Resource: Gravity and the Expanding Universe

In 1998, astonishing observational data revealed that the universe was not only expanding, but that its expansion was actually accelerating. Scientists had long thought that there were three possible shapes for the universe — closed, flat, and open — all of which included a slowing of the expansion rate. But recent research suggests that none of these descriptions was entirely right. Observations indicate that the shape of the universe is indeed flat, but that its expansion rate has increased within the past 5 or 6 billion years.

Scientists are at a loss to explain what could be causing this acceleration, but it is one of the hottest research topics of modern astrophysics. Evidently, there is a mysterious force, dubbed dark energy, which dominates the universe and counteracts the attractive force of gravity on very large scales. Astronomers suggest that 70% of the universe is dark energy and that the normal matter of stars and life composes only about 5%. The remaining 25% of the universe is thought to be dark matter — matter we cannot see, but that scientists believe exists. The repulsive effects of dark energy could possibly be explained with modified theories of gravity or string theory. However, the struggle to understand dark energy is also resurrecting the history of science. Two promising theories for dark energy are quintessence, a dynamic energy of empty space, and the cosmological constant, a steady energy first introduced by Albert Einstein.

In the early 1900s, scientists thought that the universe was static — neither contracting nor expanding. Einstein explored the question of why, with all its combined matter, the universe did not succumb to gravity and collapse in on itself. As an explanation, he introduced the idea of a universal force that opposes gravity and pushes the universe apart. He called this anti-gravity force the cosmological constant. Although he could not measure it, Einstein was convinced that this force had to exist.

Not long after Einstein proposed the cosmological constant, scientists observed that the universe was much bigger than previously thought. In fact, the astronomer Edwin Hubble concluded it was expanding. Faced with this knowledge, Einstein renounced his own idea, calling it his "biggest blunder." Today, theoretical physicists studying exploding stars, called supernovae, have found that the rate of expansion of the universe is also accelerating. Even though our understanding of dark energy is very speculative, Einstein's idea of the cosmological constant is still a real possibility.

To learn more about the discovery of the expanding universe, check out Hubble's Expanding Universe.

To learn more about the history and fate of the universe, check out History of the Universe.

To learn more about the size of the universe, check out How Big Is Our Universe?.

• What led Einstein to hypothesize the cosmological constant?
• What made Einstein wish he could "put the genie back in the bottle"?
• What is the evidence for the Big Bang Theory, as presented in the video?
• The narrator indicates that our understanding of the impact of gravity on galaxies and stars may be incomplete. What does he mean?
• Why were scientists surprised by the evidence that the expansion of the universe was accelerating? What is their explanation in this video for the expansion?