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.