Hemispherical Power Asymmetry of the Cosmic Microwave Background from a Remnant of a pre-Inflationary Topological Defect

TL;DR

This paper proposes that a pre-inflationary topological defect caused a phase variation in a boson field, leading to energy density fluctuations that explain the hemispherical power asymmetry observed in the cosmic microwave background.

Contribution

It introduces a novel mechanism linking pre-inflationary topological defects to large-scale CMB anomalies, providing a potential evidence for inflation.

Findings

Predicts a scale-dependent power asymmetry.

Explains the origin of hemispherical asymmetry via topological defects.

Connects topological defects to observable CMB anomalies.

Abstract

Observations indicate that large-scale anomalies exist in the fluctuations of the cosmic microwave background. In these anomalies, the hemispherical power amplitude asymmetry has a correlation length comparable to that of the observable universe. We propose that a topological defect created by spontaneous breaking of the U(1) symmetry prior to inflation generated an initial phase variation, $\delta \theta$, across the observable region of the universe. The amplitude of this phase fluctuation is protected by topology if the defect is inside the horizon, and is frozen by causality if the defect exits the horizon. After inflation, the phase-corresponding boson field started to oscillate, when the Hubble rate decreased to a level comparable to the mass of the boson field. The energy density of the newly created boson particles varied across the observable universe. The bosons subsequently decayed into radiation prior to the BBN epoch, and the resulting fluctuations in the energy density produced the observed power asymmetry. This scenario predicts a scale-dependent modulation amplitude power asymmetry and in addition, as topological defects created by phase transitions are a very general phenomenon, the observed hemispherical asymmetry may be seen as an evidence for the cosmological inflation.