The Big Bang explosion ushered in the birth of our universe 13.8 billion years ago. Before this unfathomably complicated event unfolded, everything in the universe was compressed in an incredibly small and infinitely dense ball of matter under conditions scientists are now beginning to understand. Quite literally, time didn’t exist before the Big Bang. It wasn’t until 1981 that a physicist named Alan Guth, among others, came up with a precursor to the Big Bang theory, namely, the theory of inflationary cosmology (Greene, 2004). In conjunction, these theories offer the best explanations we have so far about the origin of our universe. In this article, we will discuss what the Big Bang Theory is, what this theory doesn’t tell us about the universe, and how inflationary cosmology answers those questions.
Background
Simply put, the Big Bang theory tells us what happened the second after the dense ball of matter started to stretch and inflate. According to a Space article, “during this period, the universe doubled in size at least 90 times, going from subatomic-sized to golf-ball-sized almost instantaneously.” (Howell & May, 2022). This puzzled scientists because the dominant force in the universe is gravity- an attractive force. However, the Big Bang shows us that gravity could be repulsive in the right environment and that it was, in fact, responsible for the expansion of the universe. This discovery encouraged scientists to delve into the environment surrounding the Big Bang. As a result, scientists developed the study of inflationary cosmology to describe what they observed.
But first, let’s begin by describing the foundation of both theories – Einstein’s cosmological constant. According to Einstein’s theory of relativity, the cosmological constant is the balancing force that keeps the universe static and unchanging. Namely, it constitutes a force directly opposite that of gravity, which Einstein used to balance some of his equations (Mann, 2021). Like other physicists of the time, Einstein believed that the universe wasn’t expanding or contracting. Rather, he believed the universe was stationary, and the cosmological constant created a model in which the universe did just that. However, after revisiting the mathematics, Einstein removed it from his general theory of relativity (Greene, 2004). In 1929,14 years after Einstein’s theory debuted, an astronomer by the name of Edwin Hubble observed that all of the stars around us have a redshift, meaning that all of the stars are moving away from Earth, and, to Einstein’s surprise, each other. This single observation proved that Einstein’s cosmological constant was wrong (Mann, 2021) — the universe was indeed changing and expanding outward; the cosmological constant didn’t describe how the universe actually worked.
Argument
Although Einstein’s cosmological constant fell out of favor, a similar theory was popularized that explained the expansion of the universe — the theory of inflationary cosmology. In 1979, physicists Alan Guth and Henry Tye postulated that as the universe started to cool down after the Big Bang, the Higgs field responsible for the creation of matter, got trapped in high energy levels, and this catalyzed the expansion of the universe (Greene, 2004). In fact, these two physicists figured out that the supercooled Higgs field had the same strong repulsive gravitational forces as Einstein’s cosmological constant. However, there is a fundamental difference between them. On one hand, the cosmological constant describes a universe that is exerting a constant and uniform gravitational force pushing outward — a universe that doesn’t expand or contract (Mann, 2021). On the other hand, the theory of inflationary cosmology describes a universe that has a uniform gravitational force pushing outward and a universe that is expanding.
The Big Bang model was based on the idea that gravity is attractive and that the universe is constant (Greene, 2004). In contrast, inflationary cosmology grew out of the idea that gravity could be repulsive too and that it could indeed drive the expansion of the universe, which is what we observed (World Science Festival, 2014). Although these theories don’t contradict each other, inflationary cosmology allows for an initial burst of outward expansion, while the Big Bang describes what happens after this initial burst.
Likewise, inflationary theory fills in gaps that the Big Bang Theory left unanswered. For example, the Big Bang Theory doesn’t explain why the cosmic microwave background has a uniform temperature in every direction in the cosmos (World Science Festival, 2014), what scientists call the horizon problem. According to Brian Greene in his book The Fabric of the Cosmos, the “horizon” is a subjective term that describes the longest distance between two objects that can send each other light signals. From our point in the universe, our horizon is the visible universe around us. We receive light signals from these objects in space, and this is how we exert our influence on each other (Greene, 2004). However, this doesn’t explain how areas beyond our horizon, areas of space that we theoretically haven’t influenced, have the same temperature as areas we have influenced.
Before the rapid expansion of the universe, everything was condensed into a very small, very dense ball of matter. Now, picture the first moment when the universe started expanding. At that very instant, repulsive gravity burst outward and separated this ball of matter. However, we were still close enough to every part of the universe to influence each other. In fact, enough time passed between the beginning of the universe and the expansion of the universe for the extreme temperatures to even out (ESA, The cosmic microwave background and inflation) . In addition, all of the pent-up pressure from this ball of matter was released, and because we are still close to each other, the heat from the initial explosion had time to travel to all parts of the universe. Hence, different parts of the universe that aren’t in contact with each other now have the same temperature imprint from their cosmological origin.
Another problem that inflationary cosmology answers, that the Big Bang Theory doesn’t, has to do with the shape of the universe, namely — why does the universe look flat? (Greene, 2004). At first glance, this doesn’t seem so much of a problem. However, as scientists have discovered, for the universe to be flat it must have had precisely enough matter to expand the universe, but not enough for it to collapse. Scientists couldn’t understand how the universe could’ve been so fine-tuned to allow for a flat universe, which is what the Big Bang Theory postulates. This is where inflationary theory comes in. Imagine you are an ant living on the surface of a slightly inflated balloon. From the ant’s point of view, the balloon would appear round. Now, imagine making the balloon twice as big. The ant on the surface of the balloon will appear flatter. Now, let’s inflate that balloon just a bit more. At this point, the balloon will look flat to the ant no matter where it stands. This is how inflationary theory describes why the universe appears flat. Whereas a flat universe requires just the right amount of energy/matter density equal to critical density (Greene, 290), inflationary cosmology describes a universe that appears flat to us just like an expanded balloon looks flat to the ant.
Conclusion
In conclusion, the theory of inflationary cosmology is a few steps ahead of the Big Bang theory. Namely, it answers questions like: Why is the universe expanding? Is gravity attractive and repulsive? How is the CMB temperature the same throughout the universe? And was the universe fine-tuned to be flat? On one hand, the Big Bang model gives us a basic understanding of how the universe was born, which was a revolutionary discovery in 1920 (Greene, 2004). However, inflationary theory clears up the baggage that the Big Bang left behind while at the same time offering us a more accurate and dynamic explanation of how the universe behaved the way it did 13.8 billion years ago.
Citations
Greene, B. R. (2004). 10. In The Fabric of the Cosmos: Space, Time, and the Texture of Reality (pp. 272–303). essay, A Division of Random House, Inc.
Howell, E., & May, A. (2022, January 10). What is the Big Bang Theory? Space.com. Retrieved May 3, 2022, from https://www.space.com/25126-big-bang-theory.html
Mann, A. (2021, February 16). What is the cosmological constant? LiveScience. Retrieved May 3, 2022, from https://www.livescience.com/cosmological-constant.html
ESA The cosmic microwave background and inflation.(n.d.). Retrieved May 3, 2022, from https://www.esa.int/Science_Exploration/Space_Science/Planck/The_cosmic_microwave_background_and_inflation
World Science Festival. (n.d.). Alan Guth explains inflation theory – youtube. Youtube. Retrieved May 3, 2022, from https://www.youtube.com/watch?v=rEXDgpttmyc
Book(s) mentioned: The Fabric of the Cosmos: Space, Time, and the Texture of Reality by Brian Greene