University of Minnesota
School of Physics & Astronomy


Signatures of Inflation

Marco Peloso
Marco Peloso
Alex Schumann

Marco Peloso is a theorist working in the area of cosmology (the study of the early Universe). His research focuses on models of inflation, which is a period of accelerated expansion that took place very early in the life of the Universe. Observational cosmologists look for the signatures of inflation in the forms of gravitational waves and of density perturbations, which can be seen as small ripples in the Cosmic Microwave Background (CMB) data.

Peloso studies the imprints that could have been left on the CMB data from the interactions of the inflaton (this is the name given to the field that sourced the inflationary expansion) with other fields. For example, one possible interpretation of the recent BICEP result [link here], is that the inflaton interaction generated a large amount of particles during inflation, and that these particles collided among each other, generating gravitational waves. Analogously to photons, also the gravitational waves have two helicities. In the model studied by Peloso, gravitational waves of only one definite helicity are produced. This is a clear prediction of that model, that can be experimentally tested. As Peloso mentioned, "BICEP has provided a beautiful measurement with unprecedented accuracy of the CMB polarization. This gives an extra motivation to build experiments that will further characterize this signal. In fact, there are several other CMB experiments coming online. So this is a very good moment for this field."

More in general, most of Peloso’s works is on cosmological perturbations. Peloso describes the early Universe as a homogenous soup. Perturbations exist on top of the soup. These perturbations originate from quantum mechanics (and, possibly, from particle interactions) on very small scales, and then are blown to cosmological scales by the expansion of the universe. These perturbations were very small when the CMB was formed (this took place 400,000 years after the big-bang). In general, the equations of general relativity are highly nonlinear, and very hard to solve. However, as long as perturbations are small, their equations can be "easily solved" through analytical approximation techniques, in which interactions that involve an increasing number of perturbations are less and less relevant. This has allowed cosmologists as Peloso to deduce several properties of the universe with high accuracy from the CMB data.

As time progressed, the same perturbations that gave rise to the CMB started to gravitationally attract each other, and to grow in size. Matter started to inflow in regions where (by chance) the perturbations happened to be a bit larger than the surroundings. This matter inflow eventually gave rise to the galaxies that we observe today. During this process of matter accumulation (and well before the present galaxies were formed) gravity became nonlinear; in other words, the perturbations became large enough that their interactions did no longer play a minor role. Peloso says that this makes computations extremely difficult. The point he is specifically interested in is what is called the quasi-linear state, the tipping point so to speak, when gravity becomes nonlinear. The scales at which we can observe this are smaller than the CMB scales, but greater than the scales of galaxies.

The study of the Large Scale Structure of the universe is already a very important field in cosmology, and its relevance will increase further in the near future due to the strong ongoing and planned experimental improvement. "In the near future there will be much more, and much more accurate data on the large scale structures and we need to learn how to make the most out of them. The simplest analytic computations that work so well for the CMB no longer work here. You can do computer simulations of this gravitational clustering, which take a large number of particles in a box and show how they interact gravitationally with each other, and how the structures grow. These are beautiful simulations, but they require a huge computational power, and they take a lot of time. So, we are trying to develop analytical tools to study this quasi-linear regime, when non-linearity starts to be important, but it is not yet dominant, and it can be studied with improved perturbation techniques." For that, Peloso has turned to the world of quantum field theory, and uses something very similar to the Feynman diagrams that are used in particle physics. Peloso’s Feynman diagrams compute the scattering caused by interacting ripples in the early Universe in place of colliding particles. "We didn’t invent this technique ourselves, but we are trying to refine it."