Phenomenological implications of cosmic (super)strings with time-varying tension

Author(s)

Avgoustidis, Anastasios, Copeland, Edmund J., Moss, Adam, Raidal, Juhan, Revello, Filippo, Villa, Gonzalo

Abstract

Cosmic strings and cosmic superstrings could have formed in the early universe. In a particular class of models, the tension associated with these defects is time dependent, either because the strings themselves are composed of evolving moduli fields, or because they are directly coupled to evolving scalar fields leading to an effective time dependence in the string tension. Here we take seriously the possibility of this time dependence and investigate the phenomenological consequences of such an evolution, comparing the results to the more traditional case of a fixed time independent tension. In particular, for five different time varying models, motivated by different possible forms of evolution of the string tension, we evaluate their Gravitational Wave (GW) contributions as well as their impact on the Cosmic Microwave Background (CMB). Performing an MCMC analysis of the models we find there are indeed interesting spectral signatures in the GW emission at frequencies between the current PTA and interferometer data, but the signatures are not unique to these strings, making them hard to pin down using current data. However, we do find cosmic strings with evolving tension fit the GW data as well as other favoured models.

Figures

Caption

Example scatter plot of superstring VOS solutions for $g_s = 0.3$ and $m=1.3$ with randomly sampled initial conditions ($\xi_0,\,v_0$). The yellow region shows configurations where the superstrings reach the standard scaling solution, while blue region shows where they reach the ultra-relativistic attractor.

Caption

$TT$ angular power spectrum sourced by cosmic strings with time-varying tension given by Eq.~\eqref{eq:tension_power}, decomposed into scalar, vector and tensor contributions, for three values of the tension index $m$. Each panel is normalised so that the peak of the total amplitude matches the $m=0$ case at $G\mu_0=10^{-8}$; the tension required to do so is annotated in each panel.

Caption

Corner plot showing the posterior marginals for the MCMC analysis of cosmic strings to Planck+ACT CMB polarization data. The values quoted above each 1D marginal are the 95\% confidence levels or bounds. Note that Gaussian priors were used for $\beta$ and $\tilde{c}$ and the posteriors are prior-dominated, not determined by the data.

Caption

Corner plot showing the posterior marginals for the MCMC analysis of cosmic superstrings to Planck+ACT CMB polarization data. The values quoted above each 1D marginal are the 95\% confidence levels or bounds. Where a bound cannot be placed, no value is quoted. Note that Gaussian priors were used for $\beta$ and $\tilde{c}$ and the posteriors are prior-dominated, not determined by the data.

Caption

The best-fit spectra of each of the time-varying models for cosmic strings (dashed lines) with the constant tension cosmic superstring fit shown for comparison as a solid line. The NANOGrav 15-year data bins are shown as orange violins, while the LVK O3 non-detection bound is shown as a green shaded region.

Caption

The best-fit spectra of each of the time-varying models for cosmic superstrings (solid lines) with the constant tension cosmic string fit shown for comparison as a dashed line. The NANOGrav 15-year data bins are shown as orange violins, while the LVK O3 non-detection bound is shown as a green shaded region.

Caption

Posteriors for the cosmic string and superstring constant tension models fitted to NG15 and LVK O3 data. The top right panel shows the cosmic string single-parameter marginal, while the rest of the corner plot shows the superstring parameters. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic string full time varying tension model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic string case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic string truncated time varying tension model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic string case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic string window time varying tension model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic string case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic string top-hat time varying tension model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic string case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic string axion kination model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic string case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic superstring full time varying tension model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic superstring case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic superstring truncated time varying tension model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic superstring case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic superstring window time varying tension model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic superstring case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic superstring top-hat time varying tension model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic superstring case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.

Caption

Posteriors for the cosmic superstring axion kination model fitted to NG15 and LVK O3 data. The green marginal shows the constant tension cosmic superstring case for comparison. Quoted above each panel are the median and 68\% confidence level of each parameter.