The hot Big Bang is often touted as the beginning of the Universe. But there's one piece of evidence we can't ignore that shows otherwise.
The notion of the Big Bang goes back nearly 100 years, when the first evidence for the expanding Universe appeared. If the Universe is expanding and cooling today, that implies a past that was smaller, denser, and hotter. In our imaginations, we can extrapolate back to arbitrarily small sizes, high densities, and hot temperatures: all the way to a singularity, where all of the Universe’s matter and energy was condensed in a single point. For many decades, these two notions of the Big Bang — of the hot dense state that describes the early Universe and the initial singularity — were inseparable.
But beginning in the 1970s, scientists started identifying some puzzles surrounding the Big Bang, noting several properties of the Universe that weren’t explainable within the context of these two notions simultaneously. When cosmic inflation was first put forth and developed in the early 1980s, it separated the two definitions of the Big Bang, proposing that the early hot, dense state never achieved these singular conditions, but rather that a new, inflationary state preceded it. There really was a Universe before the hot Big Bang, and some very strong evidence from the 21st century truly proves that it’s so.
Although we’re certain that we can describe the very early Universe as being hot, dense, rapidly expanding, and full of matter-and-radiation — i.e., by the hot Big Bang — the question of whether that was truly the beginning of the Universe or not is one that can be answered with evidence. The differences between a Universe that began with a hot Big Bang and a Universe that had an inflationary phase that precedes and sets up the hot Big Bang are subtle, but tremendously important. After all, if we want to know what the very beginning of the Universe was, we need to look for evidence from the Universe itself.
In a hot Big Bang that we extrapolate all the way back to a singularity, the Universe achieves arbitrarily hot temperatures and high energies. Although the Universe will have an “average” density and temperature, there will be imperfections throughout it: overdense regions and underdense regions alike. As the Universe expands and cools, it also gravitates, meaning that overdense regions will attract more matter-and-energy into them, growing over time, while underdense regions will preferentially give up their matter-and-energy into the denser surrounding regions, creating the seeds for an eventual cosmic web of structure.
Our entire cosmic history is theoretically well-understood, but only because we understand the theory of gravitation that underlies it, and because we know the Universe’s present expansion rate and energy composition. We can trace out the timeline of the Universe to exquisite precision, despite the uncertainties and unknowns surrounding the very beginning of the Universe. From cosmic inflation until today’s dark energy domination, the broad strokes of our entire cosmic history are known. Credit: Nicole Rager Fuller/National Science Foundation
But the details that will emerge in the cosmic web are determined far earlier, as the “seeds” of the large-scale structure were imprinted in the very early Universe. Today’s stars, galaxies, clusters of galaxies, and filamentary structures on the largest scales of all can be traced back to density imperfections from when neutral atoms first formed in the Universe, as those “seeds” would grow, over hundreds of millions and even billions of years, into the rich cosmic structure we see today. Those seeds exist all throughout the Universe, and remain, even today, as temperature imperfections in the Big Bang’s leftover glow: the cosmic microwave background.
As measured by the WMAP satellite in the 2000s and its successor, the Planck satellite, in the 2010s, these temperature fluctuations are observed to appear on all scales, and they correspond to density fluctuations in the early Universe. The link is because of gravitation, and the fact that within General Relativity, the presence and concentration of matter-and-energy determines the curvature of space. Light has to travel from the region of space where it originates to the observer’s “eyes,” and that means:
The overdense regions, with more matter-and-energy than average, will appear colder-than-average, as the light must “climb out” of a larger gravitational potential well,
The underdense regions, with less matter-and-energy than average, will appear hotter-than-average, as the light has a shallower-than-average gravitational potential well to climb out of,
And that the average density regions will appear as an average temperature: the mean temperature of the cosmic microwave background.
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