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.

Cosmology FAQ Part 2 (The Middle)

Recently on Periscope I did a 'Cosmology 101', a two-hour long scope where I answered the 25 most common questions in cosmology. You can find a typed up version of those 25 questions and answers, broken down in to smaller chunks, in this series of posts.

There is going to be a lot of information condensed in these posts, so don't worry if you don't understand everything the first time. If you see anything mentioned here that you would like to know more about, or that you think is unclear, let me know in the comments; that will help me see what topics you want to hear about.

In Part 1 I covered the Big Bang; the beginning. In today's post I will discuss what has happened since then. This post is going to use scientific notation; a really useful way of writing long numbers. If you are familiar with how it works, you can jump ahead and ignore this section. If you are unfamiliar with it, this is how it works:

• Big numbers. A number like 'one billion' has a lot of zeros, if we write it all out we get 1,000,000,000. Scientist love saving time, so what we do is we count the zeros, see there are nine of them, and put $10^9$. This means we have a 'one' followed by 'nine zeros'. 
• Small numbers. The principle is the same: 'one billionth' would be 0.000000001. Again, we count the zeros, but this time the zeros are before the one, so we say they are negative, and write $10^{-9}$. So we have a 'one' preceded by 'nine zeros', we add a decimal place after the first zero, and we're done!

7. How old is the Universe? How do we know?
There are two ways we can answer this question. The first involves theoretically calculating the age of the universe, using the Hubble constant (which is a measure of the rate of expansion of the universe) and the Big Bang model. Using the latest data from the Planck satellite, this model-based age is currently 13.82 billion years. This calculation assumes our models are correct. The other way of answering the question is to look at model-independent evidence. Ideally, if our model is correct, the evidence will give the same result as the theory. So what evidence can we use?

• Radioactive decay. The age of chemical elements can be estimated using radioactive decay to determine how old a given mixture of atoms is. We basically look at the composition of a rock, and use the known half-life of elements (the half-life is the time it takes for half of a sample to have decayed) to estimate the age of the rock. When applied to rocks on the surface of the Earth, the oldest rocks are about 3.8 billion years old. When applied to meteorites, the oldest are 4.56 billion years old, which tells us how old the Solar System is. Applying this method to the whole universe is problematic, as we would need to find big old rocks, but we can apply this method to specific elements, like Uranium, which is present in very old, metal poor (earliest) stars. By studying the amount of uranium and thorium (the element uranium decays into) in very old stars, we can put a constraint on the age of the universe. Specifically, physicist found an estimate of $14.5\pm 2.6$, which means it is somewhere between 12 and 17 billion years. 
• Star Clusters. There are millions of stars close to us that we can study. This means we have been able to understand a lot of properties about stars, and make a nice plot known as the 'Hertzsprung-Russell Diagram' to illustrate what we know.

  

  Hertzsprung-Russell Diagram. Credit: ESO

  Most stars belong to the 'main sequence', a family of stars that follow a specific set of rules: their luminosity (amount of light they release) and their temperature are related, the mass of a star is also related to the luminosity, and the mass and the lifetime of the star are also related. This means that the luminosity of a star can give us an approximate age of the star, using some very basic relations. The brightest stars in a cluster are the oldest, and so they give us a lower limit on the age of the cluster. Obviously, the universe has to be older than these clusters. Applying this technique, physicists found that the age of the universe is greater than 12.07 billion years with 95% confidence. Other researches found the mean age of the oldest globular clusters to be 11.5 $\pm$ 1.3 billion years.
• White dwarfs. Imagine a star as massive as the Sun, compressed in a space as small as the Earth. These are called white dwarfs, and they are formed when small stars collapse at the end of their lifetime. These stars no longer experience fusion, so they have no energy source. They still have some residue heat left, but with no energy source, they gradually cool down. This means that the oldest white dwarfs will be the coldest and thus the faintest. We know more or less what temperature the star was when it collapsed (this is known by looking at a lot of stars, and seeing what they do often). If we are able to measure the current temperature of white dwarfs - by measuring their brightness - we can calculate how long they have been burning. Our same stellar models also predict how long the first white dwarfs take to form in the universe. So by searching for faint (old) white dwarfs, and combining this with how old the universe needs to be for them to exist, we can calculate the age of the universe. With this method, the age of the universe is calculated to be $12.8\pm 1.1$ billion years.

