The Big Bang Model

The Big Bang Theory is one of the pillars of modern cosmology, so it's about time I dedicated a blog post to it. The Big Bang (BB for short) is many things, but before we get into that, it's useful to first remember what it isn't.

What it isn't
A TV series. There is no laugh track included in this blog post, I'm afraid.
On a slightly more serious note, one of the most common misconceptions about the BB is that it's a theory about the origin of the universe. The BB model is often accused of "not explaining what caused the bang, or what came before the bang", but the Big Bang theory never claimed to be able to explain these things; that's not its purpose. That's like asking a coffee machine to make you toast. If you do want toast (or an answer to the question of what came before the Big Bang), you might find something useful on the FAQ post. No toast here though, only coffee.
Another misconception about the BB model is the "bang" part. Despite the misleading name, the Big Bang model does not involve an actual bang; there was no initial explosion, but there was - and still is - an expansion taking place everywhere in the universe at the same time.

Okay, so what is it?

13.8 billion years of history in one image.

The BB theory explains the evolution of the universe from an initial singularity until today, and makes predictions for what we can expect in the future. That means the BB model covers more than 13.82 billion years of cosmic expansion, which is rather impressive. In very early times, the universe was in a really hot, really dense, and really small state. We refer to this as the initial singularity, or "Big Bang". The theory doesn't start at this singularity, though, as a singularity is just a fancy way of saying "our equations break down at this point". Our model begins a fraction of a second after this initial singularity, and moves forward with it. The BB theory explains that the universe has been gradually expanding and cooling down, going from this hot, dense, and small initial state to the massive, complex, and rather chilly place that it is today.

This cooling down and expansion is a lot more significant than it might sound, though. As the universe cooled down, the first particles could begin to form. Initially, the universe was so hot that everything was a bit chaotic: the particles were constantly interacting, but not forming lasting bonds. Dark matter particles, always the more elusive of the bunch, broke away at an early time and went their separate way to start setting up the scaffolding for future structures.
Gradually, the temperature cooled down enough for "normal" (baryonic matter) particles to bind together and form the first nuclei. First came hydrogen nuclei, and through Big Bang Nucleosynthesis - which is very similar to what happens inside stars - heavier nuclei like Helium and Lithium could form. With more expansion and cooling down, electrons could combine with the nuclei to form the first atoms - which also caused the photons (particles of light) to break away, to later become the cosmic microwave background radiation.

Filaments of dark matter on large scales, with galaxy clusters
at the intersections. From Springel et al, Millennium Simulation

These initial hydrogen atoms could coalesce into the first stars, and with enough stars in one place, the first galaxies could begin to form, and later galaxy clusters. But these galaxies didn't just form anywhere; the dark matter that broke away at an earlier time had a strong gravitational effect, causing the stars and galaxies to form in a giant structure we call "cosmic web". All of this could happen thanks to the expansion and cooling of the universe, and all of this is described by the Big Bang theory.

Why should I believe it?
As any good scientific theory, the Big Bang model has a lot of observational evidence backing it up. The expansion of the universe was first measured by Hubble in the early 20th century. Hubble noticed that distant galaxies move away from us faster than closer galaxies. Since then, we have increasingly precise measurements of the movement of galaxies, which all seem to indicate an expanding universe. We've also been able to measure that this expansion is accelerating, but that's a whole other blog post.
Furthermore, the abundance of the light elements (like Lithium and Helium) should be set by the Big Bang Nucleosynthesis, and can be calculated. We can measure the amount of these light elements in really distant galaxies in early stages of evolution, where there hasn't been enough time for more of these elements to form. The results of these measurement match what we expect from the BB model.

But a theory is not complete without predictions. The biggest prediction made by the Big Bang model is the the Cosmic Microwave Background radiation (CMB), which is a bath of photons everywhere in the universe, made up of the photons that broke away when the first atoms were formed. These photons have been propagating freely ever since, growing fainter and less energetic due to the expansion of space (if their wavelength increases over time, their energy decreases). The accidental discovery of the CMB in 1964 by Penzias and Wilson, which led to a Nobel Prize in 1964, was the key piece of evidence which made the Big Bang model the prevailing cosmological theory.

Not perfect, but the best we have
With all the evidence we have, we know that the Big Bang theory is the best model we currently have to understand the universe. However, despite its overwhelming success, there are a few aspects of the universe that seem at odds with the Big Bang Model.

• Horizon problem: The photons of the CMB have the same average temperature of 2.73 Kelvin (-270 Celsius), varying by no more than one part in 10000. This means that if we measure CMB photons coming from opposite directions, they will be at the same temperature, despite being separated by more than 28 billion light years. As nothing can travel faster than the speed of light, these photons have not been able to communicate in any way, and given how far apart they are now, they weren't in causal contact (close enough to affect each other) in the early universe. This poses the question of why are all of the photons at the same temperature? 
• Flatness problem: We have very good measurements on the shape of the observable universe, and our latest data show us that the universe is flat (if you are wondering what we mean by a flat universe, check out question 18 in the FAQ page). The shape of the observable universe is given by its density, and this could take any value, yet of all the possible values, the density is exactly the one that is needed to have a flat universe. This seems like a fine-tuning issue, and rises the question of why is the universe so flat?
• Monopole problem: Electricity and magnetism present a certain duality, and can in fact be grouped together under the umbrella term "electromagnetism". We have single electric poles -or charges - in the form of electrons. However, no single magnetic charge - or monopole - has ever been found, despite being predicted by several particle physics models. Of course, it could be that the models predicting these magnetic monopoles are wrong, but it is also interesting to wonder if there is a deeper explanation to why we haven't yet found these monopoles. 

Given these questions, what do we do with the Big Bang model? Unanswered questions are usually not a good thing in scientific theories. But don't worry, the Big Bang theory is safe: we know how to solve these issues! We just need to introduce something known as inflation, which will be the focus of a future blog post.