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.
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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 is known. Credit: Nicole Rager Fuller/National
Science Foundation |
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.
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The Universe doesn’t just expand uniformly, but has tiny
density imperfections within it, which enable us to form stars, galaxies, and
clusters of galaxies as time goes on. Adding density inhomogeneities on top of
a homogeneous background is the starting point for understanding what the
Universe looks like today. Credit: E.M. Huff, SDSS-III/South Pole Telescope,
Zosia Rostomian |
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.