The Big Bang and the Mystery of the Universe's Origin
13.8 billion years ago, the universe began, and the story of what happened next is still a mystery. The question of how everything came to be is a fundamental mystery in science, crossing the boundaries of theory, experiment, physics, and philosophy. Everything we know about the universe hinges on mechanism, process, and causality, and we strive to understand the reasons things are the way they are. However, at the most fundamental level, we do not know how or why the universe came to be.
Scientists believe that a period of exponential expansion known as inflation put the bang in the big bang and set the stage for everything that followed. Some physicists suggest that inflation is not the only period of inflation in the cosmos, and inflation would continue elsewhere, eternally producing bubble universes distributed through a potent everlasting multiverse. Others suggest that our current universe is merely the latest in a long and potentially infinite series of expanding and contracting cosmos, a theory known as the big bounce.
Physicist Roger Penrose suggests that the universe loses a sense of time and scale at the infinitely small and the infinitely big and becomes, to all intents and purposes, equivalent, like the game of chess played the same whether on a board that fits in your pocket or spans an entire courtyard. String theory attempts to explain the fundamentals of reality as tiny vibrating strings within an 11-dimensional reality of hyperspace.
Despite all this searching, the origin story of our universe remains a mystery, and for now, it is a multiple-choice choose-your-own-adventure story. Nevertheless, 13.8 billion years ago, the universe began, and this is the story of what happened next.
The Hottest Temperature in the Universe
Temperature is little more than a manifestation of a particle's energy, its motion, or how fast it's vibrating. Absolute zero, marking zero kelvin, is the lowermost limit where particles hypothetically would come to a complete shuddering stop were it possible to reach. However, there is also an upper limit to an absolute heart that marks the highest temperature before the particles themselves are torn apart by their energy. It's known as the Planck temperature, and it is around 1.4 times 10 to the 32 kelvin.
The Planck temperature is the temperature at which the wavelength of thermal radiation reaches the Planck diameter. It can go no smaller, and it is the highest temperature that has ever been recorded anywhere in our vast cosmos. It is no fair reflection on the human brain to say that these numbers are beyond our true comprehension. We are evolutionarily attuned to scales that are relevant to us, and so it may seem that these Planck units are too small or, in the case of temperature, too large to be of any practical use.
However, when contemplating the immeasurably tiny moment at the beginning of our immeasurably vast cosmos, these units really come into their own. Our universe is expanding, becoming less dense, and it has been for all its history. Scientists do not know if our universe is infinite or not, but if it is, then it always has been so. Rather than starting as a single point, it would have also begun infinitely large. Our observable universe, the only part of the universe we could ever see or interact with or could ever see and interact with us, is limited by the speed of light and the distance it has been able to travel since the universe's birth 13.8 billion years ago.
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The Planck Era: When Gravity Ruled the Universe
The Planck Era, a moment 100 million trillion trillion trillionths of a second after the beginning of time and space, was a very strange time in the cosmos. This was the time when everything within it was only a single Planck unit of time old and a single Planck length in diameter, with all of our observable universe compacted into such a tiny space that the temperature of the whole cosmos sat squarely at Planck temperatures. Everything we experience today is governed by certain immutable laws of nature, ultimately controlled by the interaction of four major forces: the strong force, the weak force, the electromagnetic force, and gravity.
Gravity, as Max Planck predicted at the end of the 19th century, was the first to manifest during the Planck Era, long before any of the conventional laws of nature held sway. However, there is a problem: general relativity and quantum mechanics, which govern the dynamics of the extremely tiny, are fundamentally incompatible with each other. Researchers are still searching for a solution, a so-called quantum gravity, that is able to reconcile two of the most important theories in modern physics and properly describe how the Planck Era universe behaved.
Quantum Gravity Theories
One idea suggests that like the other fundamental forces of nature, gravity is communicated by a messenger particle, a quantum unit of gravity that has come to be known as a graviton. However, gravity as a force is so much weaker than the others than its meager messenger would be practically impossible to detect. Another theory known as loop quantum gravity imagines the smooth and ever-changing geometry of spacetime to be ultimately pixelated on the very smallest scales and by rewriting Einstein's equations of general relativity in terms of lines or loops instead of points, the calculations for gravity on a quantum scale become much more manageable. Perhaps the leading possible quantum gravity solution continues to rumble away in the background of theoretical physics: string theory imagines the point-like particles of traditional physics as one-dimensional vibrating strings, and it is the vibrational state of the strings that defines what a particle is, as well as the elusive force of gravity.
Ultimately, each of these hypotheses for how quantum gravity may actually work is more mind-bending than the last, rooted in the abstractions of mathematics and almost impossible to visualize. But that is what we are trying to do when we cast our minds back to imagine the first fraction of a second of our universe during the Planck Era.
The Four Fundamental Forces of Nature
Everything in our universe is controlled and defined by the four fundamental forces of nature:
Gravity: Keeps you pinned firmly to the seat of your chair, holds the earth's atmosphere close to the surface, and provides a habitable environment for your continued existence.
Electromagnetism: Invisible infrared light electromagnetic radiation beams across the living room from the front of the remote. The picture comes to life in front of you through millions of tiny pixels emitting visible light, another form of electromagnetic radiation which travels back across the room and into your eyes. Your eyeballs themselves are marvels of evolutionary innovation but they ultimately rely on the properties and behaviors of the atoms within, electrons and nuclei held together by the electromagnetic force.
Strong Nuclear Force: The atomic nuclei themselves are held together by the strong nuclear force, protons and neutrons forced together in the center of the atom. Not only that, these protons and neutrons are really a combination of three smaller subatomic particles called quarks, which are also bonded by the same pull. Every atom, including those in your body, the television, and existence itself, relies upon this force.
Weak Nuclear Force: This force is responsible for the decay of radioactive elements, such as potassium, which is very slightly radioactive and around 5000 potassium atoms radioactively decay every second. This is also what drives the fusion of hydrogen into helium in the core of our sun, powering the heat that keeps the earth habitable.
Each of these forces played a role in the universe during the Planck Era, but it was gravity that was the first to manifest.
