Testing General Relativity!With Astrophysical Black Holes!
University of Arizona!DIMITRIOS PSALTIS!
GRAVITATIONAL FIELDS!IN ASTROPHYSICAL SYSTEMS!
Psaltis 2008!
Dark Matter
Dark Energy
Solar System
Stars
White Dwarfs
Neutron Stars
Milky Way
X-ray !Binaries!Intermediate !Mass Black-Holes!
Active Galactic !Nuclei!
Current!tests!
Potential!
Curv
atur
e!
Thorne & Dykla 1971; Hawking 1974; Bekenstein 1974; Sheel et al. 1995 !
Brans-Dicke black holes are identical to GR ones!!
Rabc
d Rab
cd/4
8M2 r
-6
easier said than done because, e.g.,
all R2 terms ! any function of R! … in the Palatini formalism !
Let’s add:!
a dynamical vector field!
Psaltis, Perrodin, Dienes, & Mocioiu 2008, PRL, 100, 091101 !
Uniqueness: Proven so far for (i) Brans-Dicke gravity, (ii) f(R) gravity! (Sotiriou & Faraoni 2012)!
Only known exception: Chern-Simons gravity (Yunes & Pretorious 2009)!
Always get Kerr Black Holes in steady state! !
key point: Vacuum and Black Holes are solutions to the same equation:! Rµν=0!
…an über-No hair theorem!!
Black Holes are Very Simple!!
They are all expected to be described by the Kerr metric, which depends only on two parameters: !
the black-hole mass and its spin!
(no charge in astrophysical black holes)!
A formal test of the no-hair theorem:!
• expand vacuum metric in multipoles!
• use observations to measure at least 3 multipole coefficients !
Ryan 1995; Wex & Kopeikin 1999; Collins & Hughes 2004; Glampedakis & Babak 2006 !Will 2008; Brink 2008; Gair et al. 2008; Apostolatos et al. 2009; Vigeland & Hughes 2010;!
Lukes-Gerakopoylos et al. 2010; Vigeland 2010!
• investigate whether!
€
q = −a2
by writing a general spacetime with!
and measuring the deviation parameter ‘ε’.!
€
q = −(a2 + ε)
The No Hair Theorem: The Kerr solution is the only stationary,!axisymmetric metric in vacuum that has no naked singularities and!no closed timelike loops. !
Black Holes are Very Simple!!
They are all expected to be described by the Kerr metric, which depends only on two parameters: !
the black-hole mass and its spin!
(no charge in astrophysical black holes)!
Regions with pathologies in, e.g., the VH spacetime!
Johannsen, Vigeland, Yunes, Hughes, Psaltis 2012!
A metric that deviates from Kerr, but remains regular !even at very high spins (Johannsen & Psaltis 2011)!
Metrics that deviates from Kerr and are characterized by!Carter-like conserved quantities (Vigeland, Yunes, Stein 2011)!
A census of approaches to testing the No-Hair Theorem!Johannsen, Vigeland, Yunes, Hughes, Psaltis 2012!
Our two golden rules for testing GR with (non-dynamical) observations!of astrophysical objects:!
(i) Look for a phenomenon that has no GR equivalent!
(ii) Perform the same quantitative test with more than one, ! independent probe!
The quadrupole affects: !(i) the location of the Innermost Stable Circular Orbit!
Joha
nnse
n &
Psal
tis (
2010
a)!
GR!
The quadrupole affects: (ii) the amount of gravitational lensing !
Joha
nnse
n &
Psal
tis (
2010
a)!
Sgr A* is the optimal candidate for testing the no-hair theorem!!
Joha
nnse
n et
al.
2012
!
because its horizon has the largest opening angle in the sky!An
gula
r Siz
e of
Sha
dow!
What will the Event Horizon Telescope see?!
Brod
eric
k, J
ohan
nsen
, Psa
ltis,
& L
oeb
2012
!
The Photon Ring: A Ubiquitous Signal, Independent of the Details of the Accretion flow (Johannsen & Psaltis 2011)!
Dext
er e
t al
. 200
9!
Shch
erba
kov
& Pe
nna
2010
!
Mos
cibr
odzk
a et
al.
2009
!
The ring is highly circular for almost all Kerr spins and orientations!
But becomes asymmetric for non-Kerr spacetimes!
A test of the no-hair theorem with Sgr A* images!
Diameter Mass!
Displacement Spin!
Asymmetry Quadrupole!
Any potential violation of the no-hair theorem in Sgr A* can be tested with completely independent observations!
(i) frame-dragging effects in the orbits of IR stars within 1mpc (Will 2008; Merritt et al. 2009)!
GRAVITY!The adaptive optics assisted, beam combiner for the VLTI!
(ii) spin-orbit coupling effects in the timing of an orbiting radio pulsar (Wex & Kopeikin 1998; Liu et al. 2012)!
CONCLUSIONS!
• Measuring the quadrupole moment of the spacetime of a! black hole leads to a quantitative, formal test of the no-hair theorem!
• Observations of Sgr A* may lead to the first ! test of the no-hair theorem in the near future with:! (i) high-resolution images of its shadow! (ii) dynamical measurements with orbiting stars! (iii) dynamical measurements with radio pulsars!
• The Event Horizon Telescope and GRAVITY are opening a new ! window in gravitational physics!
• A well developed theoretical framework exists for performing! this test!