The whole universe was in a hot dense state, when nearly 14 billion years ago, expansions started!

Wait…

What was expanding, exactly? And where did it all come from?

The ultimate question

People often think of the big bang theory as an explanation for how the universe began; in fact, it is much more tentative than that. The big bang theory only tells us that there was a big bang, not why or how it happened. Since it was first proposed by Georges Lemaître in 1927, the theory has garnered incontrovertible support. And yet, we still know very little about the first fraction of a second of our universe's existence. Thankfully, by making observations of the cosmos, we can better understand the physics of the very early universe, and hopefully someday understand what started it all. I, in particular, am interested in the period just following the initial "bang", in which we believe the expansion of the universe briefly sped up to a nearly incomprehensible rate before slowing back down. We hope this period of "cosmic inflation" can explain why the universe looks the way it does today, and even where the matter in our universe came from. Luckily, this violent expansion would have left an imprint in the faint afterglow of the big bang — a relic we refer to as the Cosmic Microwave Background, or the CMB. To understand this relic, it is useful to take a step back, and take a look at the very fabric of the cosmos.

A (maybe not so) brief guide to the universe

The second concept of infinity is more subtle, and relates to the “quantity of things”. For instance, though “the set of all numbers between 0 and 1” has a minimum and a maximum (zero and one, respectively), it still contains an infinite number of numbers! And, even if you choose two points inside that set – say, 0.0001 and 0.0002 – I can still find an infinite number of numbers between those numbers (like 0.00015).

Is the universe infinite?

This means the question as to whether the universe is infinite is actually two separate questions. The first roughly translates to “Does the universe have an edge, or can you always go farther?” And the answer to that questions is… Maybe? Most people I’ve known would argue, in the broadest sense, that the universe probably doesn’t have an edge, but that argument is mostly an aesthetic argument, based only loosely on established scientific principles.

The other question, however, is a bit more concrete, and it can be stated as such: “Is the universe like a chess board, where you can exist on one square or another, but nowhere in-between, or are there an infinite number spots in which you can exist?” Ignoring some nuance with respect to quantum mechanics, the answer is that the universe is essentially infinite in this sense of the word.

Let’s explore the implications of this. First, think about a rubber band made of a bunch of rubber molecules (I apologize in advance to any chemists in the audience). When I stretch the rubber band, these rubber molecules get pulled apart, and the band gets deformed. Eventually, you pull hard enough that the molecules get separated too far from their nearest neighbors and the band breaks. Now suppose I have a second rubber band, but this one is made of an infinite number of infinitely small rubber molecules. When I pull the rubber band apart, every rubber molecule will get farther away from every other. But look closer! The rubber band still looks exactly the same – every point on the rubber band still has an infinite number of rubber molecules. You can keep pulling this rubber band forever, and you will never be able to break it, because there will always be an infinite number of rubber molecules at every point. If you drew two dots on this rubber band, the dots would fly away from each other, but around each dot, the rubber band is totally unchanged.

It is in this second sense that we say the universe is expanding. It’s not expanding into anything. Things on the magic rubber band we call our universe are just getting pulled farther and farther apart. You can see this if you point a sufficiently powerful telescope at the night sky and look at far away galaxies. It looks like every galaxy is flying away from us, and the farther a galaxy is, the faster it’s moving. But they’re not actually moving at all! Instead, space is just growing between us and these distant galaxies. Here's a Forbes article discussing this and the shortfalls of the ways we usually visialize it.

You can listen to Neil DeGrasse Tyson explain his existential dread about this phenomena:

What are we looking at?

Because light from farther away will take a longer time to reach us, the farther out we look, the farther back in time we peer. Of course, 14 billion years ago, there were no galaxies, stars, or any other structures we are familiar with today. For the first few hundred thousand years, the universe was just a dense plasma - electrons and protons which have so much energy that they bounce around without binding to one another. Because electrons and protons on their own are electrically charged particles, they cause light to be constantly bounced around (this is, in some sense, the definition of having electrical charge). Since the light was not able to propagate for long distances without interacting with the plasma, the information it carried was trapped in a small region of space and could not be communicated with far away points; think, for instance, about how far a kickball would travel in an open field versus a dense forest. Then suddenly, about 380,000 years after the Big Bang, the plasma cooled enough that the electrons and protons were able to join together into neutral hydrogen (meaning that the hydrogen as a whole has no net electric charge), in an event known as recombination. Once this happened, light was no longer trapped by the free charged particles, and began to travel unimpeded for the next 14 billion years, until a few of these photons collided with purpose-built detectors here on earth.

