Cosmic strings as dynamical dark energy: Novel constraints

Cosmic strings, topological defects predicted by high-energy theories, may contribute to the late-time expansion of the Universe, effectively mimicking dynamical dark energy. We investigate four phenomenological extensions of the Lambda CDM model involving a residual string network: (i) a non-relativistic component with positive energy density (Model 1), (ii) a velocity-dependent extension (Model 2), (iii) a non-relativistic string network with energy density allowed to take both positive and negative values (Model 3), and (iv) a general scenario with free energy and velocity parameters (Model 4). These models are constrained using Planck CMB data, SDSS or DESI baryon acoustic oscillations, and Type Ia supernovae observations. Models 1 and 2 yield strong upper bounds on the string density, for example, Omega(s) < 0.00901 at 95% CL from the CMB+DESI+DESY5 combination for Model 2, and mildly shift the inferred value of H-0 upward, though they are not favored by Bayesian evidence. For the same combination, the bulk velocity is bound as vs < 0.569. Models 3 and 4 exhibit a consistent preference for slightly negative values of Omega(s), with CMB-only data yielding Omega(s )= -0.038(-0.022)(+0.029) and vs < 0.574 in Model 4, and a best-fit improvement of Delta x(2) = -6.07. However, these improvements are not sufficient to overcome the Occam penalty, and the Bayesian evidence continues to favor Lambda CDM. These findings demonstrate the power of current data to constrain exotic energy components and encourage further exploration of string-inspired extensions to Lambda CDM, particularly those involving negative-tension networks.