LIGO e la Sfida delle Inafferrabili Onde Gravitazionali Laura Cadonati Massachusetts Institute of Technology LIGO Scientific Collaboration Trento, 21 Febbraio 2007 LIGO-G070030-00

Newton’s Law of Universal Gravitation (1686) Mechanics [F=ma] and the Universal law of gravitation [centripetal acceleration a=GM/d2] explained various puzzles of the time: Why things fall Orbit of planets and comets Tides Perturbation of the moon’s motion due to the gravitational attraction of the sun BUT: The gravitational field is static The attraction between two masses is instantaneous action at a distance what mechanism produces the mysterious force of attraction in Newton’s theory? LIGO-G070030-00

Einstein’s Vision: General Relativity (1916) Gravity is not a force, but a property of space-time Smaller masses travel toward larger masses, not because "attracted" by a mysterious force, but because they travel through space that is warped by the larger object. "Mass tells space-time how to curve, and space-time tells mass how to move.“ John Archibald Wheeler Einstein’s Equations: When matter moves, or changes its configuration, its gravitational field changes. This change propagates outward as a ripple in the curvature of space-time: a gravitational wave. LIGO-G070030-00

Newton’s Universal Gravitation action at a distance Einstein’s General Relativity information is carried by a gravitational wave traveling at the speed of light LIGO-G070030-00

A New Probe into the Universe Radio x-ray Gravitational Waves will give us a different, non electromagnetic view of the universe, and open a new spectrum for observation. This will be complementary information, as different from what we know as hearing is from seeing. CMB g-ray IR GRBs EXPECT THE UNEXPECTED! Gravitational Waves carry information from the bulk motion of matter. With them we can learn the physics of black holes, spinning neutron stars, colliding massive bodies, and gain further insights in the early universe. GW sky?? LIGO-G070030-00

Astrophysics with E&M vs Gravitational Waves GW Accelerating charge Accelerating aspherical mass Wavelength small compared to sources  images Wavelength large compared to sources  no spatial resolution Absorbed, scattered, dispersed by matter Very small interaction; matter is transparent Frequency > 10 MHz and up Frequency < 10 kHz Dipole Radiation, 2 polarizations (up-down and left-right) Quadrupole Radiation, 2 polarizations (plus and cross) There are some important differences between light or radio waves and gravitational waves. Light waves move through space and time as oscillations of electric and magnetic fields. The fields have directions that are perpendicular to the direction of the wave’s motion. A light wave coming toward an observer has fields that oscillate to the left and right, or up and down, or a mix of the two directions. This direction of oscillation of the fields, or polarization, determines how the wave interacts with matter. An electric charge placed at various points along the wave shown above will be forced either to the left and right or up and down depending on the polarization of the wave. The wave’s electric field causes electrons in an antenna to move back and forth along the antenna when it is aligned with the polarization of the wave. This generates an electrical signal which a radio can use to produce sound. Just as light waves have two polarizations, horizontal and vertical, gravitational waves also have two polarizations. These are vertical-horizontal, like an addition symbol "+," and at 45 degrees to these directions, like an "x." A small mass placed at various points along the gravitational wave shown above will be stretched along one axis and squeezed along the perpendicular axis, then one-half cycle later the reverse occurs and the axis that was squeezing now stretches while the stretching axis squeezes. plus cross Very different information, mostly mutually exclusive LIGO-G070030-00

of the Existence of GWs: We have Indirect Proof of the Existence of GWs: Pulsar System PSR 1913 + 16 (R.A. Hulse, J.H. Taylor Jr, 1975) A 17/sec pulsar (neutron star in rapid rotation, emanating periodic pulses of electromagnetic radiation) orbits around a neutron star with period = 8 hours Only 7kpc away General Relativity prediction: the orbital radius diminishes 3mm/orbit; a collision is expected in 300 million years Source: www.NSF.gov The rotation period diminished 14 sec in 1975-94; energy loss Optimum agreement with the predictions of general relativity: the energy is carried away by gravitational waves! LIGO-G070030-00

…But They are Hard to Find: Space-Time is Stiff! Einstein’s equations are similar to equations of elasticity: T = (c4/8πG)h T = stress tensor, G = Curvature tensor c4/8πG ~ 1042N is the space-time “stiffness” (energy density/unit curvature) The wave can carry huge energy with miniscule amplitude: h ~ (G/c4) (E/r) For colliding 1.4M neutron stars in the Virgo Cluster: M r R I =quadrupole mass distribution of source M  1030 kg R  20 km F  400 Hz r  1023 m h ~10-21 LIGO-G070030-00

