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!

Gravitational waves

"Ladies and gentlemen, we have detected gravitational waves. We did it." Dave Reitze (Executive director of LIGO)

Well that's awesome. What a day, what a moment to be alive. Today it was officially reported that the LIGO collaboration has detected gravitational waves. You've probably seen scientists everywhere shouting with joy. And we have every reason to be happy. 

A few weeks ago, rumours started circulating about a potential discovery of gravitational waves. I really think rumours should be kept out of science, but the good side of those rumours is that they got people talking. I've discussed this topic quite a few times one Periscope, you can watch the most recent one here.

Just to get you started, this video was released a few days before the announcement of detection, and I think it's great. It provides a quick three-minute summary of gravitational waves. This video is like a trailer to my post - you can get an overview, but do stay for all the juicy details.

Now that we've seen an overview, let's try to understand everything going on here.

Massive objects curve spacetime.
Simulation made by C. Joana.

What are gravitational waves?

Einstein's theory of general relativity tells us that space and time are not two separate concepts: they are intrinsically linked - spacetime. You can't have one without the other, they are two sides of the same coin. Furthermore, Einstein's theory says that gravity is just a curvature of this spacetime. Imagine you have a big sheet and you put a ball in the middle: this will curve the sheet (or spacetime), causing other objects to move towards it, following the curvature of spacetime. This means that objects will travel in a straight line on a curved surface. While this might seem strange, it's a phenomenon that we encounter daily: if you take a flight from Los Angeles to Berlin, the plane will pass over Greenland, even though if you look at a flat map this seems like a longer, curved route. If you look at a globe, however, you will see that this is the shortest path (and therefore the straightest) on a curved surface.
Einstein summarised his theory as 'massive objects tell spacetime how to curve; spacetime tells objects how to move'.

This brings us to the idea of gravitational waves.

Gravitational waves are ripples in the fabric of spacetime. Imagine you are in a pool with a friend, and you start dancing in circles around each other. You would notice ripples forming around you and moving outwards. The same thing happens in spacetime. Like all waves, these waves have an amplitude (height of wave), a frequency (how often the crests pass us by), a wavelength (distance between the crests), and a certain speed. The first three are determined by the source of the wave. Therefore, if we are able to measure these characteristics, we could get information about the source. The speed, however, is fixed: gravitational waves move at the speed of light. This should not be surprising as the speed of light is the speed at which 'spacetime talks to itself'.

When Einstein came up with his theory in 1915, he made many predictions: the perihelion precession of Mercury's orbit (the closest point between the Sun and Mercury changes every year), the deflection of light by massive objects (light travels in a straight line on a curved surface), the gravitational redshift of light (light waves are stretched or compressed by gravitational fields), and gravitational waves. The latter was the only prediction from general relativity we hadn't been able to observe. The final proof of Einstein's theory had eluded us. Until now.

What could cause them?
The mathematical formulas from Einstein's general relativity tell us that any massive accelerating object would disrupt spacetime, sending ripples through the universe. So what type of objects accelerate?

• Two objects orbiting each other, like two neutron stars or black holes, will emit gravitational waves.
• Any non-spherical planet or moon; like a planet with a big bump. If it looks more like an American football than a 'rest-of-the-world' football, it would emit gravitational waves.
• A supernova will probably create gravitational waves, unless it explodes in a perfect sphere, symmetrical in every direction.

Just to be clear, we can see some examples of objects that would not create gravitational waves:

• A lonely, non-spinning, solid object, moving at a constant speed and not interacting with the world around it will not generate gravitational waves. This is what we like to call 'conservation of linear momentum'.
• Flat objects, like disks (think galaxies) will not radiate gravitational waves. If you spin a pizza base on your finger (a skill I am yet to master), the pizza will not emit gravitational waves. This is because of something we like to call 'conservation of angular momentum'.

Simulations have existed for years to show us what the merging of two black holes would look like, as well as the gravitational waves they would produce. Like this one, from NASA's Scientific Visualization Studio:

What effects do these waves have? What would they look like?
These small, circular, ripples in spacetime would present themselves as small changes in the distance between two objects. The waves would stretch and compress spacetime; first in one direction, then in the other. If we had particles arranged in a cylinder, this is what we would see:

Here the wave is moving through the cylinder, from back-right to front-left. (A more detailed explanation of this image can be found here.)
How noticeable is this effect? It's really small. Ridiculously small. Even the most powerful gravitational waves (like those that would form in the merging of two black holes) would only change a length by a factor of $10^{-21}$ (zero point twenty zeros and a 1). That's a difference of one thousandth billionth billionth. If this wave went through you, it would change your height by one millionth billionth the width of a single hair.

