# The History of the Universe

You're going to read a lot about science on Scientopia. This post is to help you put it all in context. I'm not going to go into great depth, because that would take, well, the effort of humanity over many lifetimes. This is just a drive-by overview.

The slide below is one that I show in a substantial fraction of the talks that I give:

The first thing to notice about it is that it's a logarithmic scale. In the top line, at each tic-mark the universe is about 10 billion times older than it was a the previous tic-mark. That sounds like a lot, until you look at the labels on the tic-marks... the last mark on the top line is 109 seconds, or about 30 years.

Since the Universe is only about 14 billion years old, another factor of 10 billion past that step would take the Universe past the present day. Thus, the black circled region on the right of the top line is expanded into the bottom line, where each step is only typically a factor of 30 in age. This isn't exactly right, because really it's a step of a factor of 10 of redshift. That's what the variable "z" is in the figure, and it's very important, but I'm going to have to save it for another post. Likewise, the variable "T" along the timeline is the ambient temperature of the Universe, which also requires a lot of additional exposition, and so I will put that off to a future post.

### Here be Dragons

In the good old days, when cartographers drew maps of the Earth, they might label the places off of the edge of what was known as "Here Be Dragons". That's how I've labeled the earliest parts of the history of our Universe. At these stages, the Universe is so hot and so dense that the physics that we understand no longer fully applies. In order to estimate what was going on here, we'd need an effective theory of quantum gravity. Maybe that's what the string theorists are working on, but at the moment we really don't know what was going on in the very early stages of the Universe. That means, of course, that the Big Bang theory is named after something it doesn't even address-- the moment of Bang itself, or even if there were such a moment.

### Inflation

The earliest thing we can talk about in our Universe and have some faint confidence that we're talking about reality is this period called "Inflation". Inflation is more of a paradigm than a theory. It was introduced to answer some questions, and some more recent results have confirmed some predictions made by the Inflation paradigm, but our understanding of it remains weak. The basic idea is that there was some particle or "field" in the early Universe that has since decayed away, and that field caused the Universe to expand exponentially. In a period of less than 10-32 seconds, the Universe expanded by a factor of something like 1024. One effect of this was to take tiny random quantum fluctuations and blow them up to macroscopic size; that's the seed of all of the structure we see in the Universe today.

Because Inflation is the earliest we can really talk about, it makes sense to refer to the end of Inflation as the "beginning of our Universe". Indeed, it's possible that a whole lot went on before Inflation, and that it took much longer than the 10-34 or so seconds that my diagram gives it, but that was such a different era that it might be strange to describe the events of that era as belonging to our Universe. Indeed, you're not too far off if you think of Inflation as being the bang. Nobody will notice if you subtract that tiny fraction of a second off of the age of the Universe on all of the other times on the diagram.

### Electroweak Unification

There are four fundamental forces in the Universe: gravity, electromagnetism, the weak force, and the strong force. When the Universe was younger than 10-12 seconds old, the temperature was so high, particles were moving with enough energy, that the electromagnetic and weak forces behaved as different aspects of the same unified force, much as the electric and magnetic forces behave as different aspects of the unified electromagnetic force. As the Universe expanded and cooled off, eventually this unification was broken, and thereafter the weak and electromagnetic forces behaved as entirely different forces.

Indeed, at an earlier stage, somewhere around the end of Inflation, we expect a "Grand Unification" between the "electroweak" force and the strong force. This is not as well understood or tested as electroweak unification, however.

### Protons/Neutrons Form

Before this era, the Universe is a quark-gluon plasma. The Universe is so hot that particles move with enough energy to blast apart protons and neutrons. As such, quarks (and the gluons transmit the strong force between them) are not confined into those particles, but swarm freely about. As the Universe expands and cools off, eventually particles don't have enough energy to blast apart protons, and thereafter quarks and gluons are confined inside "baryons"-- the only two examples of which you have to deal with on a daily basis are protons and neutrons.

### Elements Form

Until about 20 minutes after the end of Inflation, the Universe remains hot enough that no elements heavier than Hydrogen can survive. A neutron might stick to a proton, making a Deuterium nucleus (a heavier form of Hydrogen), but very quickly a high-energy photon (probably), or another particle, will hit it and blast it apart. However, the Universe is expanding and cooling, and eventually particles don't have enough energy to blast apart nuclei any more. This is the stage at which the elements form.

It turns out that the Big Bang only makes Hydrogen (about 90% by number), Helium (about 10% by number), and trace amounts of Deuterium, Lithium, and Beryllium. That's it. Which begs the question: where did all the Oxygen, Iron, and so forth that we're made out of come from? The short form of the answer is: they're made in stars.

Also, somewhere around this time, the Universe becomes low enough density that neutrinos no longer interact very often with other species. The stew of neutrinos left over from the Big Bang decouple from everything else, forming a "Cosmic Neutrino Background" that people like me dream we will detect one day.

### The Cosmic Microwave Background

From the formation of the elements, on through the next several hundred years, the Universe is an opaque plasma. There are no atoms; electrons are flying freely amongst (and bouncing off of) the Hydrogen nuclei, Helium nuclei, and photons. The Universe expands, and cools, for several hundred thousand years. Eventually, it reaches a point were a few things happen. First, the Universe cools off enough that photons (and other particles) don't have enough energy to blast apart atoms that form. Thus, when a proton captures an electron, forming an atom of Hydrogen, it will stay that way. Very quickly (compared to the few-hundred thousand year age of the Universe), all the electrons get captured by nuclei, and the Universe becomes neutral. Also, at this point, the Universe becomes transparent. Before this time, photons wouldn't travel for very long before bouncing off of (probably) an electron. Hereafter, the average time between photon collisions is longer than the age of the Universe, so after their last scattering, they stream freely. We have observed these photons, encoding information about the plasma they last bounced off of a few hundred thousand years after the beginning, in the form of the Cosmic Microwave Background.

