Speaker
Description
For a discrete symmetry that is anomalous under QCD, the domain walls produced
in the early universe from its spontaneous breaking can naturally annihilate due to QCD
instanton effects. The gravitational waves generated from wall annihilation have their
amplitude and frequency determined by both the discrete symmetry breaking scale and
the QCD scale. The evidence of stochastic gravitational waves at nanohertz observed
by pulsar timing array experiments suggests that the discrete-symmetry-breaking scale is
around 100 TeV, assuming the domain-wall explanation. The annihilation temperature is
about 100 MeV, which could naturally be below the QCD phase transition temperature.
We point out that the QCD phase transition within some domains with an effective large
QCD θ angle could be a first-order one. To derive the phase diagram in θ and temperature,
we adopt a phenomenological linear sigma model with three quark flavors. The domainwall explanation for the NANOGrav, EPTA, PPTA and CPTA results hints at a first-order
QCD phase transition, which predicts additional gravitational waves at higher frequencies.
If the initial formation of domain walls is also a first-order process, this class of domain-wall
models predicts an interesting gravitational wave spectroscopy with frequencies spanning
more than ten orders of magnitude, from nanohertz to 100 Hz.