When a GW Passes Through Us… …we “stretch and squash” in perpendicular directions at the frequency of the GW: Time Leonardo da Vinci’s Vitruvian man The effect is greatly exaggerated!! If the Vitruvian man were 4.5 light years tall with feet on hearth and head touching the nearest star, he would grow by only a ‘hairs width’ To directly measure gravitational waves, we need an instrument able to measure tiny relative changes in length, or strain h=DL/L LIGO-G070030-00

Suspended Mirrors as Free Masses Interferometers: Suspended Mirrors as Free Masses Initial LIGO goal: measure difference in length to one part in 1021, or 10-18 m strain h = DL/L LIGO-G070030-00

Giant “Ears” Listen to the Vibrations of the Universe Beam patterns: F+,Fx : [-1, 1] F = F(t; a, d) Fx average F+ LIGO-G070030-00

The LIGO Observatory Hanford (WA) 4 km + 2 km interferometers Livingston (LA) 4 km interferometer LIGO-G070030-00

Hanford, Washington 4 km 2 km 4 km 2 km LIGO-G070030-00

Livingston, Louisiana 4 km LIGO-G070030-00

The LIGO Scientific Collaboration LIGO-G070030-00

An International Quest: Ground-Based Detectors AURIGA INFN Legnaro, Italy 1 Bar detector ALLEGRO Baton Rouge LA Nautilus, Italy 1 Bar detector Explorer, CERN 1 Bar detector Interferometers And Resonant Bars LIGO-G070030-00

Initial LIGO Sensitivity Limits Seismic Noise test mass (mirror) Thermal (Brownian) Noise Residual gas scattering Beam splitter LASER Wavelength & amplitude fluctuations Radiation pressure photodiode Quantum Noise "Shot" noise LIGO-G070030-00

Mitigation of Noise Sources Photon Shot Noise: 10W Nd-YAG laser Fabry Perot Cavities Power Recycling Thermal noise: Use low loss materials Work away from resonances Thin suspension wires Seismic noise: Passive Isolation Stacks Pendulum suspension All under vacuum LIGO-G070030-00

Interferometry Anti-Symmetric Strain Readout Ly ~ Lx ~ 4 km 14 kW RM BS ITMX ITMY ETMY ETMX Reflected Port Anti-Symmetric Pickoff Ly Lx lx Input Beam ly Ly ~ Lx ~ 4 km ly ~ lx ~ 9 m Strain Readout (AS_Q) ∝ (Ly-Lx) 14 kW 6 W 250 W Fabry-Perot cavities: light makes ~ 100 bounces Power recicling: another factor 40 in power ~0.2 W LIGO-G070030-00

Vacuum for a Clear Light Path LIGO beam tube (1998) 1.2 m diameter - 3mm stainless steel 50 km of weld Corner Station 20,000 m3 @ 10-8 torr; earth’s largest high vacuum system LIGO-G070030-00

Suspended Mirrors 10 kg Fused Silica, 25 cm diameter and 10 cm thick 0.3mm steel wire Local sensors/actuators for damping and control forces LIGO-G070030-00

Passive Seismic Isolation System Tubular coil springs with internal constrained-layer damping, layered with reaction masses Isolation stack in chamber LIGO-G070030-00

Active Seismic Pre-Isolation for a Special Livingston Problem: Logging RMS motion in 1-3 Hz band The Livingston Observatory is located in a pine forest popular with pulp wood cutters Spiky noise (e.g. falling trees) in 1-3 Hz band creates dynamic range problem for arm cavity control 10-6 night day Livingston 10-7 Displacement (m) Hanford 10-8 The installation of HEPI (Hydraulic External Pre-Isolator), for active feed-forward isolation (Advanced LIGO technology) has sensibly improved the stability of Livingston: can lock in day time! LIGO-G070030-00

Despite some obstacles along the way… LIGO-G070030-00

…LIGO meets its experimental challenges the design sensitivity predicted in the 1995 LIGO Science Requirements Document was reached in 2005 S2: Feb.- Apr. 2003 59 days BNS reach ~ 1Mpc S3: Oct. ‘03 - Jan. ‘04 70 days BNS reach ~ 3Mpc Science Requirement. document (1995) S1: Aug. - Sep. 2002 17 days BNS reach ~100kpc S4: Feb. - Mar. 2005 30 days BNS reach ~ 15Mpc S5: Nov 2005 – Current LIGO-G070030-00

LIGO Hanford control room Science Run 5 S5: started Nov 2005 and ongoing Goal: 1 year of coincident live-time LIGO Hanford control room 31 Mar 2006 – S5 LIGO-G070030-00

