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Gravitational physics and gravitational wave detectsion with the SKA


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Title: Gravitational physics and gravitational wave detectsion with the SKA

Gravitational physics andgravitational wave
detectsionwith the SKA
Michael Kramer
ASTRONET - Liverpool, 18th June 2008
Gravitational physics with the SKA
SKA will be a superb instrument for fundamental
physics, e.g. - Equation-of-state of
dark energy - Changes of fundamental
constants - Neutrino mass -
Gravitational physics - etc.
Browse the very rich SKA Science Book and
the many updates in the literature Here
- Detection of gravitational waves -
Tests of theories of gravity in strong fields
Strong-field tests of gravity
Definition of strong field vs. weak fields (e.g.
Will 2006)
  • Simple 1PN approximation is no longer appropriate
  • Evolution of the system may be affected by grav
  • Highly relativistic orbital motion (vc)
  • Orbital motion can have imprints from the
    strong-field internal
  • gravity of the bodies (esp. for alternative
  • Pulsars probe strong-fields

Kramer et al. (2006)
  • Already very successfully using binary pulsars
  • E.g. Double Pulsar confirms GR at 5x10-4
  • However, more extreme systems exists in the
  • ? hunt for the
    PSR-BH systems!

Kramer et al. (2006)
Pulsar Black Hole Systems
  • Various types stellar BH, intermediate mass BH
    in globular clusters
  • and pulsars around
    super-massive BH in Galactic Centre
  • Qualitatively different probe for both GR and
    alternative theories

Limits on tensor-scalar theories
a binary pulsar with a black-hole companion has
the potential of providing a superb new probe of
relativistic gravity. The discriminating power of
this probe might supersede all its present and
foreseeable competitors (Damour
Esposito-Farese 1998)
Esposito-Farese (2008)
Testing Black Hole Properties
  • Use pulsar orbit as probe for intrinsic
    properties of BHs
  • Relativistic and classical spin-orbit coupling
    as the tool
  • Applicable to wide range of masses and orbits
  • Measurable

Black Hole spin measured via relativistic
SO-coupling, i.e. as wobble
of the orbit (scales with M2)
Black Hole quadrupole moment measured via
classical SO-coupling,
i.e. as transient timing signals (scales with
  • Not easy (needs SKA!), but allows us e.g. to
    test GR concepts of BHs
  • - Cosmic Censorship Conjecture
  • i.e. Non-existence of naked
    singularities (event horizons!)
  • - No-Hair theorem
  • i.e. Determination of BHs properties by
    mass, spin and charge

The SKA Sky Revolution
  • Galactic Census will yield 30,000 pulsars
    1,000 MSPs
  • Essentially all pulsars beaming towards Earth
  • Sample includes PSR-stellar BH systems as well
    as pulsar around SGR A
  • But also 1000 millisecond pulsars to directly
    detect gravitational waves

Pulsars as Gravitational Wave Detectors
  • Pulsar timing is affected by the displacement of
    the pulsar and the Earth due to the presence of
    gravitational waves
  • Hence, pulsars or a combination of them as a
    Pulsar Timing Array act as a network of arms
    of a huge, sensitive cosmic gravitational wave
  • Perturbation in
  • space-time can be
  • detected in timing
  • residuals
  • Sensitivity as a
  • dimensionless strain

Pulsar Timing Array
Pulsars as Gravitational Wave Detectors
(Kramer et al. 2004)
  • PTA is sensitive to
  • nHz gravitational waves
  • as sensitive as LISA
  • Complementary to LISA,
  • LIGO and CMB-pol band
  • Expected sources
  • - binary super-massive
  • black holes
  • - Cosmic strings
  • - Cosmological sources
  • Types of signals
  • - stochastic (multiple)
  • - periodic (single)
  • - burst (single)

Example 1 Single source detection
  • Single binary super-massive black hole produces
    periodic signal
  • Signal contains information from two distinct
    epochs t and t-d/c
  • Characteristic signal in timing data

Pulsar timing residuals (cf. Jenet at al. 2006)
Example 2 Stochastic background
  • Strongest signal expected from binary
    super-massive black holes in
  • early galaxy evolution
  • Amplitude depends on merger rate, galaxy
    evolution and cosmology
  • Resulting stochastic background detectable from
    displacement of
  • Earth in correlation of timing residuals with
    sky position
  • Shape of correlation function
  • depends on theory of gravity
  • and GW polarization
  • Experiment rather difficult
  • today but easy with the SKA!!

Figure provided by F. Jenet
  • SKA will improve over current PTA experiments by
    at least four
  • orders of magnitude GW science at nHz-range
    becomes easy!
  • For instance
  • definite answer on existence of cosmic strings
  • stringent limit on graviton mass
  • - single source detection likelihood increases
    from lt2 (now) to gt90
  • Pulsars orbiting BHs can probe their properties
    for different masses
  • mass, spin and quadrupole moment
  • test of cosmic censorship conjecture
  • testing no-hair theorem
  • unique tests of alternative theories
  • Besides unique science, many synergies
  • and complementarities to other future
    facilities e.g. LISA, XEUS, GAIA
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