Gravitational Waves From a Dark (Twin) Phase Transition

TL;DR

The work shows that dark SU($N_d$) gauge theories that confine at a scale $\Lambda_d$ can undergo strong first-order phase transitions in the early universe, producing gravitational waves potentially detectable by missions like eLISA and pulsar timing arrays. By outlining the conditions for strong PTs, computing the GW spectra from bubble collisions and MHD turbulence, and mapping detectability to $T_*$, ${\cal H}_*$, $\beta$, and $v$, the authors connect dark-sector dynamics to observable GW signals. The paper analyzes four motivated models—CDM1, CDM2, Twin Higgs, and SIMP—showing that TeV-scale confinement yields signals in sensitive GW bands while GeV-scale scenarios may be probed by PTA/CMB-era constraints, thereby offering a complementary probe to collider and DM searches. A key caveat is the current lack of precise predictions for the PT parameters, which could be improved with lattice studies or holographic duals to sharpen the GW predictions.

Abstract

In this work, we show that a large class of models with a composite dark sector undergo a strong first order phase transition in the early universe, which could lead to a detectable gravitational wave signal. We summarise the basic conditions for a strong first order phase transition for SU(N) dark sectors with n_f flavours, calculate the gravitational wave spectrum and show that, depending on the dark confinement scale, it can be detected at eLISA or in pulsar timing array experiments. The gravitational wave signal provides a unique test of the gravitational interactions of a dark sector, and we discuss the complementarity with conventional searches for new dark sectors. The discussion includes Twin Higgs and SIMP models as well as symmetric and asymmetric composite dark matter scenarios.

Figures (3)

• Figure 1: Phase diagram of QCD at zero chemical potential (schematic). The dashed region represents our current lack of knowledge about the order of the PT in the limit of two massless flavours.
• Figure 2: Left: Peak frequencies of the GW spectra (in mHz) from bubble collisions (blue,solid) and MHD turbulence (red, dashed) in the $T_* - \beta$ plane, for $v=1.0$. Right: GW spectrum from bubble collisions (blue, solid) and turbulence (red, dashed) as well as the combined spectrum (black, thick), as a function of conformal wave number $k$, for $v=1.0$ and $\beta = 10{\cal H}_*$.
• Figure 3: GW spectra $\Omega(f)h^2$ for $T_*=0.1$ GeV (SIMP), $T_*=3$ GeV (CDM1, TH models), $T_*=300$ GeV and $T_*=10$ TeV (CDM2 models). The upper (lower) edges of the contours correspond to $\beta={\cal H}$$(\beta = 10{\cal H})$, and furthermore $v=1$ and $\Omega_{S*}=0.1$ for all curves. The red band $T_*=0.1$ GeV indicates where a signal of the QCD PT would lie if it was strong. The projected reach of several planned GW detection experiments is shown (dashed).