One μ to rule them all: CMB spectral distortions can probe domain walls, cosmic strings and low scale phase transitions

Abstract

We present a new probe of purely gravitationally coupled sectors with large anisotropies. These anisotropies are damped via gravitational interactions with the baryon-photon fluid, which is heated up in the process. The injected heat causes measurable distortions of the cosmic microwave background spectrum. We give analytic estimates for the size of the distortions and outline how to calculate them from first principles. These methods are applied to anisotropies in the form of a domain wall/cosmic string network or caused by a first order phase transition or scalar field dynamics. We find that this method can potentially probe large regions of previously unconstrained parameter space and is very much complementary to up-coming searches of gravitational waves caused by such dark sectors.

Does NANOGrav observe a dark sector phase transition?

“…We remain excited about the pending 15 yr data release of the NANOGrav collaboration, which might already confirm the Hellings-Downs curve needed to state the detection of a GWB [7,13]. While we do not expect a definite answer concerning the origin of the signal within the coming months, we are confident that additional PTA data as well as complementary constraints from cosmology [112] will help to disambiguate between different models. The earliest evidence we can hope for in favour of a cosmological origin of the signal will be any deviation from the spectral slope of γ SMBHB = 13/3 expected for SMBHBs.…”

“…Upcoming experiments like the Simons observatory and CMB-S4 measurements in combination with surveys of large-scale structure will be able to push the limits on ∆N eff by about an order of magnitude [133][134][135], contributing to the tension on the stable dark sector explanation we investigated above (and reducing the parameter space for a decaying DSPT). Measurements of CMB spectral distortions with PIXIE [136] will give additional and complementary constraints [112], which would be relevant even in the case of a decaying dark sector that avoids constraints on ∆N eff . In fact, a confirmation of spectral distortions in the CMB could in principle even provide supporting evidence for such a scenario.…”

“…Note that constraints from µ-distortions of the CMB photon spectrum[112] and curvature perturbations[113] are less relevant as they quickly lose sensitivity for transition temperatures above the MeV-scale. PBH formation due to first-order phase transitions[114][115][116] could offer novel probes, but turns out to be irrelevant for the phase transition strengths of interest in our work 9.…”

Gravitational waves from defect-driven phase transitions: domain walls

“…However, a cosmological firstorder phase transition at such low temperatures tends to produce tension with the number of effective degrees of freedom measured in big bang nucleosynthesis (see e.g. [63]). In many models, the strength and duration of the first-order phase transition are correlated.…”

Gravitational waves from defect-driven phase transitions: domain walls

“…However, a cosmological first-order phase transition at such low temperatures tends to produce tension with the number of effective degrees of freedom measured in big bang nucleosynthesis (see e.g. [66]). In many models, the strength and duration of the first-order phase transition are correlated.…”