Transcript Gravitational Waves – detectors, sources & science

The Black Hole Symphony
– listening to the Universe with
gravitational waves
Jonathan Gair
Graduate Seminar,
St.Catharine’s College, 28th November 2005
Talk Outline
• The nature of gravity
• What are gravitational waves?
• What are the astrophysical sources of gravitational
waves?
• How do we detect gravitational waves?
• What have we learnt so far?
• What do we stand to learn in the future?
The Nature of Gravity
Newton
• “Action at a distance”.
• Newton’s Law describes effect of
gravity but does not explain it.
Einstein
• Gravity is spacetime curvature.
• Any mass/energy bends spacetime
near it.
• Freely falling objects follow the
local background curvature.
What are Gravitational Waves?
• “Ripples in spacetime” – any
rapidly moving mass
generates fluctuations in
spacetime curvature.
• These fluctuations propagate
at the speed of light away
from the source. These are
gravitational waves!
• When a gravitational wave
passes through, space is
stretched and squeezed
alternately. The effect is
opposite in perpendicular
directions.
GW Sources I – Binaries
• Many stars are born in binary
systems. Each star evolves with
time and eventually leaves a
compact remnant – formation of
binaries of two WD, NS or BH.
• All such binaries emit GWs.
Most sources are
monochromatic – e.g., WD-WD
binaries in our galaxy.
• In very compact binaries, loss of
energy to GWs leads to inspiral.
Frequency of GWs increases
with time – a “chirp”.
GW Sources II – Extreme Mass
Ratio Inspirals
• Most galaxies harbour a supermassive black hole in their
centre which is surrounded by a cluster of stars.
• Encounters between stars in the cluster can put compact
objects onto orbits that pass close enough to the black
hole to be captured. Emission of GWs drives inspiral into
the BH.
• Waveforms are complex since orbits are eccentric and
the black hole is likely to be spinning rapidly. The “zoomwhirl” features of the orbit are seen in the waveform.
• Precessions encode information about the spacetime,
e.g., the black hole mass and spin and the orbital
parameters.
GW Sources III – Pulsars
• Pulsars are rapidly rotating
neutron stars.
• Many have been detected with
radio telescopes, e.g., Crab
Pulsar. Wide range of rotation
frequencies – ~1 Hz –> 1 kHz.
• The surface of a neutron star may
be bumpy, e.g., from fluid motions
or crustal deformation.
• Rapid rotation ensures such
“mountainous” neutron stars emit
GWs at twice the rotation
frequency.
GW Sources IV – Cosmological
Quantum fluctuations in the early Universe are stretched as
the Universe expands – see a “stochastic” background of
GWs. These probe back to 1s after the Big Bang.
GW Sources V – Bursts
• When massive stars reach the end of their lives, they
collapse in a supernova explosion. This collapse is likely
to be asymmetric – emits a burst of gravitational radiation.
• May also see bursts from other events, e.g., cosmic
string cusps, unknown “exotic” sources.
GW Detection I – Indirect
Binary pulsars
• Observe binary systems with
optical & radio telescopes.
• See changes in orbit due to loss
of energy to GWs.
• Best test of GR to date –
J1915+1606 (Hulse/Taylor Nobel
Prize 1993), J0737-3039.
Pulsar timing
• GWs propagating across line of
sight from a pulsar to an observer
change light travel time.
• See this time shift in pulse timing.
GW Detection II – 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)
GW Detection III – Interferometers
• Interferometers exploit quadrupole nature of GWs – send
laser beams in perpendicular directions and combine
them on return to construct interference patterns.
Ground Based Interferometers
Several ground based
interferometers are now
operating or are being built –
• LIGO – US project. Two 4km
and one 2km detector.
• GEO – British/German
project. One 600m detector.
• VIRGO – Italian/French
project. One 3km detector.
• TAMA – Japanese project.
One 300m detector.
• AIGO – Australian project.
One 80m detector.
Space Based Interferometers
• A space based interferometer,
LISA, is planned
– Joint NASA/ESA mission.
– Will consist of three satellites in a
heliocentric, earth-trailing orbit.
– Longer baseline (5 million km)
gives sensitivity to lower
frequency gravitational waves.
• Launch date is 2013.
• LISA will be a true GW
telescope – confusion between
multiple sources dominates over
instrumental noise throughout
much of the spectrum.
Interferometers - Sources
Difficulties in GW detection
• Gravitational waves are very
weak and weakly interacting.
• Events are faint, typically an
order of magnitude below the
noise.
+
• Detection will be by matched
filtering using a bank of
templates.
• Overlap of template with data
pulls signal out of the noise.
“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!
Outlook for GW Detectors
• 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.
• Launch of LISA in 2013 will usher in era of gravitational
wave astronomy.
• More advanced detectors are planned – on the ground
(LIGO III, EIGO, LCGT, VIRGO II) and in space (BBO).
What can we learn?
• “New window on the Universe” – GWs probe regions of
the Universe that are unobservable electromagnetically.
• Gravitational wave observations allow high precision
measurements of black hole masses and spins (to < 1%)
– provides a survey of astrophysical black holes.
• Observations of SMBH mergers tell us about the
properties of galaxies and the rate of galaxy mergers.
• Observations of stellar mass black holes tell us about
stellar evolution in various environments.
• GW observations provide a test of General Relativity,
e.g., may use EMRI observations to test “no-hair” theorem
or to detect exotic supermassive compact objects.
Summary
• Gravitational waves are one of the most interesting
predictions of general relativity, and provide an
unprecedented probe of extreme gravity environments in
the Universe.
• There are many potential sources of gravitational waves
for our detectors, ranging from binary star systems to
supermassive black hole mergers to cosmic string cusps.
• We are on the verge of making our first direct gravitational
wave detection. This should happen within 5-10 years,
probably using Advanced LIGO.
• Once gravitational wave detections become routine, we
stand to learn a great deal about systems that are
inaccessible to electromagnetic telescopes.