Ask Ethan #84: Where did light first come from?

Before the first star ever formed, the Universe was filled with light. But how?

“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it.” -Terry Pratchett

When we look out at the Universe today, highlighted against the vast, empty blackness of the sky are points of light: stars, galaxies, nebulae and more. Yet there was a time in the distant past before any of those things had formed, shortly before the Big Bang, where the Universe was still filled with light. This past week, chemistry professor Fábio Gozzo got a question he couldn’t answer, so he sent it in to Ask Ethan, and it goes like this:

I try to keep students updated using a lot of material from your blog. But recently a good question came up during a discussion about [the] big bang: where do the photons from CMB come from? My understanding is that the photons came from annihilation of particle/anti-particle pairs produced by quantum fluctuations after inflation. But shouldn´t this energy be “returned” as they were “borrowed” intially to produce the particle/anti-particle pairs?

There are some things that are dead-on about Fábio’s inclinations, but there are a few misconceptions in there, too. Let’s take a look at the CMB, first, and where it comes from going way, way back.

Image credit: Physics Today Collection/AIP/SPL.

In 1965, the duo of Arno Penzias and Robert Wilson were working at Bell Labs in Holmdel, New Jersey, trying to calibrate a new antenna for radar communications with overhead satellites. But no matter where they looked in the sky, they kept seeing this noise. It wasn’t correlated with the Sun, any of the stars or planets, or even the plane of the Milky Way. It existed day and night, and it seemed to be the same magnitude in all directions.

After much confusion over what it might be, it was pointed out to them that a team of researchers just 30 miles away in Princeton predicted the existence of such radiation, not as a consequence of anything coming from our planet, solar system or galaxy itself, but originating from a hot, dense state in the early Universe: from the Big Bang.

Image credit: the Cosmic Microwave Background of Penzias and Wilson, via

As the decades went on, we measured this radiation to greater and greater precision, finding that it was not at merely three degrees above absolute zero, but 2.7 K, and then 2.73 K, and then 2.725 K. In perhaps the greatest achievement related to this leftover glow, we measured its spectrum and found it was a perfect blackbody, consistent with the idea of the Big Bang and inconsistent with alternative explanations, such as reflected starlight or tired light scenarios.

Images credit: Wikimedia Commons user Sch, under c.c.-by-s.a-3.0 (L), of the Sun (yellow) vs. a perfect blackbody (grey); COBE/FIRAS, via NASA / JPL-Caltech (R), of the CMB.

More recently, we’ve even measured — from the absorption and interaction of this light with intervening clouds of gas — that this radiation increases in temperature the farther back in time (and redshift) we look.

As the Universe expands over time, it cools, and hence when we look farther back into the past, we’re seeing the Universe when it was smaller, denser, and hotter.

Image credit: P. Noterdaeme, P. Petitjean, R. Srianand, C. Ledoux and S. López, (2011). Astronomy & Astrophysics, 526, L7.

So where did this light — the first light in the Universe — first come from? It didn’t come from stars, because it predates the stars. It wasn’t emitted by atoms, because it predates the formation of neutral atoms in the Universe. If we continue to extrapolate backwards to higher and higher energies, we find some strange things out: thanks to Einstein’s E = mc^2, these quanta of light could interact with one another, spontaneously producing particle-antiparticle pairs of matter and antimatter!

Image credit: Brookhaven National Laboratory / RHIC, via

These aren’t, as Fábio alludes to, virtual pairs of matter and antimatter, which can only exist for a tiny fraction of a second thanks to Heisenberg’s uncertainty principle and the relation ΔE Δt ≥ ћ/2, but rather real particles. Just like two protons colliding at the LHC can create a plethora of new particles and antiparticles (because they have enough energy), two photons in the early Universe can create anything there’s enough energy to create. By extrapolating backwards from what we have now, we can conclude that within the observable Universe shortly after the Big Bang, there were some 10^89 particle-antiparticle pairs.

For those of you wondering how we’ve got a Universe full of matter (and not antimatter) today, there must have been some process that created slightly more particles than antiparticles (to the tune of about 1-in-1,000,000,000) from an initially symmetric state, resulting in our observable Universe having about 10^80 matter particles and 10^89 photons left over.

Images credit: E. Siegel.

But that doesn’t explain how we wound up with all that initial matter, antimatter and radiation in the Universe. That’s a lot of entropy, and simply saying “that’s what the Universe began with” is a wholly dissatisfying answer. But if we look to the solution to an entirely different set of problems — the horizon problem and the flatness problem — the answer to this one just pops out.

Image credit: E. Siegel, of how spacetime expands when it’s dominated by Matter, Radiation or energy inherent to space itself.

Something needed to happen to set up the initial conditions for the Big Bang, and that “thing” is cosmic inflation, or a period where the energy in the Universe wasn’t dominated by matter (or antimatter) or radiation, but rather by energy inherent to space itself, or an early, super-intense form of dark energy.

Inflation stretched the Universe flat, it gave it the same conditions everywhere, it drove away any pre-existing particles or antiparticles, and it created the seed fluctuations for overdensities and underdensities in our Universe today. But the key to understanding where all these particles, antiparticles and radiation first came from? That comes from one simple fact: to get the Universe we had today, inflation had to end. In energy terms, inflation happens when you roll slowly down a potential, but when finally you roll into the valley below, inflation ends, converting that energy (from being up high) into matter, antimatter and radiation, giving rise to what we know as the hot Big Bang.

Image credit: E. Siegel.

Here’s how you can visualize this.

Imagine you’ve got a huge, infinite surface of cubic blocks pushed up against one another, held up by some incredible tension between them. At the same time, a heavy bowling ball rolls over them. In most locations, the ball won’t make much progress, but in some “weak spots” the ball will make an indentation as it rolls over them. And at one fateful location, the ball can actually break through one (or a few) of the blocks, sending them plummeting downwards. When it does this, what happens? With these blocks missing, there’s a chain reaction due to the lack of tension, and the whole structure crumbles.

Image credit: E. Siegel.

Where the blocks hit the ground far, far below, that’s like inflation coming to an end. That’s where all the energy inherent to space itself gets converted to real particles, and the fact that the energy density of space itself was so high during inflation is what gives rise to so many particles, antiparticles and photons getting created when inflation ends.

This process, of inflation ending and giving rise to the hot Big Bang, is known as cosmic reheating, and as the Universe then cools as it expands, the particle/antiparticle pairs annihilate, creating even more photons and leaving just a tiny bit of matter left over.

Image credit: ESA and the Planck Collaboration, modified by me for correctness.

As the Universe continues to expand and cool, we create nuclei, neutral atoms, and eventually stars, galaxies, clusters, heavy elements, planets, organic molecules, and life. And through it all, those photons, left over from the Big Bang and a relic of the end of inflation that started it all, stream through the Universe, continuing to cool but never disappearing. When the last star in the Universe flickers out, those photons — long since shifted into the radio and having diluted to be less than one-per-cubic-kilometer — will still be there in just as great an abundance as they were trillions and quadrillions of years prior.

And that’s where the first light in the Universe came from, and how it got to be the way it is today. Thanks for an incredible question with an amazing story for an answer, Fábio, and if you’ve got a question or suggestion for the next Ask Ethan column, send yours in here, and just maybe you’ll see yours answered on the next Ask Ethan!