Abstract
Cosmic bubbles nucleated through the quantum tunneling process would expand and undergo collisions with each other. We focus on collisions of two equal-sized bubbles and compute gravitational waves emitted from the collisions. The mechanism of the collisions can be modeled by means of a real scalar field and its quartic potential. Out of this model we can compute gravitational waves from the collisions by integrating the energy-momentum tensors over the volume of the wave sources; in the quadrupole approximation. Our computational results show that the waveforms are characterized by (i) cusp-like bumps with frequency modulation during the initial-to-intermediate stage of strong collisions and (ii) smooth monochromatic oscillations during the final stage of weak collisions.
Highlights
Bubbles in the early universe and gravitational wavesAccording to the theory of cosmic “inflation”, the early universe experienced an extreme burst of expansion, which lasted a tiny fraction of a second, but smoothed out irregularities–inhomogeneities, anisotropies and the curvature of space, and made the universe appear homogeneous and isotropic.[1]
The nucleation and growth of such bubbles can be modeled by a Coleman-de Luccia (CDL) instanton, a type of quantum transition between two classically disconnected vacua at different energies; the higher energy, the lower energy.[4]
We have computed Gravitational waves (GWs) emitted from collisions of two equal-sized bubbles in time domain
Summary
According to the theory of cosmic “inflation”, the early universe experienced an extreme burst of expansion, which lasted a tiny fraction of a second, but smoothed out irregularities–inhomogeneities, anisotropies and the curvature of space, and made the universe appear homogeneous and isotropic.[1] It has been suggested that inflationary models of the early universe most likely lead to a “multiverse”.2 One such model is “eternal inflation”:3 it proposes that many bubbles of spacetime individually nucleate and grow inside an ever-expanding background multiverse. The nucleation and growth of such bubbles can be modeled by a Coleman-de Luccia (CDL) instanton, a type of quantum transition between two classically disconnected vacua at different energies; the higher energy (false vacuum), the lower energy (true vacuum).[4] A scalar field initially in the false vacuum state may tunnel quantum mechanically to the true vacuum state. The mechanism of bubble collision–GW emission can be well approximated by a two-bubble collision model
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