RACHEL BEAN: By the middle of the 20th century, it appeared that cosmology was effectively solved. Hubble's observations demonstrated that the universe was expanding in agreement with Einstein's theory of general relativity. The CMB showed that the universe changed over time, consistent with the Big Bang hypothesis. Over the next five decades, however, it was to become clear quite how little we know about the universe.
The first sense of this came from measurements performed by Vera Rubin at the Carnegie Institution in Washington in the 1970s. Newton's law of gravity tells us that the speed of rotation of planets around the sun is directly related to the sun's mass. Rubin applied the same principle to galaxies, measuring the speed of stars orbiting a galactic center to measure the mass in a galaxy. She compared this mass estimate to one that she got from essentially counting all the stars in a galaxy and assuming a reasonable mass for each of them. What you are seeing now is a graph of the rotation speed as a function of the distance from the galactic center. What she found was a startling disparity between the amount of luminous mass and the total mass from looking at the rotation speed. This implies that as well as luminous matter, there is also a significant amount of dark matter in galaxies that doesn't interact with light in any way.
In 1998, an equally puzzling discovery was made. Einstein's theory of general relativity predicts that the universe should be expanding, but that the rate of expansion should be slowing as the density of matter decreases, i.e., the expansion should be decelerating. Two groups of observers in Australia and the US were using distant supernovae, violent explosions of stars, to measure the expansion rate and therefore its rate of deceleration. What they found was astonishing. In contradiction to GR, the universe's expansion is accelerating, and not decelerating. Physicists have labeled the missing physics to explain this dark energy.
Understanding how we should modify Einstein's theory to explain this phenomenon is one of the central areas of research in cosmology today. Einstein himself may have provided the solution to the dark energy problem. Prior to Hubble's observations, scientists believed that the universe was not expanding. Einstein's equations disagreed, however. To address this, he added in a fudge factor to his equations, called the cosmological constant, to stop the universe expanding. After Hubble's observations of cosmic expansion in the 1920s, however, Einstein throughout his cosmological constant and is said to have called it his biggest blunder.
Today we are reintroducing the cosmological constant, however, to explain the accelerating universe. Whereas Einstein treated his cosmological constant as a mathematical quantity, nowadays we believe it has physical origins in quantum physics and maybe string theory. Cosmologists now realize that what we understand about the universe is significantly outweighed by what we don't know. 96 percent of the universe is made up of components we don't yet have a definitive theoretical explanation for, with 75% dark energy and 21% dark matter. Only 4% is the normal matter that planets and stars and humans are made of, matter that is within the standard model of particle physics.
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Cosmology uses observations of cosmic structures, like stars and galaxies, to understand the origin, evolution, and ultimate fate of the universe. Join Rachel Bean as she examines our current perception and evolving ideas of the universe.
This video is part 1 of 6 in The Puzzling Life of the Universe series.