According to the cosmological principle, the Universe should appear isotropic, without any preferred directions, to an observer whom we may consider to be fixed in the co-moving co-ordinate system of the expanding Universe. Such an observer is stationary with respect to the average distribution of the matter in the Universe and the sky brightness at any frequency should appear uniform in all directions to such an observer. However a peculiar motion of such an observer, due to a combined effect

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... f Doppler boosting and aberration, will introduce a dipole anisotropy in the observed sky brightness; in reverse an observed dipole anisotropy in the sky brightness could be used to infer the peculiar velocity of the observer with respect to the average Universe. We determine the peculiar velocity of the solar system relative to the frame of distant radio sources, by studying the anisotropy in the sky brightness from discrete radio sources, i.e., an integrated emission from discrete sources per unit solid angle. Our results give a direction of the velocity vector in agreement with the Cosmic Microwave Background Radiation (CMBR) value, but the magnitude (∼ 1600± 400 km/s) is ∼ 4 times the CMBR value (369± 1 km/s) at a statistically significant (∼ 3σ) level. A genuine difference between the two dipoles would imply anisotropic Universe, with the anisotropy changing with the epoch. This would violate the cosmological principle where the isotropy of the Universe is assumed for all epochs, and on which the whole modern cosmology is based upon.