The discovery that the universe is expanding changed how astronomers think about cosmic history. This lesson introduces the evidence and ideas that led to the expanding-universe picture.
Gravity attracts matter. In a finite collection of galaxies that began at rest, their mutual gravitational attraction would make them move closer together and eventually collide. To avoid collapse, the galaxies would need to be moving apart fast enough to overcome their mutual attraction—much as a rocket needs sufficient launch speed to avoid falling back to Earth.
All flashcards
Why is a static universe difficult to explain using gravity?
Gravity attracts matter. Without an opposing effect or suitable motion, galaxies would tend to move closer together rather than remain at fixed separations.
What is the cosmological constant?
It is a term Einstein added to his equations, representing a hypothetical repulsive effect that could balance gravity and allow a static universe.
What did Hubble’s observations establish?
They showed that the universe is expanding, with galaxies generally getting farther apart.
What does it mean to say that space expands?
The distances between widely separated galaxies increase as the space between them stretches; the galaxies are not merely moving through a fixed background.
Why does looking at distant objects reveal the past?
Their light takes time to reach us. The farther away an object is, the longer its light has traveled, so we observe it as it was earlier in cosmic history.
Quiz questions
If recession velocity follows the Hubble law v=Hd and the expansion time is estimated as T0=d/v, what does this estimate become?Answer: T0=1/H
In a universe whose expansion has decelerated, how does the Hubble-time estimate compare with the actual age?Answer: It overestimates the age because expansion was faster in the past.
What did the unexpectedly faint distant Type Ia supernovae indicate compared with a constant-expansion prediction?Answer: Their greater inferred distances supported accelerated expansion.
What does the term dark energy refer to in explanations of cosmic acceleration?Answer: The unknown source associated with the observed acceleration.
Why can uranium-238 measurements provide an independent check on globular-cluster age estimates?Answer: They use radioactive decay rather than distance measurements or stellar-interior models.
Based on Astronomy 2e by Andrew Fraknoi, David Morrison, Sidney Wolff (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.