what is hubble law ....................
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Expanding Universe. The distant galaxies we see in all directions are moving away from the Earth, as evidenced by their red shifts. Hubble's law describes this expansion.
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Hubble's law
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Hubble's law, also known as the Hubble–Lemaître law,[1] is the observation in physical cosmology that:
Objects observed in deep space—extragalactic space, 10 megaparsecs(Mpc) or more—are found to have a redshift, interpreted as a relative velocity away from Earth;
This Doppler shift-measured velocity of various galaxies receding from the Earth is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away.[2][3]
Hubble's law is considered the first observational basis for the expansion of the universe and today serves as one of the pieces of evidence most often cited in support of the Big Bang model.[4][5] The motion of astronomical objects due solely to this expansion is known as the Hubble flow.[6]
Although widely attributed to Edwin Hubble,[7][8][9] the notion of the universe expanding at a calculable rate was first derived from the general relativity equations in 1922 by Alexander Friedmann. Friedmann published a set of equations, now known as the Friedmann equations, showing that the universe might expand, and presenting the expansion speed if this was the case.[10] Then Georges Lemaître, in a 1927 article, independently derived that the universe might be expanding, observed the proportionality between recessional velocity of and distance to distant bodies, and suggested an estimated value of the proportionality constant, which when corrected by Hubble became known as the Hubble constant.[4][11][12][13] Though the Hubble constant {\displaystyle H_{0}} is roughly constant in the velocity-distance space at any given moment in time, the Hubble parameter {\displaystyle H}, which the Hubble constant is the current value of, varies with time, so the term 'constant' is sometimes thought of as somewhat of a misnomer. [14]Moreover, two years later Edwin Hubble confirmed the existence of cosmic expansion, and determined a more accurate value for the constant that now bears his name.[15] Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917.[16][17][18][19]
The law is often expressed by the equation v = H0D, with H0 the constant of proportionality—Hubble constant—between the "proper distance" D to a galaxy, which can change over time, unlike the comoving distance, and its velocity v, i.e. the derivativeof proper distance with respect to cosmological time coordinate. (See uses of the proper distance for some discussion of the subtleties of this definition of 'velocity'.) Also, the SI unit of H0 is s−1, but it is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away. The Hubble constant is about 70 (km/s)/Mpc. The reciprocal of H0 is the Hubble time.
Discovery
Interpretation
Determining the Hubble constant
Derivation of the Hubble parameter
Units derived from the Hubble constant
Observed values of the Hubble constant
See also
Notes
References
Further reading
External links
Last edited 8 days ago by Drbogdan

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