Complex sentence: So-called cosmic strings, hypothesized in some cosmological theories, involve very large energy densities in long, narrow lines, but all known physically reasonable cosmic-string models have positive (positive space-time warping effects) energy densities.
Split into simpler sentences: So-called cosmic strings, hypothesized in some cosmological theories, involve very large energy densities in long, narrow lines. But all known physically reasonable cosmic-string models have positive (positive space-time warping effects) energy densities.