The purpose of this page is to provide an in-depth timeline of what is known, or at least strongly presumed, about the history of our universe. I'll try to paint the most accurate picture of the past that I can here. Let's start as far back as it goes.

13.8 billion years ago there was a big bang. So big, in fact, that instead of occurring in some specific place it happened literally everywhere. The entire Universe erupted into existence as a very large (if not infinite) ocean of extremely hot plasma consisting mainly of light, with a relatively small amount of mass-bearing particles thrown in. All of those particles moved at or close to the speed of light, bouncing off of each other and making the Cosmos a little less crowded over time. An atom would be instantly destroyed in these conditions, not to mention anything more complex. However, the Universe cooled as it expanded; the light and plasma filling it became dimmer and less energetic. At first the temperature was at least 10 billion Kelvin, after which the Universe became half as hot every time it doubled in size.

It's tempting to ask what created this hot and dense baby universe. The majority of sources will tell you that the Universe began as a singularity, with everything that exists today crammed into an infinitesimal point (or otherwise infinitely compressed), and that time and space had their beginning at that very moment. Popular as this picture is, it's an outdated one based on simply extrapolating the expansion and cooling of matter backwards in time using the so-called Friedmann equations, which accurately model the expansion of the Universe throughout the majority of its history, but aren't really meant for figuring out what happened at the very beginning.

More recent theories have instead tried to determine what physical process could have produced this hot soup of particles. Our best guess at the moment, based on the overall structure of the Universe and some properties of the faint omnipresent radiation left over from the Big Bang, is that the known Universe was created in at least three stages - inflation, reheating and baryogenesis. While the history of the Cosmos after these stages is known fairly well, theories for this early time are still being developed; we don't even know for sure how long each of these stages lasted. There exist alternative hypotheses, but none seem to explain the state of our present Universe quite as well.

So, the first stage was cosmic inflation. During this time the pre-bang Universe - let's call it the Protoverse - expanded unimaginably fast, doubling in size every 10^-36 seconds, give or take. This expansion is thought to have been driven by a mind-numbingly powerful inflaton field, like a gravitational one but in reverse. Cosmic inflation ended when every distance had grown by a factor of at least 10²⁶, so that parts of the inflaton field that were smaller than a subatomic particle were now bigger than a planet. Like all fields, this one had slightly different energies in different places, and as the Protoverse blew up, places with slightly more energy would become the very early progenitors of galaxy clusters. Then the particles that made up the inflaton field gradually decayed into more familiar ones like electrons and quarks, creating the matter of our cozy universe along with possibly many others. The details of how (and if) this happened are still being explored.*

If we start our clock here, at the end of inflation, then the farthest our theories reach back in time is about 10^-34 seconds before the Big Bang*, when the region that is now the observable universe was smaller than a single atom - an invisible speck in the midst of infinity. We have no way of knowing what happened before that. Perhaps an eternity of our protoverse exponentially growing from even more unimaginably small sizes.*

In general, there are many mysteries surrounding the first moments of the Big Bang. Current knowledge only extends back to a second or two after the supposed creation of everything. Most "Big Bang timelines" you may encounter begin at 10^-43 seconds, but that is once again based on the old-fashioned model where everything starts with a singularity. Besides, we're still far from understanding the origin of newly discovered things like dark matter and dark energy, which may have influenced the early history of the Universe. Still, we can say with some confidence that the events of the second paragraph of this page happened at some point, and that the Friedmann equations more or less apply from that moment onward.

So, back to where we left off. At one second old, the Universe was over 10 billion degrees hot, and every cubic metre of it contained about half a ton of matter. The kinds of things that existed at that moment can be counted on two hands:

Electrons, the negatively charged particles that now zip around most atoms, were all freely roaming around the Cosmos back then. Well, not quite freely; they often ran into their antiparticles, positrons, which have the same mass as electrons but are positively charged. Should an electron and a positron meet, their charges cancel each other out and they become photons. This can also happen in reverse if the photons have enough energy, and they very much did at that moment, producing almost equal numbers of electrons and positrons.

