Explaining excess dipole in NVSS data using superhorizon perturbation

Abstract

Many observations in recent times have shown evidence against the standard assumption of isotropy in the Big Bang model. Introducing a superhorizon scalar metric perturbation has been able to explain some of these anomalies. In this work, we probe the net velocity arising due to the perturbation. We find that this extra component does not contribute to the CMB dipole amplitude while it does contribute to the dipole in large scale structures. Thus, within this model's framework, our velocity with respect to the large scale structure is not the same as that extracted from the CMB dipole, assuming it to be of purely kinematic origin. Taking this extra velocity component into account, we study the superhorizon mode's implications for the excess dipole observed in the NRAO VLA Sky Survey (NVSS). We find that the mode can consistently explain both the CMB and NVSS observations. We also find that the model leads to small contributions to the local bulk flow and the dipole in Hubble parameter, which are consistent with observations. The model leads to several predictions which can be tested in future surveys. In particular, it implies that the observed dipole in large scale structure should be redshift dependent and should show an increase in amplitude with redshift. We also find that the Hubble parameter should show a dipole anisotropy whose amplitude must increase with redshift in the CMB frame. Similar anisotropic behaviour is expected for the observed redshift as a function of the luminosity distance.

Superhorizon Perturbations: A Possible Explanation of the Hubble–Lemaître Tension and the Large-scale Anisotropy of the Universe

“…In particular, it will induce a small anisotropy along with a correlated isotropic shift in several other cosmological observables, such as BAO and the epoch of reionization, besides Hubble constant. At large redshifts, the model predicts an interesting redshift dependence, which can also be tested in future studies (Das et al 2021); this unique feature can potentially provide strong evidence for the existence of superhorizon modes.…”

“…This effectively introduces corrections to observed redshifts of galaxies. Das et al (2021) show that a galaxy at redshift z in the presence of a superhorizon perturbation Ψ p will be observed at redshift z obs , such that…”

“…x 3 . This kind of simple superhorizon mode has been shown to significantly affect the large-scale distribution of matter and can potentially explain (Ghosh 2014;Das et al 2021) the puzzling excess dipole signal observed in radio galaxy distribution (Singal 2011;Gibelyou & Huterer 2012;Rubart & Schwarz 2013;Tiwari & Nusser 2016;Colin et al 2017) while simultaneously explaining the alignment of CMB quadrupole and octopole (Gordon et al 2005). The superhorizon mode in Equation (2) introduces a perturbation to the gravitational potential between distant galaxies and us.…”

“…In the absence of anisotropic stress, Ψ = Φ. A single adiabatic superhorizon mode perturbation, providing initial conditions for Ψ, in its simplest form can be modeled as (Erickcek et al 2008;Ghosh 2014;Das et al 2021),…”

“…Here θ is the polar angle of the spherical polar coordinate system, γ ≡ γ(z, α, κ, ω) and is small in comparison with z , the monopole term. The monopole term z , given in Das et al (2021), is…”