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

2. What does our Universe look like?

3. Our Sun and the Solar System
300 light minutes
8 light minutes
1 light minute = 17.99e6 km
Pluto is between 4 and 7 billion km from Sun  5e9/18e6 = 280 light minutes
Graphic courtesy solarsystempictures.net

4. Our stellar neighbors Proxima Centauri (4.2 light years away)
Image courtesy STScI
Image courtesy NASA/CXC/SAO

5. Our Galaxy: The Milky Way
Solar System
Image courtesy apod.nasa.gov
Contains ~100 billion stars
~100 thousand light years (l.y.) across
Sun is ~26 thousand l.y. from center
Graphic courtesy enchantedlearning.com

6. Other galaxies
M74
M51
This composite X-ray (red)/optical (blue & white) image of the spiral galaxy M74 highlights an ultraluminous X-ray source (ULX) shown in the box. ULX sources are distinctive because they radiate 10 to 1000 times more X-ray power than neutron stars and stellar mass black holes. Chandra observations of this ULX have provided evidence that its X-radiation is produced by a disk of hot gas swirling around a black hole with a mass of about 10,000 Suns.
The ULX exhibits strong, nearly periodic variations in its X-ray brightness every two hours. These variations are likely produced by changes in the hot gas disk around the black hole. The size of the disk is related to the mass of the black hole, so more massive black holes are expected to vary over longer periods.
The observed two-hour variation suggests that this black hole has a mass of about 10,000 Suns, which would indicate that it belongs to a possible new class of black holes - intermediate mass black holes. These black holes have masses well above known stellar-mass black holes of about 10 solar masses, and well below the multimillion solar mass black holes in the centers of galaxies.
How could intermediate mass black holes form? The leading theories under consideration are that they form as dozens or even hundreds of stellar-mass black holes merge in the center of a dense star cluster, or that they are the remnant nuclei of small galaxies that are in the process of being absorbed by a larger galaxy.
(Right) A composite NASA image of the spiral galaxy M81, located about 12 million light years away, includes X-ray data from the Chandra X-ray Observatory (blue), optical data from the Hubble Space Telescope (green), infrared data from the Spitzer Space Telescope (pink) and ultraviolet data from GALEX (purple). The inset shows a close-up of the Chandra image. At the center of M81 is a supermassive black hole that is about 70 million times more massive than the Sun.

7. Billions of galaxies Hubble Ultradeep Field
The Hubble UltraDeep Field includes galaxies of various ages, sizes, shapes, and colors. The smallest, reddest galaxies, of which there are approximately 10000, are some of the most distant galaxies to have been taken by an optical telescope, existing at the time shortly after the big bang.

8. Earliest light from the Big Bang
First photons that could escape from the hot soup of photons and elementary particles in the early Universe
Cold! 2.73 K (~ -270 °C)

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

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

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

12. The many colors of light
Light particles – photons – can be emitted with different energies, depending on the emission process

13. 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

14. Galaxies collide (NGC6240)
INFRARED (SPITZER)
NGC 6240 offers a rare glimpse of a cosmic catastrophe in its final throes. The titanic galaxy-galaxy collision is located a mere 400 million light-years away in the constellation Ophiuchus. One of the brightest sources in the infrared sky, the merging galaxies spew distorted tidal tails of stars, gas, and dust and undergo frantic bursts of star formation. The two supermassive black holes in the original galactic cores will also coalesce into a single, even more massive black hole. Soon, only one large galaxy will remain. This dramatic image of the scene is a multiwavelength composite; red colors trace infrared emission from dust recorded by the Spitzer Space Telescope, with Hubble visible light images of stars and gas in green and blue hues. The view spans over 300,000 light-years at the estimated distance of NGC 6240.
OPTICAL (HUBBLE)

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

16. 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.

17. 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.

18. Some important things to know about Black Holes
Don’t get too close
They are shrouded in an “event horizon”
An imaginary exterior boundary where you would need to move at exactly the speed of light to avoid falling into the Black Hole
They come in all sizes ranging from a few times the mass of our Sun to millions and even billions of times the mass of our Sun
These super-massive Black Holes are found at the centers of galaxies

19. 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?

20. Gravity to the rescue

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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)

27. 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)

28. 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

29. 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

30. 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

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

32. 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

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

35. 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

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

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

38. The End