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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 ...
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.
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, H 0 {\displaystyle H_{0}} , is ≈70.88 km s −1 Mpc −1 (The Hubble time is 13.79 billion years).
Recessional velocity is the rate at which an extragalactic astronomical object recedes (becomes more distant) from an observer as a result of the expansion of the universe. [1] It can be measured by observing the wavelength shifts of spectral lines emitted by the object, known as the object's cosmological redshift .