Peculiar motion of Solar system from the Hubble diagram of supernovae Ia and its implications for cosmology

Abstract

Peculiar motion of the solar system, determined from the dipole anisotropy in the Cosmic Microwave Background Radiation (CMBR), has given a velocity $370$ km s$^{-1}$ along RA$=168^{\circ}$, Dec$=-7^{\circ}$. Subsequent peculiar motion determinations from the number counts, sky brightness or redshift dipoles observed in large samples of distant radio galaxies and quasars yielded peculiar velocities two to ten times larger than CMBR, though in all cases the directions matched with the CMBR dipole. Here we introduce a novel technique for determining the peculiar motion from the magnitude-redshift ($m_{\rm B}-z$) Hubble diagram of Type Ia Supernovae (SN Ia), one of the best standard candles available. We find a peculiar velocity $1.6\pm 0.5 \times 10^3$ km s$^{-1}$, larger than the CMBR value roughly by a factor of four, along RA$=173^{\circ}\pm12^{\circ}$, Dec$=10^{\circ}\pm9^{\circ}$, the direction being within $\stackrel{<}{_{\sim}}2\sigma$ of the CMBR dipole. Since a genuine solar motion would not depend upon the method or the dataset employed, large discrepancies seen among various dipole amplitudes could imply that these dipoles, including the CMBR one, might not pertain to observer's peculiar motion. However, a common direction for various dipoles might indicate a preferred direction in the universe, implying an intrinsic anisotropy, in violation of the cosmological principle, a cornerstone of the modern cosmology.