# Origins of the Universe

## Mathematics and Physical Sciences

The Origins of the Universe program was launched in 2017 to support three teams of researchers investigating three modern theories of the universe and the forces that continue to shape it. The teams operate independently, often exploring avenues of research that directly compete with those of the other teams, but meet for a joint conference once a year. Though their research is theoretical, they all hope to test their predictions at the neighboring Simons Array and Simons Observatory, the latter of which will begin operations in 2020 in Chile.

The telescopes at the observatories will be taking measurements of the cosmic microwave background radiation (CMB), which can provide clues about the early universe. Greg Gabadadze, associate director for physics at the Simons Foundation and a leader, along with Massimo Porrati, of one of the Origins research groups, describes analysis of the CMB as a kind of cosmic archaeology. “Archaeologists go back and dig out artifacts of the Roman Empire or something,” he says. “Through those artifacts, they learn how that society was organized.” Researchers studying the CMB can likewise excavate information about primordial physics.

The structure of the Origins of the Universe program is unusual in that the research groups are — intellectually — in direct competition with one another. Everyone involved is passionate about their work, and the annual meeting gives them a chance to discuss their competing ideas openly (“and peacefully,” one principal investigator affirms), and to broaden their horizons. “These interactions are beyond the interactions anyone could have within their own groups,” Gabadadze says.

Eva Silverstein, professor of physics at Stanford University, leads the largest of the groups, the Research in Modern Inflationary Cosmology group. The inflationary model of cosmology is the most widely accepted origin story for the universe. It holds that the universe underwent superluminal expansion — an expansion faster than the speed of light — almost 14 billion years ago.

This group is large and sustains many lines of research. One of the group’s goals is to unite inflationary cosmology and string theory, in the hopes of explaining quantum gravity. “String theory has led us to ideas for inflationary cosmology that we had not found using classical physics alone,” Silverstein says. The project would also move beyond analysis of the CMB and instead examine the large-scale structure of the present universe — how galaxies and other large objects are distributed — to understand the early evolution of the universe.

The structure of the Origins of the Universe program is unusual in that the research groups are — intellectually — in direct competition with one another.

An Inflationary Cosmology subgroup works to find a mathematical formulation of quantum gravity that is compatible with the accelerated expansion of space-time; such a formulation is demanded by observations indicating that the cosmological constant is positive. The researchers are building from theoretical work in the opposite case, where the cosmological constant is negative, incorporating exciting new tools in quantum theory to make the generalization to the real-world case.

The Cosmological Bounces and Bouncing Cosmologies group’s ideas upend the ideas of inflationary cosmology. Lead investigator Paul Steinhardt of Princeton University was himself one of the pioneers of the inflationary paradigm but eventually came to realize that quantum fluctuations spoil the original idea, creating deep problems for the theory. “If you look at the problems with the prevalent idea, they all trace back to the assumption that the universe — space and time — has a beginning,” he says. His group instead investigates the possibility of replacing the ‘bang’ with a ‘bounce,’ a smooth transition from an earlier period of contraction to the current period of expansion. Furthermore, such contraction and expansion may repeat every trillion years or so, resulting in a cyclic universe with no beginning or end. The group’s goal for the Origins project is to develop the mathematical, theoretical and numerical tools to put the ‘bounce’ on solid footing. After that, they hope to develop predictions that could be tested at the Simons Array or Simons Observatory.

“One of the possibilities when we do the full numerical calculations is we might discover — oops! Something destroys the idea,” Steinhardt says. “That’s the way it should be in science. You develop an idea, you learn from it whether you win or lose, and it should be killable. If it doesn’t have that property, it’s not a very good idea.”

Gabadadze and Porrati, both of New York University, lead the Cosmology Beyond Einstein’s Theory group, which investigates the mathematical foundations of theoretical frameworks that would allow for periods before inflation. “There are a multitude of scenarios which the universe could have followed in the beginning, and we don’t know which one is true,” Gabadadze says. “That’s why we’re working on theories that describe those stages and working out predictions of those theories that can be contrasted with observations.” Some aspects of their work are more compatible with inflation and some are more compatible with bouncing cosmology or other alternatives, so they work with both groups.

One of the motivations for the group’s research comes not from questions about the early history of the universe but from questions in modern cosmology: Their work could help physicists understand dark energy, the mysterious force that makes up about two-thirds of the total energy in the universe and which would also explain the present-day expansion of the universe.

The graduate students and postdoctoral researchers supported by the Origins of the Universe project are in a fortunate position working in such a dynamic area. “Young people who engage in this project get training both in the fundamentals of the field but also in how to conduct research under unusual circumstances, when things are not settled,” Gabadadze says. “I believe that is very important for the continuation of the field in general.”

You develop an idea, you learn from it whether you win or lose, and it should be killable. If it doesn’t have that property, it’s not a very good idea.”