From a Universe that went no bigger than our Milky Way to the trillions of galaxies in our expanding Universe, our knowledge increased one step at a time.
“Gamow was fantastic in his ideas. He was right, he was wrong. More often wrong than right. Always interesting; … and when his idea was not wrong it was not only right, it was new.” –Edward Teller
Exactly 100 years ago, our conception of the Universe was far different from what it is today. The stars within the Milky Way were known, and were known to be at distances up to thousands of light years away, but nothing was thought to be further. The Universe was assumed to be static, as the spirals and ellipticals in the sky were assumed to be objects contained within our own galaxy. Newton’s gravity still hadn’t been overthrown by Einstein’s new theory, and scientific ideas like the Big Bang, dark matter, and dark energy hadn’t even been thought up yet. But during each decade, huge advances were made, all the way up to the present day. Here’s a highlight of how each one moved our scientific understanding of the Universe forward.
1910s — Einstein’s theory confirmed! General Relativity was famed for making the explanation that Newton’s gravity couldn’t: the precession of Mercury’s orbit around the Sun. But it isn’t enough for a scientific theory to explain something we’ve already observed; it needs to make a prediction about something that’s yet to be seen. While there have been many over the past century — gravitational time dilation, strong and weak lensing, frame dragging, gravitational redshift, etc. — the first was the bending of starlight during a total solar eclipse, observed by Eddington and his collaborators in 1919. The observed amount of bending of starlight around the Sun was consistent with Einstein and inconsistent with Newton. Just like that, our view of the Universe would change forever.
1920s — We still didn’t know there was a Universe out there beyond the Milky Way, but that all changed in the 1920s with the work of Edwin Hubble. While observing some of the spiral nebulae in the sky, he was able to pinpoint individual, variable stars of the same type that were known in the Milky Way. Only, their brightness was so low that they needed to be millions of light years away, placing them far outside the extent of our galaxy. Hubble didn’t stop there, measuring the recession speed and distances for over a dozen galaxies, discovering the vast, expanding Universe we know today.
1930s — It was thought for a long time that if you could measure all the mass contained in stars, and perhaps add in the gas and dust, you’d account for all the matter in the Universe. Yet by observing the galaxies within a dense cluster (like the Coma cluster, above), Fritz Zwicky showed that stars and what we know as “normal matter” (i.e., atoms) was insufficient to explain the internal motions of these clusters. He dubbed this new matter dunkle materie, or dark matter, an observation that was largely ignored until the 1970s, when normal matter was better understood, and dark matter was shown to exist in great abundance in individual, rotating galaxies. We now know it to outmass normal matter by a 5:1 ratio.
1940s — While the vast majority of experimental and observational resources went into spy satellites, rocketry and the development of nuclear technology, theoretical physicists were still hard at work. In 1945, George Gamow made the ultimate extrapolation of the expanding Universe: if the Universe is expanding and cooling today, then it must have been hotter and denser in the past. Going backwards, there must have been a time where it was so hot and dense that neutral atoms couldn’t form, and before that where atomic nuclei couldn’t form. If this were true, then before any stars ever formed, that material the Universe began with should have a specific ratio of the lightest elements, and there ought to be a leftover glow permeating all directions in the Universe just a few degrees above absolute zero today. This framework is today known as the Big Bang, and was the greatest idea to come out of the 1940s.
1950s — But a competing idea to the Big Bang was the Steady-State model, put forth by Fred Hoyle and others during the same time. Spectacularly, both sides argued that all the heavier elements present on Earth today were formed in an earlier stage of the Universe. What Hoyle and his collaborators argued was that they were made not during an early, hot and dense state, but rather in previous generations of stars. Hoyle, along with collaborators Willie Fowler and Geoffrey and Margaret Burbidge, detailed exactly how elements would be built up the periodic table from nuclear fusion occurring in stars. Most spectacularly, they predicted helium fusion into carbon through a process never before observed: the triple-alpha process, requiring a new state of carbon to exist. That state was discovered by Fowler a few years after it was proposed by Hoyle, and is today known as the Hoyle State of carbon. From this, we learned that all the heavy elements existing on Earth today owe their origin to all the previous generations of stars.
1960s — After some 20 years of debate, the key observation that would decide the history of the Universe was uncovered: the discovery of the predicted leftover glow from the Big Bang, or the Cosmic Microwave Background. This uniform, 2.725 K radiation was discovered in 1965 by Arno Penzias and Bob Wilson, neither of whom realized what they had discovered at first. Yet over time, the full, blackbody spectrum of this radiation and even its fluctuations were measured, showing us that the Universe started with a “bang” after all.
