Inflationary Universe  

In General > s.a. inflationary phenomenology and scenarios; inflation and planck-scale physics.
* Idea: A universe with an accelerating expansion rate, d2a/dt2 > 0 for FLRW spacetime in proper time; The expansion may be exponential, a ∝ exp{Ht}.
* Origin in homogeneous cosmology: It arises in models where (e.g., because of a phase transition) the energy density remains constant during the expansion, like a cosmological constant; This changes drastically the expansion rate with respect to the standard big-bang model, and makes it exponential in time (inflation), and we get a de Sitter-like behavior.
* Motivation: A sufficiently long inflation could solve the horizon, flatness and monopole problems of big bang models, and possibly explain structure formation; It makes predictions, and provides a link between high-energy physics and cosmology.
* Generalized view: The universe has no beginning and no end, inflating bubbles in it keep producing new ones.
* 1980s: Inflation predicts Ω = 1, but observationally we see Ω < 0.3 (without dark matter).
* 1991: What is the inflaton? How does it become ordinary matter (reheating problem)?
* 1998: Chaotic inflation seems to be the most widely accepted; Requires the least amount of fine tuning.
* 2008: Predictions from inflation have so far been confirmed, but what drove it and how long did it last?
* 2013: The single field slow-roll inflationary scenarios (with minimal kinetic term) are favored, based on Planck satellite data.
* 2014: BICEP2 Collaboration results on the ratio of power in tensor to scalar density perturbations are seen as confirming inflation.

Specific Issues > s.a. de sitter-cft; Effective Field Theory; holography in field theory; civilizations; singularities [theorems]; arrow of time.
* Issues: Open questions are understanding whether the universe needed to be in a very smooth initial state for inflation; the quantum theory of cosmological perturbations; how these might have led to inflation; and what imprint they left on the resulting classical spacetime.
@ General references: Albrecht et al PRD(87); Goncharov & Linde JETP(87); Jensen & Stein-Schabes PRD(87); Ellis CQG(88) [with current Ω ≠ 1]; Accetta & Steinhardt GRG(91); Frolov & Kofman JCAP(03)ht/02 [de Sitter thermodynamics]; Kinney & Riotto JCAP(06)ap/05 [uncertainties].
@ Preheating and reheating: Dolgov & Linde, Abbott et al [elementary theory]; Boyanovsky et al ap/96-proc [review]; Kofman et al PRD(97)hp; Bassett & Tamburini PRL(98) [in GUTs]; Brandenberger hp/01-proc; Allahverdi et al ARNPS(10)-a1001.
@ And phase transitions: Mazenko et al PRD(85); Freivogel et al JCAP(09)-a0901 [probability distribution for bubble collisions]; Johnson et al PRD(12)-a1112 [outcome of bubble collisions]; McEwen et al a1206-proc [bubble-collision signatures in the cmb]; Pashitskii & Pentegov JETP(16)-a1510 [driven by changing scalar curvature]; Pekker & Shneider a2102 [physical processes in the transition region]; > s.a. multiverse [bubbles as universes].
@ Inhomogeneities, causality / genericity: Goldwirth & Piran PRD(89), PRP(92); Calzetta & Sakellariadou PRD(92), PRD(93); Iguchi & Ishihara PRD(97), et al PRD(97)gq/96 [curvature]; Kaloper et al PRD(99)ht/98; Vachaspati & Trodden PRD(00)gq/98; Toporensky G&C(99)gq; Sakai CQG(04)gq/03 [topological inflation]; Imponente & Montani gq/04; Carroll & Tam a1008 [and invariant measure on solutions]; Corichi & Karami PRD(11)-a1011 [the measure problem and lqc]; Greene et al PLB(11) [smooth early universe from weak gravity]; Perez & Pinto-Neto G&C(11)-a1205; Corichi & Sloan CQG(14)-a1310 [Hamiltonian dynamics, attractor solutions and the measure problem]; Easther et al JCAP(14)-a1406 [multifield]; East et al JCAP(16)-a1511; Clough et al a1608 [robustness]; Azhar a1911 [effective field theory approach]; > s.a. inflationary phenomenology [perturbations].
@ Initial conditions: Calzetta PRD(91); Kaloper et al JHEP(02)ht; Collins & Holman ht/05 [effective theory renormalization]; Handley et al PRD(14)-a1401; Carrasco et al PRD(15)-a1506 [and cosmological attractors]; Brandenberger IJMPD(17)-a1601 [rev]; Dimopoulos & Artymowski APP(17)-a1610; Linde a1710-conf [rev]; Mishra et al PRD(18)-a1801 [in a FLRW universe]; Finn & Karamitsos PRD(19)-a1812 [finite measure]; Sloan & Ellis PRD(19)-a1810 [Higgs as dilaton].
@ Likelihood of inflation: Albrecht & Sorbo PRD(04)ht [framework]; Miao & Woodard JCAP(15)-a1506 [fine tuning].

