Ben Gilliland, is a science writer and illustrator and the 2013 winner of the Sir Arthur Clarke Award for space media. He recently published "How to Build a universe: From the Big Bang to the End of the universe" (Sterling, 2015). Gilliland contributed this excerpt to Space.com's Expert Voices: Op-Ed & Insights.
How We Discovered the Big Bang
The notion that the universe was born in a "Big Bang," or indeed anything else, is a relatively new concept. In fact, even the term Big Bang was coined by someone who didn't believe in it and intended it as a disparaging putdown. Today, however, Big Bang theory is one of the most successful ideas in science, but how did we get there?
From the age of the ancient Greeks to the Scientific Revolution nearly 2,000 years later, it was believed that everything in the universe was entrapped within a series of celestial spheres that encased the Earth, which was (of course) the central pivot around which the rest of existence rotated. These celestial spheres were the Solar System, which was thought to be the full extent of the universe. [More Scrutiny? Major Big Bang Discovery Study Published ]
Science moved on a bit by the 16th and 17th centuries, with the likes of the astronomer Nicolaus Copernicus and the famous Italian polymath Galileo Galilei, who used reasoning, mathematics, and observation to prove that the Earth and the rest of the planets orbit the sun.
A crucial innovation at that time was the invention of the telescope. Originally little more than an amusing curiosity, the telescope was introduced to astronomy in 1609 by Galileo Galilei and the not-so famous English polymath, Thomas Harriot (who used his telescope to sketch the Moon four months before Galileo's celebrated observations, and who is sometimes credited with introducing the potato to England).
The telescope helped increase the size at which we could view the universe. Galileo's observations of the strange milky band that crossed the night sky revealed that it was made up of stars — the universe had now increased in size to include the Milky Way.
Beyond the Solar System
After a few decades of being used to peer at planets, moons and comets, the telescope was next put to use to seek out objects beyond our home galaxy. In the late 1700s, a Frenchman, Charles Messier — who was trying his best to discover new comets (he discovered 13 in his lifetime) — kept stumbling across strange fuzzy objects in the heavens, which he would at first mistake for comets. To avoid this confusion, Messier compiled a catalog of the strange nebulous glows. By the time he died, oblivious to what he had been cataloging, he had charted the locations of 103 of these white smudges. For the next two centuries, the identity of his "Messier objects" remained a mystery.
By the 19th century there were two schools of thought as to the identity of the nebulae. One, championed in the previous century by the great astronomer William Herschel, was that they were "island universes" located beyond our own Milky Way. The other, more popular, idea was that they were little more than clouds of gas floating within (or just outside of) the Milky Way.
In the 1860s, British astronomer William Huggins borrowed a trick from the field of chemistry, which moved things on a bit: spectroscopy. A spectroscope is an instrument that splits light into its component colors like raindrops split sunlight to create a rainbow— spreading out the spectrum of light into its different wavelengths. Hidden in the rainbow is a series of bright or dark lines (called emission and absorption lines) that are caused by chemical elements in the object the light is coming from. These lines act like a sort of chemical barcode that allows you to identify the element that created it.
Using spectroscopy, Huggins was able to identify the elements that make up the sun and compare the sun's barcode with that of other stars. He found that starlight contained pretty much the same spectral barcode as the sun — meaning that distant stars were made of the same mixture of chemical elements as the star on our doorstep.
Huggins then turned his spectroscope to Messier's nebulae and, beginning in 1864, he examined the spectra of about 70. Around a third of the clouds didn't exhibit the spectral patterns of stars but instead seemed to be "just" clouds of hot gas. But the majority showed patterns that could only have been produced by stars.
But, were these nebulae just collections of stars floating around within the Milky Way, or were they more distant? The answer wouldn't be forthcoming for several decades, so, for the time being at least, the extent of the universe remained bound up within the Milky Way.
In the 1920s, an American astronomer, Edwin Hubble, finally solved the mystery of Messier's fuzzy objects. He proved that they were actually other galaxies located outside of our own galaxy, the Milky Way. The universe was suddenly a whole lot bigger than we humans had realized.
A Day Without Yesterday
Although Hubble is often credited with thinking up the idea of an expanding universe, the true Big Bang daddy was (perhaps a little ironically) a Catholic priest from Belgium called Georges Lemaitre.
In 1927, Lemaitre proposed that distant galaxies appeared to have been shifted into the red part of the electromagnetic spectrum — i.e. redshifted — because they are moving away from us, carried away by a universe that was expanding in all directions.
The Theory of General Relativity & Hubble's Law
A man who had never touched a telescope had come to the conclusion that the universe must be expanding more than a decade earlier than Lemaitre. When Albert Einstein formulated his theory of General Relativity in 1916, which describes gravity as the result of mass, energy and the curvature of space-time, he found the equations were telling him that the universe had either to be expanding or shrinking, but it couldn't be static. Einstein thought this must be a mistake, so to balance things out he added a bit of mathematical jiggery-pokery to his equations that he called the cosmological constant — a move he would later describe as his "biggest blunder." (However, we'll discover later in this book that the cosmological constant was not the cosmological cock-up Einstein thought it was.)
In 1929, Edwin Hubble provided observational evidence of Lemaitre's theory of an expanding universe by showing that, relative to Earth, the galaxies were indeed receding. He also showed that the more distant the galaxy, the greater its redshift and the faster it appeared to be moving. From this, Hubble formulated his Redshift Distance Law of Galaxies or, as we know it today, Hubble's Law.
But how were the galaxies moving away from us? It is tempting to think that galaxies are whizzing away through space like shrapnel from a bomb, but this is not the case. Hubble's Law, when combined with Einstein's equations, showed that, rather than shooting through space, galaxies were actually being carried by the expanding fabric of space itself (like chocolate chips are carried apart on the surface of a rising cupcake).
Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Space.com.