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If a source of the light is moving away from an observer, then redshift (z > 0) occurs; if the source moves towards the observer, then blueshift (z < 0) occurs. This is true for all electromagnetic waves and is explained by the Doppler effect. Consequently, this type of redshift is called the Doppler redshift.
Hubble's law, also known as the Hubble–Lemaître law, [1] is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from the Earth, the faster it moves away.
The redshift is the relative increase in the wavelength emitted by a light source, in this case a galaxy, moving away from an observer from which its speed and then, using Hubble's law, its distance can be calculated. A 3-dimensional map of that part of the Universe could thus be produced.
In astronomy, a redshift survey is a survey of a section of the sky to measure the redshift of astronomical objects: usually galaxies, but sometimes other objects such as galaxy clusters or quasars. Using Hubble's law, the redshift can be used to estimate the distance of an object from Earth. By combining redshift with angular position data, a ...
Such observers are called "comoving" observers because they move along with the Hubble flow. A comoving observer is the only observer who will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted.
In astronomical objects with strong gravitational fields the redshift can be much greater; for example, light from the surface of a white dwarf is gravitationally redshifted on average by around (50 km/s)/c (around 170 ppm). [12] Observing the gravitational redshift in the Solar System is one of the classical tests of general relativity. [13]
The relationship between the expansion of the universe and the Doppler effect is not simple matter of the source moving away from the observer. [11] [12] In cosmology, the redshift of expansion is considered separate from redshifts due to gravity or Doppler motion. [13]
Since the relation holds in all directions it cannot be attributed to normal movement with respect to a background which would show an assortment of redshifts and blueshifts. Everything is moving away from the Milky Way galaxy. Hubble's contribution was to show that the magnitude of the redshift correlated strongly with the distance to the ...