Cosmology FAQ Part 4 (The End)

As you've probably already heard by now, I recently did a 'Cosmology 101' on Periscope, a two-hour long scope in which I addressed the 25 most common questions in cosmology. Don't worry if you missed it, you can just read these posts.

This is the final part in this series. You can read the previous sections on the Big Bang, the history of the universe, and the present state of the universe. Today we'll answer the big questions.

As always, don't worry if you don't understand everything in this post. This is meant as an overview to give you a general idea of what we do in cosmology. I encourage you to let me know if you see anything mentioned here that you would like to know more about, or that you think is unclear.

19. What is a black hole?
If you throw a tennis ball upwards, you know it's going to fall back down. This is because the gravitational force pulls things together. But if you were able to throw the ball fast enough, it would have enough energy to escape the gravitational pull of the Earth and launch into space. The speed needed to beat the gravitational force of an object is known as the escape velocity. A black hole is an object whose escape velocity is larger than the speed of light. This means that not even light is not travelling fast enough to escape the gravitational force of a black hole. And as we know, nothing can travel faster than light in a vacuum, which means nothing can escape a black hole.

But don't worry; we (probably) aren't going to be swallowed by a black hole. This is because the gravitational force decreases quadratically with distance: if you get ten times further away from an object, the gravitational force is a hundred times weaker. As such, a black hole has an event horizon: an imaginary line that marks the point of no return. If you are further away than the event horizon, you can still escape the black hole's gravitational pull. But you really don't want to cross that horizon: once you do, there's no coming back.

Simulation of a black hole, by Alain Riazuelo

Physically speaking, a black hole is generally formed by the death of a star: when a big enough star reaches the end of its life it can explode as a supernova, leaving behind a really massive - but quite small - object. If two black holes collide, they can form a bigger black hole. 'Normal-sized' black holes are generally between 5 and 35 times the mass of the Sun. And then we have their older brothers: the supermassive black holes, which can be million times more massive than the Sun, and play a really important (but not fully understood) role in the formation of galaxies.

Mathematically speaking, black holes are regions where the curvature of spacetime becomes infinite (Einstein taught us that this curvature is what causes the gravitational force). These regions are known as gravitational singularities, which leads to an important question...

20. Was the Big Bang a Black Hole?
You've heard me before describe the Big Bang as a singularity, and I've just described a black hole as a singularity, so it is logical to ask if they are the same thing. But actually, the Big Bang was nothing like a black hole. The main difference here is we believe the Big Bang was a singularity, with time and space existing inside the singularity. So the Big Bang was a singularity of space and time, whereas a black hole is a singularity in space time.

21. Is our universe unique? What is the multiverse?
There are many different theories in physics that support the existence of multiple universe. Some people claim there are an infinite number of universes, each with different laws of physics, and we just so happen to live in the region which has the conditions necessary for us. Other people claim that when the universe inflated, it did so in little 'pockets' with each pocket universe evolving with a different amount of inflation, leading to very different universes. In any case, we have no way of accessing these postulated alternative universes, or obtaining any information from them. Therefore, it is not a productive question: it deals with something we don't know nor have any ability to know, and that is not likely to change soon. It is an interesting thought exercise, but it's not really a useful question from a scientific point of view.

22. What is dark matter?

This is one of the biggest open questions in physics. We are fairly sure that 25% of the universe is made up of dark matter, but we don't yet fully understand the nature of this mysterious matter. It's a type of matter that doesn't interact with the electromagnetic force; this means it doesn't emit, absorb, or reflect light, so we call it 'dark'. We are also quite sure it doesn't interact via the strong force, which means it doesn't form big clumps: the dark matter particles don't bind together. It does, however, interact via the gravitational force. And this is how we know of its existence.
There are several main evidences for the existence of matter we can't see. One such example is the rotation curves of galaxies. In a normal galaxy, we would expect the stars further away from the central bulge to move slower; in the same way that Neptune moves around the Sun slower than the Earth does. However, when we take enough measurements of galaxies, we actually see that the outer stars are moving at the same speed as some of the closer stars. The following plot shows the velocity (vertical axis) and distance (horizontal axis) of the stars in a galaxy.

