Abstract
We study the spectrum of gravitational waves produced by a first order phase transition in a hidden sector that is colder than the visible sector. In this scenario, bubbles of the hidden sector vacuum can be nucleated through either thermal fluctuations or quantum tunnelling. If a cold hidden sector undergoes a thermally induced transition, the amplitude of the gravitational wave signal produced will be suppressed and its peak frequency shifted compared to if the hidden and visible sector temperatures were equal. This could lead to signals in a frequency range that would otherwise be ruled out by constraints from big bang nucleosynthesis. Alternatively, a sufficiently cold hidden sector could fail to undergo a thermal transition and subsequently transition through the nucleation of bubbles by quantum tunnelling. In this case the bubble walls might accelerate with completely negligible friction. The resulting gravitational wave spectrum has a characteristic frequency dependence, which may allow such cold hidden sectors to be distinguished from models in which the hidden and visible sector temperatures are similar. We compare our results to the sensitivity of the future gravitational wave experimental programme.
Highlights
A new hidden sector is a well motivated extension of the Standard Model (SM), both from top down considerations of string theory [1] and from a phenomenological perspective e.g. because it could contain the dark matter (DM) [2]
We study the spectrum of gravitational waves produced by a first order phase transition in a hidden sector that is colder than the visible sector
We have studied phase transitions in hidden sectors that are cold relative to the visible sector, and the possible resulting gravitational wave signals
Summary
A new hidden sector is a well motivated extension of the Standard Model (SM), both from top down considerations of string theory [1] and from a phenomenological perspective e.g. because it could contain the dark matter (DM) [2]. One signal that could be observed even in the limit of a vanishing coupling to the SM is a background of gravitational waves left over from a phase transition in the hidden sector which occurred early in the universe’s cosmological history. This is a worthwhile possibility to explore, since the sensitivity and frequency coverage of experimental searches for gravitational waves will improve dramatically in the near future as new instruments are developed and deployed, even though not all hidden sector models have a phase transition that leads to such a signal.
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