Testing the cosmological principle in the radio sky

Abstract

The Cosmological Principle states that the Universe is statistically isotropic and homogeneous on large scales. In particular, this implies statistical isotropy in the galaxy distribution, after removal of a dipole anisotropy due to the observer's motion. We test this hypothesis with number count maps from the NVSS radio catalogue. We use a local variance estimator based on patches of different angular radii across the sky and compare the source count variance between and within these patches. In order to assess the statistical significance of our results, we simulate radio maps with the NVSS specifications and mask. We conclude that the NVSS data is consistent with statistical isotropy.

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

“…At the same time, the direction of the velocity vector, though, was surprisingly found to be in agreement with the CMBR value. These unexpected findings of the NVSS dipole being many times larger than the CMBR dipole, have since been confirmed in a number of publications [11][12][13][14][15]. Such a difference between two dipoles would imply a relative motion between two cosmic reference frames which will be against the cosmological principle on which the whole modern cosmology is based upon.…”

“…If we now compare the dipole determined from number counts for the TGSS dataset with that determined from the NVSS dataset [3,[11][12][13][14][15], we find that the directions of the dipole from both these datasets match well with the CMBR measurements, implying that the cause of the dipoles is common and a peculiar motion of the solar system seems to be the only reasonable interpretation for that. However such a statistically significant disparity in their magnitudes, with the TGSS dipole being an order of magnitude (a factor of ∼ 10) larger than the CMBR dipole, while the NVSS dipole being ∼ 4 times larger than the CMBR dipole, is rather unsettling.…”

“…The TGSS-ADR1, henceforth called TGSS, dataset covers whole sky north of declination −53 • , a total of 3.6π sr, amounting to 90% of the celestial sphere, with an rms noise below 5 mJy/beam and an approximate resolution of 25 ′′ × 25 ′′ . Using a detection limit of 7-sigma, the TGSS catalog comprises 0.62 Million radio sources with an accuracy of about 2 ′′ or better in RA and Dec. From the spectral index data of common sources in the TGSS and NVSS catalogs [15,17,18], it has been demonstrated that the number counts in the two datasets could be compared above the flux-density limits of 100 mJy and 20 mJy respectively, using a relation S NVSS ≈ 0.2 S TGSS , and that the two catalogs are essentially complete above these respective flux-density limits.…”

“…As we mentioned above, the direction of the dipole from TGSS data is quite in agreement with that of the CMBR dipole. However, the strength of the dipole (D ≃ 5.2 × 10 −2 , the best estimate in Table I) appears an order of magnitude larger than the CMBR dipole (≃ 0.47 × 10 −2 ) [3], even though it is smaller by a factor of ∼ 1.4 than an earlier estimate (7 × 10 −2 ) [15]. When compared with the dipole determined from the sky brightness in the NVSS dataset [3], the TGSS dipole is a factor of ∼ 2.5 stronger than the NVSS dipole (≃ 2.1 × 10 −2 ).…”

“…Therefore it is imperative that an investigation of the radio source dipole be made employing some independent radio source samples. A recent estimate from the TGSS data [16], has yielded an even larger amplitude for the radio dipole [15], which is all the more disturbing. Here we investigate this radio dipole in further details, by choosing different flux-density levels, to examine the self-consistency of the dipole in the TGSS data and relate the results to those from NVSS catalog by using the spectral index information between the two datasets [17,18].…”