First developed around a century ago, the Big Bang theory reigns supreme as the sole surviving explanation for the history of the cosmos. It fits all the available evidence: the expansion of the universe, the formation of the light elements, the existence of the cosmic microwave background, the evolution of the cosmic web and more.
In a nutshell, the universe began as an extremely hot and dense point that inflated over roughly 13.8 billion years, becoming bigger and colder. That's the Big Bang theory.
But to make a cosmic omelet, you have to break a few eggs. And over the decades, the Big Bang theory has taken on some pretty heavy challengers. Let's explore those alternatives and why they didn't work.
Before the Big Bang theory was developed, the prevailing consensus among scientists (and really anybody who thought about it for any length of time) was that the universe was just … the way things were. And always had been. And always would be. Sure, there may have been some creation event at some point in the distant past if you had a religious persuasion, but that creation was of a universe that looked and acted pretty much the way it does in the present.
Sure, stars occasionally blew up and the random comet appeared, but on the whole, the universe simply was. It was one great cosmic tapestry that, at large scales at least, remained unchanged for eternity.
All that blew up when astronomer Edwin Hubble discovered the expansion of the universe. That discovery immediately threw a wrench in the idea of an eternal universe, because in an expanding cosmos, the universe is obviously different in the past than in the present, and the future will be even more different.
The evidence showed that we live in a dynamic, evolving universe
Even with the realization that the universe is expanding, many astronomers were still resistant to the concept of the Big Bang. The biggest contender in the early 20th century was a theory called the steady state model, first proposed by astronomer Fred Hoyle.
In the steady state model, the universe is always expanding, but there is always new matter appearing in the void to replace it. So, according to that theory, the cosmos gets bigger, but the density stays the same, thus rescuing the general themes of the eternal universe idea. In other words, in the steady state model, the universe is dynamic but, over long timescales, still unchanging.
Steady state came to a screeching halt with two major observations: quasars and the cosmic microwave background (CMB). Quasars are intensely bright sources of radio emission found exclusively in the distant universe, and the CMB is a source of radiation that surrounds us on all sides. In the Big Bang picture, these are easy to explain: The light comes from an earlier epoch in cosmic history, when things were different. But in the steady state model, the early universe should look like the modern universe.
With steady state done for good, another contender rose up to challenge the Big Bang, thanks to Nobel prize-winning physicist Hannes Alfvén. Alfvén was a master of understanding the forces inside electrically charged gasses, known as plasmas, and he developed an entire branch of physics known as magnetohydrodynamics.
Alfvén argued that, because electromagnetic forces were far stronger than gravitational forces, what we observe in the cosmos should be better understood as consequences of electromagnetism, not gravity. This included the evolution of the solar system, the birth of stars and the expansion of the universe.
Alfvén argued that the universe was composed of large pockets of matter and antimatter, which are constantly in competition. These bubbles expand against each other, resulting in what we perceive as the expansion of the universe, and where they meet, the light of the CMB is generated, he theorized.
Unfortunately for Alfvén, there is no way for an electric universe to match all observations — most importantly, Hubble's law. For nearby galaxies, the speed of their recession is proportional to their distance — something neatly explained by general relativity and the expansion of space. In Alfvén's version, all galaxies receded at an equal rate. Sorry, Hannes.
The Big Bang theory isn't perfect; no scientific theory is. One puzzling feature of the universe is how smooth it is at large scales. Regions of the cosmos vastly separated from each other have roughly the same temperature. There simply wasn't enough time in the early universe for all of these patches to even out.
This is called the horizon problem, and in 1969, physicist Charles Misner developed a solution to it, called mixmaster cosmology (yes, named after the brand of kitchen blenders). In a mixmaster universe, the early cosmos was incredibly chaotic, with space constantly sloshing back and forth. This chaotic action did two things: mixed up material at small scales (eventually giving rise to structures such as galaxies) and evened things out at large scales (to make the overall universe homogenous).
Despite the cool name, the math never really worked out for mixmaster models, and another description of the early universe, called inflation, was able to explain the horizon problem in a much simpler way.
One of the biggest conceptual problems with the Big Bang is that it has a beginning. There was a time with no universe and then a time with a universe. Because the Big Bang model doesn't attempt to explain the true beginning of the universe, there have been many attempts over the years to come up with some scenario that generates a "big bang" from some other physical process.
Almost all attempts at replacing the Big Bang end up delivering some sort of cyclic universe, in which the Big Bang is just one of an infinitely long string of universes, because if you replace the Big Bang with another singular occurrence, you haven't really changed anything. In essence, the cyclic models represent an eternal universe, but with more steps.
There are many cyclic models, and all of them rely on highly speculative physics. Perhaps higher-dimensional "branes" keep colliding, triggering new Big Bangs. Or maybe inflation just doesn't stop, and there's always a new universe right around the corner. Or maybe the universe will eventually collapse, reach some incredibly small quantum size and bounce right back again.
But all of these models have difficulty explaining dark energy — that the expansion of our universe is accelerating, with no signs of slowing down. So, as far as we can tell, the cosmos is a one-and-done affair.
And any sophisticated model of the cosmos must contain the Big Bang, because it describes all of the available evidence. So, no matter what, the Big Bang will always win.
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