Gravitational Wave Detection – current status & future prospects Jonathan Gair Extragalactic Group Seminar, IoA, 21st November 2005
Gravitational Waves • Fluctuations in spacetime curvature, generated by rapidly accelerating masses. • Offer an exciting new window on the Universe to complement electromagnetic observations. • No direct detections at present, but good indirect evidence from pulsars J1915+1606, J0737-3039. • We live in an exciting time, with many new detectors coming online • Resonant bars – AURIGA, ALLEGRO, EXPLORER, GRAIL, NAUTILUS, NIOBE. • Ground interferometers – AIGO, GEO, LIGO, TAMA, VIRGO • Space interferometer planned - LISA
Current Detectors – Resonant Bars • A large cylinder of metal resonates when bathed in gravitational waves of the right frequency. • Detectors must be suspended to give seismic isolation. Cryogenic cooling reduces thermal noise. • First ever GW detector was a resonant aluminium bar. Today there are several increasingly sophisticated experiments in operation – • ALLEGRO (US), AURIGA (Italy), EXPLORER (CERN), NAUTILUS (Italy), NIOBE (Australia), GRAIL (Netherlands)
Current Detectors – Interferometers • Ground based interferometers exploit quadrupole nature of GWs – space is distorted in opposite sense in two perpendicular directions – use a Michelson interferometer.
Current Detectors – Interferometers • LIGO • US project • 2x4km detectors, 1x2km detector at two sites (Louisiana and Washington) • Last science run (March 2005) was virtually at design sensitivity • Data analysis pipeline operating, but lags behind data taking • Plan one year of coincident observation time, starting 2006
Current Detectors – Interferometers • GEO 600 • UK/German project • 1x600m detector located near Hannover • Has achieved design sensitivity and is taking data • Full partner in the LIGO project. Detector is a testing ground for LIGO technology • Will take data coincident with next LIGO science run for combined analysis
Current Detectors – Interferometers • VIRGO • French/Italian project • 1x3km detector, located near Pisa • Still commissioning, ~2 years behind LIGO/GEO • TAMA • Japanese 300m detector, in Tokyo, currently operating • AIGO • Australian 80m detector, near Perth
LIGO - expected sources • Possible astrophysical sources include NS-NS and BH-BH inspirals, pulsars, bursts (e.g., from supernovae) and a stochastic background.
“GW detections” to date - Bars • In the late 60s/early 70s, Joseph Weber claimed to have made coincident detections in two detectors, 1000km apart. The claim was never verified and is regarded skeptically. • In 2002, the EXPLORER and NAUTILUS teams announced an excess of events towards the galactic centre. • These results are highly controversial, even though no claim of a “detection” was actually made • The statistics used in analysing the data are extremely suspect
“GW detections” to date - LIGO Storms! Logging! Aeroplanes! No astrophysical detections so far!
Future Prospects on the ground • LIGO/GEO aim to take one year of coincident data at current sensitivity levels. Detections will only be made • If we are lucky, e.g., nearby supernova, nearby BH-BH merger • If exotic sources exist, e.g., cosmic string cusps • LIGO will be taken offline in 2007 and upgraded – Advanced LIGO (~2009) • Order of magnitude improvement in strain sensitivity • Even pessimistic event rate estimates predict several a month • Likely to make first robust direct detection of GWs • Third generation detectors planned (LIGO III, EIGO, LCGT, VIRGO II) – 20-30 years in the future • Allows GW astronomy from the ground
Future Prospects in Space • Space based interferometer, LISA • Joint NASA/ESA mission • Will consist of three satellites in heliocentric, earth-trailing orbit • Longer baseline (5 million km) gives sensitivity to lower frequency gravitational waves • Precursor mission, LISA Pathfinder, in 2008 • LISA is currently funded in both Europe and the US (Phase A). Launch date is 2013, but likely to slip • Efforts to scope out data analysis are already underway (DAST, AMIGOS) • LISA will be a true GW telescope – confusion between multiple sources dominates over instrumental noise throughout much of the spectrum
Extreme mass ratio inspirals • Inspiral of a stellar mass compact object (WD, NS, BH) into a SMBH in the centre of a galaxy. • Exciting LISA source since the small body acts as a test particle in the SMBH background – gravitational waves encode a map of the spacetime structure. • Allow accurate source parameter determination • Δ(S/M2), ΔM ~ 10-4, Δ(ln D) ~ 0.05, ΔΩS ~ 10-3, Δe ~ 10-4 • Waveforms are well understood thanks to Carter, Teukolsky etc. – allows detection by matched filtering. • Data analysis is difficult, but with best current algorithm, SNR at detection threshold is ~35, setting maximum reach at z~1. • Astrophysical rates uncertain, but can estimate from stellar cluster simulations.
EMRI formation • Standard picture • two-body scattering in the stellar cusp puts COs onto orbits that pass close to the BH • energy is lost to GWs as CO passes the BH, changing the orbit • if GW inspiral timescale is sufficiently short, CO is not scattered onto a different orbit before plunging • Simulate this process to estimate event rates (Freitag) • Results are extremely uncertain and trend is to lower numbers
Improving EMRI rate estimates • Codes treat orbits as Keplerian, but most captures have rp ~ few x GM/c2, in strong field of BH spacetime • Can use radial geodesic equation to reparameterise orbit • Better approximations are obtained by evaluating the standard GW expressions for these relativistic parameters • Accurate results require BH perturbation theory and solution of Teukolsky equation – computationally expensive
Improving EMRI rate estimates • Results have been tabulated for parabolic orbits in Schwarzschild (Martel 2004). Use geodesic properties to derive suitable fit – Keplerian as rp→∞, logarithmic in limit rp→4GM/c2 • Decay timescale dominated by eccentricity change on first pass • Use fit to parabolic emission to improve timescale computation (Gair et al. astro-ph/0508275)
Improving EMRI rate estimates • Standard expressions quote orbital averaged fluxes. Clear breakdown for 1-e « 1, specifically when • Better model changes orbit discretely at periapse. In fact, enough to do this for first pass only. • These improvements might enhance the rate by a factor of a few, but is it enough to give a decent EMRI rate? • Fortunately, other mechanisms to seed EMRIs exist • Formation of stars in an accretion disc near a BH (Levin 2003) • Tidal stripping of binaries (Miller et al. 2005) • Triaxiality (Holleybockelmann et al.)
Summary • We are on the verge of making our first direct gravitational wave detection. Should happen within 5-10 years, probably using Advanced LIGO. • LISA will mark the beginning of GW astronomy and will teach us much about galactic binaries, black holes and general relativity. • EMRI detections provide a unique probe of galactic cores. We will learn much about galactic SMBHs, and in principle could detect exotic supermassive objects, if they exist. • Astrophysical rate of EMRIs is very uncertain, but efforts to improve these estimates are underway. Should still have sufficient events for EMRI science.