What happened before the Big Bang?

Although cosmic inflation is well known for having solved some important mysteries concerning the structure and evolution of the universe, other very different theories can also explain these mysteries. In some of these theories, the state of the universe preceding the Big Bang – the so-called primordial universe – was shrinking instead of expanding and so the Big Bang was part of it. a Big Bounce.

To help choose between inflation and these other ideas, the question of falsifiability – that is, whether it is possible to test a theory to show that it is false – has inevitably arisen. Some researchers, including Avi Loeb from the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, Mass., Have expressed concerns about inflation, hinting that its seemingly infinite adaptability makes any test impossible.

"Falsifiability should be a hallmark of any scientific theory.The current situation with regard to inflation is that it is such a flexible idea that it can not be falsified on an experimental basis," Loeb said. . "No matter what value people measure for an observable attribute, there are still patterns of inflation that can explain it."

Now, a team of scientists led by Xingang Chen of CfA, along with Loeb and Zhong-Zhi Xianyu from Harvard University's Physics Department, have applied an idea that they call a "primordial standard clock" non-inflationary theories, and exposed a method that can be used to falsify inflation experimentally. The study will appear in Letters of physical examination as a suggestion of the editors.

In order to find a characteristic that can separate inflation from other theories, the team began by identifying the property that defines the different theories – the evolution of the size of the primordial universe.

"For example, during inflation, the size of the universe increases exponentially," said Xianyu. "In some alternative theories, the size of the universe shrinks.Some do it very slowly, others very quickly.

"The attributes that people have proposed up to now to measure are generally difficult to distinguish the different theories because they are not directly related to the evolution of the size of the primordial universe", he continued. "So we wanted to find what observable attributes can be directly related to this definition property."

The signals generated by the primordial standard clock can serve such purpose. This clock is any type of heavy elementary particle in the primordial universe. Such particles should exist in any theory and their positions should oscillate at a regular frequency, much like the clock ticking of a clock.

The primordial universe was not entirely uniform. Small irregularities of density on tiny scales have become the germs of the large scale structure observed in today 's universe. This is the main source of information on which physicists rely to learn about what happened before the Big Bang. The standard clock ticks generated signals that were printed in the structure of these irregularities. Standard clocks in different theories of the primordial universe predict different signal patterns because the evolutionary histories of the universe are different.

"If we imagine all the information we have learned up to now about what happened before the Big Bang comes in the form of rolls of film footage, the" " 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 back or forth, fast or slow, just as we are not sure if the primordial universe is swelling or shrinking, and The problem lies in the fact that the standard clock has timestamped each of these images when the movie was shot before the Big Bang and tells us how to play the movie. "

The team calculated how these standard clock signals should appear in non-inflationary theories and suggested a search in astrophysical observations. "If a signal structure representing a shrinking universe were found, it would distort the entire inflationary theory," Xianyu said.

The success of this idea lies in experimentation. "These signals will be very subtle to detect," said Chen, "and we may have to look in many different places, cosmic microwave background radiation is one, and the distribution of galaxies is another. look for these signals and there are already interesting candidates, but we need more data. "

Many future galaxy surveys, such as the US LSST, Euclid and the recently approved NASA SphereX project, are expected to provide high quality data that can be used for this purpose.

The pre-printing of this document is available at the following address: arxiv.org/abs/1809.02603. You will find a similar earlier work in: arxiv.org/abs/1509.03930.

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More information:
Unique fingerprinting of alternatives to inflation in the prime power spectrum, arXiv: 1809.02603 [astro-ph.CO] arxiv.org/abs/1809.02603