|  Higher-Order Theories of Quantum Gravity | 
In General > s.a. classical higher-order theories;
  covariant quantum gravity [stability of Minkowski space].
  * Results: With the usual quadratic action
    the theory is renormalizable and asymptotically free [@ Tomboulis], but non-unitary (the
    quartic Γ terms in R2 give spin-2 ghosts
    in the propagators) and with H unbounded below [@ Stelle, etc]; However, it has been
    conjectured that the ghost does not appear in the physical spectrum, based on a QCD analogy;
    A theory with infinitely many derivative terms in the action is super-renormalizable.
  @ General references:
    Asorey et al IJMPA(97);
    Mielke PRD(08)-a0707 [topological action];
    Accioly et al PRD(18)-a1707 [features].
  @ Renormalization: Utiyama & DeWitt JMP(62);
    Stelle PRD(77);
    Mazzitelli PRD(92);
    Fukuma & Matsuura PTP(02);
    de Berredo-Peixoto & Shapiro PRD(05)ht/04 [Gauss-Bonnet term, 4−ε];
    Chaves a0808 [with quadratic terms];
    Modesto PRD(12)-a1107 [super-renormalizability];
    Modesto a1305-MG13 [super-renormalizable higher-derivative theories];
    Modesto & Rachwal NPB(14)-a1408 [super-renormalizable and finite theories];
    Modesto & Shapiro PLB(16)-a1512 [superrenormalizable, with complex ghosts];
    > s.a. asymptotic safety.
  @ Related topics: Accioly et al IJTP(00) [computing the propagator];
    Kleidis et al PLB(02)ht [with massive scalar];
    Bodendorfer & Neiman PRD(14)-a1304 [loop quantization, and Wald entropy formula];
    Bonezzi et al PRD(14)-a1407 [as a Chern-Simons theory].
  > Related topics: see Hierarchy
    Problem; path-integral quantum gravity; quantum gravity;
    semiclassical quantum gravity; Stückelberg Trick.
Specific Types of Theories
  > s.a. types of higher-order theories.
  @ Quadratic gravity: Mazzitelli PRD(92),
    Tomboulis PLB(96)ht [relationship with general relativity, and renormalization];
    Holdom & Ren PRD(16)-a1512 [quadratic and Einstein-Hilbert terms, quantum phase transition],
    IJMPD(16)-a1605-GRF;
    Álvarez et al JCAP(17)-a1703 [first-order formalism];
    Lehners & Stelle a1909 [and inflation].
  @ f(R) theories:
    Cognola et al JCAP(05)ht,
    Cognola & Zerbini JPA(06)in
      [one-loop covariant, around de Sitter spacetime];
    Ahmed a1112 [2D];
    Ohkuwa & Ezawa CQG(12)-a1203,
    CQG(13) [third quantization];
    > s.a. unimodular gravity.
  @ f(R) theories, lqg approach:
    Fatibene et al CQG(10)-a1003;
    Zhang & Ma PRL(11)-a1101,
    PRD(11)-a1107;
    Ma JPCS(12)-a1112;
    Amorós et al PRD(14)-a1402 [lqc];
    Chen PRD(19)-a1811 [R2 lqc, effective dynamics].
  @ 3D: Deser PRL(09) [ghost-free, UV-finite theory];
    Helayël-Neto et al EPJC(10)-a1002.
  @ Infinite-derivative theories: Talaganis et al CQG(15)-a1412,
    Talaganis & Mazumdar a1704 [UV behavior].
  @ Other types: Narain & Anishetty PLB(12) [fourth-order derivative gravity, perturbatively renormalizable and unitary];
    Modesto AR-a1202,
    a1402,
    Modesto & Rachwal NPB(15)-a1503 [super-renormalizable theory in any spacetime dimensionality];
    Christiansen a1612 [Euclidean four-derivative theory].
And Quantum Cosmology
  @  General references:
    Hawking & Luttrell NPB(84);
    van Elst et al CQG(94)gq [R + R3 action];
    Pimentel et al CQG(97) [pure R2 action];
    Davis GRG(00) [string-motivated];
    Fabris & Reuter GRG(00);
    Sanyal & Modak PRD(01)gq,
    CQG(02)gq/01 [R + R2 action];
    Shojai & Shojai GRG(08)-a0801 [spatially flat];
    Tkach MPLA(09)-a0808 [ghost-free theory and hierarchy problem].
  @ FLRW minisuperspace: Sanyal PLB(02)gq
      [Schrödinger equation and interpretation];
    Vázquez-Báez & Ramírez AMP(17)-a1706 [quadratic f(R) theories].
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