By 1948, our view of the Universe had changed drastically from 1909. Instead of space being run by Newton's Gravity, where our Milky Way comprised the entire Universe, we had learned that space and time are governed by Einstein's General Relativity, that our Universe contained at least many thousands of other galaxies, and that the farther these galaxies were from us, the faster they moved away from us. In other words, the Universe was not only vast and full of interesting stuff, it was also expanding as it got older!

The prevailing idea of the day was the Perfect Cosmological Principle, stating that -- on average -- the Universe was the same at all places and at all times. In other words, even though galaxies close to us were expanding away from us, the Universe would make more in their place as they moved away. This way, the Steady-State Theory of the Universe would be just fine, even in an expanding Universe.

Why was this idea so pervasive? Because, with the exception of small events (e.g., supernovae, the slow drift of nearby stars), the Universe doesn't appear to change over the hundreds and hundreds of years we've been observing it! And this is true. Compare this image of Andromeda (M31) taken in 1887 by Isaac Roberts:

with this one of Andromeda taken by this guy with this telescope, just a few years ago:

See? The Universe doesn't change.

End of story.

What, you're still here? You aren't convinced?

What kind of contrarian are you?

Perhaps you're like George Gamow: smart and a smart-ass. Gamow wasn't buying this "steady-state" garbage for a minute. He did a little bit of extrapolating instead. If the Universe has the density it has now and the expansion rate that it has now, and we know the Laws of Gravity, what was it doing in the past?

Well, things were closer together (denser), things were moving at a faster rate (larger velocities), and the temperature was higher (hotter). The small deal was that this meant we lived in an expanding, cooling, slowing Universe. But the big deal was that the Universe used to be hotter, denser, and faster. In principle, this means that if we looked back, billions and billions of light years away, we'd see the Universe as it was when it was billions of years younger. Would it be hotter, denser, and faster?

Well, the answer is yes, but telescopes weren't even close to good enough at that point to measure temperature or density at those distances. Of course they were moving faster; that's Hubble's Law! But Gamow didn't let this stop him. He simply decided to extrapolate further back in time. He realized that high temperatures and densities made the early Universe very interesting in two special ways, in addition to the hotter/denser/faster consequence above!

Well, if the Universe was hot, dense, fast, and cooling in the past, there must be some point where we formed neutral atoms for the first time. When you combine an electron and a nucleus together, they emit light (right). If you combine all the electrons and nuclei in the Universe together at the same time, you should be able to see the afterglow of this light everywhere in the Universe. Because the Universe has expanded and cooled, Gamow estimated that the temperature of this light should only be a few (~5 was his guess) degrees Kelvin. And, he said, this cosmic background would be evidence of this hot, dense, early phase to the Universe.

2. At high enough temperatures, nuclei dissociate into protons and neutrons! Going even further back in time, at some point the temperatures and densities were so high that protons and neutrons -- much like in the Sun today -- fused together to make heavier atomic nuclei! It's very hard to get heavy elements like Carbon, Nitrogen, and Oxygen, much less Iron or Nickel, but it's easy to get Helium-4, Helium-3, Deuterium, and Lithium-7. In fact, it's easier to get these elements here than it is in stars! Gamow calculated what the abundances of the light elements should have been, and got that the Universe should be about 76% Hydrogen, 24% Helium, and everything else should be less than 1%. And this is exactly -- to the best of their observations -- what we saw.

It wasn't until the 1960's that they found the Cosmic Microwave Background, confirming the third of the three pillars of Big Bang Cosmology, and relegating the Steady-State Theory to the history bin. But Gamow, for masterminding this new theory of the Universe, and getting it right, you are totally my hero of the decade for the 1940s. After all, we wouldn't be Starts With A Bang if it weren't for you!