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In Conversation With Alan Guth

Bell experiments and primordial black holes

In Conversation With Alan Guth
In Conversation With Alan Guth

16/03/2018 - Trieste

The Salam Distinguished Lecture Series is an annual tradition at ICTP, comprised of three talks given over three days to the whole ICTP community by an active, renowned scientist. Not only do these lectures showcase recent advances in a particular field, but also they allow an eminent scientist to provide a visionary take on their field, in line with the legacy provided by ICTP’s founder Abdus Salam. The 2018 Salam Distinguished Lectures featured cosmologist Alan Guth, a professor at MIT and an ICTP Dirac Medal winner. Guth explored fundamental questions about the beginnings and structure of the universe in his talks. He took time while he was in Trieste to speak about primordial black holes, energy densities, and the state of cosmology.

In 1979, Guth was a post-doctoral researcher focusing not on cosmology but on particle physics. He was in his eighth year as a postdoc, struggling to find a job, when he heard two lectures by visiting cosmologists, one of which introduced him to the flatness problem. Our universe has no perceptible space-time curvature, hence it is ‘flat’, a situation that seemingly requires incredibly precise initial conditions. This leaves cosmologists to wonder how these initial conditions came to be so precisely fine-tuned. Guth was fascinated, even though it had little to do with his previous work. It seemed suspicious that such precise initial conditions would just be a coincidence.

Guth was soon drawn into cosmology, however, by another postdoc, Henry Tye, who convinced Guth to join him in investigating how many magnetic monopoles, (magnets typically come with two poles, north and south; these are one alone) could have been created in the big bang. They soon concluded that the conditions in the very early universe, where it seemed that there would be so many monopoles that it would have collapsed. Tye and Guth wanted to publish their potential solutions, but another researcher got there first.

This led him to consider a separate problem about the very early universe: the magnetic monopole problem, with another postdoc, Henry Tye. The two were considering conditions in the very early universe, it seemed that there would be so many monopoles that it would have collapsed. Tye and Guth wanted to publish their potential solutions, which included supercooling, but another researcher got there first. Then one night, Guth had what he described as a "spectacular realization": he realized that the same mechanism that he and Henry had proposed to cure the monopole problem would lead to a solution to the flatness problem as well. His idea, inflation theory, describes an exponential expansion explosion of space from about 10-36 seconds to 10-33 seconds after the Big Bang, which accounts for the density distribution of the universe, the large-scale structure of the universe.

That was the inspiration that launched Guth’s career into cosmology and earned him the professorship at MIT. Since then, he has continued to elaborate the theory of inflation and delve into other topics. His three Salam Distinguished Lectures covered the possibility of our universe being part of a multiverse, the implications of eternal inflation of the universe, and the possible origin of the arrow of time. Work by Guth and other physicists on eternal inflation has led to the idea of a multiverse, where space is full of patches, with physical properties differing in each. “In most versions of inflation theory, inflation is eternal into the future— it stops in places, but always continues in other places. Where inflation stops, universes form, which we call pocket universes,” Guth explains. Increasing amounts of data are helping refine theories of inflation, but Guth has said that it’s difficult to have a theory of inflation that does not lead to the existence of a multiverse.

Guth is still investigating the early universe in a variety of other ways. One of his lines of research is an exploration of primordial black holes, “which are black holes that could have formed immediately after the Big Bang,” Guth says. “We're looking at a scenario where the black holes form directly as a consequence of the density perturbations created by inflation.” Not much is known about how black holes merge and evolve over time, but eventually, theories about primordial black holes could provide predictions about distributions and masses of observable black holes today, providing more clues to the development of the universe.

Guth is also now involved in the first experimental physics project of his career, a cosmic Bell experiment. “It's an experiment to test the fundamentals of quantum mechanics,” Guth says. Quantum mechanics predicts a universe that is inherently probabilistic, but some physicists have argued that this seemingly random behavior is perhaps controlled deterministically by hidden variables, forces as yet unknown. A Bell experiment seeks to test for the presence of those possible hidden variables, by entangling, or linking, two particles and then separating them, and checking to see if a particular quality of each accords with what quantum mechanics predicts, or what hidden variable theory predicts. “The basic idea is to create an entangled pair of photons,” Guth explains, and then measure a particular characteristic of each. The results can be compared to what is predicted by hidden variable theory and what is predicted by quantum mechanics. This particular Bell experiment uses two photons traveling from hugely distant corners of the galaxy, an even more stringent test than previous Bell experiments: their local hidden variables are likely completely unrelated.

“The amazing thing about the current state of cosmology is both how much we know and how much we don’t know,” Guth says. “We know how to build inflationary models that make spectacular predictions for all the properties we can measure, like the measurements of the cosmic background radiation. Those measurements have grown more and more precise and still continue to fit perfectly with what simple models of inflation predict— I find that amazing.” Guth adds, “But at the same time, there are very fundamental things that we don’t understand, so there are some very important loose ends.”