|  Phenomenology of Higher-Order Gravity Theories | 
In General
  > s.a. cosmology in higher-order theories.
  * Key features: On cosmological scales
    modified-gravity theories deviate from general relativity in that they can cause cosmic
    acceleration without a physical, negative-pressure fluid, a gravitational slip between the
    two metric potentials, and a  propagation speed for gravitational waves that differs from
    that of light.
  * Stability: Certain f(R)
    theories can stabilize solutions which are unstable in general relativity, such as the
    Einstein static universe.
  * Viability: Only the f(R)
    models which have a built-in chameleon screening mechanism are viable. 
  @ Reviews and general references:
    Capozziello & Francaviglia GRG(08);
    Sotiriou JPCS(09)-a0810;
    de la Cruz-Dombriz & Sáez-Gómez Ent(12)-a1207.
  @ Stability: Schmidt PRD(94);
    Ezawa et al CQG(99) [higher-dimensional, semiclassical];
    Dolgov & Kawasaki ap/03 [R−1 theories];
    De Felice et al JCAP(06)ap [superluminal mode propagation];
    Faraoni PRD(06)ap,
    Sotiriou PLB(07)gq/06,
    Sawicki & Hu PRD(07)ap [\(f(R)\) theories];
    Sokołowski CQG(07)gq [interpretation and viability];
    Böhmer et al PRD(07)-a0706 [Einstein static universe];
    Lee a0710;
    Bertolami & Carvalho Sequeira PRD(09) [and energy conditions];
    Saltas & Kunz PRD(11)-a1012 [and anisotropic stress];
    > s.a. types of theories
      [Hamiltonian perturbation theory; infinite-derivative gravity].
  @ R−1 theories:
    Flanagan PRL(04)ap/03,
    Vollick CQG(04)gq/03,
    Kremer & Alves PRD(04)gq [Palatini form];
    Soussa & Woodard GRG(04)ap/03 [force of gravity];
    Vollick PRD(05)gq/04 [with Dirac field];
    Shao et al PLB(06)gq/05 [as correction term].
  @ Other theories: Clifton & Barrow PRD(05)gq [R1+a Lagrangian];
    Navarro & Van Acoleyen JCAP(06)ap [ln f(R), dark energy and MOND];
    Olmo AIP(10)-a0910 [general, esp \(1/R\) and \(f(R) = R + R^2/R_{\rm P}^{~}\) theories].
  @ Non-local theories: Koivisto PRD(08)-a0807 [newtonian limit];
    > s.a. theories of gravitation.
Solutions > s.a. black-hole solutions;
  schwarzschild spacetime; Universal Spacetimes.
  * In general: All solutions of
    the ordinary Einstein equation still extremize S, and there are other
    solutions; However, if S does not include the usual R term,
    the ordinary solutions do not couple to positive-definite matter sources.
  * Features: In some theories,
    torsion and its conjugate momentum play an important role.
  * Results: Birkhoff's
    theorem in general is not valid, but it is in some cases; There is massive
    radiation (2 Yukawa potentials, 8 degrees of freedom, the usual spin-2 graviton,
    a massive one – negative energy – and one scalar field); It is
    claimed that in theories with purely quadratic terms in the action, the energy
    is identically zero, but this seems strange (RDS).
  @ Gravitational waves: Campanelli & Loustó PRD(96)gq/95 [shock waves];
    de Rey et al CQG(03)gq,
    CQG(03)gq,
    CQG(04)gq/03;
    Ananda et al PRD(08)-a0708 [4th-order gravity in FLRW models];
    Corda & De Laurentis proc(08)-a0710 [\(R^{-1}\) theory];
    Capozziello et al PLB(08)-a0812 [massive, and LISA];
    Kausar et al PRD(16)-a1606,
    Liang et al PRD(17)-a1701 [polarizations];
    Moretti et al a1906
      [f(R), gauge-invariant formalism];
    > s.a. gravitational waves and background.
