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The Black Hole Symphony – listening to the Universe with gravitational waves. Jonathan Gair. Graduate Seminar, St.Catharine’s College, 28 th November 2005. Talk Outline. The nature of gravity What are gravitational waves? What are the astrophysical sources of gravitational waves?

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the black hole symphony listening to the universe with gravitational waves

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
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 “zoom-whirl” 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.


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
“GW detections” to date - LIGO




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.
  • 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.