Jonathan Gair

Extragalactic Group Seminar,

IoA, 21st November 2005

Gravitational Wave Detection – current status & future prospects

• 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

Gravitational Waves

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

No astrophysical detections so far!

Logging!Storms!

Aeroplanes!

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

• 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

Future Prospects in Space

LISA – expected sources

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

M•

(M๏)

Space density

(10-3 h652Mpc-3)

Merger rate (Gpc-3 yr-1)

0.6 M๏WD 1.4 M๏ NS 10 M๏ BH 100 M๏ IMBH

106.5±0.25 1.7 8.5 1.7 1.7 1.7x10-3

106.0±0.25 1.7 6 1.1 1.1 10-3

105.5±0.25 1.7 3.5 0.7 0.7 7x10-4

M• m Optimistic

0.6 10

300000 10 700*

100 1*

0.6 100

1000000 10 1100*

100 1*

0.6 70

3000000 10 1700*

100 2*

Pessimistic Rates

0

10

1*

<1

70*

1*

0

15

1*

Pessimistic DA

<1

90

1*

10

660*

1*

2

130

1*

• 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

• 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

Improving EMRI rate estimates

• Use fit to parabolic emission to improve timescale computation (Gair et al. astro-ph/0508275)

• Standard expressions quote orbital averaged fluxes. Clear breakdown for 1-e « 1, specifically when

Improving EMRI rate estimates

• 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.