The cosmic microwave background: a peephole into the past
In 2013, the Planck collaboration released the data acquired by the Planck satellite, including the following image of the Cosmic Microwave Background or CMB. What you’re looking at is a map of the entire universe as it was more than 13 billion years ago. The image might look like a simple heat map (which it actually is) but it is so much more important than that. In fact, it may highlight the reason for our very existence.
What is the Cosmic Microwave Background?
The idea that the universe is expanding may seem like common sense now, but the Big Bang theory was only popularized in the 1920s after astronomer Edwin Hubble first noticed that distant galaxies were moving away from us faster than the nearer galaxies. This expansion is interesting enough in itself, because according to our current model of physics, gravity should have overcome the initial force from the Big Bang by now, causing the expansion to stop, but I’ve already written about that here:
Today I’m going to talk about the very early stages of the universe, which is where the CMB originates from. There are several topics that I need to explain before I get to the CMB, so please bear with me.
The early universe
The Big Bang theory states that the entire universe was once concentrated onto a single, infinitesimally small point. This point then expanded, forming our universe (I wrote about an explosion here first, but we will see why that is wrong in a bit). The universe at this point was nothing more than a hot, seething plasma full of energy and chaotic particles, often called the ‘primordial soup’.
During this time most photons were so energetic that upon colliding with electrons, they would knock those electrons out of their orbits around atomic nuclei, which meant that atoms could simply not exist. Since the electrons were not bound to any nuclei, they would float around freely, constantly colliding with photons. These collisions happened to be so frequent that a photon could not travel any significant distance before being bounced back due to a collision.
The early universe was thus nearly opaque.
However, things would not remain like that forever.
The expansion of the universe
An interesting thing about the expansion of the universe is the way in which it happens. You might imagine two galaxies moving away from each other as if due to an explosion, but that is only partially true. Yes, the galaxies are growing apart, but not in the way you’d think. In fact, the galaxies are not actually moving (P.S.). Instead, they grow apart because the distance between them increases. The empty space between the two galaxies itself gets stretched or, in other words, more empty space is added between them.
So even though the galaxies are not moving, they get further and further apart because the empty space between them itself expands. This might sound like a minor technicality since the end result is the same, but it is actually very important to understand for the following reason.
If we had considered the expansion of the universe to just be things moving away from each other as if due to an explosion, this expansion would have no effect on light, since it is a wave and moves at a constant speed at all times. However, when space itself is getting stretched, things start getting weird.
The last scattering
Light is basically electromagnetic waves traveling through empty space, and if this space itself gets stretched, so will the light waves. Their speed does not change, but since the waves are getting stretched, their wavelength increases. This in turn leads to a decrease in the frequency of the light wave (P.S.S.).
The energy carried by a wave is directly proportional to its frequency (Regarding the energy of a light wave). So when light gets stretched, it loses its energy. This means that the photons which were once energetic enough to knock electrons out of their orbits, can no longer do that. Atoms could finally exist, and all of the free electrons were now bound to atoms.
As a result, the universe was no longer opaque and the photons could escape the plasma. This event is known as the photon decoupling, and the part of space where this happened is called the surface of last scattering. This is where the CMB image comes from.
Shown below is the evolution of the universe, where the photon decoupling can be seen marked in red:
Does the universe expand faster than the speed of light?
Everything I’ve said so far still leaves the question, how did we capture this if it all happened more than 13 billion years ago? The answer once again lies in the expansion of the universe. The rate of expansion is not constant, but directly proportional to the distance from the reference point, which is why, as Hubble observed, distant galaxies move away from us faster than closer ones.
Due to this, the bigger the universe got, the faster the expansion became, eventually crossing the speed of light. This meant that the light from the last scattering could no longer keep up and would instead have to catch up to us billions of years later. So the light from the CMB we see today is the same light that escaped the plasma nearly 13.8 billion years ago, it’s just reaching us now.
Over this period of time, the wavelength of the light has increased so much that it is now a part of the microwave spectrum, which is where the name comes from.
The CMB was first discovered by Arno Penzias and Robert Wilson when they detected a constant noise that they could not explain, when designing a sensitive radio antenna for Bell labs.
They also observed that this radiation came from everywhere in the sky, and not from any particular direction.
Why do we see the CMB everywhere?