1970s — At the very end of 1979, a young scientist had the idea of a lifetime. Alan Guth, looking for a way to solve some of the unexplained problems of the Big Bang — why the Universe was so spatially flat, why it was the same temperature in all directions, and why there were no ultra-high-energy relics — came upon an idea known as cosmic inflation. It says that before the Universe existed in a hot, dense state, it was in a state of exponential expansion, where all the energy was bound up in the fabric of space itself. It took a number of improvements on Guth’s initial ideas to create the modern theory of inflation, but subsequent observations — including of the fluctuations in the CMB, of the large-scale structure of the Universe and of the way galaxies clump, cluster and form — all have vindicated inflation’s predictions. Not only did our Universe start with a bang, but there was a state that existed before the hot Big Bang ever occurred.
1980s — It might not seem like much, but in 1987, the closest supernova to Earth occurred in over 100 years. It was also the first supernova to occur when we had detectors online capable of finding neutrinos from these events! While we’ve seen a great many supernovae in other galaxies, we had never before had one occur so close that neutrinos from it could be observed. These 20-or-so neutrinos marked the beginning of neutrino astronomy, and subsequent developments have since led to the discovery of neutrino oscillations, neutrino masses, and neutrinos from supernovae occurring more than a million light years away. If the current detectors in place are still operational, the next supernova within our galaxy will have over a hundred thousand neutrinos detected from it.
1990s — If you thought dark matter and discovering how the Universe began was a big deal, then you can only imagine what a shock it was in 1998 to discover how the Universe was going to end! We historically imagined three possible fates:
- That the expansion of the Universe would be insufficient to overcome everything’s gravitational pull, and the Universe would recollapse in a Big Crunch.
- That the expansion of the Universe would be too great for everything’s combined gravitation, and everything in the Universe would run away from one another, resulting in a Big Freeze.
- Or that we’d be right on the border between these two cases, and the expansion rate would asymptote to zero but never quite reach it: a Critical Universe.
Instead, though, distant supernovae indicated that the Universe’s expansion was accelerating, and that as time went on, distant galaxies were increasing their speed away from one another. Not only will the Universe freeze, but all the galaxies that aren’t already bound to one another will eventually disappear beyond our cosmic horizon. Other than the galaxies in our local group, no other galaxies will ever encounter our Milky Way, and our fate will be a cold, lonely one indeed. In another 100 billion years, we’ll be unable to see any galaxies beyond our own.
2000s — The discovery of the Cosmic Microwave Background didn’t end in 1965, but our measurements of the fluctuations (or imperfections) in the Big Bang’s leftover glow taught us something phenomenal: exactly what the Universe was made of. Data from COBE was superseded by WMAP, which in turn has been improved upon by Planck. In addition, large-scale structure data from big galaxy surveys (like 2dF and SDSS) and distant supernova data has all combined to give us our modern picture of the Universe:
- 0.01% radiation in the form of photons,
- 0.1% neutrinos, which contribute ever so slightly to the gravitational halos surrounding galaxies and clusters,
- 4.9% normal matter, which includes everything made of atomic particles,
- 27% dark matter, or the mysterious, non-interacting (except gravitationally) particles that give the Universe the structure we observe,
- and 68% dark energy, which is inherent to space itself.
2010s — The decade isn’t out yet, but so far we’ve already discovered our first potentially Earth-like habitable planets, among the thousands and thousands of new exoplanets discovered by NASA’s Kepler mission, among others. Yet, arguably, that’s not even the biggest discovery of the decade, as the direct detection of gravitational waves from LIGO not only confirms the picture that Einstein first painted, of gravity, back in 1915. More than a century after Einstein’s theory was first competing with Newton’s to see what the gravitational rules of the Universe were, general relativity has passed every test thrown at it, succeeding down to the smallest intricacies ever measured or observed.
The scientific story is not yet done, as there’s so much more of the Universe still to discover. Yet these 11 steps have taken us from a Universe of unknown age, no bigger than our own galaxy, made up mostly of stars, to an expanding, cooling Universe powered by dark matter, dark energy and our own normal matter, teeming with potentially habitable planets and that’s 13.8 billion years old, originating in a Big Bang which itself was set up by cosmic inflation. We know our Universe’s origin, it’s fate, what it looks like today, and how it came to be this way. May the next 100 years hold just as many scientific advances, revolutions, and surprises for us all.
Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.