Cosmic strings, domain walls and environment-dependent clustering

TL;DR

The paper investigates late-time, environment-dependent clustering arising from SIPT models in a non-minimally coupled scalar sector, using norns to evolve a complex scalar field and compare global U(1) strings against domain-wall analogues. By analyzing matter power spectra, halo statistics, and defect dynamics, it finds that attractive fifth forces can suppress growth in voids while enhancing it near overdensities, producing modest changes to $P_m(k)$ but strong signatures in the matter-density PDF and marked halo statistics. The work emphasizes environment-sensitive observables, such as the low-density tail of the PDF and marked halo spectra, as effective probes to distinguish SIPT scenarios from ΛCDM in low-redshift data. Overall, SIPT-type models offer a controlled framework to connect dark-sector physics with nonlinear structure formation and to develop targeted, environment-dependent observational tests.

Abstract

Recent cosmological data favour phantom-crossing dark energy, motivating models with non-minimal couplings that induce a fifth force on structure formation. Reconciling these models with local tests often requires strong screening, leading to environment-dependent clustering. We investigate such effects via a late-time structure-induced phase transition driven by a non-minimally coupled scalar field. For this purpose, we introduce norns, a fully relativistic cosmological particle-mesh code that self-consistently evolves a complex scalar field - a generalisation of the symmetron producing global U(1) strings rather than domain walls. Using simulations, we compare string and wall-forming models, quantifying impacts on the matter power spectrum, halo mass function, and defect dynamics. Strong environment-dependent effects can generate significant departures from LCDM in underdense regions while keeping the overall power spectrum changes modest (~ 4-15% at k~0.3-0.5 h Mpc^-1, sub-percent for z > 0.2). We find that an attractive fifth force can locally suppress structure growth in voids while enhancing it in surrounding overdense regions by driving outflows from the voids. These effects leave distinctive signatures in the matter density probability density function and in marked halo power spectra, which are likely detectable in low-redshift data.

Figures (18)

• Figure 1: Schematic from Vachaspati:1984dz, indicating the complex phase at four different vertices enclosing a) a string, or b) nothing. Right panel indicates how the path integral goes in either case. When the phase varies continuously there are no phase defects.
• Figure 2: Top: Fraction of the simulation volume where the global $U(1)$ or $\mathbb{Z}_{2}$ symmetry is broken, approximated by $|\Re (\chi)|>10^{-2}$. Bottom: Fraction of simulation volume where phase discontinuities or sign switches are found, respectively in the case of $\phi \in \mathbb{C}$ or $\phi \in \mathbb{R}$.
• Figure 3: The defect structures that were identified at redshift $z=0$ for simulations (from top left to bottom right) $1_{\mathbb{R}}$, $2_{\mathbb{R}}$, $1_{\mathbb{C}}$, and $2_{\mathbb{C}}$.
• Figure 4: String defects in model $4_{\mathbb{C}}$ at three different redshifts. Strong screening/pinning increases the time for the strings to resolve themselves and assume global structure.
• Figure 5: Histograms of the matter overdensity $\delta_{m}$, at the locations of the identified defects, for different simulations and redshifts.