Improved estimation of radiated axions from cosmological axionic strings

Cosmological evolution of axionic string network is analyzed in terms of field-theoretic simulations in a box of 512³ grids, which are the largest ever, using a new and more efficient identification scheme of global strings. The scaling parameter is found to be ξ=0.87±0.14 in agreement with previous results. The energy spectrum is calculated precisely using a pseudo power spectrum estimator which significantly reduces the error in the mean reciprocal comoving momentum. The resultant constraint on the axion decay constant leads to fₐ≤3×10¹¹ GeV. We also discuss implications for the early Universe.