Phenomenology of Scalar-Tensor Theories of Gravity |

**In General**
> s.a. fifth force; higher-order
gravity phenomenology; (post-)newtonian gravity.

* __Idea__: These theories in
general agree with general relativity in the weak-field, slow-motion regime,
but may differ significantly from it in strong-field situations; So far, the
best constraints come from binary neutron stars; In the future we expect
better binary-object constraints, and gravitational-wave ones.

* __Scalar fields__: In order to
be astrophysically and cosmologically relevant they would have to be light.

* __Weak equivalence principle__:
It is violated if there are types of matter that couple to different combinations
of *φ* and *g*_{ab}.

@ __General references__: Esposito-Farèse AIP(04)gq [test, rev];
Lindroos et al PRD(16)-a1512 [propagation of scalar waves];
Alonso et al PRD(17)-a1610 [future cosmological experiments];
Sakstein & Jain PRL(17)-a1710,
Baker et al PRL(17)-a1710,
Langlois et al PRD(18)-a1711 [constraints from GW170817].

@ __Lorentz symmetry violations__: Bezerra et al PRD(05)ht/04 [with torsion];
Brax PLB(12) [environmentally dependent].

@ __Solar system tests__: Devi et al PRD(11)-a1104 [Dirac-Born-Infeld action];
Anderson & Yunes PRD(17)-a1705;
Faraoni et al EPJC-a1906.

@ __Binary systems__: Freire et al MNRAS(12)-a1205 [the pulsar-white dwarf binary PSR J1738+0333];
Mirshekari & Will PRD(13)-a1301 [compact binaries to 2.5 PN order];
> s.a. neutron stars.

@ __Quantum theory__:
Shojai et al MPLA(98),
MPLA(98);
> s.a. brans-dicke theory; quantum-gravity
renormalization and asymptotic safety.

@ __Other effects and results__: Faraoni & Gunzig A&A(98) [light amplification];
Burton et al PLA(08)-a0711 [spinning particles];
Armendáriz-Picón & Penco PRD(12) [equivalence-principle violations];
Farajollahi et al PRD(11)-a1201;
> s.a. equivalence principle;
gravitational-constant variation; lensing.

> __Cosmology__:
see cosmology in modified gravity;
dark-energy models; quantum
cosmology models.

> __Gravitational waves__: see gravitational
radiation; gravitational-wave propagation.

**Solutions**
> s.a. astrophysics [Buchdahl inequality]; Birkhoff
Theorem; multipole moments; Q-Stars.

* __Compact objects__:
Scalar-tensor theories can be compatible with Solar System experiments and
still produce large modifications in the observable properties of neutron
stars, such as masses and radii (Damour and Esposito-Farèse, 1990s);
Black holes in these theories have no hair, but could grow "wigs" supported
by time-dependent boundary conditions or spatial gradients;
> s.a. neutron stars.

@ __Black holes, hair__: Bekenstein PRD(95);
Cardoso et al PRL(13)-a1308 [and instabilities];
Sotiriou & Zhang PRL(14);
Sotiriou & Zhou PRD(14)-a1408.

@ __Black holes, other__: Jacobson PRL(99)ap [primordial];
Stefanov et al MPLA(07)-a0708 [and non-linear electrodynamics];
Sotiriou & Faraoni PRL(12)-a1109 [stationary];
Cardoso et al PRD(13)-a1305 [scalarization and superradiant instability];
Rupert & Woolgar CQG(14)-a1310 [properties of horizons];
Bronnikov et al IJMPD(16)-a1603-conf.

@ __Numerical models__: Gerosa et al CQG(16)-a1602 [simulations of stellar collapse].

@ __Other solutions__: Moffat gq/07 [spherically symmetric, non-singular];
Sobreira et al JMP(09) [Einstein-Maxwell, static cylindrically symmetric];
Obukhov & Puetzfeld PRD(14)-a1404 [dynamics of extended test bodies, covariant multipolar approach];
> s.a. gravitating bodies [relativistic stars];
wormhole solutions.

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