We were fortunate to be there a day or two before ‘the big bang’
and then we got the heck out of town.
~Scotty Moore

A few weeks ago, +Matthew Villaneuva asked the following question on Google+:

Does anybody else find the Big Bang (the scientific explanation on how the universe got created) weird?

Actually, yes! Just a century ago, everyone believed that the universe was static—i.e., that it had always existed and that it would always continue to exist. Even Albert Einstein held this view. I previously explained why we know the universe is expanding, so I’m going to continue that story now.

Preliminaries

The mathematics behind the Big Bang theory of cosmology relies on general relativity. I’ve posted on general relativity a number of times before. If you’re interested, reading my previous posts might enrich your reading of this one. Here are the links:

• General Relativity Lets Us Take Shortcuts
• A Spacetime Cocktail: Minkowski Space and Special Relativity
• For There We Are Captured: The Geometry of Spacetime
• Rock Me, Einstein—Some Questions on Special and General Relativity
• More on Special and General Relativity

And the article that I’ll be building off of most is

• Receding Horizons: Dark Energy and The Expanding Universe

Alexander Friedmann

Einstein’s theory of general relativity predicts that spacetime is warped and curved by mass and energy, and that this is what causes gravity. But this raises a fascinating question: What shape does the universe take?

In 1922, Russian physicist Alexander Friedmann sought to answer exactly this question. Einstein’s theory takes the configuration of stuff—matter and radiation, like light—as input, so Friedman had to guess what the distribution of mass in the entire universe might look like.

The Universe and Gazpacho

Friedman took a hint from the air around him. By the late 1800s, physicists knew that a gas like air is actually made of many little particles zooming around. (Air is mostly made out of diatomic nitrogen, a molecule made out of two nitrogen atoms, and diatomic oxygen, a molecule made out of two oxygen atoms.) But on the human scale, we can’t tell that air is made of many little molecules. We see it as a continuous fluid that fills space and can flow from place to place.

Friedmann guessed that perhaps the universe is the same way, but upside-down. On human scales, the universe is made up of single, discrete “particles”—planets, stars, moons, etc. If we zoom a little further out, it seems to be constructed of galaxies. If we zoom a little further out, galaxy clusters. If we zoom out even further, we might see a network of galaxy clusters. (In fact, we do. It’s called the cosmic web.) But if we zoomed out as far as we can, perhaps the universe would appear to be made of a single, continuous fluid. In other words, Friedman imagined that the Earth is one electron orbiting one atomic nucleus in one molecule in a universe-sized gas cloud, which is spread out evenly everywhere. In modern lingo, we say that Friedman assumed the universe is homogeneous.

We can think of this like gazpacho soup. To make the soup, we throw in chunks of tomato, some cucumber, pepper, onions, garlic, maybe some salt. Each ingredient is a solid bit of stuff, and when we mix it together, it clumps together a bit. But when we stir thoroughly and then look at the soup as a whole, it just looks like a homogeneous fluid.

And More of the Same

In a bowl of gazpacho, all directions are not created equal. Imagine that you were shrunk to chopped-onion size and left to drown in a bowl of gazpacho. If you swim up, you’ll reach air; if you swim down, you’ll reach the bottom of the bowl. In other words, the “down” direction is fundamentally different from the “up” direction. On the other hand, when we look up at the night sky (and ignore things in our solar system), it looks essentially the same in every direction. Stars in one corner of the sky look no different from stars in another corner.

Friedmann took this sameness into account when he guessed how the stuff in the universe was distributed. In modern terminology, we say he assumed that the universe is isotropic.

Einstein’s Greatest Blunder

When Einstein made his theory of relativity public, the scientific community believed that the universe was static, neither expanding nor contracting. However, when Friedmann plugged in homogeneity and isotropy into Einstein’s equations, he discovered that the universe cannot be static. If you stick a reasonable amount and distribution of mass into the equations, the universe must either expand or contract—it can’t stay still.

(There’s some messy details about what’s “reasonable” here. You can make the universe static with the right mix of matter and dark energy, but this would be quite the cosmic coincidence… and it’s not what we observe. Also, in Friedmann’s time, no one knew about dark energy.)

Friedmann took his discovery to Einstein himself, who was very skeptical. Even after Friedmann was able to convince Einstein that his calculations were correct, Einstein rejected the physics. Instead, Einstein assumed there must be something wrong with his theory. He began developing a new theory of gravity with an added term in its equations, which he called the cosmological constant, in order to keep its predicted universe static.

In 1925, while Einstein was working on his static theory of gravity, Friedmann died mostly unrecognized. In 1927, Georges Lemaitres independently rediscovered Friedmann’s dynamic universe. He, too, took his discovery to Einstein–but by this time, Einstein was quite convinced of his static theory. He told Lemaitres: “Your calculations are correct, but your physics is atrocious.”

In 1929, Edwin Hubble observed that the universe is expanding, validating Friedmann and Lemaitres. The universe they discovered is now called the Friedmann–Lemaître–Robertson–Walker metric (a mouthful, I know) and is a central part of modern cosmological theory. Einstein eventually called the cosmological constant his “greatest blunder.”

If you want to read more about how Hubble made his discovery, I wrote about it in my article on the expanding universe:

Receding Horizons: Dark Energy and The Expanding Universe

Hubble’s discovery made waves in the scientific community—nearly everyone was as surprised by the expanding universe as Einstein was. The implications were (and are) fantastic. As Stephen Hawking said:

We observe that distant galaxies are moving away from us. They must have been closer together in the past.