The only way we can explain this with our current laws of physics is by assuming there is more mass located in the halo of the galaxy: a huge amount of matter that we can't see.
Other evidences for dark matter include merging of galaxies, and weak lensing, which I'll elaborate on in a future post, but the idea is the same: if our theories of gravity are correct, the visible matter in the universe is not enough to explain the observed phenomena. There are hundreds of experiments currently trying to find what we have not yet been able to see.

23. But what if we are wrong about gravity and DM is not real (MOND)?
There are some physicists who believe that instead of assuming dark matter is real, we should assume our laws of gravity are wrong. There are some who argue that instead of looking for 'invisible' matter, we should instead modify our theories of gravity (MOND). While these models are able to make some predictions, they can't explain observations we have made in galaxy clusters, nor can these models be used (yet) to build a complete cosmological model. They have often been described as being 'ad-hoc', with more elements added to an ever-complicated model to try to fit the evidence. These models have lost favour in recent years, but some people still pursue them.

24. How will the universe end?
There was recently a great review about this topic on the BBC. There are several different possibilities as to how the universe might end, although the data seem to favour the first two.

• Big freeze/heat death. The expansion of the universe is currently accelerating. If this carries on, the universe could expand forever. This means everything would gradually grow further and further apart, while cooling down and approaching absolute zero. Entropy would reach its maximum, which means there would be no exchange of information nor exchange of heat. Everything would just freeze. In this scenario the universe ends up very cold, dead, and empty.
• Big rip. If the acceleration of the universe increases, perhaps caused by an even stronger type of dark energy, the rate of acceleration could increase so much that it could overcome the pull of gravity on ever smaller scales. As a result, all material objects in the universe, starting with galaxies and eventually all forms of mass, no matter how small, will be ripped apart; reverting back to unbounded elementary particles and radiation, shooting away from each other.
• Big Crunch. This is a very nice, philosophical view of the universe. In this scenario the universe is cyclic; it begins with a Big Bang, pushing everything outwards. At some point, this expansion will stop and go the other way; collapsing back inwards, causing everything to crash together until a singularity is formed - which would then be the singularity for a new Big Bang, a new universe. Like a phoenix rising from the ashes of an older universe. This is a scenario that many people like, unfortunately the universe doesn't care what we like: the current accelerated expansion of the universe seems to disfavour this model. 
• False vacuum. A lot of things in the universe like to be in their 'least energetic state': the configuration that needs less energy. This is known as minimum potential energy principle, and it affects diverse things like electrons in an atom, atoms in a crystal, a marble in a bowl, and those lazy Sunday mornings when we just want to stay in bed and not spend any energy. Going back to the idea of the multiverse, if our universe is just one among billions of expanding bubbles, we could imagine that there are universes out there at a lower energetic state than ours. This would mean our universe is in a 'false vacuum': a local minimum of energy, but not a global one. If our universe came close to one of these other pocket universes, we could 'collapse' to their lower energy level. Imagine if you are heading to the gym but your friends are planning to spend the day sunbathing on a beach. It's quite likely that you would skip your gym plans and adopt their plan of 'least energy'. We can't really fault the universe for doing the same. If this happened, it could fundamentally alter our universe by changing some constants of nature, or even the very nature of space and time. Structures could be destroyed instantaneously, without any forewarning. As scary as that may seem, by studying the particles in our universe physicists believe this couldn't happen for at least a couple of billions of years.

In any case, the Sun will expand in four or five billion years and probably destroy the Earth in the process, so it's unlikely we'll be around for the end of the universe.

But don't worry, we live in a universe with puppies, chocolate, and coffee, so we don't need to concern ourselves with something so many years into the future.

