Gravity with the SKA Michael Kramer Jodrell Bank Observatory University of Manchester With Don Backer, Jim Cordes, Simon Johnston, Joe Lazio and Ben Stappers Gravity Meeting ’06, Birmingham Strong-field tests of gravity using Pulsars and Black Holes
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Gravity with the SKA
Michael KramerJodrell Bank ObservatoryUniversity of Manchester
WithDon Backer, Jim Cordes, Simon Johnston,
Joe Lazio and Ben Stappers
Gravity Meeting ’06, Birmingham
Strong-field tests of gravity using Pulsars and Black Holes
Outline• Pulsars• Past and present successes• A giant leap - the power of the SKA• Transforming our knowledge offundamental physics, particularly:
- Strong-field tests of gravity- Gravitational wave astronomy
Searching & Timing: see JimGravitational Waves: see Rick
A frequently asked question, still…
Testing EinsteinExperiments made in Solar System provide accurate tests
…but only in weak gravitational field!
In strong gravitational fields, physics may be different!
Compute energy in gravitational field:
Radiative aspects need to be tested too!
Neutron stars &Black Holes:
5.015.0
≈≈
BH
NS
εε
2mcEgravity=ε
Solar system:
10000000000.00000000001.0
000001.0
≈≈≈
Moon
Earth
Sun
εεε
Pulsars are extreme objects…• Cosmic lighthouses …• Precise clocks …• Almost Black Holes …• Objects of extreme matter …
– 10x nuclear density– B ~ Bq = 4.4 x 1013 Gauss– Voltage drops ~ 1012 V– FEM = 1010-12 Fg– High-temperature & superfluid superconductor
• Massive stable flywheels superb cosmic clocks e.g. period of B1937+21:
P = 0.0015578064924327±0.0000000000000004 s
Precision tools for a wide range of fundamental physics and astrophysics
The importance of pulsar testslog(
Self-e
nerg
y)
log(Orbital energy)
Strong-field tests using pulsars…
Various approaches:
Theory-independent or phenomenological:Parameterized Post-Keplerian approach
Parameterized Post-Newtonian (PPN)
Binary pulsars
Theory-dependent:Beyond usual post-Newtonian parameters, used forclasses of tensor-scalar theories
Parametrized post-Newtonian (PPN)γ How much space
curvature per unit mass?1 γ-1<2 10-3
γ-1<3 10-4
Viking ranging (time delay)VLBI (light deflect.)
β How much “non-linearity” in the superposition law?
1 β-1<3 10-3 Perihelion shift
ξ Preferred-location effects? 0 | ξ |<3 10-3 Earth tides, Pulsars
α1Preferred-frame effects?Orbital polarization
0 α1 < 10-4
α1 < 1.2 10-4
Lunar ranging (weak) Pulsars (strong)
α2Preferred-frame effects?Spin precession
0 α2 < 4 10-7 Solar alignment
α3Preferred-frame effects?Violation of conversation of total momentum
0 α3 < 4.0 10-20 Pulsars
Will (2002), Stairs (2005)
Plus: limits on violation of SEP: |Δ|<5.5x10-3
…already a success story:The first binary pulsar
Weisberg & Taylor (2004)
• Orbit shrinks every day by 1cm• Confirmation of existence of gravitational waves
Hulse & Taylor (1974)
Tests of General RelativityElegant method to test any theory of gravity: (Damour & Taylor ’92)
• Relativistic corrections to Keplerian orbits measured
• All corrections can bewritten as function ofthe pulsar masses
• Only one true combination of mA and mB should exist
Single unique point in
a mA - mB diagram!
PK1
PK2PK3
Pass!
Tests of General RelativityElegant method to test any theory of gravity: (Damour & Taylor ’92)
• Relativistic corrections to Keplerian orbits measured
• All corrections can bewritten as function ofthe pulsar masses
• Only one true combination of mA and mB should exist
Single unique point in
a mA - mB diagram!
PK3
PK2
PK1
Fail!
Npk – 2 tests possible
…already a success story: The first double pulsar
Kramer et al. (in prep.)
Orbital Phase (deg)
Del
ay (µ
s)
s = sin(i)=0.9997±0.0002
001.0000.1obs
exp
±=ss
Kramer et al.(in prep)
Testing GR:
• most relativistic systems• most over-constrained system:
• Five PK params• Theory independentmass ratio
Double Pulsar: Tests of GRDecember 2003 (Lyne et al. 2004)
Double Pulsar: Tests of GRKramer et al. in prep.