If we extrapolate, all matter must have been infinitely close together at some point in the distant past. Indeed, Friedmann’s solution to Einstein’s equations predicts that the universe itself was infinitely compact, such that distance meant nothing—and thus the Big Bang theory was born. Lemaitres even predicted it when he advocated his and Friedmann’s idea:

If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time. (source).

Echoes Of the Big Bang

But alas, all was not well in Big Bang cosmology. Scientists were so uncomfortable with the idea of a huge initial explosion that they tried very hard to make the math work differently. Einstein advocated a universe that oscillated and undulated, alternatively growing and shrinking but always some fairly large size. (Friedmann actually suggested this at first because he couldn’t accept his own prediction of an eternally growing universe.) Fred Hoyl compromised and suggested that matter was constantly created to fill the void as the universe expanded, so that the universe always looked the same.

In 1941, George Gamow (who was a professor at the University of Colorado, where I got my Bachelor’s degree!) and Ralph Asher Alpher had a fantastic idea. If Lemaitres was correct, all the mass in the entire universe must have been scrunched together in a very small space. This extreme density would have produced extreme heat…so much that no atoms could form. Instead, all the subatomic particles must have swum together in a boiling soup of particles.

Even after it expanded and cooled a little bit, the universe would have been hot enough to ram protons, neutrons, and electrons together to form the first atoms. It would have been easiest to produce very light elements, however, which would explain why the majority of the mass in the universe seems to be hydrogen—the lightest element there is. We now call this idea Big Bang Nucleosynthesis, and it was one of the first hints of how stars are fuelled.

Alpher and Gamow planned on publishing a paper based on their discovery. But Gamow, ever the joker, clandestinely added his friend Hans Bethe to the list of authors. This way, the paper was the Alpher-Bethe-Gamow paper, which sounded like “Alpha-Beta-Gamma,” the first three letters of the Greek alphabet.

Alpher continued this work and, with his colleague Robert Herman, made a stunning discovery about Big Bang Nucleosynthesis:

It should have left an echo.

In the hot early universe, everything was moving very fast. But charged particles emit light when they move. So every moving proton and every moving electron, everywhere in the entire universe, was glowing intensely. The light was everywhere, moving in every direction. After atoms began to form, where did this light go? The staggering answer is that it’s still here. As the universe expanded and cooled, the light became dimmer and dimmer, redder and redder (thanks to cosmic redshift), but it never disappeared.

Everywhere in the universe, space is filled with a sea of microwaves (remember, microwaves are a very long wavelength of light) that is the echo of the Big Bang.

Radio Static

Although Alpher and Herman predicted this sea of light as early as 1948, no one bothered to look for it for a long time. In fact, when they were vindicated almost twenty years later, it was a complete accident.

In 1964, Arno Penzias  and Robert Wilson were trying to bounce radio waves off of satellites. To do this, they built a super-sensitive radio antenna, cooled it to within four degrees of absolute zero with liquid helium, and aimed it at the sky. To their surprise, however, they kept hearing static. It was loud, low-frequency, and constant. Whether they listened day or night, summer or winter, the static persisted. At first, Penzias and Wilson assumed their equipment must be faulty, but the static remained even after they had inspected and cleaned the array from top to bottom.

At the same time, a team at Princeton was finally planning to look for Alpher and Herman’s Big Bang echoes: Robert Dicke, Jim Peebles, and David Wilkinson. The Princeton team’s plans eventually reached the ears of our heroic radio scientists, Penzias and Wilson. When Penzias heard that people were looking for radio waves left over from a massive explosion at the beginning of time, he realized that his and Wilson’s noise was exactly this. Penzias immediately contacted the Princeton team and the five collaborated on the discovery. We now call these Big Bang echoes the cosmic microwave background.

This is the greatest triumph of the Big Bang theory and the biggest reason we believe it today. Alpher, Bethe, and Gamow used the Big Bang to predict the cosmic microwave background twenty years before we observed it. In this history, we see a beautiful interplay between theory and observation. From Einstein’s theory of relativity, Friedmann predicted a dynamic universe. From his own observations, Edwin Hubble discovered that the universe is expanding. From the expanding universe, Lemaitres extrapolated back to a time when everything was clumped together, then inferred that this point of infinite density must have constituted a transition between nothingness and everythingness. And from this inferred explosion, Alpher and Herman predicted a sea of particles left over from the beginning of time itself, which Penzias and Wilson accidentally observed.

But is it Really True?

Well, for the most part. There are some problems with the Big Bang theory, which are addressed by cosmic inflation. I’ll describe these problems, and how inflation solves them, in a later post.

Further Reading

• The Big Bang set to music.
• This is an excellent video on Big Bang Nucleosynthesis and the cosmic microwave background.
• Ethan Siegel has a wonderful blog on cosmology and he’s posted about Big Bang Nucleosynthesis and inflation.
• NASA has a nice page on the Big Bang.

Feedback?

Although I do think I know a fair amount about general relativity, I’m not an astrophysicist (yet!), so I may have gotten some of the details of Big Bang nucleosynthesis wrong. If I have and you know better, please correct me!

If you have any questions, comments, or insults, please don’t hesitate to let me know!