25. How can I become a cosmologist? What good books do you recommend to get me started?
There is still so much we don't know about the universe; we can only see 5% of it, and we probably understand much less. Cosmology is a really active field, with a lot of discoveries waiting for us. If you are considering a career in cosmology, I strongly recommend it. To become a cosmologist first you need a Bachelor/undergrad in Physics, and take as many maths courses as possible. Follow up with a postgraduate in Astrophysics or Cosmology: some countries allow you to directly do a PhD, others require you to first do a Master's. In either case, the postgraduate studies will take about 5-6 years. All in all, to become a Doctor of Physics, you need about 9-10 years of study. It is worth it.
Also, go to as many seminars as possible, try to read scientific articles, and don't be afraid to email the authors if you don't understand things: they are generally happy to discuss their work!

If you want to get involved without all the studying, there are plenty of citizen science projects you can help out with.

Finally, if you want some good books on the topic, check out this list. If you know of any books I forgot to include in the list, let me know!


We did it! We got through the 25 most common questions in cosmology. I will elaborate on most of these topics in the future, but for now we have a good starting point.

Cosmology FAQ Part 3 (The Present)

Some time ago, in a not-too-distant place, I did a 'Cosmology 101' on Periscope, an exciting endeavour in which I tried to answer the 25 most common questions in cosmology. It was long, it was dense, and it was great fun. If you missed it, don't worry, you can read this series of posts, which will cover the same topics.

In Part 1 we covered the Big Bang; the beginning. Part 2 dealt with what's happened since then: 13.8 billion years reduced to one post. Today we will discuss what the universe looks like now.

This series of posts will have a lot of information. If you don't understand everything the first time, don't worry. 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 give me ideas for things to talk about or write about in the future.

So what is the current state of out universe?

9. What is happening to the universe now? 
Ever since the Big Bang the universe has been growing and cooling down. Our evidence shows us the universe is 13.8 billion years old, and the current temperature of the CMB (Cosmic Microwave Background) is T$=2.725$K, which is -270° Celsius, and if you use the weird scale, it's -455° Fahrenheit (seriously though, why do you use this scale?).
Our best model for the universe is the $\Lambda$CDM model. This means the universe is governed by dark energy ($\Lambda$ - pronounced 'lambda'), and cold dark matter. When we say cold dark matter we mean the dark matter is moving slowly - so slow that we don't need to consider relativistic effects. The most recent data tell us the universe contains 5% normal matter, 25% dark matter and 70% dark energy. And that leads us to a very obvious question.

10. What is dark energy, and what is the cosmological constant?
We are not sure what exactly this dark energy - or cosmological constant - is. We are very sure the expansion of the universe is accelerating (I'll come back to this), and we know something must be causing everything to go outwards, despite the attractive pull of gravity, which means it has to have negative pressure. The first person to come up with a valid, and testable, explanation for dark energy could win a Nobel Prize.

The Einstein Field Equations, with $\Lambda$

Historically, the cosmological constant dates back to Einstein. When he formulated general relativity, the universe was thought to be static: neither growing nor shrinking. If this was the case, why didn't the universe collapse due to the gravitational force? Einstein's response was to put a cosmological constant in his equations.

Turns out this guy was right

This fixed the issue, but a few years later the expansion of the universe was discovered, meaning the cosmological constant was no longer needed. Einstein removed it from his equations, and often referred to it as his 'biggest blunder'. In the 1990s it was seen, however, that the expansion of the universe was accelerating. It wasn't just expanding, it was doing so quicker than before. This meant we once again needed some form of 'anti-gravity' in our equations, and so we put the cosmological constant back in. Seems like Einstein was right again.


11. What do we mean by expansion? Are objects moving away from us, or are our definitions of length and time are changing?
By expansion we mean that everything is moving away from everything else; the distance between galaxies is increasing (see question 14).

Case 1: Grid points get further apart

We could consider expansion from two different points of view. Imagine we draw a grid on the universe, and mark galaxies at different points. In our first scenario we fix the galaxies at specific grid points, and these grid points get further apart, causing redshift as they do. If before there was 1 million lightyears between the galaxies, after a certain amount of time there are 1.5 million lightyears.