From a physical point of view, one of the main effects of gravitational wave is that they carry energy away from the source. This means that the waves carry a lot of information about the source objects. In the case of two bodies orbiting around each other, this causes their orbits to decrease: the two objects get closer.

You might be wondering at this point if the Sun-Earth system emits gravitational waves. The answer is yes, it does. This means that the Earth and the Sun are gradually getting closer. Fortunately, gravitational waves become more significant as you increase the mass of the objects, and decrease the distance between them. The Earth and the Sun are really small (on cosmic scales), and quite far apart. The effect of the waves produced in the Earth-Sun system is the Earth gets $10^{-12}$ meters closer to the Sun every year. That means in $10^{12}$ years (a trillion years), we would be one metre closer to the Sun. Considering that is more than the current age of the universe, it's not something we should worry about.

This decrease in orbit has led us to see indirect evidence for gravitational waves before. There is a system of two orbiting stars, known as the Hulse-Taylor system, that we have been able to observe for more than thirty years. Physicists R. Hulse and J. Taylor measured the decay in their orbit: the gradual decrease in the distance between the two stars. This decrease is directly related to the energy carried away by gravitational waves. They were able to show that the decay in the orbit and the decay predicted by Einstein's general relativity were in agreement with less than a 0.2% error. They were awarded the 1993 Nobel Prize in Physics for this indirect detection.

How can we directly detect them?
We can infer their existence from decaying orbits, but we would like to see them directly. So how do we detect something so small and unnoticeable? With lasers!

Recently, Julia Majors PhD (@Feynwoman), everyone's favourite laser physicist, wrote a great article explaining the LIGO set-up and how they used lasers to find these tiny waves. You should definitely read it.

The ideal way to detect gravitational waves would be to measure the small changes in distance between two points. But this presents a problem: a gravitational wave would also make our ruler longer or shorter. We need something that would not change with the gravitational wave. And so we go back to Einstein. His theory of relativity shows us that the speed of light is constant, it would not be affected by gravitational waves. Speed is defined as distance travelled over the time of travel. If the speed of light is constant, changes in the distance of travel would change the time it takes light to cover this distance. All we need to do is use light as a stopwatch.

Enter LIGO, the Laser Interferometer Gravitational-Wave Observatory. LIGO is an experiment currently running in the USA, designed to find gravitational waves using lasers. Like any interferometer (device that measures the interference of light), it relies on constructive and destructive interference.

The red wave is the result of adding the two blue ones together

When two waves overlap, different things can happen. Let's assume the waves have the same frequency and amplitude, as seen in the picture on the right. If the waves meet with the crests overlapping, we have constructive interference, and a bigger wave is created. If, on the other hand, the waves meet with the crests of one wave overlapping with the valleys of the of the other wave, we have destructive interference, and the waves annihilate.

LIGO starts by sending a laser beam to a 'beam splitter', where the light is split in two and sent down LIGO's two arms, each 4 km long. This light is then bounced off of mirrors and sent back to the start, where it is recombined. It is set up so that the beams coming back should produce destructive interference: the light waves cancel out and no signal is produced. However, if a gravitational wave passes through LIGO, it would stretch or contract one of the arms, causing a difference in distance. A difference in distance implies a difference in time: the returning light waves would no longer by synchronised, creating interference that can be measured. If this sounds confusing, don't worry; LIGO made a nice video showing this effect:

Wow, what an incredible system. We use lasers, bounced off of mirrors, to detect minuscule differences in distances, caused by a wave created thousands of miles away. Human ingenuity has no limits.

You might be thinking that such a small difference could be caused by any number of things. What if a train goes past and vibrates the laser? What if there is an earthquake? Wouldn't that create background noise? LIGO has you covered: they made two detectors; one in Louisiana and one in Washington. They are separated by 3000 km, and they serve as a check. A real wave would be detected in both places, with a difference of ten milliseconds (the time it takes the wave to travel that distance), so we can discriminate between real waves and noise. Take that, background noise!

