Cosmological Expansion Rate – Hubble's Constant  

Observational Values blue bullet s.a. issues and effects [including global-local discrepancy]; friedmann equation; relativistic cosmology.
* Hubble's constant: Defined as H0:= R ·/R, from Hubble's Law; It is the most important number in extragalactic astronomy.
* Status: 2000, The main source of uncertainty in its value (about 10%) comes from distances to galaxies, determined mainly using Cepheid variables in the case of nearby galaxies; 2015, Many object-based measurements of H0 give 72-74 km/s/Mpc with typical errors of 2-3 km/s/Mpc, while cmb-based measurements give 67-68 km/s/Mpc with typical errors of 1-2- km/s/Mpc, so accuracy rather than precision may be the cause the discrepancy.
* Remark: The values below are estimated values of the current expansion rate; H actually varies in time.
* 1990: H0 = 75 – 100 km/s/Mpc [@ Fukugita & Hogan Nat(90)sep]; Seems to require Λ ≠ 0, otherwise the universe is too young.
* 1991: H0 = 50 ± 17 km/s/Mpc [gravitational lensing, @ Rhee Nat(91)mar].
* 1996: W Freedman et al estimate 68–78 (from Cepheids, supernovae, the Tully-Fischer relation, etc), A Sandage et al 57.
* 1999: H0 = 66 ± 6 km/s/Mpc (Cepheids), 72 ± 4 (type Ia supernovae), 65–80 (lensing), 71 ± 6 (combined).
* 2000: H0 = 74 ± 7 km/s/Mpc (Cepheids), H0 = 65 km/s/Mpc (favored?), H0 = 72 ± 8 km/s/Mpc (var).
* 2001: H0 = 64 + 14 – 11 ± 14sys (std CDM) or 66 + 1 4 –11 ± 15sys (Λ-cdm) km/s/Mpc (from SZ effect).
* 2002: H0 = 70 km/s/Mpc (cmb and 2dF galaxy redshift survey).
* 2003: H0 = 70 + 4 – 3 km/s/Mpc (WMAP, SDSS, and weak lensing).
* 2006: H0 = 71 km/s/Mpc accepted value; Distance to Triangulum galaxy by Bonanos et al, and Cepheid distance to NGC 4258 give higher values.
* 2010: H0 = 66 – 4 + 6 km/s/Mpc from 18 galaxy lenses with time-delay measurements; Another estimate is 73 ± 2 ± 4 km/s/Mpc.
* 2012: H0 = 79 – 17 + 23 km/s/Mpc (68%CL) using type IIn supernovae; 74.3 ± 2.1 km/s/Mpc, using Spitzer's measurements of Cepheid variables.
* 2013: H0 = 67.15 ± ... km/s/Mpc, from Planck mission data.
* 2014: H0 = 69.6 ± 0.7 km/s/Mpc (cmb, baryon acoustic oscillations and cosmic distance ladder); 68.11 ± 0.86 km/s/Mpc (acoustic oscillations).
* 2015: H0 = 67.8 ± 0.9 km/s/Mpc, from Planck mission cmb data.
* 2016: H0 = 73.03 ± 1.79 km/sec/Mpc in the local universe, from improved observations of Cepheid variables and other uncertainty reductions.
* 2017: The discrepancy between supernova and quasar data (73 and 72 km/s/Mpc) and cmb data (67 km/s/Mpc) persists [@ news sn(17)feb].

References > s.a. history of relativistic gravity; redshift [measurement].
@ General: Ferrarese et al ASP-ap/99 [combined]; Freedman et al ApJ(01)ap/00 [HST], AS(03) [high-precision cosmology]; Mason et al ApJL-ap/01 [Sunyaev-Zeldovich effect]; Chen et al PASP(03)ap [errors]; Tammann RMA(06)ap/05-conf [history]; Wang et al ApJ(06)ap; Macri et al ApJ(06)ap [NGC 4258]; Jackson LRR(07)-a0709; Gorecki et al a1001-MG12 [very-high-energy gamma rays]; Freedman & Madore ARAA(10)-a1004 [accuracy]; Riess et al ApJ(11); Suyu et al a1202-rp [motivation for better measurements]; news jpl(12)oct [Spitzer]; Livio & Riess PT(13)oct; Bennett et al ApJ(14)-a1406; Klöckner et al a1501-conf [using the HI signal]; Pritychenko a1506 [recommended value]; Singh et al JCAP(16)-a1506 [random errors in the data set]; Jackson LRR(15) [rev].
@ In the local universe: Ekholm et al A&AL-ap/01 [near the Local group]; Peñarrubia et al MNRAS(14)-a1405 [dynamical model]; Hess & Kitaura MNRAS(16)-a1412 [and cosmic flows]; Riess et al ApJ(16)-a1604 [2.4% determination].
@ In the intermediate universe: Moresco et al JCAP(12)-a1201 [0.15 < z < 1.1]; Verde et al a1403/PDU [0.1 < z < 1.2].
@ From Cepheids: Hendry & Rauzy, et al MNRAS(01)ap/99-conf; Abdel-Sabour et al RAJ(09)-a1409; Casertano et al a1609/A&A; Follin & Knox a1707 [insensitivity to Cepheid calibration].
@ From supernovae: Richtler & Drenkhahn LNP-ap/99; Blinnikov et al JETPL(12)-a1206 [type IIn]; Campbell et al MNRAS(16)-a1602 [and SN Ia host-galaxy properties]; L'Huillier & Shafieloo JCAP(17)-a1606 [and BOSS data, 68.49 ± 1.83 km/sec/Mpc]; Zhang et al MNRAS-a1706 [low-redshift type Ia supernovae]; Dhawan et al a1707 [as near-infrared standard candles].
@ From lensing: Bernstein & Fischer AJ(99)ap; Schechter IAU-ap/00, IAU(04)ap; Saha et al ApJL(06)ap; Paraficz & Hjorth ApJ(10)-a1002; Barnacka et al ApJ(14)-a1408, Bonvin et al MNRAS(17)-a1607 [time delays].
@ From large-structure: Lee JCAP(14)-a1307; Wang et al a1706 [galaxy redshift surveys].
@ From gravitational waves: Del Pozzo PRD(11)-a1108; Nissanke et al a1307/ApJ [merging compact binaries].
@ Other methods: Qin MPLA(99)ap/00; Lew et al ap/05-wd [from proper motions of radio jets]; Cheng & Huang SCPMA(15)-a1409 [from baryon acoustic oscillations]; Onuchukwu & Ezeribe a1507 [from the relation between distance modulus and redshift].


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