Another thing about the CMB that might seem confusing at first is that we receive the signals from every direction. This would not be possible if the universe had a centre from which it expanded, but that’s the thing. The expansion doesn’t actually have any centre.
Imagine a balloon that you are blowing air into. The balloon keeps expanding, similarly to the universe. For us, the balloon has a centre, which is not on its surface but inside it. Now imagine two-dimensional beings living on the surface of the balloon. They have no understanding of up or down, there is no outside of the balloon or inside of the balloon for them, there is just the surface that they live on. For them, there would be no centre to the expanding balloon, because every point on the surface is moving away from every other point.
Similarly, we may be the three dimensional beings living on the ‘surface’ of a four (or higher) -dimensional universe. We can’t perceive the centre of the universe, because for us all points are moving away from each other. This is why the CMB can be seen in all directions.
I would highly recommend this video for a better explanation:
This may also answer another fairly common question, what is outside the universe? We could not possibly know, because we cannot perceive the ‘outside’ of the universe, because it is not in the third dimension.
The significance of the CMB
You may have seen heat-maps before, like the one below. They are actually infrared images that show differences in temperature, the blue regions being colder and the orange or red ones being warmer.
The CMB image is also a sort of heat map of the early universe, from the point of last scattering. However, since the light we see today has been traveling through expanding space for billions of years, it is now extremely cold and thus does not represent the actual temperature from that period, but only the relative differences in temperatures between different regions. However, it is these differences that we care about.
Since even atoms could not exist back then, these differences definitely did not arise due to the formation of matter. Instead, they may have come from the tiny quantum fluctuations associated with the Big Bang itself, or more specifically with the inflaton field.
These differences are absolutely essential for our existence, because without them, nothing would form in the universe. If the entire universe was perfectly smooth, all of the forces would be in a perfect equilibrium. No nebulae would form, no stars, no planets, nothing would exist except a vast, uniform sea of particles.
Luckily for us, the quantum fluctuations did exist, which caused a non-uniform distribution of matter in the universe. This meant that some regions had more mass than others, leading to gravitational fluctuations which would eventually lead to the formation of nebulae, stars, planets and everything else we see today.
This can allow us to predict the distribution of matter in the universe today, potentially even facilitating a simulation of the Big Bang.
By confirming whether what we observe in the universe is in line with what the simulation predicts, we can verify our current model of physics and several important hypotheses, including the presence of dark matter and dark energy.
A simulation could also help solve some long standing mysteries in astrophysics, but it may also give rise to some new ones. Either way, it’s a win for science! Here’s a great video about one such fascinating simulation:
Even without a simulation, the CMB allows us to understand the expansion of the early universe better, potentially revealing flaws in the Big Bang theory. It is a critical requirement for any new model of the universe to be able to explain and predict the value of the CMB correctly. It also reveals the composition of the universe, which led to the discovery that a majority of the universe consists of dark energy. It is thus an invaluable resource for cosmologists.
P.S.
Technically the galaxies are moving as they rotate as well as revolve around other galaxies or clusters of galaxies, but they do not move due to the expansion of the universe
P.S.S
The speed of light is related to its wavelength and frequency according to the following equation:
V (the speed of light) = f (frequency) x ƛ (the greek letter ‘lambda’ denoting the wavelength) / V = f x ƛ
This means that if the speed is the same, and the wavelength is increasing, the frequency must decrease.
Regarding the energy of a light wave
The energy of a light wave can be calculated from the following equation:
E = hf
Where E is the energy of the wave, f is the frequency and h is the Planck’s constant. The part about the frequency being directly proportional to the energy is quite obvious, since the faster a wave oscillates, the more energy it should have. The need for Planck’s constant is not so obvious. You don’t necessarily need to understand it in order to understand the CMB but it is such a fundamental concept that I thought it deserved some attention.
Planck’s constant, named after the german physicist Max Planck, lies at the heart of quantum mechanics, which is why Planck is often called the forefather of quantum mechanics. This constant is responsible for only allowing certain quantised energy states.
These concepts are quite complex, and I have not really been able to wrap my head around them yet. I may write about it in the future, once I’ve somewhat understood it, but for now I leave you with some sources:
This is the best explanation I’ve found so far but it does not go into too much detail about the math involved or where the equation comes from:
This one talks a lot about the math, but it’s a pretty long one:
