If there's one thing a layperson knows about the physical universe, it's the idea of a Big Bang. Also called the inflation model of cosmology, consensus holds it as the most viable explanation for the origin of everything.
However, many scientists think the idea of a single colossal explosion of light doesn't hold up to snuff for a theory of universal origin. Part of this is because science is an evidence-based empirical process of amassing knowledge of the objective world, which means accepted theories will always go where the evidence points.
Current estimates for the age of the universe places it at about 13.8 billion years, with homo sapiens (us) showing up a mere 200,000 years ago. After building a fire and inventing civilization, it took us until 100 years ago to theorize the Big Bang. But while evidence supports the Big Bang model while also casting doubt on rival theories, a new model may someday arise capable of besting consensus.
Unpacking the universe's origin with a big bang
Questions of how the universe came to be eventually raised more about its end. But philosophers and physicists have furrowed their brows at these questions for thousands of years. At its core, the Big Bang theory suggests all of the Universe's matter came into existence at the same time, roughly 13.8 billion years ago.
While it's difficult to speak of a "before" when discussing the beginning of all space and time (how can there be a "before," before "before?"), the totality of matter exploded out of a compacted ball with density and intense heat approaching infinity — called a singularity. For unknown reasons, this singularity suddenly expanded in an unimaginably violent explosion, and the universe as we know it began.
1. The steady-state model
One of the Big Bang's old rival theories is the Steady-State Universe model. This one denies the possibility of beginning and end for the universe — opting instead for a continually expanding universe that nonetheless maintains the same overall density.
In other words, a steady-state universe is forever identical, but continually growing bigger in scale.
In this model, galaxies, planets, and other forms of matter are locked in continual recreation, and since the density remains the same, old astronomical objects become unobservable as new creations take their place.
This theory was initially proposed by Sir James Hopwood Jeans in 1928, and was further developed in the late-1940s by Hermann Bondi, Fred Hoyle, and Thomas Gold.
The late cosmologist Geoffrey Burbidge championed steady-state cosmology early in his career, and was one of the last serious cosmologists to eventually abandon the theory — even after new evidence directly falsified it.
However, instead of jumping on the Big Bang train, Burbidge developed a new model — called the oscillating universe, according to which the universe undergoes many small big bangs, like the universe is on repeat.
2. The bouncing cosmological model
If or when the Big Bang happened, the universe expanded from a single point whose gravity and density approached infinity, and has continued to expand since then. This is called the inflationary model.
The Bouncing Cosmology, Big Bounce, or Cyclic Universe model involves a rapid, Big Bang-like expansion of the universe. But it adds a rewind function, accounting not only for expansion, but also contraction of the universe.
A cyclical universe shrinks to a minimum volume, and then "bounces" back into a subsequent expansion. Another variant proposes a cosmos that only bounced once. In this one, the present-day universe came into being after an earlier contraction (extending into an infinite past of infinite dispersion). According to the single-bounce theory, the universe of today will expand forever.
This theory was first propounded by Juliano Cesar Silva Neves in a paper published in the journal General Relativity and Gravitation. Neves is a researcher at the Mathematics, Statistics & Scientific Computation Institute (IMECC-UNICAMP) of the University of Campinas, Brazil.
3. Plasma, or electric universe theory
In the Electric Universe model, gravity takes a backseat to plasma and electromagnetism. In this theory, plasma takes up an integral role in cosmological events and the fundamental order of the universe, proposing electric currents that flow along plasma filaments — capable of shaping and powering galaxies. These currents stream into stars, powering them like fluorescent bulbs. They induce the births of planets.
The Electric Universe theory was first proposed by Nobel-winning physicist Hannes Alfvén in the 1930s. Alfvén argued that if plasma pervaded the universe, it could carry electric currents capable of generating a galactic magnetic field. He later won a Nobel Prize for his work in magnetohydrodynamics.
However, there's no evidence for the Electric Universe theory, and it fails to meet the National Academy of Sciences' definition of theory, since it has no predictive power in the empirical world of natural events — with which we could witness physical signs of its presence. But regardless, it's gained popularity via a number of lay-scientists and YouTubers in search of a simpler way to explain the Universe.
4. The black hole origin theory
The Black Hole theory suggests the universe popped out of a black hole from another universe. In this model, we're living beyond its event horizon.
This theory has been around for some time. Initially, researchers at Perimeter University presented a paper detailing how black holes might be the origin of our universe. In a different and more recent version from theoretical physicist Nikodem Poplawski at Indiana University, the observable universe is the interior of a black hole existing as one of possibly many inside a larger parent universe, or multiverse.
If true, we could say every black hole is a doorway to a new universe. But since we can't cross the event horizon (alive), there's likely no way to confirm or falsify the theory.
5. Simulation theory
A lot of people in Silicon Valley are fond of an idea that if true seems to trump all others: the possibility of living in a colossal simulation. This idea gained mainstream attention from movies like "The Matrix" and off-the-cuff comments from Tesla CEO Elon Musk — who said in an interview that it could be entirely possible, despite the absence of evidence.
Scientists later researched this idea in 2017, but results suggest our universe is not the creation of a computer program. The study was done by theoretical physicists from Oxford University in the U.K.
Simulated universe needs computer bigger than its creation
The Oxford researchers looked into the possibility of creating a computer capable of computing everything in this universe. But such a hypothetical computer would need to be powerful enough to calculate the motion of every particle.
Needless to say, this is a difficult calculation. For the computer to record the data of even a few electrons, its memory needs more atoms than there are in the entire universe, and with the addition of only a few more particles, the complexity goes up exponentially.
Hence, it's probably safe to say we're not the product of an alien Playstation.
Scientific worldviews will always evolve
The Big Bang theory remains the most widely accepted explanation for the origin of the universe. But in empirical science, no theory can last forever.
The scientific model of the universe is constantly changing as advances in fields like quantum mechanics and gravity struggle to unify theories about the fundamental laws of the universe. But while the Big Bang model isn't completely proven, for now, it's the best possible explanation, using all available scientific evidence.