“Scalarisation”
…already a success story: Scalar-Tensor Theories
ScalarCharge(for β0=-6)
Damour (priv. comm.)
Damour (priv. comm.)
The is more to do:
• The remaining “holy grail”: a pulsar – black hole system!• Wanted: millisecond pulsar around black hole
“…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)
Galactic Census with the SKA• Blind survey for pulsars will discover PSR-BH systems!• Timing of discovered binary and millisecond pulsars to very high precision:
• “Find them!”• “Time them!”• “VLBI them!”
Benefiting from SKA twice:• Unique sensitivity: many pulsars, ~10,000-20,000
incl. many rare & exotic systems!• Unique timing precision and multiple beams!Not just a continuation of what has been done before
- Complete new quality of science possible!
• Galactic probes: Interstellar medium/magnetic fieldStar formation historyDynamicsPopulation via distances (ISM, VLBI)
Pulsar Science with the SKAVery wide range of applications:
Electrondistribution
Magnetic field
Galactic CentreMovement in potential
• Galactic probes• Extragalactic pulsars: Formation & Population
Turbulent magnetized IGM
Pulsar Science with the SKAVery wide range of applications:
Giant pulses
Reach the local group!
Search nearby galaxies!
• Galactic probes• Extragalactic pulsars• Relativistic plasma physics: Radio emission
Resolving magnetospheresRelation to high-energies
Pulsar Science with the SKAVery wide range of applications:
• Galactic probes• Extragalactic pulsars• Relativistic plasma physics• Extreme Dense Matter Physics: Ultra-strong B-fields
Equations-of-StateCore collapsesVariation of G
Pulsar Science with the SKAVery wide range of applications:
• Galactic probes• Extragalactic pulsars• Relativistic plasma physics• Extreme Dense Matter Physics• Multi-wavelength studies: Photonic windows
Pulsar Science with the SKAVery wide range of applications:
• Galactic probes• Extragalactic pulsars• Relativistic plasma physics• Extreme Dense Matter Physics• Multi-wavelength studies: Photonic windows
Non-photonic windows
Pulsar Science with the SKAVery wide range of applications:
• Galactic probes• Extragalactic pulsars• Relativistic plasma physics• Extreme Dense Matter Physics• Multi-wavelength studies• Exotic systems: planets
millisecond pulsarsrelativistic binariesdouble pulsarsPSR-BH systems
Pulsar Science with the SKAVery wide range of applications:
3.Holy Grail: PSR-BH
Double PulsarsPlanets
• Galactic probes• Extragalactic pulsars• Relativistic plasma physics• Extreme Dense Matter Physics• Multi-wavelength studies• Exotic systems• Gravitational physics: - Strong-field tests
- BH properties- Gravitational Waves
Pulsar Science with the SKAVery wide range of applications:
Black Hole spin:
Black Hole properties
•Astrophysical black holes are expected to rotate
2MS
Gc
≡χ S = angular momentum
• Result is relativistic spin-orbit coupling• Visible as a precession of the orbit:
Measure higher order derivatives of secularchanges in semi-major axis and longitude of periastron
• Not easy! It is not possible today!• Requires SKA sensitivity!
See Wex & Kopeikin (1999)xx ,,ω
Black Hole quadrupole moment:
Black Hole properties
•Spinning black holes are oblate
Q = quadrupole moment32
4
MQ
Gcq ≡
• Result is classical spin-orbit coupling
• Visible as transient signals in timing residuals
• Even more difficult! • Requires SKA!
Wex &
Kopeikin (1999):
• For all compact massive, BH-like objects, we’ll be able to measure spin very precisely
• In GR, for Kerr-BH we expect:
1≤χ• But if we measure
χ > 1 ⇔ Event Horizon vanishes ⇔ Naked singularity!
• Then: either GR is wrongor Censorship Conjecture is violated!
Cosmic Censorship Conjecture
No-hair theorem
2χ−≠q• If we measure
either GR is wrong, i.e. “no-hair” theoremis violated
or we have discovered a new kind of object, e.g. a Boson star with
210χ−≤q
Was Einstein right? What are the Black Hole properties? Do naked singularities exist? Do Black Holes have “hairs”?
Fundamental Physics with the SKA Pulsars discovered and observed with the SKA…
• complement studies in other windows • probe wide range of physical problems• New science possible – not just the same:
Work to be done: 2PN timing formulaCalibration techniquesMeans to handle blind surveyEfficient timing methods(LOTS of data, LOTS of pulsars)
The SKA sky!
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