When most of us think of the idea of the “big bang”, a massive explosion emerging from nothingness comes to mind. While much of the physics community agrees that such a description is relatively accurate for the start of the universe as we know it, much of the context around the Big Bang remains unknown, namely what came before and how our universe evolved to what we see today. Three Harvard scientists, Avi Loeb, Xingang Chen, and Zhong-Zhi Xianyu, set out to narrow down the list of possible explanations for how the Big Bang happened (1).
One description of the Big Bang is the theory of cosmic inflation, according to which an infinitesimal point inflated almost instantaneously to create space itself (2). This theory is in accordance with all experimental data and observation, but we have yet to see definitive proof in the form of primordial gravitational waves (ripples created by the rapid expansion of the fabric of space in the early universe) for this theory (2). While it could be that these waves are just too weak to observe, others, taking advantage of the lack of direct evidence, have proposed alternate theories, one of them being the “big bounce” (3).
The theory of the big bounce posits that the universe had been contracting for a long time until it reached the smallest possible size and then “bounced back” in what we now call the Big Bang (3). Distinguishing between these two theories that describe the universe at a time when time itself was essentially meaningless is not an easy task, yet Loeb, Chen, and Xianyu recently published a paper that elucidates a possible method for doing just that (1).
According to quantum uncertainty, there is no such thing as truly empty space, which means that quantum fields in the early universe were filled with ripples of varying wavelength. The constructive and destructive interference of these ripples over time together with their interaction with the expanding or shrinking space determined the distribution of matter throughout the universe (2-3). We can observe this today by looking at the matter density of different parts of the universe on different scales. Loeb, Chen, and Xianyu postulate that the variations of matter density can tell us about the nature of space at the time when the “ripple”, from which the matter density pattern emerged, was formed (1). Specifically, they believe that by analyzing these scale-dependent matter density variation patterns, they can determine “whether the primordial universe was actually expanding or contracting, and whether it did it inflationary fast or extremely slowly,” according to Chen (1).
While their idea appears theoretically sound, the oscillating density patterns they hope will be observed may not be pronounced enough to detect with today’s technology. Even if this turns out to be the case, Loeb, Chen, and Xianyu are pushing the field of cosmology in the right direction, creatively fostering the use of direct evidence to fill in the gaps in our story of the universe’s history.
Lucia Gordon is a first-year in Weld Hall and is planning to concentrate in Physics and Mathematics.
 Chen, X.; Loeb, A.; Xianyu, Z. Fingerprints of Alternatives to Inflation in the Primordial Power
Spectrum. Cornell University Library, Sep. 16, 2018. arXiv:1809.02603 (accessed Oct. 8, 2018).
 Natalie Wolchover. A New Test for the Leading Big Bang Theory. Quanta Magazine, 9/11/18.
https://www.quantamagazine.org/a-new-test-for-the-leading-big-bang-theory-20180911/ (accessed Sept. 28, 2018).
 Natalie Wolchover. Big Bounce Models Reignite Big Bang Debate. Quanta Magazine, 1/31/18.
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