Photons, the massless particles of light, made up the vast majority of matter (and still do). Back then space was too dense for them to get around without constantly bumping into charged particles and changing direction, so if you were transported to that moment and somehow survived, you wouldn't be able to see anything. Not that there was much to see anyway - the Universe was almost perfectly uniform and literally bursting with light; you can imagine a solid white colour and you'll have an accurate visual picture of the Big Bang.

Neutrinos are fairly similar to photons, but also share some similarities with more "material" particles. They have some very small mass that still isn't precisely known and are nearly as abundant as photons, outnumbering all other particles by a factor of more than a billion. Like electrons, these neutrinos were constantly produced from light alongside their antiparticles, antineutrinos.

Finally, less than a billionth of all particles were so-called baryons - protons and neutrons. They were the most massive particles present, each nearly 2000 times the mass of an electron. Unlike the other particles listed here, which are elementary (fundamental) particles, baryons are actually clusters of smaller things called quarks; protons and neutrons each contain three quarks.*

In that first second or so, baryons constantly collided with neutrinos, antineutrinos, electrons and positrons, which caused protons to become neutrons and vice versa. But around the end of that second, the Universe had exploded enough for neutrinos and antineutrinos to roam freely instead of bouncing between other particles, thus forming the so-called cosmic neutrino background (CNB*), which could be detected today with careful enough measurements.

A few seconds later, photons no longer had enough energy to produce electrons and positrons. The ones that already existed readily annihilated each other. For reasons unknown, a relatively small amount of electrons survived this annihilation. This kind of mysterious imbalance that led to particles outnumbering antiparticles presumably also led to all antiquarks and antibaryons being eliminated sometime much earlier. Many explanations for this imbalance have been proposed, including the idea that there's an antimatter universe on the "other side" of the Big Bang evolving backwards from it, but none of them have found any confirmation yet.

After a couple of minutes, there were about as many electrons as there were protons. In the future this would allow them all to come together and form atoms, but it was still too hot for that - one billion degrees. Baryons had stopped reacting with neutrinos and swapping types. One out of every 7 baryons was a neutron; this imbalance was due to their slightly bigger mass compared to protons, which led to them becoming protons more often back when they were still reacting with neutrinos. In addition, free neutrons are unstable and decay into protons, electrons and antineutrinos after an average time of 15 minutes. This eventually lowered the neutron population to 1/8 of all baryons.

It was now cold enough for atomic nuclei to form. Protons and neutrons combined with each other to form deuterium, a heavier form of hydrogen (protons are themselves the nuclei of "ordinary" hydrogen). Deuterium would then combine to form either helium-3 (a lighter form of helium) or tritium (an even heavier, unstable hydrogen), losing a neutron or a proton, respectively. Finally, those nuclei would combine to form "ordinary" helium, helium-4, once again with a baryon breaking off. By the end of the fourth minute about a quarter of everything by mass* was helium, with most of the remainder being protons. Almost all neutrons either decayed or became part of helium nuclei. Very small amounts of heavier elements like lithium also got to form before the Universe cooled below 100 million K, too cold to produce any more elements. This period of nuclear fusion is called Big Bang nucleosynthesis (BBN). The Universe still contains roughly the same amount of hydrogen and helium as it did back then.

As you may have noticed, the expansion and cooling of the Universe slowed down over time. In cosmology, the rate of expansion of any decently-sized part of space is called the Hubble parameter and is measured in km/s per megaparsec of distance. In other words, for every megaparsec between two distant places, they'll recede from each other that many kilometres per second quicker. Today the Hubble parameter is roughly 70 km/s/Mpc, meaning galaxies 1 Mpc apart will move apart at ~70 km/s, galaxies 2 Mpc apart will move apart at ~140 km/s, and so on. During the Big Bang this parameter was much greater than today, but decreased rapidly as particles attracted each other, so over time they rushed apart ever slower.

