31/08/2021
What Came First: Inflation Or The Big Bang?
13.8 billion years ago, all the matter and energy contained within our Universe was concentrated into a volume of space about the size of a soccer ball. Even with all that energy in such a small space, however, we didn’t collapse into a black hole. Instead, the Universe expanded at a rapid rate that balanced the energy density so precisely that, for all of our measured cosmic history, we’ve walked that fine line between expanding and recollapsing.
Today, all we can see within the Universe extends for some 46 billion light-years in all directions, and scientists can trace this origin back to a hot, dense, more uniform and more rapidly expanding state. Like many theorists, you might be tempted to extrapolate this back even farther, to an arbitrarily hot and dense state: a singularity. But that temptation is the root of most of our misunderstandings surrounding the birth of the Universe. The Big Bang wasn’t the beginning, after all. Instead, that honor goes to cosmic inflation, and everyone should understand why.
According to the big idea of the Big Bang, the Universe was hotter, denser, and more uniform in the past, and that it evolved into what it is today by expanding, cooling, and gravitating to form a great cosmic web. The fabric of space itself expands as time goes on, as the laws of General Relativity demand for a Universe that’s filled with roughly equal amounts of matter and energy in all directions and locations, causing the wavelengths of photons to stretch, the kinetic energy of massive particles to decrease, and enabling gravitational imperfections to steadily grow.
In the framework of the Big Bang, each of the observable phenomena mentioned earlier get a physical explanation: distant galaxies appear to redshift because the expanding Universe stretches the light’s wavelength; more distant galaxies really are younger and less evolved; the Universe was less clustered in the past; the primeval atomic ratios are 75% hydrogen, 25% helium, and 0.00000007% lithium; the leftover radiation was discovered in the mid-1960s.
That last discovery pretty much killed every one of the Big Bang’s alternatives, and installed the Big Bang as the cosmic origin story for everything within our observable Universe. The Universe emerged from this early hot, dense, and uniform state, and over time it expanded and cooled.
When it cools below a specific energy threshold, it becomes unable to spontaneously generate particles whose mass (via E = mc2) is too great; over the first few fractions-of-a-second, every antimatter particle other than positrons and anti-neutrinos annihilate away.
About 1 second after the Big Bang, neutrinos and antineutrinos “freeze out,” meaning their (energy-dependent) interaction rates drop to such a low frequency that they effectively never interact again.
And as we move forward, nuclear reactions occur and then cease; neutral atoms stably form, releasing that primeval radiation; gravitational imperfections grow on progressively larger and larger scales, leading to the formation of the first stars, then galaxies, and then the enormous cosmic web.
But what about the Big Bang’s origin story? Where did the Big Bang itself come from?
If you extrapolate the expanding and cooling Universe all the way back as far as theoretical physics allows you to go, you’ll come to an event in the past known as a singularity. Essentially, you’d be packing all the matter and energy in the Universe into a single point. (The laws of physics break down, and stop giving sensible answers, once you reach an extremely high energy of ~1019 GeV per particle, which corresponds to an “age of the Universe” of ~10-43 seconds after the Big Bang.)
A singularity, from the perspective of General Relativity, is the only event that can correspond to the beginning or ending point of space and time. Therefore, we could extrapolate all the way back to a singularity in the framework of the Big Bang, and arrive at a point that we could legitimately refer to as “the beginning.”
In physics, we have two ways of dealing with questions like these. Because all of these questions are about initial conditions — i.e., why did our system (the Universe) begin with these specific conditions and not any others — we can take our pick of the following:
Although it’s not clear to everyone, the first option is the only one that’s scientific; the second option, often touted by those who philosophize about the landscape or the multiverse, is tantamount to giving up on science entirely.