What has LIGO found? 
LIGO had its first run between 2002-2010. They didn't detect anything in that time (not surprising, due to the low sensitivity), so they shut the machine down for five years. In that time, they upgraded everything; better lasers, smoother mirrors, better measuring devices. They switched the Advanced LIGO (we are really not imaginative when it comes to naming our experiments) on in September 2015, with high expectations. In February 2016, they called a press conference, and everyone got excited. Had they finally found the elusive gravitational waves predicted by Einstein a hundred years ago?

Not ones to beat around the bush, LIGO representatives opened their press conference with a dramatic 'We did it.' They found the waves. They had really seen them. This is the official plot they released:

Now we've had a hundred years to prepare for this moment. Physicists have made lists of what interference pattern gravitational waves from different sources would create. And now we have something to compare it to. LIGO compared their signal with all of the predictions (notice the white line in the plots listed as 'prediction'), and found that their signal (received in both detectors, as expected) was compatible with the merging of two black holes. Two black holes, with masses 36 and 29 times the solar mass, merged to create a black hole of 62 times the solar mass. The three missing solar masses were radiated away in the form of gravitational waves. They released a nice simulation showing what they think happened:

Let's just think about this for a moment. 1,3 billion years ago, 12 thousand million million kilometres away, two black holes started a deathly dance, resulting in them merging and sending waves throughout the universe. In September last year, these waves were detected by humans using lasers and mirrors. A hundred years ago, Einstein came up with a way of understanding the universe. He made lots of predictions, and every single one of them has come true. General relativity has stood up to every test we could think of.
That is truly amazing.

For many years mankind has studied the universe. The universe talks to us in different ways, but until now we only listened to one main form of communication: the electromagnetic spectrum. We now have a new way to listen, to uncover the secrets of the universe: gravitational waves. We are the first humans to ever be able to say this. This is a great moment for us.

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.

Welcome

Hello and welcome to 'A Cup of Cosmology'

Cosmology is the branch of physics that aims to study the origin, evolution and eventual fate of the universe and the large scale structures it contains. It is a fascinating and intriguing subject that endeavours to answer some of the biggest questions we can ask: 'how did the universe begin?', 'how will it end?', 'are we alone?'.

Mankind has always been driven by the need to know more and some of the oldest known civilizations already looked to the stars and the cosmos to answer their questions. As science and technology have advanced, cosmology has become more and more sophisticated: we now have equations to explain phenomena our ancestors could never even dream of, we have credible and testable theories about the origin of the universe, we have technology that allows us to probe the furthest and darkest realms of the universe looking for answers. And the more we find out, the more we realise we have yet to learn.

Somewhere along the way it seems cosmology stopped being accessible to all and became a secluded discipline, reserved for "those crazy physicists" who understand the equations. This is the case with many sciences; scientists communicate in their own jargon, scientific papers are expensive and difficult to access, and people are often scared away from the world of academia. But understanding the universe is something everyone wants to do, and everyone should be able to. All you need is a bit of patience, a lot of curiosity, and a willingness to learn.

I'm a theoretical physicist: one of "those crazy physicists" who tries to understand the universe. I spend my days battling with equations, writing codes and simulating the universe on my computer. And drinking lots of coffee. There really is a lot of coffee involved in science. Some of you might already know me from my weekly Periscope discussions about the universe. 

This blog is aimed at anyone who likes cosmology; whatever your level is I hope this blog will be useful and ideally enjoyable. I will try to explain cosmological concepts in a clear and concise way, without assuming any background knowledge.
If you are new to cosmology, I hope to provide you with an understandable introduction to one of the most intriguing fields of science. If you are well-versed in the subject, perhaps these posts will be an entertaining review.

I aim to cover a range of topics; cosmology allows us to probe the whole universe; everything from the origin of the universe to black holes, galaxies, structure formation, dark matter, the death of the universe, and everything in between. Even hypothetical concepts like time travel, wormholes, and parallel worlds can be discussed in the framework of cosmology.

I will try my best to keep this blog alive, but I can't promise daily updates, or updates every 'x' days. I commit to publishing new posts often, but I am purposefully leaving the definition of 'often' up to interpretation.

Science is awesome; it's rewarding, exciting, tiring, complex, and fun. But most of all science is understandable. Let's understand it together.