input: For me the really satisfying things I do are offered me, free, for nothing. Ever go out in the fall and do a little hunting? See the frost on the grass and the leaves turning? Spend a day in the hills alone, or with good companions? Watch a sunset and a moonrise? Notice a bird in the wind? A stream in the woods, a storm at sea, cross the country by train, and catch a glimpse of something beautiful in the desert, or the farmlands? Free to everybody ... Cooper's twenty-year friendship with Ernest Hemingway began at Sun Valley in October 1940. The previous year, Hemingway drew upon Cooper's image when he created the character of Robert Jordan for the novel For Whom the Bell Tolls. The two shared a passion for the outdoors, and for years they hunted duck and pheasant, and skied together in Sun Valley. Both men admired the work of Rudyard Kipling--Cooper kept a copy of the poem "If--" in his dressing room--and retained as adults Kipling's sense of boyish adventure. As well as admiring Cooper's hunting skills and knowledge of the outdoors, Hemingway believed his character matched his screen persona, once telling a friend, "If you made up a character like Coop, nobody would believe it. He's just too good to be true." They saw each other often, and their friendship remained strong through the years. Cooper's social life generally centered on sports, outdoor activities, and dinner parties with his family and friends from the film industry, including directors Henry Hathaway, Howard Hawks, William Wellman, and Fred Zinnemann, and actors Joel McCrea, James Stewart, Barbara Stanwyck, and Robert Taylor. As well as hunting, Cooper enjoyed riding, fishing, skiing, and later in life, scuba diving. He never abandoned his early love for art and drawing, and over the years, he and his wife acquired a private collection of modern paintings, including works by Pierre-Auguste Renoir, Paul Gauguin, and Georgia O'Keeffe. Cooper owned several works by Pablo Picasso, whom he met in 1956. Cooper also had a lifelong passion for automobiles, with a collection that included a 1930 Duesenberg. Cooper was naturally reserved and introspective, and loved the solitude of outdoor activities. Not unlike his screen persona, his communication style frequently consisted of long silences with an occasional "yup" and "shucks". He once said, "If others have more interesting things to say than I have, I keep quiet." According to his friends, Cooper could also be an articulate, well-informed conversationalist on topics ranging from horses, guns, and Western history to film production, sports cars, and modern art. He was modest and unpretentious, frequently downplaying his acting abilities and career accomplishments. His friends and colleagues described him as charming, well-mannered, and thoughtful, with a lively boyish sense of humor. Cooper maintained a sense of propriety throughout his career and never misused his movie star status--never sought special treatment or refused to work with a director or leading lady. His close friend Joel McCrea recalled, "Coop never fought, he never got mad, he never told anybody off that I know of; everybody that worked with him liked him."

Answer this question "What's an important fact in this article?"
output: Hemingway believed his character matched his screen persona,

Question: Andrei Dmitrievich Sakharov (Russian: Andrei Dmitrievich Sakharov; 21 May 1921 - 14 December 1989) was a Russian nuclear physicist, dissident, and activist for disarmament, peace and human rights. He became renowned as the designer of the Soviet Union's RDS-37, a codename for Soviet development of thermonuclear weapons. Sakharov later became an advocate of civil liberties and civil reforms in the Soviet Union, for which he faced state persecution; these efforts earned him the Nobel Peace Prize in 1975.

After 1965 Sakharov returned to fundamental science and began working on particle physics and physical cosmology. He mainly tried to explain the baryon asymmetry of the universe; in that regard, he was the first to propose proton decay and to consider CPT-symmetric events occurring before the Big Bang: We can visualize that neutral spinless maximons (or photons) are produced at t < 0 from contracting matter having an excess of antiquarks, that they pass "one through the other" at the instant t = 0 when the density is infinite, and decay with an excess of quarks when t > 0, realizing total CPT symmetry of the universe. All the phenomena at t < 0 are assumed in this hypothesis to be CPT reflections of the phenomena at t > 0. Sakharov was the first scientist to introduce twin universes he called "sheets". He achieved a complete CPT symmetry since the second sheet is populated by invisible "shadow matter" which is antimatter (C-symmetry) because of an opposite CP-violation there, and the two sheets are mirror of each other both in space (P-symmetry) and time (T-symmetry) through the same initial gravitational singularity. In his first model the two universes did not interact, except via local matter accumulation whose density and pressure become high enough to connect the two sheets through a bridge without spacetime between them, but with a continuity of geodesics beyond the Schwarzschild radius with no singularity, allowing an exchange of matter between the two conjugated sheets, based on an idea after Igor Dmitriyevich Novikov. Novikov called such singularities a collapse and an anticollapse, which are an alternative to the couple black hole and white hole in the wormhole model. Sakharov also proposed the idea of induced gravity as an alternative theory of quantum gravity.

Using a quote from the above article, answer the following question: What happen in the 70s
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Answer:
Sakharov was the first scientist to introduce twin universes he called "sheets".