Direct Searches for Stochastic Gravitational Waves with LIGO: status and prospects Albert Lazzarini California Institute of Technology On behalf of the LIGO Scientific Collaboration The 11th International Symposium on Particles, Strings and Cosmology 30 May 2005 Gyeongju, Korea  Black Holes courtesy of NCSA

Outline of Presentation Stochastic gravitational waves Characterization of strain Sources LIGO Concept Principle of operation Sites, facilities Sensitivity Recent observational results Prospects LIGO Laboratory at Caltech

Stochastic gravitational wave background GWs are the able to probe the very early universe Analog from cosmic microwave background -- WMAP 2003 Detect by cross-correlating interferometer outputs in pairs US-LIGO : Hanford, Livingston Europe: GEO600, Virgo Japan: TAMA Good sensitivity requires: GW > 2D (detector baseline) f < 40 Hz for LIGO pair over 3000 km baseline Initial LIGO limiting sensitivity (1 year search):  <10-6 The integral of [1/f•GW(f)] over all frequencies corresponds to the fractional energy density in gravitational waves in the Universe LIGO Laboratory at Caltech

Cosmological Gravitational Waves  Terrestrial LIGO Laboratory at Caltech

Characterization of a Stochastic Gravitational Wave Background GW energy density given by time derivative of strain Assuming SGWB is isotropic, stationary and Gaussian, the strength is fully specified by the energy density in GWs f) in terms of a measurable strain power spectrum, Sgw(f): Strain amplitude scale: Allen & Romano, Phys.Rev. D59 (1999) LIGO Laboratory at Caltech

Stochastic signals from cosmological processes Inflation -- flat spectrum Kolb & Turner, The Early Universe Phase transitions -- peaked at phase transition energy scale Kamionkowski, Kosowski & Turner, Phys.Rev. D49 (1994) Apreda et al., Nucl.Phys. B631 (2002) Cosmic strings -- gradually decreasing spectrum Damour & Vilenkin , Phys.Rev. D71 (2005) Pre big-bang cosmology Buonanno, Maggiore & Ungarellli , Phys.Rev. D55 (1997) LIGO Laboratory at Caltech

Stochastic signals from astrophysical processes Incoherent superposition of many signals from various signal classes Coalescing binaries Supernovae Pulsars Low Mass X-Ray Binaries (LMXBs) Newly born neutrons stars Normal modes - R modes Binary black holes Gaussian -> non-Gaussian (“popcorn” noise) depending on rates Drasco & Flannigan, Phys.Rev. D67 (2003), Spectra follow from characteristics of individual sources Maggiore, Phys.Rept. 331 (2000) LIGO Laboratory at Caltech

The Cosmological Gravitational Wave “Landscape” Armstrong et al., ApJ 599 (2003)  Range of Interferometers (Ground & Space-Based Kolb & Turner, The Early Universe (1990) Lommen, astro-ph/0208572 n & Koranda, PRD 50 (1994) LIGO Laboratory at Caltech

The LIGO Laboratory Sites Interferometers are aligned along the great circle connecting the sites Caltech MIT 3002 km (L/c = 10 ms) Livingston, LA Hanford, WA LIGO Laboratory at Caltech

The LIGO Observatories Livingston Observatory Louisiana One interferometer (4km) GEODETIC DATA (WGS84) h: -6.574 m X arm: S72.2836°W : N30°33’46.419531” Y arm: S17.7164°E : W90°46’27.265294” <- Livingston, LA  Hanford Observatory Washington Two interferometers (4 km and 2 km arms) GEODETIC DATA (WGS84) h: 142.555 m X arm: N35.9993°W : N46°27’18.527841” Y arm: S54.0007°W : W119°24’27.565681” Hanford, WA -> LIGO Laboratory at Caltech

LIGO Laboratory at Caltech Light makes Nb bounces h = L/L => / = 2 Nb hL/ Detector concept The concept is to compare the time it takes light to travel in two orthogonal directions transverse to the gravitational waves. The gravitational wave causes the time difference to vary by stretching one arm and compressing the other. The interference pattern is measured (or the fringe is split) to one part in 1010, in order to obtain the required sensitivity. LIGO Laboratory at Caltech

