More than a century of scientific research has established the Big Bang model, often referred to as the standard model of cosmology, as the evolutionary theory of the observable universe. It describes in detail how the universe evolved from an extremely hot and dense state, when its age was less than a second, to the one we observe today, nearly 14 billion years later.
The Big Bang model is supported by key experimental evidence, including the expansion of space, the abundance of light chemical elements, and the remnant glow from the Big Bang that pervades all of space—the cosmic microwave background (CMB).
Using the Big Bang model, scientists can dial back in time and peek into the status of the universe at the moment of its creation. They found several mysterious puzzles. On the one hand, the infant universe was extremely uniform beyond reasonable expectation; on the other hand, there were tiny irregularities—whose density contrast gradually grew under gravitational attraction and became the seeds of the galaxies—that exhibit very special patterns.
Both properties strongly suggest that the initial state of the Big Bang universe was the result of a very different epoch preceding it—the so-called primordial universe. This discovery started a four-decade long, and still ongoing, search for what happened before the Big Bang.
The leading candidate theory is cosmic inflation, which predicts that the primordial universe was dominated by some form of dark energy and expanded in an accelerated rate in a fleeting fraction of a second. The inflation scenario has become the most widely accepted theory. For many researchers, it provides the simplest explanation for the puzzles. And several predictions from the simplest inflation models have been verified by experiments measuring the properties of CMB and galaxy distribution.
However, a number of alternative theories have also been put forward, with some suggesting that the state of the universe before the Big Bang was contracting—and the Big Bang was really part of a Big Bounce. These alternatives serve as eye-openers and reminders that several key predictions of inflation may not be unique to the inflation theory. There may be other ideas that need to be explored and tested.
Physics is an experimental science, and theories have to be tested by experiments. During the process, the issue of falsifiability—that is, whether a theory can be tested to be potentially shown wrong—has inevitably arisen.
Inflation theory is a large framework under which there are numerous models. The critics of inflation have pointed out that although some predictions of the simplest models match the observational results, there are also others that have been refuted—again by CMB experiments. In fact, there are so many different predictions from different models of inflation, it appears there are some that can explain the experiments, whichever way the experimental results go.
These controversies have been the subjects of active debates over the years, culminating in one in 2017 which involved more than 30 leading scientists including four Nobel Prize laureates and the late Stephen Hawking.
Testing inflation theory as a whole
In new work published in Physical Review Letters as an Editors' Suggestion, myself and two colleagues from Harvard University—Avi Loeb, Chair of the Astronomy Department, and Zhong-Zhi Xianyu, a postdoctoral researcher of the Physics Department—proposed how we could use the future experimental data to find out which theory is the correct one.
Our main goal is not to distinguish various models within the inflation theory, rather to test the inflation theory as a whole against the alternative frameworks.
In our view, inflation theory—or any other alternative theory—is more than just a mathematical framework which can be correct as long as it is self-consistent. The theory describes a physical process whose defining properties need to be tested and potentially falsifiable in experiments, like any other physical theories such as quantum mechanics or galaxy formation.
While the critics are right that inflation theory has not made falsifiable predictions against the above-mentioned alternative theories, it does not mean such predictions do not exist. In fact, each of these theories has a very clear defining property—the evolution of the size of the primordial universe. For example, during inflation, the size of the universe grows exponentially; while for Big Bounce, the size contracts before the bounce. The conventional observable attributes people have proposed so far—although valuable for distinguishing various models within a theory—have trouble distinguishing the different theories because they are not directly related to this defining property.
How can we retrieve the direct information about the evolution of the size of the primordial universe? We propose to use the signals generated by the "primordial standard clocks." These clocks are any types of heavy elementary particles present in the energetic environment of the primordial universe. They exist in any theory because they are building blocks of the unification theory, whatever it is; and they oscillated at some regular frequency, much like the ticking of a clock's pendulum.
To explain how the primordial standard clocks work, let us use the following analogy.
About 100 years ago, the map of stars made by astronomers was still two-dimensional in nature, because it was challenging to figure out the distances of stars from us. This ignorance led to intense debates on questions such as: Is the sun at the center of the Galaxy? Are there stars beyond the Milky Way? These issues were not settled until the discovery of the "standard candles"—the type of stars whose absolute brightness is known. Using these standard candles, it is straightforward to figure out how far the stars are, because the further they are, the fainter the associated standard candles appear. Therefore, the standard candles helped turning a merged stack of 2D star maps into a 3D map.
Ticking clocks and time stamps
Our current knowledge about the evolution of the primordial universe is at a similar stage. Through observing distributions of CMB and galaxies which were seeded by the primordial irregularities, we have obtained a series of snap shots of what happened before the Big Bang—like the series of images in a roll of film frames.
Now, what we do not know is the time coordinates of these snap shots. Without any clock information, we do not know how to play the film. Should it be played forwardly or backwardly, fast or slow? This led to debates on questions such as: Was the primordial universe inflating or contracting? And how fast did it do so?
The ticking standard clocks put time stamps on each of these frames when the film was shot before the Big Bang. If we could observe these time stamps, we would know what this film is about.
These time stamps—that we call "clock signals"—take different patterns for different theories of the primordial universe. Predicting the detailed patterns and suggesting how they should be searched for is the main result of our paper. If a pattern of signals representing a contracting universe were found, it would falsify the entire inflation theory, regardless of what detailed models one constructs; and vice versa for alternative theories.
However, we cannot predict the overall strength of the signals, which may be very weak and hard to detect. This means we will have to search in many different places. We have already started the search in the CMB and there are some interesting candidate signals. To test if they are genuine signals, we need more data from future observational projects on CMB and galaxy distribution. There are many experiments being planned in the next decade or two—originally with other scientific goals in mind. With our proposal, these data will also be used to search for a direct answer to the question: what exactly happened before the Big Bang?
Dr. Xingang Chen is a theoretical cosmologist at the Harvard-Smithsonian Center for Astrophysics and a senior lecturer at the Astronomy Department of Harvard University.
Views expressed in this article are the author's own.
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