(2) The discovery in the late 1990s that the expansion of the universe is accelerating, apparently because of a tiny but nonzero energy density of the vacuum, upending many of our ideas about the cosmos. (1) The discovery in October 1974 of the J/psi particle, interpreted in terms of a new quark, the charmed quark, which gave dramatic confirmation to the then emerging Standard Model of particle physics. The most spectacular discoveries in fundamental physics since I started graduate school in 1973 have been the following: Tracy Slatyer, Massachusetts Institute of Technology Charmed Quarks and Accelerating Cosmos I’ve read references published around 1990 that talk confidently about how we will soon use supernovae to measure the rate at which the expansion of the universe is decelerating and the curvature of the cosmos and how this will tell us about the ultimate fate of our universe (because closed matter-dominated universes undergo a ‘ big crunch,’ while open ones expand forever)-very little of which applies to the dark-energy-dominated, spatially-flat cosmos that we appear to actually live in! I think this also qualifies because even with the benefit of hindsight, it still seems very surprising that the dark energy/cosmological constant has its measured value. I think the accelerating expansion of the universe has to be a strong contender. This cosmological constant problem is thought by many to be the deepest unsolved problems in all of modern physics.” - Katherine Freese, University of Texas at Austin Expanding Universe Now with the discovery of dark energy, however, the number must be driven down to a particular tiny value, which is much harder to explain. Editor’s Note: If the constant was this large, the universe would have expanded much, much faster than it did.] This conundrum has been known for some time, and theorists conjectured that there must be some physics that drives the number down to zero instead. The trouble is that the predicted value for the cosmological constant from calculations using quantum field theory produces a number that is too large by a factor of 10 120. The simplest explanation would be a cosmological constant originally introduced by Albert Einstein as a possible term in the equations of the general theory of relativity but then abandoned by him as his “biggest blunder.” Now it seems he may have been right after all. This discovery of dark energy in particular created a paradigm shift. I, for one, am working to identify the nature of these mysterious components. Most of the universe is “the dark side”: the universe is thought to consist of 25 percent dark matter and 70 percent dark energy. We now have a standard model of cosmology in which the ordinary matter and energy that we experience in our daily lives-our body, the air we breathe, the walls around us, and all the stars and planets-add up to only 5 percent of the content of the universe. After the initial discovery, many other observations of different types confirmed this result, such as studies of the cosmic microwave background, which is the leftover light from the big bang, and studies of clusters of galaxies. The name “dark energy” was given to the material that causes this acceleration. These objects are very well understood, no matter how far back in time they are observed, so alternative explanations just don’t work. None of us working in physics saw that coming! The observations that distant supernovae are dimmer than expected led to the idea that the universe is not just expanding but accelerating. One of the most shocking findings in the history of physics was the discovery of dark energy just before the turn of the millennium. Here’s a selection of physicists’ responses on the most amazing, stunning and flabbergasting findings. And many of these events occurred relatively recently, showing that the field of physics continues to astound us. ![]() A few discoveries, such as the accelerating expansion of the universe, were so groundbreaking that multiple experts picked them as top choices. The surprises in the history of physics are far too many to comprehensively describe, but we polled a variety of physicists for some of their favorites. ![]() And it was less than 100 years ago that researchers found out the Milky Way wasn’t the only galaxy in the cosmos but rather one of billions. For instance, scientists once thought atoms were the smallest bits of matter in existence until they split atomic nuclei to find protons and neutrons, which in turn proved to be made of even smaller fundamental particles, called quarks. Many truths about the universe we live in and the particles that make up ourselves and the world around us, as well as the forces that drive them, seemed to come out of left field when they were first discovered. Ever since Isaac Newton and the falling apple, surprises have often pushed physics forward.
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