BSM Meets PTAs
 

Synopsis

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We are involved in the work of the NANOGrav collaboration, the North American Nanohertz Observatory for Gravitational Waves. NANOGrav is one of several existing pulsar timing arrays (PTAs) and as such part of IPTA, the International Pulsar Timing Array. NANOGrav and PTAs in general search for gravitational waves at nanohertz frequencies by monitoring an array of pulsars in the Milky Way: a background of gravitational waves permeating our Galaxy will stretch and squeeze the spatial distances between NANOGrav's radio telescopes on Earth and the pulsars in the array and will thus cause tiny distortions in the arrival times of the individual pulses. Einstein's theory of general relativity allows us to precisely describe this effect on the pulsar timing data and points to a unique signature in the cross-correlated data for pairs of pulsars that would be the telltale sign of gravitational waves.

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Within NANOGrav, we are resposible for the search for signals from new physics beyond the Standard Model of particle physics. These efforts are motivated by the fact that numerous extensions of the Standard Model predict the generation of a stochastic gravitational-wave background (GWB) in the early Universe. Such a primordial GWB, if it exists, would correspond to the gravitational analog of the CMB, the cosmic microwave background, which is composed of relic photons from the early Universe that traveled freely through the cosmos since roughly 380,000 years after the Big Bang. In our work, we analyze the most recent NANOGrav data addressing two key questions:

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    1. Model fits: Given a model of new physics predicting the existence of a GWB signal from the early Universe, how well does it allow to fit the most recent NANOGrav data? What are the preferred regions in the model parameter space? And how does the interpretation of the NANOGrav data in terms of this model compare to the interpretation in terms of other models? Notably, does the cosmological model under consideration provide a better fit of the data than the astrophysical interpretation in terms of inspiraling supermassive black-hole binaries?

    2. Model constraints: Given a model of new physics predicting the existence of a GWB signal from the early Universe, which regions of parameter space can be safely ruled and declared unviable in view of the most recent NANOGrav data? That is, independently of the origin of any signal in the data, which parameter values predict a GWB signal that would be clearly in conflict with the observed data, e.g., because it is too strong and should have been seen in the data long ago?

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In addressing these two questions, we explore a new frontier, the PTA frontier, in the hunt for new physics that is complementary to other frontiers of particle physics, such as the energy and intensity frontiers. We use the NANOGrav data in particular to search for clues about the physics of the early Universe at very early times and hence extremely high energies that are unattainable in terrestrial laboratory experiments. This includes ideas related to grand unification and even string theory that may be responsible for a GWB signal from the early Universe. Specifically, we have a strong interest in primordial gravitational waves from cosmic inflation, enhanced scalar perturbations, cosmological phase transitions, and cosmic defects such as cosmic strings and domain walls. While the focus of research area (1) "New physics at the PTA frontier" is on data analysis and phenomenology, we also work on theoretical questions regarding these gravitational-wave sources, which is the focus of research area (2) "Primordial gravitational waves".