimized interferometer response  signal recycling

36. Space observatories
LISA DECIGO

37. 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

38. LISA (2020?)

39. Outlook
Most sensitive GW telescopes with user community of few 100 scientists operational
Over 30 search pipelines implemented
Advanced LIGO begins construction in 2011
R&D for third generation detectors underway (needs to be done now)
Gravity gradients
Underground observatories?
Active cancellation?
Optics and quantum nondemolition techniques
High laser power systems
Clever optical designs
(Non)linear optics
Thermal noise mitigation
Super-materials?
Cryogenic operation
NO GW EVENTS DETECTED (YET)

40. 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

41. The End