Impact of string and monopole-type junctions on domain wall dynamics:
  implications for dark energy
  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,
$v^2 \lsim (HL)^2 \lsim (\rho_\sigma + \rho_\mu)/\rho_m$ where $H$ is the
Hubble parameter and $\rho_\sigma$, $\rho_\mu$ and $\rho_m$ 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 $ L_0 \lsim
10 \Omega_{\sigma 0}^{-2/3} {\rm kpc}$ and a characteristic velocity $v_0 \lsim
10^{-5} \Omega_{\sigma 0}^{-2/3}$ where $\Omega_{\sigma 0}=\rho_{\sigma
0}/\rho_{c 0}$ and $\rho_c$ is the critical density. In order to satisfy these
constraints with $\Omega_{\sigma 0} \sim 1$, $\rho_{m 0}$ would have to be at
least 10 orders of magnitude larger than $\rho_{\sigma 0}$, 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.
