Primordial GWs
 

Synopsis

Observations of our Universe in the electromagnetic spectrum can only extend up to the cosmic microwave background, the ultimate horizon at a distance of roughly 15 Gpc from beyond which no photon signal can reach us. To peek beyond this cosmic veil and probe the earliest moments of cosmic evolution, we must rely on other messengers from the early Universe. A prime candidate in this respect are primordial gravitational waves, i.e., gravitational waves generated during the stage of cosmic inflation or during the Hot Big Bang, which propagate nearly unperturbed through the early Universe after their production and which hence, if registered in our detectors, can provide us with a pristine image of the birth of our Universe.

In our work, we are interested in various physical processes that can give rise to the production of primordial gravitational waves. A common feature of these processes is that all of them require new physics beyond the Standard Model of particle physics. In this sense, any search for gravitational waves from the early Universe is at the same time a search for new physics at very high energies. Our research activities focus on three possible sources of primordial gravitational waves:

   1. Cosmic strings: Cosmic strings are effectively one-dimensional topological defects that can be produced in the early Universe as a consequence of symmetry-breaking phase transitions. They are closely related to vortex solutions in condensed-matter systems, such as Abrikosov vortices in superconductors. Once produced in the early Universe, cosmic strings organize themselves in a network consisting of long (potentially infinitely long) strings and smaller closed loops. Each individual loop radiates off gravitational waves, which in superposition results in a stochastic gravitational-wave background (GWB) signal. In our work, we are interested in (A) a better theoretical description of this signal as well as in (B) more realistic microscopic cosmic-string models that go beyond the simplest description in terms of one-dimensional Nambu-Goto strings. In the latter case, we especially focus on current-carrying cosmic strings and metastable cosmic strings, which are well-motivated by certain grand unified scenarios.
   
   2. Phase transitions: Cosmological phase transitions correspond to changes in the vacuum configuration of our Universe triggered by the decreasing temperature during the Hot Big Bang. In everyday life, similar transitions can, e.g. be observed when supercooled water (maybe in a bottle you forgot in the freezer) suddenly turns from liquid water to solid ice. In the context of cosmology, strong first-order phase transitions are an attractive source of primordial gravitational waves. During the phase transition, bubbles filled by the new vacuum configuration expand in the ambient old vacuum configuration. When these bubbles collide and percolate, energy stored in the bubble walls is released in the form of gravitational waves. Similarly, the true-vacuum bubbles sweeping through the hot plasma can excite sound waves and, ultimately, turbulent plasma motion, both of which can also source gravitational waves. In our work, we study (A) the experimental sensitivity of upcoming gravitational-wave experiments to signals induced by cosmological phase transitions; we contribute (B) to refinements of the theoretical description of the expected signal; and we study (C) well-defined benchmark models that can give to a cosmological phase transition, such as the B-L extensions of the Standard Model, which promotes baryon-minus-lepton number B-L to the charge of a new Abelian gauge symmetry.

   3. Inflation: Cosmic inflation denotes a stage of exponential expansion in the early Universe prior to the Hot Big Bang. Today, the paradigm of cosmic inflation is a central element of inflationary cosmology, which incorporates a dynamical explanation of the initial conditions of old-school Big Bang cosmology. Indeed, cosmic inflation can explain the size, homogeneity, and isotropy of our Universe on cosmological scales, while at the same time, it sources small primordial density fluctuations that later act as the seeds of the nonlinear large-scale structure that we observe in the present Universe. In addition, inflation also stretches primordial tensor perturbations to super-horizon size, which then re-enter the Hubble horizon during the Hot Big Bang after inflation in the form of gravitational waves. The simplest models of inflation, standard single-field slow-roll inflation, only predict a weak gravitational-wave signal that is hard, if not impossible, to detect in pulsar timing arrays or interferometer experiments. The situation, however, drastically changes in many nonminimal models of inflation that predict a blue-tilted primordial gravitational-wave spectrum well within the reach of upcoming experiments. In our work, we are particularly interested in this class of models. In fact, research area (3) "Axion cosmology" is centered around models of axion inflation, an intriguing class of inflationary models with exciting predictions for the spectrum of primordial gravitational waves.