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!