Science with LIGO: Sources Lurking in the Dark Binary systems Neutron star – Neutron star Black hole – Neutron star Black hole – Black hole “Burst” Sources Supernovae Gamma ray bursts Residual Gravitational Radiation from the Big Bang Cosmic Strings Periodic Sources Rotating pulsars ????? BANG! LIGO-G070030-00

Binary Neutron Stars: a Measure of Performance The inspiral waveform for BNS is known analytically from post-Newtonian approximations. We can translate strain amplitude into (effective) distance. 1 parsec = 3.26 ly 10 Mpc = 33 Mly S5 BBH horizon distance peaks at 25+25, 130 Mpc => range ~ 50 Mpc ~150 Mly Range: distance of a 1.4-1.4 M binary, averaged over orientation/polarization Predicted rate for S5: 1/3year (most optimistic), 1/30years (most likely) LIGO-G070030-00

Astrophysical Sources: Binary Inspirals S5 binary neutron star horizon Credits: John Rowe Animation Colliding black hole movie downloaded from the NASA web site: http://www.nasa.gov/vision/universe/starsgalaxies/gwave.html This visualization shows what Einstein envisioned. Researchers crunched Einstein's theory of general relativity on the Columbia supercomputer at the NASA Ames Research Center to create a three-dimensional simulation of merging black holes. This was the largest astrophysical calculation ever performed on a NASA supercomputer. The simulation provides the foundation to explore the universe in an entirely new way, through the detection of gravitational waves. (7.4 Mb - no audio). Credit:Henze, NASA R. Powell LIGO-G070030-00 S5 binary black hole horizon Simulation of gravitational waves produced by colliding black holes. Credit:Henze, NASA

Astrophysical Sources: Bursts ? Uncertainty of waveforms complicates the detection  minimal assumptions, open to the unexpected For sine-gaussians: E_GW is proportional to r^2*f^2*hrss^2 S5 sensitivity: ~0.1M from 20MPc at 153 Hz Swift/ HETE-2/ IPN/ INTEGRAL RXTE/RHESSI LIGO-LHO LIGO-LLO LIGO-G070030-00

Astrophysical Sources: Stochastic Background Cosmological background: Big Bang and early universe Astrophysical background: unresolved bursts NASA, WMAP cosmic GW background CMB (10+12 s) S5 sensitivity: Cosmic GW background limits expected to be near GW~10-5 below the BBN limit! LIGO-G070030-00

Landscape (W0) Log(WGW) Log (f [Hz]) -2 Pulsar Timing -2 Pulsar Timing LIGO S4: Ω0 < 6.5x10-5 (newest) BB Nucleo- synthesis -4 Cosmic strings (W0) -6 Initial LIGO, 1 yr data Expected Sensitivity Log(WGW) -8 Log CMB Adv. LIGO, 1 yr data Expected Sensitivity ~ 1x10-9 -10 Pre-BB model Accuracy of big-bang nucleosynthesis model constrains the energy density of the universe at the time of nucleosynthesis  total energy in GWs is constrained integral_f<1e-8 d(ln f) Omega_GW Pulsar timing  Stochastic GWs would produce fluctuations in the regularity of msec pulsar signals; residual normalized timing errors are ~10e-14 over ~10 yrs observation Stochastic GWs would produce CMBR temperature fluctuations (Sachs Wolfe effect), Measured Delta_T constrains GW amplitude at very low frequencies Kamionkowski et al. (astro-ph/0603144-1) Use Lyman-alpha forest, galaxy surveys, and CMB data to constrain CGWBkgd, i.e. CMB and matter power spectrums. Assume either homogeneous initial conditions or adiabatic. Use neutrino degrees of freedom to constrain models. Adiabatic blue solid  CMB, galaxy and Lyman-alpha data currently available (Kamionkowski) Homogeneous blue dashed  CMB, galaxy and Lyman-alpha data currently available Adiabatic CMBPol (cyan solid)  CMB, galaxy and Lyman-alpha data when current CMB is replaced with expected CMBPol data Homogeneous CMBPol (cyan dashed)  CMB, galaxy and Lyman-alpha data when current CMB is replaced with expected CMBPol data -12 Inflation -14 Slow-roll EW or SUSY Phase transition Cyclic model -18 -16 -14 -12 -10 -8 -6 -4 -2 2 4 6 8 10 Log (f [Hz]) LIGO-G070030-00