Case 2: Grid points are fixed

In our second scenario, we keep the grid fixed, and let the galaxies flow freely. As time passes, we notice the galaxies move away from each other, also causing redshift. Like before, the distance between the galaxies has increased (and if my two drawings are correct, you can see that the increase in distance is the same in both scenarios). Both of these points of view are equally valid, and in fact general relativity explains how to transform from one view to the other, and the observable effects like the redshift are the same in both views.

Notice that in the above example, in both scenarios the definitions of length and time are not changing: one small square on the paper is always 0.5 million light years. This is not like when we changed our definition of a planet to kick Pluto out; the definitions of length and time are constant. The distance between two galaxies, however, is not constant: it is gradually increasing.

12. Why do we think that the expansion of the Universe is accelerating?
Imagine you are looking at a 60 watt light bulb. The light bulb always emits the same amount of energy, but if you walk away from it, you notice it gets fainter and fainter. This is because the amount of light you see depends on the distance, even though the amount of light emitted by the bulb is the same. With this idea, you could calculate the distance to the light bulb based on how much light you see. In astronomy we do something similar; we look for objects that emit a well-known amount of light, which we call standard candles. Specifically, we use supernovae: the violent deaths of stars. The amount of energy emitted by a supernova is extremely well know. This means that if we measure how bright the supernova appears to us, we can measure its distance from us, the same as the light bulb.

Redshift and apparent magnitude of supernova. Credit: U. Alberta

The next thing we do with these supernovae is measure their redshift (using their spectral lines), which tells us how much the universe has expanded since the light left the supernova. If the expansion of the Universe is accelerating, the expansion was slower in the past, and the distance to a supernova now would be larger than it would be in a non-accelerating case. If the expansion is decelerating, it was faster in the past and the distance now would be smaller. What we found is that in far away galaxies supernovae appear fainter than expected; therefore they are further away than they would be if they were moving at a constant speed.
If you are travelling in a car at a constant speed of 50 kilometres an hour, after one hour you should be 50 kilometres away. However, if you are 80 kilometres away, it means at some point you increased your speed: you accelerated. The same is true for the universe: if the distant galaxies are further away than expected, they must have accelerated.

13. Can objects move away from us faster than the speed of light?
As always when relativity is involved, the first thing we have to ask is 'from whose point of view?'. Things are relative, after all. Special relativity tells us that two objects cannot pass by each other with relative velocities faster than the speed of light. This is fairly straightforward, the problem comes when we try to apply this to objects very far apart. Then we have to ask who is measuring the distance between the objects, who is measuring their velocity, and if this observer is moving. I'll dedicate a whole post to special relativity in the future. To answer this question, let's assume that the distance to a galaxy is the distance between us at a specific time, measured by a set of observers moving with the expansion of the universe (so in the rest frame of the moving galaxy), and all making their observations when they see the universe as having the same age (the age of the universe also depends on who is looking). In this specific case, velocity of this receding galaxy can definitely be larger than the speed of light. This does not contradict special relativity, because we are not talking about the relative velocity between two objects as they pass each other.

Related to this question, we could ask if the universe is (or was at some point) expanding faster than the speed of light. The short answer is no, the long answer can be found on Sean Carroll's blog.

14. Why doesn't the Solar System expand if the whole Universe is expanding?
The objects in the Solar system are under the constant influence of gravity. Gravity pulls objects together, but it doesn't have effect across large distances. Mathematically, the gravitational force decreases with the distance squared. This means that if we get ten times further away, the gravitational force is a hundred times weaker. Imagine there is a constant battle between the gravitational force pulling objects inwards, and the expansion of the universe pushing objects away. On small scales, such as solar systems (or indeed galaxies), gravity wins, but on large scales the expansion of the universe wins. This also explains why galaxies can collide: on the 'small' scales of galaxies gravity wins, on large scales of the whole universe dark energy wins.

As an analogy, imagine two marathon runners: Bob (gravity) and Lisa (expansion). Bob decides to sprint and try to last as long as possible, while Lisa decides to maintain a calm and steady pace. At the first checkpoint, Bob will be in front of Lisa, but as the race goes on, Bob will lose energy and slow down. By the end of the race, Lisa will likely be winning. On small scales, Bob wins, but on large scales Lisa wins.

