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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 ...
In this case, the above expression for the scale factor is not valid and , where the constant H is the Hubble parameter. More generally, the expansion of the universe is accelerating for any equation of state w < − 1 / 3 {\displaystyle w<-1/3} .
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.
k = +1, 0 or −1 depending on whether the shape of the universe is a closed 3-sphere, flat (Euclidean space) or an open 3-hyperboloid, respectively. [10] If k = +1, then a is the radius of curvature of the universe. If k = 0, then a may be fixed to any arbitrary positive number at one particular time.
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 .
In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is uniformly isotropic and homogeneous when viewed on a large enough scale, since the forces are expected to act equally throughout the universe on a large scale, and should, therefore, produce no observable inequalities in the large-scale structuring over the course ...