University of Minnesota
School of Physics & Astronomy


Marco Peloso

Marco Peloso
Marco Peloso
Jenny Allan

Professor Marco Peloso started his research career in theoretical particle physics. He soon became interested in the application of these studies to the field of cosmology. The physics of the early Universe deals with very high energies and, Peloso says, a lot of the tools and concepts from particle physics are useful in cosmology. The first area of Peloso’s interest is in the so-called “re-heating phase” of the early Universe.

In the first 10-35 seconds after the big bang, a very unconventional expansion, named “inflation,” occurred. The field that drove this phase had very distinct properties from ordinary matter and radiation. The re-heating phase describes how the constituents of the present universe were created by the decay of this field. Re-heating was surely completed within the first second (and probably much earlier) after the big bang. At this time, nucleosynthesis,
or the formation of light nuceli, occurred. Physicists have a clear knowledge of this later “hot big bang phase” because ordinary matter and radiation were driving it, and because the processes that characterize it involve physics which we can test on Earth.
The mysterious force that drove the inflationary phase is conventionally described by a scalar field, named the “inflaton.” Although fundamental scalar fields have not yet been observed, they play an important role in particle physics, the most known example being the Higgs field of the Standard Model. The inflaton is required to have a very high mass (about 10-13 times greater than the one of the proton, in the simplest models), so there is no hope at present or in the near future to produce it in terrestrial accelerators. The energies required to create such a massive particle could only have occurred in the very early Universe. Consequently physicists look to the skies, where the remnants of the big bang processes are still observable.
For years, theoretical physicists investigated re-heating as a perturbative phase, or one in which single inflaton quanta decayed individually into ordinary matter and radiation. More recent theoretical studies have shown that in many cases the decay occurs through a non-perturbative process, in which the particles behave in an ordered manner. To explain the difference between perturbative and non-perturbative systems, Peloso uses the example of a laser which, due to its order, behaves completely differently from ordinary light, which is instead disordered and incoherent. Peloso said that the non-perturbative processes involved at re-heating are extremely more efficient than the perturbative ones, and often harder to study. “To me, the model which displays non-perturbative effects is the most natural, but others might disagree,” Peloso says, adding, “We don’t have a working theory from start to finish.”
Another area that Peloso studies is the physics of extra dimensions. Extra dimensions have been predicted by string theory, but Peloso is not a string theorist as such. His main interest is in understanding whether or not extra dimensions can ever be measured. One possibility
relies in detecting the dynamical degrees of freedom which are associated with the perturbations of the extra space itself. These studies require theoretical techniques which are very close to the ones developed for cosmological
perturbations. It is possible that extra-dimensions have never been observed simply because they are small. This begs the question, why didn’t they expand with the rest of the universe? A natural possibility is the presence of a sort of a “rubber band” that prevents the expansion of the additional coordinates, although many different implementations of this idea have been proposed. It is therefore important to study the specific properties of the different realizations with the goal of identifying distinct phenomenological signatures.
Peloso also studies what he calls “the biggest mystery in theoretical physics,” the acceleration of the universe. A few years ago it was observed that the expansion of the universe is actually accelerating and theoretical physicists are trying to understand why this is happening,
“all of a sudden.” Sudden in cosmological terms, can be defined as the last few billion years. There are two schools of thought on the causes of the acceleration. The most conventional one is that there is some sort of vacuum energy, which Peloso termed as “absurdly small” by any conceivable particle physics scale. A second possiblity
attributes the acceleration to a modified behavior of gravity at the largest observable scales. In the best case scenario, this puzzle may result in a profound change of our understanding of gravity. Peloso admits that this approach to gravity is highly unconventional, but says that the problem is a radical one, which possibly requires an equally radical solution.