  @ Cosmological solutions: Clifton & Barrow PRD(05)gq [Gödel, Einstein and de Sitter],
    CQG(06) [Kasner-type],
    CQG(06)gq [cosmological, 4th-order theories];
    Goswami et al PRD(08)-a0804 [Einstein universes, 4th-order theories];
    Hindmarsh & Saltas PRD(12) [f(R) gravity from the renormalization group];
    > s.a. types of spacetimes [Einstein static universe].
  @ Axisymmetric: Capozziello et al CQG(10)-a0912;
    De Laurentis a1111;
    Suvorov & Melatos PRD(16)-a1608 [Ernst formulation, and applications].
  @ Solutions with symmetries:
    Azadi et al PLB(08) [static cylindrically symmetric];
    Sharif & Farasat Shamir MPLA(10) [plane-symmetric];
    > s.a. spherical symmetry.
  @ f(R)  theories, gravitational collapse:
    Cembranos et al JCAP(12)-a1201 [dust collapse];
    Borison et al PRD(12),
    Guo et al PRD(14)-a1312 [spherical scalar collapse];
    Montes Núñez et al AIP(11)-a1210;
    Astashenok et al IJGMP(19)-a1812 [spherical, \(R^2\) theories].
  @ Other types of solutions: Campanelli et al PRD(94)gq [perturbative method];
    Barraco & Hamity GRG(99)
      [asymptotically flat, and general relativity solutions];
    Mazharimousavi et al EPJC(12)-a1110 [f(R) theory with an electromagnetic field];
    > s.a. Birkhoff's Theorem; Israel's Theorem;
      Lichnerowicz Theorem; types of higher-order
      theories [3D]; gödel solution; wormhole solutions.
Newtonian Limit
  * Remark: Its existence cannot work as a selection
    rule among metric f(R) theories, because it is implied by stability.
  @ General references:
    Quandt & Schmidt AN(91)gq/01;
    Domínguez & Barraco PRD(04)gq [Palatini];
    Capozziello gq/04-ch [rev];
    Capozziello & Troisi PRD(05)ap [4th-order, PPN limit];
    Sotiriou GRG(06)gq/05;
    Bertolami et al PRD(07)-a0704 [extra force];
    Capozziello et al PRD(07)-a0708;
    Corda IJTP(08) [extra repulsive force];
    Stabile PhD(08)-a0809;
    Sokołowski APPB(08)-a0810 [and stability];
    Capozziello et al MPLA(09)-a0901 [presence of Yukawa correction];
    Capozziello & Stabile CQG(09)-a0903 [quadratic Lagrangians],
    a1009 [fourth-order theories];
    Capozziello et al PLB(10)-a1002 [compared to scalar-tensor gravity];
    Brandhuber & Travaglini a1905 [particle scattering and bending];
    Burzillà et al PRD-a2012 [fourth-derivative gravity],
    a2012 [higher than fourth derivatives].
  @ f(R) gravity:
    Stabile & Capozziello a1010-proc;
    Eingorn & Zhuk PRD(11)-a1104 [three and more spatial dimensions].
  @ Newtonian limit, R−1:
    Dick GRG(04)gq/03;
    Rajaraman ap/03;
    Navarro & Van Acoleyen PLB(05)gq [and acceleration];
    Cembranos PRD(06)gq/05 [as limit at intermediate energy];
    Navarro & Van Acoleyen JCAP(06)gq/05 [large distance].
  @ Post-Newtonian framework:
    Allemandi et al GRG(05);
    Capozziello et al MPLA(06)gq [and experimental constraints];
    De Laurentis et al AdP(10)-a0911 [and cosmological gravitational waves];
    Capozziello et al IJTP(10)-a1001 [post-Minkowskian limit].
Other Phenomenology
  > s.a. cosmology in higher-order theories [including cluster
  density profiles]; phenomenology of gravity; tests of general relativity.
  @ Galaxy rotation curves:
    Capozziello et al PLA(04),
    AIP(05)ap/04;
    Frigerio & Salucci MNRAS(07)ap;
    Harko PRD(10)-a1004;
    Capozziello et al APP(13)-a1307 [hybrid metric-Palatini gravity];
    Shojai & Shojai GRG(14) [f(R) gravity];
    Salucci et al IJMPD(14)-a1405-GRF [Rn gravity].
