# Red Shift, Critical Density and Dark Matter

Red shift

As in sound waves, Doppler’s effect is also observed in case of light. If a light source is moving towards the observer its wavelength decreases and vice-versa. According to Hubble’s law every heavenly body is receded with one another. So the light emitted by a star is observed to be of longer wavelength compared to its real value. Red shift is defined as the ratio of change in apparent wavelength to the real wavelength.

i.e. red shift  receded with one another. So the light emitted by a star is observed to be of longer wavelength compared to its real value. Red shift is defined as the ratio of change in apparent wavelength to the real wavelength.

i.e. red shift =  (λo-λ) /λ = v/c

Where,  λo = observed wavelength

λ = real wavelength of a heavenly body emitting light.

Critical density

During big bang all the heavenly objects were set in motion due to which the size of universe is expanding till date but their gravitational force tries to collect them back. If the density of the universe is large enough, the masses of the universe will recollect to the size of a proton again. But if the density is not large enough the universe will keep expanding. This density which can collect all the mass of the universe is known as critical density. It is about 6.4×10-27 kgm-3. It is about 4 H atoms per cubic meter.

Dark matter

We cannot explain different physical phenomenon of the universe because the calculation requires some extra mass. This mass is supposed to be missing from our observations. Such mass are called dark matter. It is assumed that only 1% of mass have been observed and remaining 99% of the universe is unseen which is called as dark matter.

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