Transcript Gravitational Wave Detection – current status & future prospects Jonathan Gair Extragalactic Group Seminar, IoA, 21st November 2005

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 BHBH 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
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
M•
• Standard picture
m
Optimistic
Pessimistic
PessimisticRates
DA
0.6
10
<1
0
– two-body
300000 scattering10in the stellar 700*
10
90
cusp puts COs onto 100
orbits that pass 1*
1*
-3
close
BH density
M• to the
Space
Merger rate (Gpc yr-1)
0.6
100
<1
10
-3
2
(M
(10
Mpc as0.6
๏)
65 GWs
M๏passes
WD 1100*
1.4 M๏ NS 10 M๏ BH660*
– energy
is
losthto
CO
1000000
10
70*100 M๏ IMBH
3)
the BH, changing the orbit
100
1*
1*
6.5±0.25
10
1.7
8.5
1.7
1.7
1.7x10-3
– if GW inspiral timescale
0.6 is
70
0
2
sufficiently
is not scattered
106.0±0.25
1.7CO10
6
1.1
1.1 130
10-3
3000000 short,
1700*
15
onto a different orbit before plunging
5.5±0.25
2* 0.7
10
1.7 100
3.5
0.7 1* 7x10-4
• 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.