1. The search for those elusive gravitational waves
The LIGO detectors
Precision measurement
Search for the elusive waves
Nergis Mavalvala
(the LIGO Scientific Collaboration)

2. 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

3. Gravitational waves
Ripples of the space-time fabric
Act like tides  for objects that are free to move, tides change lengths by fractional amounts
Stretch and squeeze the space transverse to direction of propagation
Emitted by non-spherical massive objects

4. 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

5. Astrophysical sources of GW
Ingredients
Lots of mass (neutron stars, black holes)
Acceleration (orbits, explosions, collisions)
Not spherically symmetric (not round)
Colliding star corpses
Coalescing binaries
The big bang
Earliest moments
The unexpected
GWs
neutrinos
photons
now

6. 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

7. Gravitational waves -- the Evidence
Hulse & Taylor’s Binary Neutron Star System
(discovered in 1973, 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

8. 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

9. Gravitational Wave Interferometers
GW from space
Effect of GW on ‘test’ masses
Interferometric measurement
Very small!
1000 times smaller than the nucleus of an atom

11. 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

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

14. Initial LIGO – Sept
Initial LIGO

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

16. Enhanced LIGO
Enhanced LIGO (2008)

17. Advanced LIGO
Advanced LIGO (2011)

18. An example of a GW search
Primordial Stochastic Background

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

20. 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

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

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

23. The End