Harvard study proposes new way to probe the universe before the Big Bang – Harvard Gazette

Almost everyone knows the Big Bang – the notion that an extremely hot and dense universe has exploded to become the one we know today. But what do we know about what preceded?

In order to solve several puzzles discovered in the initial conditions of the Big Bang, scientists have developed a number of theories to describe the primordial universe, the most successful of which – known as cosmic inflation – describes how the universe has grown considerably in size. a fraction of a second before the Big Bang.

But despite the success of the inflationary theory, controversies have led to active debates over the years.

Some researchers have developed very different theories to explain the same experimental results that have hitherto underpinned the inflationary theory. In some of these theories, the primordial universe was contracting instead of expanding and the Big Bang was therefore part of a Big Bounce.

Some researchers – including Avi Loeb, science professor Frank B. Baird and director of the astronomy department – have expressed concern about this theory, suggesting that its seemingly infinite adaptability makes it virtually impossible to test.

"The current inflation situation is that this idea is so flexible … it can not be falsified on an experimental basis," Loeb said. "Whatever the result of the observable that it was proposed to measure, some inflation models can still explain it." Therefore, the experiments can only help to clarify some model details as part of the inflation strategy. theory, but can not test the validity of the frame itself. However, falsifiability should be the hallmark of any scientific theory.

This is where Xingang Chen enters the scene.

Lecturer in astronomy, Chen and his collaborators have been developing for many years the idea of ​​using something that he has called a "primordial standard clock" as the probe of the primordial universe. In collaboration with Loeb and Zhong-Zhi Xianyu, a postdoctoral researcher in the physics department, Chen applied this idea to non-inflationary theories after learning that there had been intense debate in 2017 on whether theories inflationists were making forecasts. In an article published in the form of a suggestion by the editor in Physical Review Letters, the team outlined a method that can be used to falsify experimentally the theory of inflation.

In order to find a characteristic that could separate inflation from other theories, the team began by identifying the property that defines the different theories – the evolutionary history of the size of the world. primordial universe. "For example, during inflation, the size of the universe increases exponentially, by definition," said Xianyu. "In some alternative theories, the size of the universe shrinks – in some very slowly and in others very quickly.

"The classical observables proposed up to here by people have difficulty distinguishing the different theories because these observables are not directly related to this property," he continued. "So we wanted to find what are the observables that can be related to this defining property."

The signals generated by the primordial standard clock can be used for this purpose.

This clock, Chen said, is any type of massively heavy elementary particle in the primordial energetic universe. These particles should exist in all theories and oscillate at a regular frequency, much like the swinging of a pendulum of the clock.

The primordial universe was not entirely uniform. Quantum fluctuations have become the seeds of the large-scale structure of today's universe and an essential source of information that physicists can rely on to learn about what happened before the Big Bang. The theory expounded by Chen suggests that standard clock ticks generate signals that have been printed in the structure of these fluctuations. And because standard clocks in different primordial universes would leave different signal patterns, he added, they could perhaps determine which theory of the primordial universe is the most accurate.

"If we imagine all the information that we have learned so far about what happened before the Big Bang comes in the form of rolls of film footage," he said. Standard clock tells us how these images should be read, "explained Chen. "Without clock information, we do not know if the film should be played backward or forward, fast or slow – just as we do not know if the primordial universe is inflated or s & rsquo; Is contracted, and how fast. That's the problem. The standard clock has timestamps on each of these images when the movie was shot before the Big Bang, and explains what this movie is about. "

The team calculated the appearance of these standard clock signals in non-inflationary theories and suggested a search method in astrophysical observations. "If we find a configuration of signals representing a shrinking universe," said Xianyu, "it would falsify the entire inflationary theory, whatever the detailed models that one builds".

The success of this idea lies in experimentation. "These signals will be very subtle to detect," said Chen. "Our proposal is that there should be some kind of massive fields that have generated these footprints and we have calculated their models, but we do not know how large is the overall amplitude of these They may be very weak and very difficult to detect, which means that we will have to search in many places.

"Cosmic microwave background radiation is a place," he continued. "The distribution of galaxies is another. We have already started looking for these signals and there are already some interesting candidates, but we still need more data. "

This research was funded by the Black Hole Initiative of Harvard University and the Center for Mathematical Sciences and their applications from Harvard University.

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