Chapter 23 Flashcards
What do we mean by dark matter and dark energy?
Dark matter and dark energy have never been directly observed, but each has been proposed to exist because it seems the simplest way to explain a set of observed motions in the universe. Dark matter is the name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds. Dark energy is the name given to the form of energy thought to be causing the expansion of the universe to accelerate.
What is the evidence for dark matter in galaxies?
The orbital velocities of stars and gas clouds in galaxies do not change much with distance from the center of the galaxy. Applying Newton’s laws of gravitation and motion to these orbits leads to the conclusion that the total mass of a galaxy is far larger than the mass of its stars. Because no detectable visible light is coming from this matter, we call it dark matter.
What is the evidence for dark matter in clusters of galaxies?
We have three different ways of measuring the amount of dark matter in clusters of galaxies: from galaxy orbits, from the temperature of the hot gas in clusters, and from the gravitational lensing predicted by Einstein. All these methods are in agreement, indicating that the total mass of a galaxy cluster is at least 40 times the mass of its stars.
Does dark matter really exist?
We infer that dark matterexists from its gravitational influence on the matter wecan see, leaving two possibilities: Either dark matter exists or there is something wrong with our understanding of gravity. We cannot rule out the latter possibility, butwe have good reason to be confident about our current understanding of gravity and the idea that dark matter isreal.
What might dark matter be made of?
Some of the dark matter could be ordinary (baryonic) matter in the form of dim stars or planetlike objects, but the amount of deuterium left over from the Big Bang and the patterns in the cosmic microwave background both indicate that ordinary matter adds up to only about one-sixth of the total amount of matter. The rest of the matter is hypothesized to be exotic (nonbaryonic) dark matter consisting of as-yet-undiscovered subatomic particles called WIMPs.
What is the role of dark matter in galaxy formation?
Because most of a galaxy’s mass is in the form of dark matter, the gravity of that dark matter is probably what formed protogalactic clouds and then galaxies from slight density enhancements in the early universe.
What are the largest structures in the universe?
Galaxies appear to be distributed in gigantic chains and sheets that surround great voids. These large-scale structures trace their origin directly back to regions of slightly enhanced density early in time.
What is the evidence for an accelerating expansion?
Observations of distant supernovae show that the expansion of the universe has been speeding up for the last several billion years. No one knows the nature of the mysterious force causing this acceleration. However, its characteristics are consistent with models in which the force is produced by a form of dark energy that pervades the universe.
Why is flat geometry evidence for dark energy?
Observations of the cosmic microwave background also support the existence of dark energy because they demonstrate that the overall geometry of the universe is nearly flat. According to Einstein’s general theory of relativity, the universe can be flat only if the total amount of mass-energy it contains is equal to the critical density, but measurements of the total amount of matter show that it represents only about 30% of the critical density. We therefore infer that about 70% of the total mass-energy is in the form of dark energy—the same amount implied by the supernova observations.
What is the fate of the universe?
If dark energy is indeed what’s driving the acceleration of the universe’s expansion, then we expect the expansion to continue accelerating into the future, as long as the effects of dark energy do not change with time and there are no other factors that affect the fate of the universe.