LIGO First Generation Detector Limiting noise floor Interferometer sensitivity is limited by three fundamental noise sources seismic noise at the lowest frequencies thermal noise (Brownian motion of mirror materials, suspensions) at intermediate frequencies shot noise at high frequencies Many other noise sources lie beneath and must be controlled as the instrument is improved LIGO Laboratory at Caltech

The LIGO is Approaching its Design Sensitivity  factor 2X of design goal throughout LIGO band LIGO Laboratory at Caltech

LIGO Laboratory at Caltech Detection strategy Spectrum: Cross-correlation statistic: Optimal filter (assume gw(f) = const): Make many (N~ 104) repeated short (T = 60s) measurements to track instrumental variations: LIGO Laboratory at Caltech

(f) - Overlap reduction factor g(f) LIGO Laboratory at Caltech

S2 H1-L1 results using previously outlined method Distribution of  over S2 run Scatter plot of normalized residuals vs.   LIGO Laboratory at Caltech

S3 Expected Sensitivity: H1-L1 Estimated error of measurement (+3) plotted for the H1-L1 pair as a function of run time. Preliminary LIGO Laboratory at Caltech

Current and expected results on gw h1002 H-L H1-H2 Freq range Observation Time S1 (upper limit) PRD 69, 122004, 2004 < 23 +/- 4.6 (H2-L1) see instrumental noise 64-265 Hz 64 hours (08/23/02 – 09/09/02) S2 (upper limit) Preliminary < 0.018 +0.007- 0.003 (H1-L1) 50-300 Hz 387 hours (02/14/03 – 04/14/03) S3 (sensitivity) Expected based on power spectra ~5 x 10-4 potentially ~10x lower than S3 H1-L1 ~240 hours (10/31/03 – 01/09/04) Design sensitivities LIGO I ~1x 10-6 ~1.5x 10-7 30-300 Hz 1 year LIGO Advanced nominal tuning low-freq tuning ~1.5x 10-9 ~3.5x 10-10 ~3x 10-10 ~2.5x 10-10 10-100 Hz 10-50 Hz LIGO Laboratory at Caltech

The Cosmological Gravitational Wave “Landscape” LIGO Laboratory at Caltech

Frequency range for GW Astronomy Audio band Dynamic range of Gravitational Waves Terrestrial and space detectors complementary probes Provide ~8 orders of magnitude coverage Space Terrestrial LIGO Laboratory at Caltech

Initial and Advanced LIGO LIGO Laboratory at Caltech

The Cosmological Gravitational Wave “Landscape” LIGO Laboratory at Caltech

Growing International Network of GW Interferometers Operated as a phased array: - Enhance detection confidence - Localize sources - Decompose the polarization of gravitational waves - Precursor triggers from low frequency LISA GEO: 0.6km On-line VIRGO: 3km 2005 - 2006 LIGO-LHO: 2km, 4km On-line TAMA: 0.3km On-line LIGO-LLO: 4km On-line AIGO: (?)km Proposed LIGO Laboratory at Caltech

LIGO Laboratory at Caltech Summary The current best published IFO-IFO upper-limit is from S1: h2 < 23+/-4.6 S2 result: 0.018 (+0.007- 0.003) PRELIMINARY The S3 data analysis is in progress. Expect: < few x 10-4 H1-H2 is the most sensitive pair, but it also suffers from cross-correlated instrumental noise. Also working on: Set limits for gw(f) ~ n(f/f0)n Targeted searches (astrophysical foregrounds, spatially resolved map) Expected sensitivities with one year of data from LLO-LHO: Initial LIGO h2 < 2x10-6 Advanced LIGO h2 < 7x10-10 LIGO Laboratory at Caltech

LIGO Laboratory at Caltech FINIS LIGO Laboratory at Caltech

What is known about the stochastic gravitational wave background? 2 Allen & Koranda, PhysRevD Lommen, astro-ph/0208572 Kolb & Turner, TheEarlyUniverseAddisonWesley1990 LIGO Laboratory at Caltech