Gravitational physics and gravitational wave detectsion with the SKA

1
Gravitational physics andgravitational wave
detectsionwith the SKA
Michael Kramer
ASTRONET - Liverpool, 18th June 2008
2
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
3
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
  radiation
• Highly relativistic orbital motion (vc)
• Orbital motion can have imprints from the
  strong-field internal
• gravity of the bodies (esp. for alternative
  theories)

• Pulsars probe strong-fields

Kramer et al. (2006)

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

Kramer et al. (2006)
4
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)
5
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
M3)

• 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

6
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

7
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
  detector

• Perturbation in
• space-time can be
• detected in timing
• residuals

• Sensitivity as a
• dimensionless strain

Pulsar Timing Array
8
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)

9
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)
10
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
11
Summary

• 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