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.