Where did all the heat go?

What does it mean for this primordial plasma to have cooled down? Where did all the heat go? Isn’t energy supposed to be conserved?

Much of the energy in the early universe was stored in what is sometimes referred to as the “photon gas”, which really just means a bunch of light bouncing between charged particles. Photons get “stretched out” when space expands, and when this stretching happens, they become less energetic. Whether energy is “conserved” in this situation is a semantic question. You can define your equations so as to store that lost photon energy inside space-time itself, but that doesn't really concern us here. If you are interested in the details, prominent cosmologist Sean Carroll wrote a short blog post with his take on the issue.

This cooling happened mostly uniformly throughout the universe, and that means that the hydrogen formed everywhere all at once, which means the light was released all at once. The speed of light, multiplied by the time since this release occurred, gives us a distance (again, roughly 14 billion light-years). If you draw a shell around you at 14 billion light years in all directions, you can see some of the light that was released 14 billion years ago. We call this shell the “surface of last scattering”, meaning the surface where we are seeing the photons make their last bounce of the plasma before they finally escaped. While we observe this as a hollow shell, it is in fact uniform throughout the entire universe. We can’t see the last scattering that happened here in our solar system, even though it did happen here, 14 billion years ago, because all that light has now traveled 14 billion light years away from us. A clever analogy that was shared with me (thanks Steve!) is a long line of people standing behind you that all shout at once in your direction. Because the sound takes time to reach your ears, you'll first hear the person standing directly behind you (and maybe feel their saliva, too). As you stand there, you'll then hear the person two spots behind you, then three, etc. Thus, even though the entire line yelled at once, you only ever hear a single person at a time, coming from farther and farther away as their voices finally reach you. Since it takes the same amount of time for your voice to reach them as it does theirs to reach you, at the exact moment you hear a given person's yell - the edge of your observable yell-a-verse - they are also hearing you, and so you are at the edge of their observable yell-a-verse too!

Blackbody Radiation

We sometimes refer to extremely hot objects as “red hot” - that is, so hot, they begin to glow red. In fact, nearly all macroscopic objects in the universe are glowing. This is why IR cameras are able to differentiate warm bodies from their surroundings. The peak wavelength (or “color”) at which an object radiates is related to its temperature by an equation known as Wein’s Displacement Law. By measuring the brightest color of light from an object, you can figure out what temperature that object is. Since the plasma at the time neutral hydrogen formed was about 4000K (7000°F), it was glowing a bright red. Since this glow emerged from every point in the universe, one would expect to see this red glow in all directions, even today. However, as space expanded, the wavelength of the photons also got stretched out, as we discussed earlier (ironically, this process is called redshift, though in this case, the photons are shifting away from red, toward non-visible wavelengths). Today, the peak wavelength has been stretched from 700nm all the way to 3mm (a factor of about 4000), which is right in the middle of the microwave range.

So, we can look at the earliest light from the universe – we just need a fancy microwave detector. But there’s a problem! If there’s one thing we’ve learned from re-heating our day-old coffee in the microwave, it’s that water is really good at absorbing microwaves (and also, day-old coffee is gross). Thus, after these photons travel 14 billion light years on their way to us, they reach the earth’s atmosphere and get absorbed by all the moisture in the air, and all information about their energy and origin is lost. That is what brings us to…

The Ends of the Earth

One obvious way to avoid moisture in the atmosphere is to avoid the atmosphere altogether – Go to Space! This strategy has been adopted by a series of groundbreaking experiments, including WMAP, COBE and Planck. The problem with satellites is that they are small, expensive, and, because of the difficulty of maintenance, use old, well-proven technologies.

Balloon based experiments, such as SPIDER and EBEX, provide an intermediate between ground and space based telescopes. They greatly reduce the amount of atmosphere through which they peer, and can fly newer technologies as a proof of concept for space-based missions. However, such experiments typically only operate for a week or two each year (if they even launch in a given year) and thus provide a rather limited data set.