As expansion slowed, so did the evolution of matter. BBN fizzled out over a few hours or days, after which the hydrogen-helium plasma simply continued gradually dissipating. Years, decades, centuries and millennia passed without much happening. Some theories, however, say there could have been some interesting events occurring in this era, such as a "Dark Bang" that created dark matter in a similar fashion to the Big Bang creating baryonic matter, only a month or so later.

This dark matter would cluster in slightly denser regions of the Universe (which, to remind the reader, were created right before the Big Bang!), making them even denser. Baryonic matter would follow its lead, condensing those regions even further. Gradually, concentrations of plasma thousands of light-years across would form, still expanding from the sheer force of the Big Bang, but fighting that expansion with their gravity.

Some 50,000 years after the beginning (T = 8000 K), the energy density of matter was starting to surpass that of light. This changed how the Universe expanded - its rate of expansion would now decrease a little slower than before, since light loses more energy than matter as it expands. In the language of cosmology, the Universe was radiation-dominated (RD) for the first 50,000 years or so and matter-dominated (MD) for about the next 10 billion years.

Some millennia before that, it was finally cold enough (10,000 ~ 20,000 K) for helium nuclei to start capturing the free-flying electrons and becoming helium atoms. First they captured one electron each at about 10,000 years, then a second one around the end of the RD era, balancing the positive charge of their two protons and becoming uncharged helium atoms. 300,000 years later (T = 3000 K) protons would capture electrons as well, becoming hydrogen atoms. This allowed photons to travel large distances without constantly bumping into those electrons and nuclei, and the Universe turned from an opaque plasma to a transparent gas. The vast amount of photons that were trapped in the Big Bang for so long were now free, and are still roaming the Cosmos to this day as a faint radio signal known as the cosmic microwave background (CMB), one of our best pieces of evidence for a hot and dense early Universe.

Space didn't turn dark immediately, though. Back when it was emitted, the CMB had a much higher energy density and a much shorter wavelength, so it would appear to human eyes as a light yellow haze. Over millions of years the CMB photons rushed apart and became redshifted, going from yellow to orange to red and then fading into infrared.

Math section

For starters, it's very easy to find the size and density of the Universe by the time it cooled to a given temperature - just divide the current temperature of the CMB (2.7255 K) by that given temperature to get the scale factor - the size then compared to now - then multiply by at least 92 billion light-years. Thus, at 10 billion K the Universe was at least 25 light-years across. It was probably much bigger than that, of course, if not infinite. To find the density, you have to divide the current average density of everything (~10^-26 kg/m³) by the scale factor cubed, since density is mass per volume:

So at 10 billion K the Universe was about half as dense as water.

But how do we know when it reached a given temperature? As mentioned far above, this can be estimated using the Friedmann equations. Those are pretty complex, though, but we can use something much simpler. During the radiation-dominated era, which is pretty much all the interesting parts of the Big Bang, the temperature is known to have fallen as the square root of the age of the Universe. Taking 10 billion K to be the temperature at 1 second, we get:

where t is the time in seconds. This works well for times up to about 1 trillion seconds (~30,000 years). The lowest you can reasonably go is a time of 10^-34 seconds and a temperature of 10²⁷ K, above which matter as we know it stops existing, so this more or less corresponds to the beginning of the Big Bang. Beyond 1 trillion seconds this equation no longer works - if you solve it for the present day (t = 4.35 × 10¹⁷ s), it gives about 15 K, which is five times too high. Over thousands of years the Universe gradually became matter-dominated, and for times between 300,000 and a few billion years the equation would be more like this:

The gap between 30,000 and 300,000 years is where the densities of matter and light were both significant, so you'd have to use the actual Friedmann equations instead of these simplified ones. The same happens beyond something like 10¹⁷ seconds (3 billion years), where the matter-dominated era transitions to the present dark-energy-dominated one. That transition is in fact still in progress - dark energy now dominates the expansion of the Universe, but matter still makes a non-negligible contribution.

Useful links

Wikipedia: Chronology of the universe, and most importantly this topic on its discussion page which led to my disillusionment with the hot Big Bang model. Too many people think we know precisely what happened in the first second.

Paper on interpreting cosmological redshift as a Doppler shift (click "View PDF" on the right to read it)