### The Dark Ages

The Universe has become transparent. There's no longer any plasma to absorb, emit, or interact with photons. There's nothing emitting light. There's nothing absorbing light. It's hard to imagine a better definition of something you'd call "The Dark Ages".

### The First Stars

Exactly when the first stars formed isn't well known, but from studies of the Cosmic Microwave Background we can determine that middle of the period where the first stars were dominating things happened around where this arrow is on my diagram. The first stars were almost certainly huge, much more massive than just about any star you see today, and were very very hot. They emitted enough ultraviolet radiation that they reionized the gas around them, knocking electrons off of Hydrogen atoms and turning the gas back into plasma. Somehow, in ways that astronomers are really just now beginning to address, these first stars seeded what would eventually be the supermassive black holes that you find at the cores of galaxies. Structure began to form. After this, we get stars, galaxies, quasars (which are galaxies whose core black holes are being fed), and all the other wonderful things that eventually lead to you and me.

### Acceleration Begins

It hasn't mattered much in everything above, but it turns out that there is this substance permeating the Universe called Dark Energy. The density of Dark Energy is (as best we can tell) constant. That is, when you expand the Universe, it doesn't spread out, but stays at the same density. Regular matter (including Dark Matter) doesn't behave this way; as the Universe expands, the matter spreads out and gets less dense. At some point something like 7 billion years ago, the matter spread out enough that the density of matter dropped below the constant density of Dark Energy. At that point, Dark Energy starts to take over the expansion of the Universe. The gravity from all of the matter (including mostly Dark Matter) has up to this point been trying to pull the Universe back on itself, slowing down the expansion that was left over from the Big Bang. Now, however, Dark Energy takes over, and Dark Energy has the odd property that its gravity causes the expansion of the Universe to speed up.

Also, around this time, you don't really see the formation of new galaxy clusters. Galaxy clusters are still getting more dense, but at this point everything that an astronomer would identify as a distinct galaxy cluster is already in place. The Universe isn't dense enough to form more of them, and the acceleration of the Universe means that they start getting farther apart faster and faster, making it harder for new structures to form.

### Sun Forms

4.5 billion years ago, an unremarkable yellow G-type star formed in the Milky Way galaxy. The vast majority of the Universe didn't really notice, but I bet you are quite grateful that this happened.

### You Are Here

13.7 billion years after the end of Inflation, you read this blog post.

• Rick says:

Sounds like and interesting tour! I await the posts...

R

• NoAstronomer says:

Could you relate T (measured in eV) on the line to T (measured in K) on the bottom line for me?

• Dan Dilling says:

Thanks for this quick synopsis, a good overview for a layman like myself. Reading it over, I did see one obvious question. Since Inflation was something that expanded the very early universe very fast, and the dark matter acceleration is causing the current universe to expand ever-faster, the two phenomena look similar if not related.
Are they clearly dissimilar due to something I don't understand (most likely)?
Are they potentially similar but no credible theory exists to link them?
Is understanding of both of them skimpy enough to make comparisons a moot point?

• Ken says:

Alright, you convinced me.

You can have my liver.

• Candid Engineer says:

Fascinating stuff. When I was a young teenager, I was convinced that I wanted to be an astrophysicist... sigh, never happened though, somehow I became an engineer.

• jimmy in brooklkyn says:

i appreciated what you wrote here
i've been trying to wrap my mind around some of these concepts
this is the first explanation of cosmic microwave background that made sense
also the dark matter explanation helps

send more like this

• rknop says:

NoAstronomer -- it's related by E = kT -- E is energy (that's what's quoted in eV), k is Boltzmann's constant, and T is temperature. Temperature tells you the distribution of kinetic energies of particles in a gas that's at that temperature. Folks who deal with very high temperatures typically stop talking about kelvins, and start talking about the typical energy available. It's in eV, because that's how you usually quote the mass energy of particles. You can compare the temperature of the Universe to the rest mass energy of a particle (or hypothetical particle) to see if it will be regularly created in collisions of other particles.

Dan -- all a very good question. The properties of the Inflaton and Dark Energy are basically the same-- they have negative pressure, and as a result their gravitational effect is repulsive. Are they the same thing? Probably not, because the exponential rate of expansion during Inflation was much, much faster than the exponential rate that Dark Energy is currently driving us towards. (We're not there yet, because the matter density is still high enough, but we will get there in time.) It's possible that Dark Energy is weak left-over Inflatons, it's possible that they're two entirely different things. We really don't know enough about either to say for sure. I also don't know myself the state of the theory, if there are theorists out there who have tried to come up with a combined model for the two. I *suspect* that there aren't any really serious efforts at that, because our understanding of both is too week.

Candid Engineer -- when I went to college, I thought I was going to be an engineer. That lasted until about the second or third week of my second semester Freshman year, when I realized that I really was a physicist....

• bedroom says:

You should really re-design that slide and make the t-axis vertical, with time going from top to bottom. You'd have enough space to put the labels and the description of events on each side, it would look less cluttered and more readable.

• jeremy says:

Win. I'm so happy you're here Rob. This will be my morning read tomorrow over coffee before work.