Averaging the data from all the methods, we obtain an age for the universe of 13 billion years, with an uncertainty of 0.9 billion years, which is very similar to the age predicted by our theoretical models.

8. What has happened to the universe since the Big Bang?

Quite a lot, actually. The history of the universe is a beautiful topic, but it is quite expansive (like the universe itself!). So much has happened in the universe's history that I decided to dedicate a whole post to it, which I should post in the next few days/weeks. So here I'll just review the key points in the universe's timeline. The main idea to take away is the universe used to be very small and very hot, it has gradually been growing and cooling down, so today it's very cold and very big.

A brief history of time. Credit: ESA - C. Carreau

The earliest time we can conceive of is the Planck time: $10^{-43}$s after the Big Bang. We believe that the four forces (electromagnetic, strong nuclear, weak nuclear, and gravity) were united, but our understanding of this time is very limited. A region about $10^{-33}$cm across is homogeneous and isotropic (it looks the same in every direction wherever you look from), the temperature is T=$10^{32}$K, which is ridiculously hot.

Not long after the Big Bang ($10^{-36}$s), gravity breaks away from the other three forces, and the strong force decides to do the same, but electromagnetism and weak nuclear force are still bound together, like two inseparable friends. When the strong force breaks away, it triggers a period of inflation: rapid accelerated expansion. Everything in the universe shoots away from everything else, extremely quickly. This period of inflation ends about $10^{-30}s$ after the Big Bang. The temperature is still very hot ($10^{27}$K), but due to the quick expansion the homogeneous region is now about 1m across.

Everything we see is made of these little things.

Our understanding of the universe is much clearer from here on. After inflation, the universe is filled with a very hot quark-gluon plasma. As a quick review of the standard model; fermions - quarks and leptons - are particles that occupy space, they are what normal matter is made of. Bosons, on the other hand, are what matter uses to interact. The strong force uses gluons to interact, the electromagnetic force uses photons, and so on. As the universe gradually cools to T$ = 10^{18}$K, the electromagnetic and the weak force finally separate, and the Higgs Mechanism appears. This means that at about $10^{-10}$s after the Big Bang, the universe is governed by the same forces as today.

A millionth of a second after the Big Bang, the temperature is 'low' enough (T$ = 10^{15}$K) for quarks to clump together to form hadrons, known as mesons (two quarks), and baryons (three quarks). For an as-of-yet unknown, but fortunate, reason, there is a small difference between matter (quarks) and antimatter (antiquarks), with 100,000,001 quarks for every 100,000,000 antiquarks and 100,000,000 photons. When the temperature reaches about T$ = 10^{13}$K, quarks and antiquarks annihilate in equal parts, leaving only the small excess of quarks. The homogeneous region of the universe is now at least $10^{18}$m across. Similarly, as the temperature continues to decrease, leptons and antileptons also annihilate into photons, meaning most of the universe is made up of photons (light).

A few minutes after the Big Bang, when the temperature has cooled down to a billion degrees, nucleosynthesis takes place. This means atomic nuclei (atoms without the charged electrons) can be created via nuclear fusion: protons and neutrons fuse together to form hydrogen, helium, lithium, beryllium, and boron. The quantity of these elements is fixed at this stage, so measuring the abundance of these elements in distant galaxies is a great proof of the Big Bang theory.

CMB as seen by Planck. Credit: ESA and the Planck Collaboration

After a busy start, the universe decided to chill out for a bit. Although considering its dramatic start, who can blame it? Not much happened for the next three hundred thousand years or so. The universe got colder and bigger. And then, 380,000 years after the Big Bang, one of the most important events took place: the Cosmic Microwave Background (CMB) is released. When the temperature of the universe has cooled to a couple thousand degrees, protons and electrons combine to form neutral hydrogen. This means the photons are 'left out', and so they start to move around freely, making the universe more transparent. This CMB is everywhere in the universe, and the best part is we can measure this. This is like a window through time: by studying the CMB we can see what the universe looked like nearly fourteen billion years ago. We have put amazing satellites (COBE, WMAP, PLANCK) in orbit to measure the light left over from the Big Bang, to understand where we come from. Isn't that just amazing?

The rest of the history of the universe is less dramatic: 150 million years after the Big Bang, the first stars start to form, with heavier elements forming inside. The stars explode as supernovas, scattering these elements throughout the universe. More stars form, they clump together to form galaxies, and the galaxies clump together to form clusters, which in turn clump together to form super clusters. It's like things suddenly realised the universe was a big scary place, so they decided to all stick together.

