Large disparity in cosmic reference frames determined from the sky distributions of radio sources and the microwave background radiation

Abstract

The angular distribution of the cosmic microwave background radiation (CMBR) in the sky shows a dipole asymmetry, ascribed to the peculiar velocity of the observer (or rather of the Solar system), where peculiar velocity is defined as motion relative to the local comoving coordinates. The peculiar velocity thus determined turns out to be $370\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ in the direction $\mathrm{RA}=168\ifmmode^\circ\else\textdegree\fi{}$, $\mathrm{Dec}=\ensuremath{-}7\ifmmode^\circ\else\textdegree\fi{}$. On the other hand, a dipole asymmetry in the sky distribution of radio sources in the NRAO VLA Sky Survey (NVSS), carried out by the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO), comprising a catalog of 1.8 million sources, yielded a value for the observer's velocity approximately four times larger than the CMBR value, though the direction turned out to be in agreement with that of the CMBR dipole. This large difference in the observer's speeds with respect to the reference frames of NVSS radio sources and of CMBR, confirmed since by many independent groups, is rather disconcerting, as the observer's motion with respect to local comoving coordinates should be independent of the technique used to determine it. A genuine difference in relative speeds of two cosmic reference frames could jeopardize the cosmological principle; thence, it is crucial to confirm such discrepancies using independent samples of radio sources. We investigate here the dipole in the sky distribution of radio sources in the recent TIFR GMRT Sky Survey (TGSS), carried out by the Giant Metrewave Radio Telescope (GMRT) of the Tata Institute of Fundamental Research (TIFR), comprising a dataset of 0.62 million sources, to determine the observer's motion. We find a significant disparity in the observer's speeds relative to all three reference frames, determined from the radio source datasets and the CMBR, which does not fit with the cosmological principle, a starting point for the standard modern cosmology.