Physicist - Research (Physics)

Research Highlights

Primordial Gravitational Waves

Cosmic Inflation, a short period of accelerated expansion in the very early moments of the universe, has become one of the main pillars of modern cosmology [1]. Leaving aside its success in addressing the puzzles of the standard hot Big Bang cosmology, it provides an explanation for the quantum mechanical origin of structures such as galaxies (including our own!) and the anisotropies in the Cosmic Microwave Background (CMB) radiation [2]. The recent advances on the observations of the CMB and of the large scale structure (LSS) of our universe have so far confirmed the predictions of simplest realizations of inflation, and arguably established its status as the main theoretical framework describing the very early universe [3]. Remarkably, all the information gathered from these cosmological probes still places only rather qualitative bounds on the microphysics of inflation. Key questions such as those on the nature of the inflationary particle content remain unanswered.

The unprecedented array of cosmological probes set to become operational in the next two decades holds the potential to address such fundamental questions. For example, probes of the cosmic microwave background such as CMB-S3 and later LiteBIRD and CMB-S4 will improve by almost two order of magnitudes existing bounds on primordial gravitational waves (PGW). Perhaps the most exciting opportunities will be afforded by current (e.g LIGO, Pulsar Timing Arrays) and upcoming gravitational wave probes (LISA, Einstein Telescope) at intermediate and small scales. These have opened up an entire new window (spanning multiple decades in frequency!) on the physics of the early universe. Undoubtedly, this is the ideal time to put our most compelling inflationary models to the test, focusing in particular on their the PGW spectrum.

The standard lore for the generation of GWs from inflation lies on the excitement of the quantum fluctuations of space-time which are then amplified by the accelerated expansion. In simplest realizations of inflation (such as a scalar field equipped with a flat potential), properties of the PGW spectrum expected to exhibit certain properties, such as scale invariance, absence of non-Gaussianity and parity violation. However, presence of extra degrees of freedom may invalidate these results by producing additional "synthetic" component of GW sources induced by the interactions of a (possibly) rich particle content during inflation.

This is a research topic that I have spend some time exploring with my collaborators over the past few years, see e.g [4,5,6]. In a recent work, we have taken a further step in this direction to guide experimentalists in the search for the origin of PGWs. In particular, focusing on a multi-field inflationary setup (which often naturally arise in UV complete theories such as string theory), we have found that transient particle production events that occur in a hidden (axion-)gauge sector can lead to a unique shape for the GW spectrum (see Figure below).

Fig 1. Total GW spectrum (solid dark curves) and its positive/negative helicity components (dashed curves) together with the sensitivity lines of current and forthcoming GW interferometer missions.

More interestingly, the resulting spectrum also exhibits scale dependent chirality as the GW signal evolves from IR to UV scales around its peak frequency value(s). In other words, the gap between the power of different GW polarization states is increasing as the signal climbs through its peak value while becoming maximal following the peak frequency. This unique behavior provide an interesting polynomial relation that ties the spectral index and the chirality of the GW signal (see below):

Fig 2. Evolution of the GW signal in the "excess chirality" vs spectral index plane: in the IR, chirality is vanishing but eventually it grows while GW signal is in the positive spectral index branch, reaching its maximal value during the fall of the spectrum.

where the expression inside the logarithm denotes the "excess" chirality of the GW spectrum and the coefficients of this third order relation can be found in our paper.

Considering the forthcoming efforts on the observation of GWs, our work highlights the importance of such unique relation as it would potentially help break the degeneracy between various astrophysical and primordial sources and provide compelling evidence for "synthetic" GWs generated by axion-(Abelian) gauge field dynamics.