15. What is the Universe expanding into?
Nothing! The idea that the universe is expanding into something is a common misconception; we imagine the universe to be in something. I often use the analogy that the expansion of the universe is like a cake in the oven rising. However, the universe is not in anything. The universe is defined as everything in existence: if we were expanding 'into' something, it would imply there is something outside of the universe, but by definition, this would also be part of the universe. This is why we can't talk about 'the edge of the universe'. On a similar note, if the universe has no edges, we say it's infinite (see question 17).

Even if there was something that we are expanding into, it's not something we would have any access to; we have no way of exchanging information with it, so it's not a profitable thing to think about. We are asking about the 'external' geometry of an object in which we are 'trapped'. It's like putting wheels on a tomato: time consuming and completely unnecessary.

16. What is the observable universe? How big is it?
Einstein taught us that the speed of light is not infinite; it has a clearly defined value of 300,000 kilometres per second. The age of the universe is also quite well known. This means that light has only been able to travel a certain distance since the beginning of the universe. This brings us back to the idea of observable universe. The observable universe is everything we are able to see in the universe; it's a sphere centred around each observer. Everyone is the centre of their own observable universe. The light from objects beyond our observable universe hasn't yet had time to reach us.

The size of the observable universe often leads to misconceptions: one could naively assume that as the universe is 13.7 billion years old, the observable universe would have a radius of 13.7 billion light years. This is wrong! The light that we see from a star emitted 5 billion years ago was 5 billion lightyears away when the light was emitted, but the star has moved away from us since then, due to the expansion of the universe. Taking into consideration this expansion, current estimates put the size of the observable universe at 46.5 billion light years in radius, so 93 billion light years in diameter.

Earth's size compared to the observable universe. Image by Andrew Z. Colvin

17. How big is the universe? How can something that started of so small be infinite?
We believe the universe is infinite, but we really have no way of testing what's outside our observable universe. It would take light over 93 billion years to cross the observable universe (even more if we take into consideration expansion), which is a lot. The observable universe might not be infinite, but it is ridiculously big, and we think our observable universe is only a small fraction of the whole thing.

Now we need to make a slight distinction: I've said before that at the Big Bang, everything was contained in a small point. Of course, what we mean is that everything in the observable universe was concentrated into a small point: we can't make the same claim for things outside of the observable universe, at least not without a model to tell us exactly what the universe looked like when gravity and quantum mechanics worked together; for that we need a theory of quantum gravity.
But what we do know is the universe is big. Very big. So big that we can justify saying it's infinite.

18. Is the universe flat? What does that mean?

The universe is homogeneous (same in every direction) and isotropic (same from wherever you look). Mathematically, there are only three possibilities for a universe like this; flat like a sheet of paper, hyperbolic like a Pringle, and spherical like a ball.
This can be seen by drawing triangles: on a flat surface the sum of the angles in any triangle is always 180°. On a negatively curved surface (like a crisp/potato chip) the sum of the angles in a triangle is smaller than 180°. On a positively curved sphere the sum of the angles in a triangle is more than 180°. Our universe is flat; triangles in the universe will look the same as triangles on a sheet of paper.

From a more physical point of view, general relativity explains that mass and energy bend the curvature of spacetime, so the curvature of the universe is related to its density (mass over volume). We use something called the density parameter ($\Omega$), which is defined as the density of the universe divided by the critical energy density: the mass energy needed for a universe to be flat. So our three cases reduce to the following: if $\Omega>1$ the universe looks like sphere, if $\Omega<$ the universe looks like a crisp, and if $\Omega = 1$ the universe is flat. Our measurements show that the density parameter is really close to 1. In fact we find that the universe is flat with only a 0.4% margin of error.


So we've seen that our universe is flat, undergoing accelerated expansion, and filled with a lot of stuff we don't understand. We've covered quite a few questions today; but there is a lot to say about the universe.
In the fourth, and final, part of this series we will discuss the big questions like the fate of the universe.

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