Eras of the Universe

Eras of the Universe

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A black hole evaporating due to Hawking Radiation
A black hole evaporating due to Hawking Radiation, visualized by Melodysheep. Source: Melodysheep (opens in a new tab)


As the universe slowly dies off, like all bodies, it continuously changes, putting different cosmic bodies into the foreground of it's eras. As professors Fred Adams and Gregory P. Laughlin described it in their book 'The Five Ages of the Universe', there are, as the title suggests, 5 eras the universe goes through before eventually coming to a halt. In this chapter, we want to explore where we currently are, how we got there and how the universe will change in the near to distant future.

Primordial Era

The primordial era began with the Big Bang, a momentous event that marked the birth of the universe as we understand it. At this point, the universe was extremely hot, dense, and filled with an intense burst of energy. As the universe expanded and cooled, elementary particles such as protons, neutrons, and electrons started to form.

Within a few minutes of the Big Bang, nuclear reactions occurred, creating the first atomic nuclei, primarily hydrogen and helium. This process is known as Big Bang nucleosynthesis (opens in a new tab) and is responsible for the abundance of these elements in the universe today. However, the universe was still too hot and dense for light to travel freely, leaving the universe filled with a hot, dense, and ionized gas known as plasma.

Over the next several hundred thousand years, the universe continued to expand and cool, allowing electrons to combine with atomic nuclei, resulting in the formation of neutral atoms. This event, known as recombination (opens in a new tab), marked a crucial turning point in the primordial era. It was during this time that the universe became transparent to light, and the cosmic microwave background radiation was emitted. This radiation, which is still detectable today, provides valuable insights into the early universe.

During the primordial era, tiny fluctuations in the density of matter began to emerge. These fluctuations served as the seeds for the formation of cosmic structures. Under the influence of gravity, regions with slightly higher densities attracted more matter, leading to the formation of galaxies, stars, and clusters of galaxies over time.

Stelliferous Era

The Stelliferous Era, as the name suggests, marks a point in the life of the universe where - and still are, since we are currently experiencing this era - stars were the most dominant cosmic structures in the universe, powering solar systems and potentially giving life to planets inside of their systems.

A white dwarf, the collapsed and cooling core of an earlier star
A white dwarf, the collapsed and cooling core of an earlier star, visualized by Melodysheep. Source: Melodysheep (opens in a new tab)

It is the Era with the best chance for life to form, given that the Primordial Era wasn't stable enough and structures like planets and solar systems haven't formed yet. Eventually with time, stars will use up their fuel and most of them - over 97% of them (opens in a new tab) - will turn into white dwarfs. You may ask yourself "Why, though?". Well...

Death of a star

To understand the end of this Era and the following Eras, we have to understand what a star is, how it works and why it dies. Stars are, simply put, spheres of hydrogen gas, having come together from nebulas. Nebulas are created from mass that hasn't been attracted by any solar system or galaxy in the Primordial Era.

Stars generate energy by nuclear fusion: Hydrogen atoms are combined and helium atoms are formed, which is exactly what happens inside a Hydrogen Bomb. Similarly to what fusion reactors are supposed to do, they generate lots of energy by doing so, most of which is radiation. When some day, the hydrogen in the Star is used up, it will fuse the created helium into carbon until eventually, the helium supplies run out aswell. The energy and therefore radiation created and emitted during the process of nuclear fusion created an outward pressure that stabilized the star and it's gravity.

The lifecycle of a star
The lifecycle of a star. Source: BYJU'S (opens in a new tab)

As soon as nuclear fusion stops, that outward pressure can't be maintained anymore and the star collapses on it's core due to inward acting gravity. All that is left is a slowly cooling white dwarf that doesn't produce any more heat, similarly to bread taken out of the oven, cooling down towards room temperature - absolute zero in our case.

Degenerate Era

Although the name suggests we're currently living through this very Era of the universe, the Degenerate Era is yet to come in the not too distant future. As a study (opens in a new tab) suggests, 95% of the stars that will ever be created are already around, indicating the universe is hardly going to create any more potential for life than there currently is. There is a lot of reasons to this, but the easiest way to imagine it is to compare the Big Bang to a regular explosion of fireworks:

Most of the heat and mass can be found near the center of where the explosion takes place whereas to the outermost parts of it, less and less debris can be found.

Throughout the Degenerate Era, according to the hypothesis of proton decay, protons might decay into pions and positrons. This would make life beyond the Degenerate Era nearly impossible as planets (and other solid bodies for that matter) would slowly decay and fall apart, leaving nothing but black holes behind.

Black Hole Era

As planets and other solid bodies decay due to proton decay, black holes take the spot of the most dominent form of matter in the universe. All matter eventually will either decay or get consumed by black holes until absolutely nothing but them remains. Until a couple of centuries ago, we would have now assumed that the black hole era is not only the last step in the history of the universe but also an eternal state, because for a long time, black holes were thought to be immortal.

However, as Stephen Hawking showed, Hawking Radiation makes black holes evaporate away at a rate that's incomprehensibly slow, making it impossible to detect and very impressive to discover.

The collapse of a wave function upon interaction with the outside world
The collapse of a quantum wave function upon interaction with the outside world. Source: Veritasium (opens in a new tab)

📘 Hawking Radiation

In the most simplified way to put it, particles in the quantum field theory have a chance to be at any given spot in space at any specific time instead of verifiably being in one certain spot. This is called a superposition. The particle's probabilities to be at any given spot, also called the quantum wave activities, are cancelling each others out and are adding up to a probability of 1, meaning there's always just one particle with an uncertain location. Upon interaction with the outer world, for example an observation by us, the wave function describing the particles probabalistic location collapses into a single state, as you can see in the image above, giving the particle a certain position and erasing all chances of being somewhere else.

A black hole's event horizon works somewhat similarly but only eliminates some parts of the quantum wave activity during collapse of the wave function. Due to this, the quantum field activity does not cancel itself out anymore, creating a particle-rich field instead of a "void of probability".

Since all particles have energy and energy has to come from somewhere, it is taken from the black hole, slowly decreasing the amount of energy within it. This ultimately leads to a decrease in the amount of mass it has, making it also shrink in size. This was just a very simplified illustration, so feel free to check out Stephen Hawking's original paper (opens in a new tab) if you want to learn more about Hawking Radiation.

The last era of the Universe

For the very last era of our universe, we are going to look at how it will end, although we will come to realize that the term "end" is not correct at all, because it's eventually not going to end anytime soon. To learn more, flip the page to our last chapter.

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Additional Resources

  1. Professors Gegory P. Laughlin and Fred Adams wrote a book about the 'The Five Ages of the Universe', which I took great inspiration from in this article.
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  3. If you have a somewhat scientific background, you might want to checkout Hawking's original paper on Hawking Radiation if you found it interesting so far.