Impact of string and monopole-type junctions on domain wall dynamics: Implications for dark energy

Abstract

We investigate the potential role of string and monopole-type junctions in the frustration of domain wall networks using a velocity-dependent one-scale model for the characteristic velocity, v, and the characteristic length, L, of the network. We show that, except for very special network configurations, v 2 ∼ < (HL) 2 ∼ < (ρσ + ρµ)/ρm where H is the Hubble parameter and ρσ, ρµ and ρm are the average density of domain walls, strings and monopole-type junctions. We further show that if domain walls are to provide a significant contribution to the dark energy without generating exceedingly large CMB temperature fluctuations then, at the present time, the network must have a characteristic length L0 ∼ < 10 Ω −2/3 σ0 kpc and a characteristic velocity v0 ∼ < 10 −5 Ω −2/3 σ0where Ωσ0 = ρσ0/ρc0 and ρc is the critical density. In order to satisfy these constraints with Ωσ0 ∼ 1, ρm0 would have to be at least 10 orders of magnitude larger than ρσ0, which would be in complete disagreement with observations. This result provides very strong additional support for the conjecture that no natural frustration mechanism, which could lead to a significant contribution of domain walls to the dark energy budget, exists.

Detecting Domain Walls of Axionlike Models Using Terrestrial Experiments

“…This choice can be self-consistent within the cosmological scenario for the formation of the domain-wall network from randomly distributed initial a in , assuming that the network is "frustrated", and exhibits ρ DW ∼ R −1 scaling, where R is the cosmological scale factor. As a word of caution, we add that the numerical simulations of domain walls in some scalar field theories have shown much faster redshifting of ρ DW , and never achieved the frustrated state [7]. In light of this, some unorthodox cosmological/astrophysical scenarios for the formation of domain walls may be required.…”

Cosmic microwave background anisotropies generated by domain wall networks

“…They are, however, often overlooked in cosmology since the energy scale of the phase transition that originates the domain walls is restricted to be smaller than 1 MeV [2]. Nonetheless, there is still room for domain walls to play a relevant role in cosmology: for instance, they have been suggested as a possible significant dark energy contributor [3]-albeit they have since been ruled out as a major dark energy component [4][5][6] -and as a possible explanation for the spatial variations of the fine structure constant hinted at by HIRES/Keck and VLT/UVES data [7][8][9].…”

Phantom domain walls

“…[21,22], the amplitude of the CMB temperature fluctuations generated by domain walls are (conservatively) constrained around the present time to be smaller than 10 −5 down to scales of the order of L V = H −1 0 /100 (otherwise they would generate strong signatures on the CMB). This means that the fractional density fluctuations associated with domain walls on a physical scale L V (much larger than L),…”

“…Therefore, if phantom domain walls are to significantly contribute to the dark energy budget, their RMS velocity should be extremely small and the network should therefore be frustrated (or frozen in comoving coordinates). The dynamics of frustrated domain wall networks have been thoroughly studied in the literature [17,[21][22][23] and all studies indicate that such networks cannot realistically be (or significantly contribute to) dark energy. The frustration of domain wall networks can be achieved with additional damping mechanisms, however the energy necessary to decelerate the walls is so large that these mechanisms would make a much larger contribution than the walls themselves to the energy budget of the universe.…”