Continuous Waves Preliminary S5 expectations: J. Creighton M. Kramer Preliminary Wobbling neutron stars Pulsars with mountains Dana Berry/NASA Credits: A. image by Jolien Creighton; LIGO Lab Document G030163-03-Z. B. image by M. Kramer; Press Release PR0003, University of Manchester - Jodrell Bank Observatory, 2 August 2000. C. image by Dana Berry/NASA; NASA News Release posted July 2, 2003 on Spaceflight Now. Rotating stars produce GWs if they have asymmetries or if they wobble. Accreting neutron stars: Neutron stars spin up when they accrete matter from a companion Observed neutron star spins “max out” at ~700 Hz. Gravitational waves are suspected to balance angular momentum from accreting matter Wobbling neutron stars: The wobbling (or precession) causes the rotation axis of the pulsar to follow a circle-like motion in time (see yellow and green axes at different epochs). The motion is very much like the wobble of a top or gyroscope. As a result, we see the cone-like lighthouse beam of the radio pulsar under different angles, resulting in the observed changes in pulse shape and arrival times. (Image by M. Kramer) Surface asymmetries in neutron stars: a “mountain”, mm high Observed spindown can be used to set strong indirect upper limits on GWs. GWs (or lack thereof) can be used to measure (or set up upper limits on) the ellipticities of the stars. There are many known pulsars (rotating stars!) that produce GWs in the LIGO frequency band (40 Hz-2 kHz). Targeted searches for 73 known (radio and x-ray) systems in S5: isolated pulsars, binary systems, pulsars in globular clusters… There are likely to be many non-pulsar rotating stars producing GWs. All-sky, unbiased searches; wide-area searches. Search for a sine wave, modulated by Earth’s motion, and possibly spinning down: easy, but computationally expensive! S5 expectations: Best limits on known pulsars ellipticities at few x10-7 Beat spin-down limit on Crab pulsar Hierarchical all-sky/all-frequency search Accreting neutron stars LIGO-G070030-00

The Einstein@home Project http://www.physics2005.org Thur Nov 9 2006 15:14 UTC LIGO-G070030-00

How do we avoid fooling ourselves How do we avoid fooling ourselves? Seeing a false signal or missing a real one Require at least 2 independent signals: e.g. coincidence between interferometers at 2 sites for inspiral and burst searches, external trigger for GRB or nearby supernova. Apply known constraints: Pulsar ephemeris, inspiral waveform, time difference between sites. Use environmental monitors as vetos Seismic/wind: seismometers, accelerometers, wind-monitors Sonic/acoustic: microphones Magnetic fields: magnetometers Line voltage fluctuations: volt meters Understand the detector response: Hardware injections of pseudo signals (actually move mirrors with actuators) Software signal injections LIGO-G070030-00

LIGO timeline S5 S6 ~2 years 3.5 yrs 4Q ‘05 ‘06 ‘07 ‘08 ‘10 ‘09 Adv LIGO ~2 years Other interferometers in operation (GEO and/or Virgo) 3.5 yrs The first science run of LIGO at design sensitivity is in progress Hundreds of galaxies now in range for 1.4 M neutron star binary coalescences Enhancement program In 2009 ~8 times more galaxies in range Advanced LIGO Construction start expected in FY08 1000 times more galaxies in range Expect ~1 signal/day - 1/week in ~2014 100 million light years LIGO today Advanced LIGO ~2014 Enhanced LIGO ~2009 The science from the first 3 hours of Advanced LIGO should be comparable to 1 year of initial LIGO LIGO-G070030-00

Advanced LIGO: President Requests FY2008 Construction Start Seismic ‘cutoff’ at 10 Hz 10-22 10-23 Quantum noise (shot noise + radiation pressure) dominates at most frequencies 10-24 10 Hz 100 Hz 1 kHz 10 kHz LIGO-G070030-00

Science Potential of Advanced LIGO Binary neutron stars: From ~20 Mpc to ~350 Mpc From 1/30y(<1/3y) to 1/2d(<5/d) Binary black holes: From ~100Mpc to z=2 Known pulsars: From e = 3x10-6 to 2x10-8 Stochastic background: From ΩGW ~3x10-6 to ~3x10-9 Kip Thorne LIGO-G070030-00

These are exciting times! We are searching for GWs at unprecedented sensitivity. Early implementation of Advanced LIGO techniques helped achieve goals: HEPI for duty-cycle boost Thermal compensation of mirrors for high-power operation Detection is possible, but not assured for initial LIGO detector We are getting ready for Advanced LIGO Sensitivity/range will be increased by ~ 2 in 2009 and another factor of 10 in ~2014 with Advanced LIGO Advanced LIGO will reach the low-frequency limit of detectors on Earth’s surface given by fluctuations in gravity at surface Direct observation: Not If, but When LIGO detectors and their siblings will open a new window to the Universe: what’s out there? www.ligo.caltech.edu www.ligo.org LIGO-G070030-00