References > s.a. entropy bounds; gravitational instantons; history; cosmological models [alternatives to inflation].
@ I: Guth & Steinhardt SA(84)may; Lindley Nat(90)may; Steinhardt Nat(90)may; Linde SA(94)nov; Lidsey 00; Steinhardt SA(11)apr [issues].
@ II: Huggins TPT(13) [and vacuum energy].
@ Books: Abbott & Pi 86; in Kolb & Turner 94; Linde 90-ht/05; Guth 97; Liddle & Lyth 00; Lemoine et al 08; Lyth & Liddle 09; Gorbunov & Rubakov 10 [and perturbations].
@ Reviews: Linde RPP(84), PT(87)sep, gq/96-conf, gq/96-conf; Turner APPB(87), in(88); Olive PRP(90); Brandenberger ap/96, ap/97, hp/99-ln; Starobinsky ap/98-conf; Peebles ap/99-proc; Guth PRP(00)ap, ap/00-proc, ap/01-proc; Lyth hp/00; Brandenberger hp/01-proc, ap/02; Linde IJMPA(02)ht/01-proc [vs ekpyrotic]; García-Bellido NPPS(03)hp/02; Turok CQG(02); Turner AHP(03)ap/02-proc; King Pra(04)hp/03-conf; Guth ap/03-in, ap/04-in; Linde PS(05)ht/04-conf; Guth & Kaiser Sci(05)ap; Bassett et al RMP(06)ap/05 [especially reheating]; Guth JPA(07)ht-conf [especially eternal]; Linde LNP(08)-a0705-conf; Kinney IJMPA(07); Baumann & Peiris ASL(09)-a0810; Sriramkumar a0904-CS [and perturbations]; Baumann a0907-ln [TASI lectures]; Linde a1402-ln [after Planck 2013]; Clesse a1501-proc, Martin a1502-ln [after Planck 2015]; Rangarajan a1506-conf [after Planck and BICEP2]; Martin a1807-proc, a1902-in [status].
@ Intros: Liddle ap/99-proc; Faraoni AJP(01)mar-phy/00; Albrecht ap/00-ln; Watson ap/00; Tsujikawa hp/03-ln; Lazarides JPCS(06)hp; Langlois a0811-conf; Kinney a0902-ln; Enqvist a1201-ln; Senatore a1609-ln; Vázquez et al a1810-ln.
@ Conceptual: Earman & Mosterin PhSc(99)mar; Knobe et al BJPS(06)phy/03 [implications, and doomsday]; García-Bellido a1003-wd [epistemological foundations]; Ijjas et al PLB(14)-a1402 [alternative inflationary paradigm after Planck2013]; Horgan blog(14)dec [Paul Steinhardt's position]; Singal a1603-conf [questioning the motivation].
@ Issues: Hawking et al PRD(01)ht/00 [and trace anomaly]; Ijjas et al PLB(13) and response Guth et al PLB(14), SA(17) and rebuttal SA(17) [need or new ideas]; > s.a. Boltzmann Brains; chaos in general relativity; energy conditions; tachyons.

Scientific ideas should be simple, explanatory, predictive. The inflationary multiverse as
currently understood appears to have none of those properties. – Paul Steinhardt, 2014.

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