“Twinkle, twinkle, quasi-star
Biggest puzzle from afar
How unlike the other ones
Brighter than a billion suns
Twinkle, twinkle, quasi-star
How I wonder what you are.” -George Gamow
The Big Bang is one of the greatest, revolutionary ideas in all of modern science, but it’s also one of the most successful ideas as far as making predictions that have aligned with our observations of the Universe. Despite all of that, there are a whole bunch of people who’ll read this (and more who won’t) who aren’t sure that the Big Bang is correct. Some people, in fact, don’t even understand the definition of the Big Bang. I’ve written about this many times before, of course, including a great introduction to the Big Bang for beginners, and a couple of lengthy explanations that the Big Bang isn’t, in fact, the beginning of the Universe, but I don’t think I ever told you where it came from in the first place.
Let’s go way back in scientific history to the 1940s. General Relativity had come along about 30 years before, replacing space, time, and Newtonian gravity with a more complete, comprehensive and geometrical picture of spacetime. Rather than gravitational forces, spacetime was a geometrical entity that was curved due to the presence of matter and energy, and all particles, both massive and massless, moved according to the curvature of that spacetime.
In 1923, Edwin Hubble discovered variable stars in the Andromeda Galaxy, and used their properties to deduce that Andromeda — and later, many other spiral and elliptical galaxies — lay far beyond the extent of our own Milky Way. The work he did with variable stars allowed him to figure out not only how far away these galaxies were, but also, by combining this with data obtained from the spectra of these galaxies, to find a curious relationship: the farther away a galaxy was from us (on average), the faster it appeared to be receding away from us!
In the context of General Relativity, this fit in perfectly with the concept of spacetime in an expanding Universe! If we imagine the galaxies as coins glued onto the surface of a balloon (or, for those of you who need a 3D analogy, raisins in an unbaked ball of dough), then what happens as you inflate the balloon (or as you bake the bread)? If you’re one of the galaxies, you’ll see each and every other galaxy look like it’s speeding away from you, and the farther away a galaxy is, the faster it appears to recede!
But this has nothing to do with you, or your galaxy in particular. Every observer in any, arbitrary galaxy would see the same thing! This was worked out by a number of people in the late 1920s (Lemaitre) and, in more detail, in the 1930s (Robertson and Tolman, among others).
This was the starting point for where the Big Bang came from; George Gamow, working in the 1940s, had a very important realization.
The Universe wasn’t just filled with matter. Yes, there was obviously plenty of matter, and as the Universe expanded, the amount of mass stayed constant while the volume increased, meaning that the matter density was diluting as time went on. This also meant that the Universe was denser in the past, of course. But the Universe was also filled with light: light from stars, light from hot plasmas, light from gravitational contraction, and light produced from the collision of particles.
And light — a form of massless radiation — behaved differently.
As the Universe expands, the number of photons stays the same, and so the energy density dilutes similarly to matter. But for radiation, its energy is defined by its wavelength, and as the Universe expands, the wavelength of light, according to General Relativity, also stretches!
Longer wavelengths means lower energies, so as we go forward in time, the amount of energy in radiation continues to decrease relative to the amount of energy in the form of matter. But in the past, the opposite was true!
Radiation was more important in the past, both relative to matter and also just absolutely. The light in the Universe was more energetic in the past. And, as Gamow realized, that has some very important consequences for matter.
Matter, of course, is made up of atoms, which in turn is made up of electrons and atomic nuclei. In our Universe today, there are only a very few select regions of space where the energy of photons are high enough to kick electrons off of atoms: active star-forming regions, where the hottest, youngest and newest stars can ionize the neutral atoms around them.
But in the young Universe, if you go back far enough, the photons/radiation had enough energy to prevent neutral atoms from forming at all! Imagine a Universe so hot and so dense that as soon as an electron and a nucleus found each other, a high-energy photon comes along and kicks the electron off, ionizing the system once again.
The Universe needed densities about a billion times higher than today and photon wavelengths about 1000 times shorter than today to make that happen, but according to General Relativity and the expanding Universe, this was all but inevitable.
But there’s more! If we then go back even farther, to earlier times, shorter distances, and higher densities, there was a time when even atomic nuclei couldn’t have formed. The Universe was so hot that it would have blasted any nucleus apart into the individual protons and neutrons that made it up!
And if there are more fundamental states of matter inside protons and neutrons (and we now know that there are), going back even farther — to when the energies were even higher — would have broken them apart, too!
But the Universe started from this hot, dense, state — what Gamow called the Primeval Fireball (so much more awesome a name than the Big Bang) — and expanded and cooled! For the first time, atomic nuclei formed (after a few minutes), and then, after hundreds of thousands of years, neutral atoms finally formed.
The explicit prediction of this hot Big Bang, or of the Primeval Fireball, would be that there was a cold, leftover, relic radiation in all directions, of the same cold temperature (just a few degrees above absolute zero), with a blackbody spectrum to it.
No other competing theory — not Steady State, not Tired Light, not the Milne Cosmology, not the Plasma Universe, not the Godel Universe — had this feature. And in the mid-1960s, that leftover radiation was detected!
Arno Penzias and Bob Wilson — using the Holmdel Horn Antenna, above — discovered the signature of this relic radiation from the Primeval Fireball, and in the early 1990s, the COBE satellite accurately measured the entire spectrum of the radiation to find out whether it was a perfect blackbody, as predicted, or whether there were any departures.
There were none. And check out those error-bars; 400-sigma statistical significance!!! Ever since, there have been no serious competitors to the Big Bang, and a plethora of other predictions, such as big-bang nucleosynthesis, the details of structure formation, the presence of pristine gas in the early Universe, and the increase of temperature with redshift have all been observed to be consistent with Gamow’s theory’s predictions.
And that’s what it’s all about!