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Section 1: Introduction

We want to know what the universe is made of and how it has changed over time. Astronomers have been exploring these questions, and have discovered some surprising results. The universe is expanding, and the expansion is being accelerated by a "dark energy" that today apparently makes up more than 70 percent of the universe.

Type Ia supernovae, like the one shown here in the outskirts of galaxy NGC 4526, have been used to trace the influence of dark energy on the expansion of the universe.

Figure 1: Type Ia supernovae, like the one shown here in the outskirts of galaxy NGC 4526, have been used to trace the influence of dark energy on the expansion of the universe.

Source: © High-Z Supernova Search Team, NASA/ESA Hubble Space Telescope. More info

The universe that contains our planet, our star, and our galaxy, as well as 1011 other galaxies and their stars and planets, obeys the same physical laws that we have uncovered in our exploration of nature here on Earth. By applying these laws, we have learned the scale of the universe, and the surprising fact that the other galaxies appear to be moving away from us as the universe stretches out in all directions. Astronomers detected this cosmic expansion in the 1920s. They understood it by applying Einstein's general relativity—the theory of gravity—to the universe as a whole. Recent work using exploding stars to measure the history of cosmic expansion shows that the universe is not slowing down due to the familiar braking action of gravity we know from everyday experience. The expansion of the universe has actually sped up in the last 5 billion years. In Einstein's theory, this can happen if there is another component to the universe. We call that mysterious substance "dark energy," and we seek to discover its properties through astronomical observations.

Today's astronomical measurements show that dark energy makes up about 70 percent of the universe. So our ignorance is very substantial. This deep mystery lies at the heart of understanding gravity, which is no simple matter, as we saw in the first part of this course. Future observations will trace the growth of lumpy structures in the universe. These measurements can help distinguish between the effects of dark energy and possible imperfections in our understanding of gravity. We may face more surprises ahead.

This unit spells out how astronomers measure distance and velocity and describes how an expanding universe best explains these measurements. We show how recent observations of exploding stars, the glow from the Big Bang, and the clustering of galaxies make the case for dark energy as the largest, but least understood, component of the universe.