Where do we come in to all this? Just over 9 billion years after the Big Bang, so 4.6 billion years ago, the Sun formed. Not long after, the rocks orbiting the Sun formed planets. On the third planet from the Sun, after a few billion years, lifeforms evolved enough to be sat here, reading a blog about cosmology, and probably drinking coffee. Hello, humans!

Only two questions today, but considering we just covered more than 13 billion years, I think we can call it a day. Remember to check out Part 3!

Cosmology FAQ Part 1 (The Past)

Recently on Periscope I did a 'Cosmology 101'. It was an ambitious scope in which I answered the 25 most common questions in cosmology. I had nearly ten pages of lecture notes, and it took me over two hours. It was great fun, and it motivated me to start this blog.

This series of posts will be a typed up version of those 25 questions and answers, with a few more details and comments. I'll also try to incorporate the comments made by everyone during the scope.*

There is going to be a lot of information condensed in these posts. You might not understand everything the first time, and that's okay. This is just to give you an idea of the type of topics we can discuss in this blog. If you see anything mentioned here that you would like to know more about, or that you think is unclear, let me know in the comments; that will help me see what topics you want to hear about.

So let's get started with the first 6 questions.

1. What is cosmology? How is it related to astrophysics?
Cosmology is the study of the origin, evolution and eventual fate of the universe as a whole. Astrophysics deals with the components of the universe (stars, planets, galaxies, clusters) while cosmology takes on the whole system.

2. What is the currently most accepted model for the universe?
Currently our best model is a flat, Big Bang, $\Lambda$CDM ($\Lambda$ is pronounced 'Lambda') model, which means we believe the universe originated from the Big Bang 13.7 billion years ago, it is dominated by Cold Dark Matter and a cosmological constant ($\Lambda$). The universe is flat, it is expanding, and this expansion is accelerating. All of this will be elaborated on in the following questions.

3. What is the Big Bang theory?
The Big Bang theory is the leading effort to explain what happened in the very early universe, and everything that has happened since. At its simplest, the Big Bang model starts with the universe as a singularity; the entire universe was inside a point that was thousands of times smaller than a pinhead. It was an extremely small, hot, dense something - a singularity.

This singularity went through a rapid period of inflation (extremely quick expansion), it expanded and cooled, going from very small and very hot to the size and temperature of our current universe (very big and very cold). It continues to expand and cool to this day.

There are a few common misconceptions with the Big Bang theory. First, it was not an explosion, it was an expansion; this is an important distinction (see question 6). Also, the singularity didn't appear in space; space began inside of the singularity. 'Before' the singularity, nothing existed: there was no space, no time, no matter, no energy - nothing. Questions like 'where did it come from' or 'what caused the singularity' are thus not considered in cosmology: we have no access to this information, and therefore it's not productive.

Because current instruments don't allow us to peer back at the universe's birth, much of what we understand about the Big Bang theory comes from mathematical theory and models, which leads to the question: why do we believe it?

4. What is the evidence for the Big Bang? (What is redshift? What is the CMB?)
There are many many pieces of observational data that are consistent with the Big Bang. Individually, none of these prove the Big Bang Theory, and many of these facts are also consistent with other cosmological models, but taken together these observations show that the Big Bang theory is the best model we currently have to understand the universe.

• Redshift/Hubble's law: one of the main evidences that the universe is expanding is given by the redshift of galaxies. This is similar to the Doppler effect: a sound wave will be compressed by an object moving towards us or stretched by an object moving away from us (think of the noise an ambulance makes as it comes towards us and as it moves away). The light wave from a galaxy does the same: if a galaxy is moving away from us, the light wave is stretched, moving the light more towards the red end of the spectrum. If a galaxy moves towards us, the light will be blueshifted. This redshift of light gives us information on the speed and direction that a star or galaxy is moving. By measuring the redshift we have seen that galaxies are not only moving away from us, but they are also moving away from each other. This was originally thought of by Hubble, who said 'objects in deep space have a Doppler shift associated with their movement away from us, and their velocity is proportional to their distance from us'. This is known as Hubble's law, and is expressed as $v=H_0*D$, where $H_0$ is the Hubble constant and can be used as a measure of expansion.
• Olber's paradox states that if we consider an infinitely old, infinitely large, static universe with an infinite amount of stars then looking in any direction in the night sky we should see a star, meaning that the night sky should be extremely bright. This is considering that while the brightness decreases with distance, the number of stars would increases with distance. This is obviously not what we see. This paradox is solved in two ways. First we must remember that we only see a small part of the universe, as we are limited by the finite speed of light. Secondly, if we consider an expanding universe, the light from very distant stars will be red-shifted into wavelengths that are no longer visible to us.

