1. Listening to the cosmos with Gravitational Waves
Nergis Mavalvala
Department of Physics
Massachusetts Institute of Technology

2. Our Universe in one Viewgraph

3. How have we learned all these amazing things about our Universe?

4. We point telescopes into the cosmos
Spitzer Space Telescope
Hubble Space Telescope
Keck Observatory
Fermi Gamma-ray Observatory
Chandra Xray Observatory
Magellan Telescope

5. What do telescopes see? Light (electromagnetic radiation)
Emission of waves due to time-varying electric and magnetic fields
Essential ingredients
Charge (usually electrons)
Motion
Charge at rest
Moving at constant velocity
Accelerating

6. What do the colors of light tell us?
They tell us about what kinds of atoms emitted the light
Examples
Visible light from our nearest star  the SUN
Xrays and gamma rays from cosmic explosions  SUPERNOVAE
Radio waves from rapidly rotating, very dense stars  PULSARS

7. But can light tell the whole story?
What about dark objects?

8. A Black Hole (GRO J )
April Hobart (Chandra). In the center of a swirling whirlpool of hot gas is likely a beast that has never been seen directly: a black hole. Studies of the bright light emitted by the swirling gas frequently indicate not only that a black hole is present, but also likely attributes. The gas surrounding GRO J , for example, has been found to display an unusual flickering at a rate of 450 times a second. Given a previous mass estimate for the central object of seven times the mass of our Sun, the rate of the fast flickering can be explained by a black hole that is rotating very rapidly. What physical mechanisms actually cause the flickering -- and a slower quasi-periodic oscillation (QPO) -- in accretion disks surrounding black holes and neutron stars remains a topic of much research.

9. What are Black Holes?
They are stars that shrink until their gravity is so strong that even light cannot escape
Our earth has a mass of 6 x 1024 kg (that’s 6 trillion trillion kg) and a radius of km
To become a black hole it would have to shrink to about 1 cm
(It’s not going to)
Photons always travel at the speed of light, but they lose energy when travelling out of a gravitational field and appear to be redder to an external observer. The stronger the gravitational field, the more energy the photons lose because of this gravitational redshift. The extreme case is a black hole where photons from within a certain radius lose all their energy and become invisible. Indeed, light in the vicinity of such strong gravitational fields exhibits quite bizarre behavior.
To understand fully why a black hole can trap light but the light still always travels at constant velocity requires an understanding of the General Theory of Relativity, but the essential point is that the black hole curves spacetime back on itself, so that all paths in the interior of the black hole lead back to the singularity at the center, no matter which direction you go (an analogy in two dimensions is that no matter which direction you go on the surface of the Earth in a "straight line" (what mathematicians call a "geodesic" or a "great circle"), you never escape the Earth but instead return to the same point. Imagine extending that analogy to the 4 dimensions of spacetime and you have a rough explanation for why light travels at light speed, but cannot escape the interior of a black hole.

10. How do we “see” Black Holes?
We can see high energy (X-ray) light from jets of hot gas as material from its surroundings falls onto the Black Hole
We can see stars zooming around some very dense, massive object. Their motion can only be explained by the presence of a Black Hole that they are orbiting
But how could we directly observe a Black Hole?

11. Gravity to the rescue

12. 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

13. 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

14. 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

15. 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 (pretty pictures)
Waveforms (pretty sounds)
Absorbed, scattered, dispersed by matter
Very small interaction; matter is transparent
100 MHz and up
10 kHz and down

16. 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

17. 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

18. In our galaxy (21 thousand light years away, 8 kpc)
Strength of GWs
Typical 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

19. Gravitational wave detectors

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

21. Global network of detectors
GEO
VIRGO
LIGO
TAMA
AIGO
LIGO
LISA

22. 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

23. 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

24. LIGO listened… And had something to say

25. The search for GRB070201 GRB 070201 Looked for a GW signal in LIGO
Very luminous short duration, hard gamma-ray burst
Detected by Swift, Integral, others
Consistent with being in M31
Leading model for short GRBs: binary merger involving a neutron star
Looked for a GW signal in LIGO
No plausible GW signal found
Can say with >99% confidence that GRB was NOT caused by a compact binary star merger in M31
Conclusion: it was most likely a Soft Gamma Repeater giant flare in M31
25%
50%
75%
90%
DM31
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”
Abbott et al., Ap. J 681, 1419 (2008)
Mazets et al., Ap. J 680, 545 (2008)
Ofek et al., Ap. J 681, 1464 (2008)

26. Farther away

27. Strain sensitivity

28. New eyes observe the Universe
and ears
New eyes observe the Universe ^

29. The End