1.5 Reading: A Tour of Cosmic History
Before you start exploring cosmic history in more depth, it might be useful to look at an overview first. This Chronozoom tour, based on Fred Spier's ideas about complexity in big history, will give you such an overview. It will also show you how the videos of this module fit into the big picture.
At the beginning of space and time, the universe would have emerged with a big bang. An infinitely small singularity would have exploded that contained all the still undifferentiated cosmic matter and energy. This big bang produced the expansion of the universe that we can measure today with the aid of redshifted electromagnetic radiation emitted by cosmic objects. Based on these data, it is estimated that the primordial explosion took place around 13.8 billion years ago.
The cosmic expansion led to both a rapid cooling and a decrease of pressure. During a very short period of time this produced Goldilocks circumstances that made possible the emergence of the basic atomic building blocks, namely first protons (hydrogen nuclei) and neutrons, and a little later also electrons and neutrinos. After few minutes, however, while the embryonic universe kept expanding and cooling down, the Goldilocks circumstances that favored this process disappeared and never returned. As a result, these elementary particles only emerged during the very early phase of cosmic history.
Thanks to the continued expansion, favorable circumstances soon emerged that allowed the formation of the nuclei of some heavier chemical elements, most notably helium and deuterium as well as a little lithium. This lasted about 15 minutes. Yet cosmic expansion happened so fast that most matter remained in the form of hydrogen, about 70 percent, while about 27 percent evolved into helium. During this early phase of cosmic evolution, only a few percent of heavier chemical elements emerged. Had the universe expanded much more slowly, almost all matter would have turned into iron, the most stable chemical element. Because very little complexity can be built with the aid of only iron as building blocks, this would have severely limited the emergence of later forms of complexity. Here we see a very powerful demonstration of the importance of Goldilocks circumstances for the history of the universe.
It took about 400,000 years of cosmic expansion until the temperature had decreased to about 3000 K. This provided Goldilocks circumstances for the pairing of the positively and negatively charged particles, which thus canceled out each other's charges. As a result, electromagnetic radiation could suddenly travel through the universe virtually unimpeded, because it was no longer scattered by all these formerly charged particles. This radiation diluted over time as a result of the ongoing cosmic expansion, thus producing the cosmic background radiation that can be observed today all across the sky.
During the period between about five hundred thousand and two billion years after the big bang, Goldilocks circumstances existed that favored the emergence of stars and galaxies out of the primordial matter that had formed earlier, mostly hydrogen and helium. By that time, the universe had cooled down sufficiently, while the matter density was also just right. Up until about five hundred thousand years after the big bang, the universe had been homogeneous to a very high degree. Yet after five hundred thousand years of expansion, under the influence of gravity spontaneously occurring tiny irregularities began to produce large galactic structures. This led to a differentiation between areas with large matter concentrations (galaxies) and areas with very little matter, namely intergalactic space. The unrelenting universal expansion accentuated these differences. Also within galaxies a differentiation took place between areas with large matter concentrations (stars and black holes) and interstellar space.
This separation into areas with and without matter was extremely important for the rest of cosmic history. Had this not happened, no further complexity could have emerged, not least because there would not have been any empty space where entropy could have been dumped in the form of low-level radiation. This type of entropy is an inevitable by-product of the emergence of greater complexity. Eric Chaisson emphasized that had this growing cosmic dumping ground not existed, no greater complexity would have emerged. After about two billion years, no new galaxies were formed in the known universe. Apparently, the circumstances were never Goldilockian anymore for this to happen.
Within stars, new Goldilocks circumstances came into being that favored the emergence of the nuclei of heavier chemical elements, all the way up to iron. This was the result of nuclear fusion processes that ignited as a result of the stars' gravitational contraction. These Goldilocks circumstances in stellar cores are very similar to the conditions that reigned during the early universe. However, there are two major differences. First of all, the early cosmos had been more or less homogenous, while stars and their surroundings are very different indeed. The large matter and energy gradients that had developed between the very dense stars and mostly empty interstellar and intergalactic space allowed stars to get rid of their entropy and keep their complexity going. In the second place, the infant universe changed so very quickly that there was very little time for nuclear fusion to take place. All stars, by contrast, even the shortest shiners, live a great deal longer. As a result, over the course of time stars became the major cosmic furnaces for forging greater complexity at very small scales, thus producing increasing amounts of heavier chemical elements, thanks to the specific Goldilocks circumstances that reign in their cores.
When stars that are at least eight times the size of our sun reach the end of their lives they may detonate. These explosions are called supernovae because they appear to be “large new stars” that suddenly shine very brightly for a short period of time. Indeed, some supernovae produce almost as much light as the entire galaxy they form part of. During these explosions, heavier chemical elements are formed all the way up to uranium. Because these processes last for only very short periods of time, heavy chemical elements are rare.
Stellar blasts disperse the heavier chemical elements and thus seed their cosmic surroundings with these new elementary building blocks. As a result, over the course of time galaxies come to contain increasing amounts of more complex chemical elements. When such a galactic dust cloud subsequently contracts to form new stars and planets, the new solar system may contain the building blocks that allow the emergence of forms of greater complexity such as life and culture. It is thought that our solar system emerged from such a galactic dust cloud around 4.6 billion years ago.
Life may have emerged on our planet as early as 3.8 billion years ago. It is not yet certain whether life emerged spontaneously on Earth or whether it emerged elsewhere in the universe and was transported to our home planet later. Whatever the case may have been, both the emergence of life and its continued existence must have required very specific Goldilocks circumstances. For instance, scientists have defined a galactic habitable zone in our Milky Way in which the conditions for life (as we know it) are just right. This zone is defined by its distance from the galactic center. Close to this center, a great many stars exist that end their lives with a bang, which would destroy any life that had formed in their vicinity. Yet these supernovae also forge and spread more complex chemical elements that are needed for life. This means that life could not have emerged very close to its core. But it could not have emerged very close to the edge of the galaxy either, because in such places there were too few supernovae events to accumulate sufficient numbers of heavier chemical elements that are needed for life. As a result, the galactic habitable zone is characterized by sufficient amounts of supernovae that produce the needed heavier chemical elements, while there are not too many star bursts that would flush out life. Calculations show that our galactic habitable zone would have emerged about eight billion years ago as a zone situated between 23,000 and 30,000 light years from the Galactic center (the radius of our galaxy is about 50,000 light years). Since astronomers think that over the course of time fewer supernovae explosions would have taken place while the amounts of heavy chemical elements increased, over the course of time the galactic habitable zone has widened towards both the galactic center and its outer edge.
Within our solar system, a similar habitable zone is thought to exist. This Goldilocks region is first of all defined by the amount of radiation our sun produces. The planets that are too close to the sun, Mercury and Venus, are too hot and are thus unable to support life. Not very surprisingly, our planet Earth finds itself in a Goldilocks position, while Mars may just be outside of the planetary habitable zone, because it is too cold while it does not have any other energy sources that could support life. Yet it is thought possible that on some of the moons of Jupiter and Saturn life may exist, sustained by the energy emanating from within or perhaps even by the tidal forces generated as a result of the fact that these moons orbit large planets.
There are a more Goldilocks circumstances that needed to be met before life could emerge, most notably liquid water, and thus also an atmosphere surrounding a planet that is large enough so that its gravity keeps the water and the atmosphere there for billions of years. Because it is unknown where and how life emerged, scientists are still seeking to define the very specific Goldilocks circumstances within which this would have happened. Yet it is clear that for more than three billion years after life emerged on Earth, our planet has provided Goldilocks circumstances that allowed it to flourish.
