One possible hypothesis involves a new, as-yet-undiscovered form of energy calleddark
energy. We know even less about dark energy than we know about dark matter. However,
some scientists believe that dark energy makes up more than half the total content of the
universe. Other scientists have other hypotheses about why the universe is continuing to
expand; the causes of the universe’s expansion is another unanswered question that scientists
are researching.
Lesson Summary
- The universe contains all the matter and energy that exists now, that existed in the
past, and that will exist in the future. The universe also includes all of space and time. - Redshift is a shift of element lines toward the red end of the spectrum. Redshift occurs
when the source of light is moving away from the observer. - Light from almost every galaxy is redshifted. The farther away a galaxy is, the more
its light is redshifted, and the faster it is moving away from us. - The redshift of galaxies tells us that the universe is expanding.
- The current expansion of the universe suggests that in the past the universe was
squeezed into a very small volume. - The Big Bang theory proposes that the universe formed in an enormous explosion
about 13.7 billion years ago. - Recent evidence shows that there is a lot of matter in the universe that we cannot
detect directly. This matter is called dark matter. - The rate of the expansion of the universe is increasing. The cause of this increase is
unknown; one possible explanation involves a new form of energy called dark energy.
Review Questions
- What is redshift, and what causes it to occur?
- What is Hubble’s law?
- What is the most widely accepted scientific theory of the formation of the universe
called? - How old is the universe, according to the Big Bang theory?
- Describe two different possibilities for the nature of dark matter.
- What makes scientists believe that dark matter exists?
- What observation caused astronomers to propose the existence of dark energy?
Further Reading / Supplemental Links