Does NANOGrav observe a dark sector phase transition?

Abstract

Gravitational waves from a first-order cosmological phase transition, at temperatures at the MeV-scale, would arguably be the most exciting explanation of the common red spectrum reported by the NANOGrav collaboration, not the least because this would be direct evidence of physics beyond the standard model. Here we perform a detailed analysis of whether such an interpretation is consistent with constraints on the released energy deriving from big bang nucleosynthesis and the cosmic microwave background. We find that a phase transition in a completely secluded dark sector with sub-horizon sized bubbles is strongly disfavoured with respect to the more conventional astrophysical explanation of the putative gravitational wave signal in terms of supermassive black hole binaries. On the other hand, a phase transition in a dark sector that subsequently decays, before the time of neutrino decoupling, remains an intriguing possibility to explain the data. From the model-building perspective, such an option is easily satisfied for couplings with the visible sector that are small enough to evade current collider and astrophysical constraints. The first indication that could eventually corroborate such an interpretation, once the observed common red spectrum is confirmed as a nHz gravitational wave background, could be the spectral tilt of the signal. In fact, the current data already show a very slight preference for a spectrum that is softer than what is expected from the leading astrophysical explanation.

Hunting WIMPs with LISA: correlating dark matter and gravitational wave signals

“…LISA therefore raises new hopes to detect dark sectors that are otherwise unobservable. Over the past few years, first-order PTs in dark sectors have been studied in great detail [8][9][10][11][12], and various correlations between GW signals and the phenomenology of DM have been explored [13][14][15][16][17][18][19][20][21][22][23][24][25]. The conclusion of these studies is that it is difficult to robustly predict the expected amplitude of the GW signal for a given DM model, because strong PTs often only happen in special regions of parameter space.…”

“…Indeed, even if the two sectors have the same temperature initially, the first-order PT in the dark sector will change the temperature ratio, as the vacuum energy in the dark Higgs field is converted to rest mass and kinetic energy. This additional energy needs to be rapidly transferred to the SM in order to avoid a dilution of GW signals from late-time entropy injection [11,12]. We calculate the dilution of the GW background and derive a lower bound on the portal coupling from the requirement that no significant dilution occurs.…”

Supercooling in radiative symmetry breaking: theory extensions, gravitational wave detection and primordial black holes

“…[94] for a PT interpretation of the detected signals performed by the NANOgrav collaboration and relevant for our study and refs. [95][96][97][98][99][100][101][102][103][104][105][106][107][108][109][110] for other independent discussions of PT interpretations).…”

Can the Gravitational Wave Background Feel Wiggles in Spacetime?

“…Due to the weakness of the gravitational interaction, gravitational waves (GWs), and the GWB as well, are thought to be insensitive to any environmental physics they propagate through, unless such physics has strength beyond the linear regime, which is typically only true near the GW source. As a result, most theoretical studies of the PTA signal have focused on the possible sources such as supermassive black holes (see Agazie et al 2023b;Antoniadis et al 2023b for some up-todate constraints, and also Huang et al 2023;Konoplya & Zhidenko 2023;Yang et al 2023b), inflation (Starobinsky 1979;Rubakov et al 1982;Guzzetti et al 2016;Vagnozzi 2021Vagnozzi , 2023Borah et al 2023;Choudhury 2023;Datta 2023;Firouzjahi & Talebian 2023;Niu & Rahat 2023;Unal et al 2023), scalar-induced GWs (Tomita 1967;Matarrese et al 1993Matarrese et al , 1994Domènech 2021; Abe & Tada 2023;Cai et al 2023;Ebadi et al 2023;Franciolini et al 2023;Liu et al 2023a;Wang et al 2023a;Yi et al 2023;Zhu et al 2023), and collision of bubbles of first-order phase transitions (Kosowsky et al 1992;Caprini et al 2008;Huber & Konstandin 2008;Arzoumanian et al 2021;Li et al 2021;Ashoorioon et al 2022;Addazi et al 2023;Bringmann et al 2023;…”