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Strong field tests of gravity using pulsars and black holes
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Galaxy evolution, cosmology and dark energy
Recommendations
for the Key science projects (PDF), Bryan Gaensler
General relativity (GR) has to date passed all observational
tests with flying colours. One of the most fundamental questions remaining
is whether Einstein's theory is the last word in our understanding of
gravity or not. Solar system tests of GR are made under weak-field conditions,
and even the existing binary pulsar tests only begin to approach the strong-field
regime.
Important questions are yet unanswered: can GR correctly describe the
ultra-strong field limit, are its predictions for black holes correct,
and is the cosmos filled with a gravitational wave background? About
50 years after the discovery of pulsars marked the beginning of a new
era in fundamental physics, pulsars observed with the SKA will transform
our understanding of gravitational physics.
Through
its sensitivity, sky and frequency coverage, the SKA offers the possibility
of probing the strong-field realm of gravitational physics by finding
and timing pulsars. The SKA will discover -- besides extragalactic pulsars
-- a very large fraction of the pulsars in the Galaxy, resulting in 10,000
to 20,000 pulsars, including the discovery of more than 1,000 millisecond
pulsars.
In addition to probing the equation-of-state at extreme limits, this impressive
yield also effectively samples every possible outcome of the evolution
of massive binary stars, thereby guaranteeing the discovery of very exciting
systems. We expect at least 100 compact relativistic binaries, providing
exciting test grounds for gravitational physics. We should find double
pulsar systems and perhaps even exotic or strange stars. Finally, the
SKA should provide us finally with ``the holy grail'' of pulsar-stellar
black hole
(PSR-BH) systems.
High precision timing observations of a pulsar orbiting a black hole -only
possible in the radio band and with the SKA - will make a PSR-BH system
an amazing probe of relativistic gravity with a discriminating power that
surpasses all its present and foreseeable competitors. It will provide
extreme limits on the most general, Lorentz-boost invariant, deviations
from GR to a level thousand times tighter than present solar-system limits
and at least an order of magnitude better than expected from any future
satellite mission.
Most
importantly, SKA observations will finally address the fundamental question
of whether GR can describe nature in the ultra-strong field limit. One
can not only study stellar black holes but also apply the same timing
techniques to pulsars around the super-massive black hole in the Galactic
Centre. This allows a direct comparison of the properties of these objects:
one can determine mass, spin and quadrupole moment of black holes to test
their description in Einstein's theory (the "no-hair"-theorem)
for the first time - obviously a major achievement in the history of physics!
The SKA will also produce a dense array of millisecond pulsars across
the sky. Being timed to very high precision (~< 100 ns), they act as
multiple arms of a cosmic gravitational wave (GW) detector. This ``device'',
with the SKA at its heart, will be sensitive to GWs at frequencies of
nHz. Thereby complementing the much higher frequencies accessible to Advanced
LIGO (~100Hz) and LISA (~mHz), the SKA is crucial in answering the question
about the existence, nature and composition of a GW background which is
expected from a variety of sources such as coalescence of massive black
hole binaries during galaxy evolution or the evolution and decay of cosmic
strings as predicted in grand unified theories.
These fundamental questions about our understanding of gravity can be
answered by finding and timing radio pulsars to very high precision. Pulsars
and their high-precision observations are unique to the radio band and
to use them to their full potential, we require the sensitivity and resulting
timing precision of the SKA. Whilst the scientific yield goes much beyond
the study of radio pulsars. Pulsars are also the only astrophysical sources
that are observable in every astronomical window, including that of GW.
With future sensitive telescope in other astronomical windows pulsars
promise to be a useful tool for crucial identification of many potential
sources in the radio band.
Read "
Strong field tests of gravity using pulsars and black holes "
for general public.
Animations:
>
Multibeam
>
Pulsars
More: Strong-Field
Tests of Gravity Using Pulsars and Black Holes
- M. Kramer, D. C. Backer,
J. M. Cordes , T. J. W. Lazio, B. W. Stappers, S. Johnston - " Science
with the Square Kilometre Array", 2004.
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