• CMB: the Cosmic Microwave Background is a key prediction of the Big Bang model, and the most important observation that discriminates between this and other models.

  

  CMB as seen by Planck. Credit: ESA and the Planck Collaboration

  
  The Big Bang model tells us that when the universe was very young it was dense, hot, and filled with hydrogen plasma. As the universe expanded the plasma and the radiation (photons) filling it grew cooler. When it had cooled enough, protons and electrons combined to form neutral hydrogen atoms (known as recombination). This hydrogen stopped interacting with the photons. Imagine there are three friends together; a proton, an electron and a photon. If the electron and the proton start dating, the photon would leave to stop being a third wheel. These photons started to travel freely through space (decoupling), making the universe more transparent. 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).
  These photons have been redshifted so much that with a traditional optical telescope, the space between stars and galaxies is completely dark; however a sufficiently sensitive radio telescope shows a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum, hence the name CMB. The accidental discovery of the CMB in 1964 by Penzias and Wilson led to a Nobel Prize in 1964.
  The photon decoupling happened 380000 years after the Big Bang, so by observing this light we can study the early universe.

• The cosmological principle states that the universe is homogeneous and isotropic. Homogeneous means the universe should look the same at all points: wherever you are, you should see the same. Isotropic means it should look the same in every direction: wherever you look, you should see the same. This means there is no privileged point; the universe does not revolve around us. Data show us that our vantage point is not special and the universe looks the same in all directions to 1 part in 100,000.
• Abundances of lighter elements: as the universe expanded and cooled down, some of the elements that we see today were created, namely Lithium, Beryllium and Boron, in a process known as nucleosynthesis. The Big Bang theory predicts how much of each element should have been made in the early universe. If we measure distant galaxies (and because the speed of light is finite, the further away something is, the older it is), we can measure the abundance of these elements before much more was created. The results of these measurement match the predictions made by the Big Bang model.

5. What came before the Big Bang?
There are different ways we can approach this question, depending on how we view the early universe, but we reach the same conclusion.

Many people, such as Hawking, believe that time began at the Big Bang. This means that time didn't exist until then. When we say 'before', we need a time scale. But if time didn't exist, it doesn't make sense. It's like asking what comes after the end.

Some people, however, think this stance is too harsh, and argue that we can't know what happened on such small time scales, as our understanding of the universe breaks down when we go to the Planck scale (smallest time and highest energy we can conceive of), and we would need a theory of quantum gravity. In this perspective, we can't affirm that time didn't exist before the Big Bang, but we can instead say that anything before the Big Bang would have no impact on the universe we live in (we say it would be causally disconnected) - it's information that doesn't affect us, and which we don't have access to. So again, it's not something we can answer with today's cosmology.

Finally, in some models of the multiverse and 'perpetual inflation', the Big Bang is just one of many inflating bubbles in a spacetime foam. In this scenario, time could exist 'before' the Big Bang, but we wouldn't be able to get information from outside our own bubble.

In all cases we reach the conclusion that this is not a question we can address using the current cosmological models.

6. Where was the centre of the Big Bang?

This is an explosion, the Big Bang was not

This brings us back to the difference between expansion and explosion. We all know what an explosion looks like. Imagine a firework, radiating outward from one point: it has a clearly defined centre. An expansion is different in the sense that every point sees itself as the centre. At any point of an expansion, you would see everything moving away from you. If every point is its own centre, there can't be only one centre. This ties in with the cosmological principle: there is no centre because all positions in the universe are equivalent; the universe is homogeneous.


I think that's enough for now. Be sure to check out part 2.

The UCLA has a nice (but outdated) FAQ about the universe. See it here.

*Due to the live nature of Periscope, it is unavoidable that I will sometimes make mistakes. If at any point you notice a discrepancy between what I said live and what is written here, trust what is written here. I will try to keep the information in these posts as accurate and up-to-date as possible.