  @ Other galactic dynamics: Borka et al PRD(12) [\(R^n\) theories and precession of stellar orbits].
  @ Solar system constraints: Olmo PRL(05)gq,
    PRD(05)gq
    + gq,
    Jin et al gq/06 [f(R)];
    Faraoni PRD(06)gq;
    Ruggiero & Iorio JCAP(07)gq/06 [planetary orbits];
    Chiba et al PRD(07)ap/06;
    Zakharov et al PRD(06);
    Faulkner et al PRD(07)ap/06,
    Saffari & Rahvar PRD(08)-a0708 [and cosmology];
    Hu & Sawicki PRD(07)-a0705 [models that evade solar system tests];
    Allemandi & Ruggiero GRG(07);
    Bertolami & Páramos PRD(08)-a0709;
    Iorio & Ruggiero SRE(08)-a0711;
    Capozziello & Tsujikawa PRD(08)-a0712;
    De Felice & Tsujikawa PRD(09)-a0907;
    Bisabr PLB(10)-a0907;
    Näf & Jetzer PRD(10)-a1004 [1/c expansion];
    Bisabr G&C(10)-a1005 [power-law f(R) theories];
    Faraoni PRD(11)-a1106 [\(R^n\) theories severely constrained];
    Guo IJMPD(14)-a1306 [f(R) theories];
    Xie & Deng MNRAS(13)-a1312 [f(R) theories];
    Schärer et al PRD(14)-a1410 [tests in Earth orbit];
    Bonino et al a2011;
    > s.a. gravity tests with orbits.
  @ Solar system constraints, R−1:
    Erickcek et al PRD(06)ap [theory ruled out];
    Exirifard a0810 [theory not ruled out].
  @ Stars, astrophysics: Kainulainen et al PRD(07)-gq/06;
    Bustelo & Barraco CQG(07)gq/06;
    Henttunen et al PRD(08)-a0705;
    Capozziello et al PRD(11)-a1101
      [f(R), hydrostatic equilibrium and stellar structure];
    Goswami et al PRD(14)-a1409 [collapsing spherical stars];
    > s.a. gravitating bodies; neutron stars.
  @ Cosmological scales: Boubekeur et al PRD(14)-a1407 [clustering];
    de Martino et al Univ(15)-a1507,
    Achitouv et al PRD(16)-a1511 [large-scale structure, halos];
    Capozziello et al MNRAS-a1711 [clustering].
  @ Propagating degrees of freedom: Magnano & Sokołowski AP(03)gq/02 [massive spin-2 field generated];
    Núñez & Solganik PLB(05)ht/04
      [IR modifications not viable, ghosts or light gravity scalars];
    Capozziello et al MPLA(07) [gravitational-wave background];
    De Laurentis et al MPLA(12)-a1206 [massive ghost modes and gravitational Cherenkov radiation];
    Myung a1608
      [polarization modes of gravitational waves in f(R) gravity];
    Bueno et al PRD(17)-a1610.
  @ Related topics: Accioly et al NCB(00),
    Accioly & Blas PRD(01)gq [light deflection, quadratic];
    Gabadadze & Iglesias PLB(06) [precession];
    Li et al CQG(09)-a0801 [indistinguishable macroscopic behavior];
    Bisabr PLB(10)-a1003 [and time variation of α];
    Morais et al JCAP(15)-a1507 [f(R) gravity and observed radiation];
    Giacchini a1612-MG14
      [R2 and
      \(R_{ab}^2\) theories, short-range laboratory experiments].
  > Related topics:
    see black-hole formation and laws;
    causality violation; Chameleon Field;
    energy [virial theorem]; energy conditions;
    equivalence principle; gravitating bodies
    [including two-body systems]; gravitational radiation;
    singularities and types of singularities
    [isotropic, naked]; thermodynamics.
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