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Comment to the reader.
In the previous entry I began recording events unfolding during my lifetime — conflicts between nations and the uncertainties of the present moment. Those events belong to the immediate history and concerns of our species, which I will continue to document in future logs. In this entry, however, I step much further back in time to describe what we currently understand about our origins, beginning with the origins of the universe.
Because Earth Logs will move between subjects that exist on vastly different time scales — from the birth of the universe billions of years ago to events occurring in the present day — readers may sometimes wish to keep track of the timeframe associated with the topic being discussed.
For this reason, each Earth Log includes a metadata field called Subject Timeframe. Readers who wish to quickly orient themselves within the timeframe of the topic of an Earth Log's may refer to this tag.
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Human beings did not appear suddenly in the universe. Our species emerged from a long chain of cosmic events stretching back billions of years. By the early twenty-first century, scientists had developed models that attempt to explain how the universe itself began and how the structures within it gradually formed.
What follows is a brief description of the dominant scientific understanding of my time regarding the origins of the universe and the formation of the world on which I live.
According to the dominant scientific understanding of humanity at this time, the universe began approximately 13.8 billion years ago in an event commonly referred to as the Big Bang. Contrary to what the name might suggest, this was not an explosion occurring at a particular point in space. Rather, space and time themselves began expanding from an extremely hot and dense initial state. In this scientific view, time itself did not exist before this event; it began as part of the universe's expansion. In those earliest moments there were no stars, no galaxies, and no planets — only energy, elementary particles, and the physical laws that govern their interactions.
As space expanded, temperature and density gradually decreased. Within a few minutes, nuclear reactions created the first atomic nuclei — mostly hydrogen and helium. For hundreds of millions of years afterward the universe remained dark, filled with enormous clouds of gas but no stars. Slowly, gravity began drawing matter into denser regions. These regions eventually collapsed, giving birth to the first stars and galaxies.
From these first stars and galaxies would emerge the later generations of stars and planetary systems that eventually made possible the existence of worlds like Earth — and, billions of years later, observers capable of asking how the universe itself began.
Galaxies are immense collections of stars, gas, dust, and dark matter bound together by gravity. Some galaxies contain hundreds of billions of stars.
According to the scientific models developed during my era, galaxies began forming a few hundred million years after the Big Bang. Small clumps of matter gradually merged and grew larger through gravitational attraction. Over billions of years these structures evolved into the galaxies we observe today.
Our own galaxy is known as the Milky Way. It is a spiral-shaped galaxy containing hundreds of billions of stars. The Sun — the star that provides energy to Earth — is only one of those many stars.
From the perspective of an individual human life, galaxies appear permanent and unchanging. In reality they evolve slowly over immense periods of time, merging with other galaxies and forming new generations of stars.
Within one of the spiral arms of the Milky Way galaxy, in a region containing clouds of gas and dust left behind by earlier generations of stars, another process of formation began billions of years later. From this material, a new star and its surrounding planetary system would eventually emerge. This system would become the environment in which the planet Earth — and later human civilization — would arise.
Roughly 4.6 billion years ago, a region within the Milky Way galaxy containing gas and dust began to collapse under the force of gravity. This collapsing cloud formed what scientists call a protoplanetary disk — a rotating disk of material surrounding a growing star.
At the center of this disk, pressure and temperature increased until nuclear fusion began. This marked the birth of our star, the Sun.
The remaining material in the surrounding disk gradually began to collide and combine into larger bodies. Through countless impacts over millions of years, these bodies grew into planets, moons, asteroids, and other objects.
This system of objects orbiting the Sun is known as the Solar System.