Probing the primordial universe with Gravitational waves

• Applicant: Dr. Guillem Domenech
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields ; Astrophysics and Astronomy
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496592360

The scientific community has new means available to probe the unexplored universe - gravitational waves. Within the next decades, not only astrophysical but cosmological models will be put to new stringent tests. New physics may be discovered. One of the most ambitious targets will be to complete our picture of the early universe. At the moment, by observations with electromagnetic waves, we only have access to physics since one second after the Big Bang. In contrast, gravitational waves may reach us from the depths of our past universe, as they travel barely scattered by matter. With several planned future gravitational wave detectors, the time to probe the pre-Big Bang universe is nearing. Now, theoretical research must go in par with the immense discovery potential of future gravitational waves observations. I aim to fill the gap and explore the primordial universe with gravitational waves that are indirectly generated from primordial fluctuations (the secondary gravitational waves or 2GWs in short), fully utilising my newly derived analytical formulas.2GWs are a guaranteed signal, since we have precise observations of primordial fluctuations. In fact, these primordial fluctuations are the seed of the large scale structure in the universe like galaxies we observe today. We also have very strong evidence that these fluctuations were generated in the infant universe during a period called cosmic inflation. However, only a small fraction of the inflationary stage has been probed by observation. A large fraction of the inflationary stage has been unexplored. It has been shown that 2GWs are the most promising means to explore that piece of the primordial universe's history, as they fall in the frequency range of various future gravitational wave detectors. In addition, 2GWs are a crucial counterpart to primordial black holes which may have formed in the early universe from large primordial fluctuations. In recent years, primordial black holes have gathered considerable attention from both the theoretical and experimental communities as plausible explanations to several observations, such as some black hole mergers seen by LIGO. Notably, the absence of 2GWs has the potential to completely rule out such a scenario.I will use 2GWs to probe the theory and particle content of the primordial universe, and derive new predictions with my recent advancements, ready to be contrasted when new data is available. In this way, I will prepare the ground for cosmology with 2GWs in the new era of multimessenger cosmology where gravitational wave observations play an indispensable role in addition to the other traditional observational means. Pulsar Timing Array experiments are expected to bring us more information on cosmological gravitational waves within the next five years, and a space based gravitational wave detector LISA is expected to launch within a decade. The proposed research is very timely and fits well with the length of the Emmy Noether programme.

• DFG Programme: Independent Junior Research Groups