Top 10 cosmological discoveries

quasar 3C 273

Quasar 3C 273, shown here in an image from Hubble's Wide Field and Planetary Camera 2, was first identified in the 1960s by its radio signals. The unusually bright object is about 2.5 billion light-years away in the constellation Virgo. The detection of quasars comes in at No. 7 in the Top 10 cosmological discoveries.

ESA, Hubble, NASA

Talk about making a cosmic ripple. This week’s report of gravity waves from the Big Bang is the biggest cosmological news of the century. It’s the science news equivalent of a Category 5 hurricane or F5 tornado. If the new results hold up, the understanding of the universe will have taken a bigger leap for humankind than hopping around on the moon. And the BICEP2 result will join an illustrious list of Page One–worthy discoveries that have advanced modern science’s knowledge about the cosmos. My top 10 (P.S. Theories, for example general relativity or heliocentrism, do not count as discoveries):

10. Cosmic microwaves are black body radiation (COBE, 1990)

To most experts, the faint glow of microwave radiation pervading the universe seemed like good evidence for the Big Bang. If so, though, that radiation should show a precise pattern for its intensity at various wavelengths. If the radiation deviated from that pattern, known as a black body spectrum, then perhaps some non-Big Bang cause was responsible. In 1990, though, measurements by the COBE satellite showed that the cosmic microwave radiation matched the black body spectrum perfectly. Opposing the Big Bang was no longer tenable.

9. Comic microwave background anisotropies (COBE, 1992)

Everybody knew that the microwave background couldn’t be completely smooth — otherwise there would be no galaxies today. The small seeds of matter that eventually grew into galaxies would have left an imprint in the temperature of the radiation, causing slight temperature differences (anisotropies) between different points on the sky. COBE was the first satellite to detect those differences and measure how big they were, crucial clues to piecing together the history of the universe. In presenting the images during the announcement of the finding, George Smoot of the COBE team said it was like looking at God.

8. Space is flat (BOOMERanG, 2000)

In a great victory for balloon science, the BOOMERanG project (which flew around the South Pole and came back to its starting place) measured the angles between ripples in the cosmic microwaves and concluded that the geometry of space was very close to perfectly Euclidean — in technical terms, “flat.” Showing that space on the whole was flat helped confirm the discovery of dark energy (see No. 5). (Without dark energy, there wasn’t enough matter and energy in space to make it flat.)

7. Quasars (Maarten Schmidt, 1963)

It was a great surprise at the time to find bright objects literally on the outskirts of the visible portion of the universe. Quasars seemed like stars (quasistellar) but were too far away to see unless they harbored some enormously energetic phenomena. Quasars beamed radio signals or other electromagnetic radiation across the universe, providing a new source of information on events transpiring in the cosmos.

6. Dark matter (Fritz Zwicky, 1931)

Zwicky noticed that the motions of galaxies in the Coma Cluster could not be explained by gravity if the only matter in the cluster was visible (that is, giving off light). He deduced that a lot of the matter in the universe was dark. “If this … is confirmed,” he wrote, “we would arrive at the astonishing conclusion that dark matter is present with a much greater density than luminous matter.” Only much later did scientists realize that not only was most of the matter in the universe invisible, it was also of some type totally unlike the ordinary matter, composed primarily of protons and neutrons, found on Earth.

5. Dark energy (Saul Perlmutter et al, Brian Schmidt et al, 1998)

Another shock to the cosmocommunity came from two independent teams in 1998. Data from distant supernovas showed their brightness wasn’t quite right if the universe’s expansion had slowly been decelerating, as most experts had long believed. Instead, the universe is expanding faster and faster. Accelerated expansion implies that something in space — now called dark energy —is pushing the cosmos apart. A new entry to this Top 10 list will have to be made whenever anybody figures out what the dark energy actually is.

4. Primordial gravity waves (BICEP2, 2014)

By far, the top cosmological discovery of the 21st century. So far, at least. Besides confirming that gravity waves really do exist (thereby further validating general relativity), this discovery provides as sure a sign as you can get that inflation instantly after the Big Bang set the stage for the future evolution of the cosmos. This discovery may also turn out to imply the existence of an infinity of parallel universes, as most versions of inflation require that our universe is just one of many big spacetime bubbles — a multiverse. Ours would surely still be the best of all possible bubbles, though.

3. Cosmic microwave background radiation (Arno Penzias and Robert Wilson, 1964)

It was discovered by accident when Penzias and Wilson found out that no matter how much they cleaned their radio antenna, it still recorded static from everywhere in space. That static was the echo of the birth of the universe, literally the smoke from the Big Bang gun, in the form of microwaves. That radiation was leftover from the Big Bang fireball — very hot at the beginning, but now less than 3 degrees above absolute zero (kelvins). Very cold.

2. Island universes (Edwin Hubble, 1925)

In a sort of historical precedent for the notion of the multiverse, Hubble showed that some of the fuzzy patches of light known as nebulae were not clouds within the Milky Way galaxy, but entire galaxies (“island universes”) unto themselves. Previously the standard view, articulated most forcefully by Harlow Shapley, held that the Milky Way, home to Earth and sun, made up virtually the entire universe. But Hubble, using the most powerful telescope available, detected a star in the Andromeda nebula that varied in brightness on a regular schedule. Such “Cepheid variables” had been used by Shapley himself to gauge the distance scale of the Milky Way, so he had to concede when Hubble showed that Andromeda was vastly far beyond the Milky Way’s outskirts.

1. Universe is expanding (Hubble, 1929)

Others had figured out that the universe might be expanding. But Hubble, using data collected by Vesto Slipher and Milton Humason, published the definitive analysis establishing that the cosmos actually is growing bigger. It was the greatest intellectual upheaval in the human conception of the cosmos since Copernicus. Displacing this one from Number One on the list would require something really radical — like maybe the multiverse.

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Tom Siegfried is a contributing correspondent. He was editor in chief of Science News from 2007 to 2012 and managing editor from 2014 to 2017.

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