Humans love putting things neatly into boxes and categories. We like these regular divisions. So, it is not surprising that even the chronology of the universe is divided into eras, epochs, and ages, corresponding to periods where our cosmos was doing very different things.
We know where we are and what the universe has been doing for most of its time. We also have a good indication of where it is going for the next 80 billion years. But go far enough in each direction and you will find big uncertainties and many unknowns.
YOU ARE HERE – THE STELLIFEROUS ERA
Depending on which physicist you asked, this could be the second era of the universe, the penultimate era of the universe, or one of the many epochs that have gone by and are yet to come. No matter the definition, this is where we are: the stelliferous era. The time of stars, of galaxies, and of life.
This period of the universe started about 150 million years after the Big Bang and it is expected to last perhaps up the point that the universe can celebrate its 100 billionth birthday, 86 billion years from now.
This epoch is marked by several important events that have shaped the universe and will continue to do so for a long time. The first one is the formation of the first star and the assembly of the first galaxies. We have many uncertainties on how that happened, but future telescopes should help us understand. These stars were the second light in the universe, after the Big Bang, and contributed to the re-ionization of the cosmos. The light ripped apart the electrons from the hydrogen atoms (ionizing them) found in clouds everywhere in the universe. This made the universe transparent.
Then for billions of years, those clouds collapsed on themselves forming stars after stars, a period called the cosmic high-noon where the universe was at its most active. The universe is still making stars but at a much lower rate and it will continue to do so for many many billions of years.
The other notable event is that roughly four billion years ago, the expansion of the universe shifted gear. It began expanding at an accelerated rate. The leading hypothesis is that due to the existence of dark energy, a mysterious substance that makes up for most of the energy-matter content of the cosmos.
THE STORY SO FAR...
In the words of Julie Andrews, “Let's start at the very beginning, A very good place to start.” Only there is a problem. When we talk about the universe, we really can’t. Our starting point needs to be 10-43 seconds after the Big Bang. That’s the Planck Epoch and our physics doesn’t work there at all. Maybe a theory in the future will explain it but currently, this is our starting point.
Another minuscule fraction of a second later, we find ourselves in the Grand Unification Era lasting up to 10-36 after the Big Bang. That’s a billionth of a billionth of a billionth of a billionth of a second.
At this point, the universe has only two forces: gravity and the electrostrong interaction which is the combination of electromagnetism, and the two nuclear forces. Gravity might have been unified with the other three forces, but it separated sooner than the others. After this epoch, the strong nuclear force (the one that keeps protons together) also separated, leading to the electroweak epoch. And then expansion.
At 10-32 seconds after the big bang, the universe experience a (yet-to-be explained or confirmed) expansion known as cosmic inflation. The volume of the universe expanded at least 1078 times. This expansion would see something a thin as a hair becoming 10 wider than the Milky Way in a minuscule time.
Despite the incredible expansion, the universe remains very hot for a little bit longer. At around 100 billionths of a second after the Big Bang, the universe begins to form the first particles and it begins to exhibit the peculiar quark-gluon plasma state of matter, which laboratories like CERN have only recently started to investigate.
At about one second after the Big Bang, we have the formation of particles we are a bit more familiar with. First neutrinos, then electrons, and between two and 20 minutes after the Big Bang the first atoms. Despite the formation of these particles, matter will take a long time to begin to dominate the universe. For thousands of years, radiation in the form of photons will reign supreme culminating in the Cosmic Microwave Background 370,000 years after the Big Bang. That is the moment that the universe has now expanded enough that the photons can move freely without interacting with matter. Until the formation of stars, these 150 million years are the Cosmic Dark Ages.
The dominion of matter started a bit earlier though. From 47,000 years after the Big Bang until 9.8 billion (when dark energy took over), the matter was the crucial ingredient that shaped the 6universe on its largest scale.
WHERE DO WE GO FROM HERE?
If you thought that wrapping your heads around tiny time intervals where so much happens was difficult, hopefully, you will enjoy the other side. From 100 billion years in the future onwards we have huge swathes of times where the universe is slowly dying.
After the universe stops producing stars, objects like white dwarfs, neutron stars, and black holes will be taking over: the degenerate era. Because they are made of degenerate matter.
The end of the cosmos in detail is as you would have guessed by now nebulous at best. It depends on the exact knowledge of cosmology which we are currently still lacking. But there are several reasonable scenarios and some less-so but quite fun. They should keep shining until about 1040 years into the future.
Then it's time for the black holes to be the only object that matters. Until 10100 years after the Big Bang, black holes will slowly evaporate losing energy, one way or another. After that, darkness, and even more doubts about the universe's ultimate fate.
First, one suggestion that has been popularized a lot is the Big Crunch. At some point, the expansion of the universe will stop, and it will start contracting. Getting smaller and smaller until it goes back to being a singularity. This might lead to the formation of a new Big Bang. That scenario is known as the Big Bounce. But unfortunately, this is unlikely to happen based on current data.
Another cool scenario is the Vacuum Instability. The idea is that the universe at its fundamental level is not stable but that there exists a more stable version of all particles and forces in the universe. At some point, by chance, a bit of the universe will reach this state and that change will spread. Eventually, all known forces and particles will be replaced and everything that makes us and keep us together will change.
It is also possible that dark energy is pushing the universe apart at a rate that will eventually destroy the fabric of space-time. The Big Rip is just that. First clusters of galaxies are separated, then galaxies, then star system, then atoms, until the continuum is completely gone. While this dramatic end seems to match well with the incredible beginning of our cosmos, it is currently unclear if dark energy could achieve this. Some of the current data does not consider this scenario viable in the grand scheme of things.
The most credited end to everything is the heat death of the universe. Eventually, around the year 101000 the universe will have expanded enough that no structure exists. An empty, dark, and cold cosmos.
The Vacuum Instability is not looking too bad, am I right?