Since the discovery of the cosmic microwave background (CMB), our understanding of the cosmos has been rapidly evolving. Detailed measurements of the CMB temperature fluctuations have led to a standard cosmological model, which traces the origin of the large-scale structure of the universe to quantum fluctuations during inflation. Although the basic framework of inflationary cosmology is now well-established, the microphysical mechanism responsible for the accelerated expansion remains a mystery. In this thesis, we describe how the physics underlying inflation can be probed using two cosmological observables: higher-order correlations of primordial density perturbations (non-Gaussianity) and primordial gravitational waves (tensor modes).

In the first part of the thesis, we explore novel signatures of high-energy physics in higher- order correlation functions of inflationary perturbations. First, we use causality and unitarity to make connections between cosmological observations and the underlying short-distance dynamics of single-field inflation. We obtain a constraint on the size and the sign of the four-point function in terms of the amplitude of the three-point function. We then study the imprints of extra massive particles of arbitrary spin on the three-point function. We classify the couplings of these particles to inflationary scalar and tensor perturbations and derive explicit shape functions for their three- point functions that can serve as templates for future observational searches. Establishing the particle content during inflation would provide important hints for the microscopic theory of inflation.

In the second part, we study ways of testing the nature of inflation using inflationary tensor modes. We consider effects of gravitational corrections to Einstein gravity in models of high-scale inflation. We show that these scenarios can lead to a violation of the tensor consistency condition (i.e. the relation between the amplitude and the scale-dependence of the tensor two-point function) that is satisfied by canonical single-field inflationary models. Finally, we consider the prospects for measuring the inflationary superhorizon signature in future observations. We define an estimator that captures superhorizon correlations and present forecasts for the detectability of the signal with future CMB polarization experiments.