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Instead of working with Hubble's constant, a common practice is to introduce the dimensionless Hubble constant, usually denoted by h and commonly referred to as "little h", [29] then to write Hubble's constant H 0 as h × 100 km⋅s −1 ⋅Mpc −1, all the relative uncertainty of the true value of H 0 being then relegated to h. [46]
Even light itself does not have a "velocity" of c in this sense; the total velocity of any object can be expressed as the sum = + where is the recession velocity due to the expansion of the universe (the velocity given by Hubble's law) and is the "peculiar velocity" measured by local observers (with = ˙ () and = ˙ (), the dots indicating a ...
The observational result of Hubble's law, the proportional relationship between distance and the speed with which a galaxy is moving away from us, usually referred to as redshift, is a product of the cosmic distance ladder. Edwin Hubble observed that fainter galaxies are more redshifted. Finding the value of the Hubble constant was the result ...
Evaluating the Hubble parameter at the present time yields Hubble's constant which is the proportionality constant of Hubble's law. Applied to a fluid with a given equation of state , the Friedmann equations yield the time evolution and geometry of the universe as a function of the fluid density.
Using the Planck units, and the value evaluated in 2025 for the Hubble constant H 0 = 76.5 ± 2.2 (km/s)/Mpc = (2.48 ± 0.07) × 10 −18 s −1, [18] Λ has the value of = = = where is the Planck length. A positive vacuum energy density resulting from a cosmological constant implies a negative pressure, and vice versa.
The current density of the observable universe is of the order of 9.44 · 10 −27 kg m −3 and the age of the universe is of the order of 13.8 billion years, or 4.358 · 10 17 s. The Hubble constant, , is ≈70.88 km s −1 Mpc −1 (The Hubble time is 13.79 billion years).
where is the Hubble constant, is the proper distance, is the object's recessional velocity, and is the object's peculiar velocity. The recessional velocity of a galaxy can be calculated from the redshift observed in its emitted spectrum. One application of Hubble's law is to estimate distances to galaxies based on measurements of their ...
For example, 7 × 10 13 h −1 M ☉ = 10 14 h −1 0.70 M ☉. Our best measurement, as of 2013, for the Hubble parameter is h = 0.6780 ± 0.0077 from the Planck mission. In early 2011 it was 0.704 +0.013 −0.014 from WMAP 7-year data. [1] See Hubble's law#Determining the Hubble constant for the most recent value of H 0.