The first image of a black hole, seen in silhouette against a background of hot gases. (EHT)
The 1969-72 Apollo moon landings took place in the era when humankind was just beginning to explore outer space with robotic probes and space-based observatories.
It was a time when we took the cosmos more at face value, with a "what you see is what you get" attitude. Black holes, for instance, were mind-bending, hypothetical objects whose existence was yet to be verified. And we still wondered if our sun might be the only star in the universe with planets.
In the decades since the Apollo missions, a long list of fresh discoveries has reshaped our understanding of the universe, from the cosmic to the subatomic. Here are eight of the most important of those since humans last landed on the moon.
Black Hole Confirmed
In 1971 strong emissions of X-rays were detected from a point in the constellation Cygnus. Like smoke from an unseen gun, the X-rays were believed to emanate from the first-ever detected black hole, though this wasn't confirmed for over 30 years.
The notion of a massive object with gravity so strong that even light cannot escape goes back to at least 1784, when the Englishman John Michell first published the idea. Einstein's theory of general relativity in the early 20th century predicted black holes, though the theoretical objects had such bizarre properties that Einstein himself was not convinced they could exist.
Life on the Ocean Floor
In 1977, a thriving ecosystem of living organisms was found on the floor of the deep ocean, surrounding a hydrothermal vent and subsisting entirely on heat and chemical energy emerging from Earth's interior. An NSF-funded team of marine geologists made the discovery in the geothermal hot spot of the Galapagos Rift.
This find provided a first example of life that thrives without sunlight in the cold, dark environment of the ocean floor, encouraging scientists to imagine how extraterrestrial life might form and prosper under very alien conditions on other worlds.
Dinosaur Killer Identified
In 1980, the Nobel-prize-winning physicist Luis Alvarez implicated an asteroid hitting Earth as the culprit responsible for the demise of the dinosaurs. This extinction event at the end of the Cretaceous geological period was a mystery that had gone unsolved for more than a century.
Alvarez's team found an unusually high concentration of the element iridium in the worldwide geologic layer of sediment marking the end of the Cretaceous period. Iridium is rare in Earth rocks, but abundant in asteroids, suggesting that a global asteroid- impact catastrophe was the logical source.
In the 1990s, a 100-mile wide impact crater was identified at the northern tip of the Yucatán Peninsula in Mexico, with its center near the town of Chixulub. Mostly buried under jungle and sea floor sediment, the crater was chemically dated to around 66 million years old, coinciding with the dying off of the dinosaurs.
Today, Chixulub crater is widely accepted as the fatal wound that ended the 200 million year dynasty of Earth's most famous extinct creatures.
First Planets Outside Our Solar System Found
The first confirmed discovery of a planet outside our solar system occurred in 1992, when two extrasolar planets were detected orbiting a pulsar, which is the remnant core of a dead star, in the constellation Virgo. The first detection of an exoplanet orbiting a star that is still active and burning fuel took place three years later.
Before these events, the existence of planets orbiting other stars was only speculation. To date, a total of 4,096 planets in almost 3,000 planetary systems outside our solar system have been confirmed, most of them in our general neighborhood of the Milky Way galaxy.
The Expansion of the Universe is Speeding Up
In 1998, the scientific community was stunned to discover that our universe is not only expanding, a fact known for decades, but expanding at an accelerating rate. Conventional wisdom dictated that gravitational attraction by matter within the universe should be slowing the expansion, but careful observations of a special type of supernova that serves as a precision tool for measuring distances across the universe revealed the opposite.
So the idea of "dark energy" was born, a strange form of energy thought to permeate the universe and exert a repulsive force on all large-scale structures -- galaxies and clusters of galaxies -- driving them farther apart at an ever-faster rate. Though its nature remains largely unknown, it is estimated that at least 68% of the universe's overall composition is made up of dark energy.
Factoring in another invisible substance called "dark matter," it turns out that the objects in the universe that we can see -- the type of stuff we and our planet and the stars are made of -- make up only about 4% of the universe's mass.
Images of Europa's cracked surface suggested that it is a shell of ice floating on top of an ocean that may be up to 100 miles deep and contain twice the water in Earth's ocean.
The existence of an ocean on a nearby world is reason for celebration by astrobiologists interested in finding life beyond Earth, and has compelled NASA and the ESA to mount space missions to conduct further exploration of Europa.
Not long after Europa's ocean was discovered, NASA's Cassini spacecraft detected plumes of water vapor spewing from Saturn's tiny moon Enceladus, further upping the stakes in the search for extraterrestrial life.
Black Holes Collide
In 2016, researchers at the Laser Interferometer Gravitational-wave Observatory, or LIGO, made the first-ever detection of gravity waves. Gravity waves are disturbances, or ripples, in the fabric of space-time, caused by the acceleration of massive objects in space. The detection of these waves allows us to perceive events in the universe that cannot be observed by conventional instruments like telescopes.
It was LIGO that detected the disturbance caused by two black holes colliding and merging, an event whose possibility was hypothesized but never observed. But because LIGO's highly sensitive laser-and-mirror array enables it to measure distortions in space-time smaller than the nucleus of an atom, it was able to catch the collision.
First Image of a Black Hole
In 2019, an international array of coordinated telescopes, collectively called the Event Horizon Telescope, or EHT, achieved what was conventionally thought to be impossible: It captured an image of the silhouette of one of the most elusive objects in the universe, a black hole.
The supermassive black hole caught on camera lies 53 million light years away, at the heart of the galaxy Messier 87, and contains the equivalent mass of 6.5 billion stars the size of our sun.
Black holes have long been famed as the ultimate dark object in the universe, impossible to capture in pictures by virtue of their strong gravity, which prevents any light from escaping. While it is a fact that light cannot get out of a black hole from inside its event horizon -- the distance at which the black hole's gravity becomes strong enough to prevent light from escaping -- it had long been thought that a black hole might be viewed in silhouette against the glow of hot gas surrounding it.
But black holes are too small and distant for conventional telescopes to observe. The EHT array, however, is not a conventional telescope; it's a collection of multiple millimeter-wavelength radio telescopes stationed at observatories from Antarctica to Greenland, Spain to Hawaii, and throughout the Americas. When their collective observations of a target object are synchronized, the EHT achieves imaging resolutions equal to an imaginary telescope that would measure half the size of Earth's diameter.
Going forward, what can we imagine will be discovered in the next 50 years?
Life forms swimming in Europa's remote ocean? A fresh and unexpected picture of the universe seen through the lens of dark energy telescopes? The long-sought radio signals from distant, intelligent civilizations?
If recent history is a guide, we can imagine now what we may soon no longer need to.
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