Misaligned, tilted and distorted: the hard life of audible axions
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Abstract
The year is 2022DC. Observations in high energy physics, astrophysics and cosmology
are entirely explained by the standard model (SM) of particle physics together with the
theory of gravity and the ΛCDM model. Well not entirely... There is still a “small”
number of unexplained phenomena and tensions on top of dissonances of theoretical nature
within the respective theories. Particle accelerators acted as the driving force behind the
development of the SM. However, they are seemingly reaching the limits of available
technology. Therefore, the search for new messengers of high energy physics is as timely
as ever.
Guided by some of these dissonances, namely the strong CP problem and the hierarchy
problem, we first take a look at their respective dynamical solutions, the QCD axion and
the relaxion. Of particular interest is the phenomenon of axion fragmentation, which is
build into the relaxion mechanism but also arises in general axion scenarios. The dynamics
of this process feature an instability that leads to the initially homogeneous axion field
developing large inhomogenities that eventually come to dominate the axions energy, in
particular the initial kinetic energy. Using a lattice simulation allows us to take the
inhomogenities fully into account and we are able to show that this energy transfer is
even more efficient than previously estimated. We furthermore explore the subsequent
cosmology of the axion, in particular the possibility that multiple vacua are populated by
the axion.
We then turn our attention to one possible new messenger, gravitational waves (GWs),
and present in detail the dynamics of an axion scenario that leads to an observable signal.
In this scenario the axion is coupled to a new U (1) gauge boson, the so-called dark photon.
The dark photon develops an instability, similar to the one discussed above, due to the
coupling to the axion, which results in the exponential production of dark photon quanta.
This process is associated with a large anisotropic stress, which sources the GWs. We
again study the non-linear dynamics of this process with a lattice simulation to confirm
and refine our previous results concerning the viability of this scenario as well as the
amplitude of the signal. While the minimal model can indeed lead to a signal detectable
by pulsar timing arrays (PTAs), extensions are needed in order to be detectable by planned
interferometers. We study two such extensions in detail, one where the axion has a non-
vanishing initial velocity and one where the axion is identified with the relaxion that by
construction features a time dependent potential.
In the last part of the thesis we focus purely on the phenomenology of new physics with
sizeable fluctuations in energy density. Apart from the examples discussed above, phe-
nomena like networks of scaling seeds or first order phase transitions come to mind. We
discuss the ability of these new physics scenarios to explain the recent hint of GWs from
PTAs such as NANOGrav. The temperature of the SM plasma when these signals are
induced is ≈ 1 GeV and below. Since couplings to the SM are highly constrained at such
low energies, we primarily focus on purely gravitationally coupled sectors. While we find
that currently limits stemming from CMB observations and BBN significantly constrain
scenarios with a signal this strong, the prospect of being able to probe such sectors with
the constantly culminating PTA data in the near future is exciting. Finally we study
the ability of such a dark sector to cause distortions of the CMB spectrum by inducing
acoustic waves in the baryon photon fluid. We find that future experiments could detect
new physics over large regions of parameter space, some of them also accessible by PTAs,
which might mark the dawn of multi-messenger cosmology.