How has scientific understanding about the composition of the universe changed over time? Select the two correct answers.(1 point)
Responses
The percentage of unknown material has grown in scientists' estimates since the 1970s.
The percentage of unknown material has grown in scientists' estimates since the 1970s.
The percentage of dark matter has shrunk in scientists' estimates since the 1980s.
The percentage of dark matter has shrunk in scientists' estimates since the 1980s.
The percentage of dark energy has shrunk in scientists' estimates.
The percentage of dark energy has shrunk in scientists' estimates.
The percentage of ordinary matter has grown in scientists' estimates.
The percentage of ordinary matter has grown in scientists' estimates.
The two correct statements that reflect changes in scientific understanding about the composition of the universe are:
1. The percentage of dark matter has shrunk in scientists' estimates since the 1980s.
2. The percentage of unknown material has grown in scientists' estimates since the 1970s.
Explanations for the correct responses:
1. Dark Matter Estimates: Initially, when dark matter was proposed to account for missing mass in galaxies and galaxy clusters, the estimates of its abundance were more uncertain. As observational techniques and theoretical models have improved, scientists have been able to make more precise measurements and refine their estimates of how much dark matter exists. The percentage of dark matter relative to the other components of the universe is now thought to be around 27%. While this figure isn't necessarily "shrunk," the uncertainties around it have been reduced, leading to more confidence in this estimate.
2. Increase in Unknown Material: The concept of dark energy was introduced in the late 1990s after observational evidence from Type Ia supernovae suggested that the expansion of the universe is accelerating. This led to the realization that there must be a component of the universe driving this acceleration, which isn't accounted for by ordinary matter or dark matter. As a consequence, the amount of unknown material—in this case, dark energy—in the universe has increased according to scientific understanding. The most recent estimates suggest that dark energy constitutes about 68% of the total energy content of the universe.
The responses that are not correct:
- The percentage of dark energy has not shrunk in scientists' estimates; on the contrary, the discovery and subsequent measurements have led to an increase in the estimated amount of dark energy in the universe.
- The percentage of ordinary (baryonic) matter has not grown in scientists' estimates. The generally accepted model of the universe, the Lambda-CDM model, maintains that ordinary matter comprises about 5% of the total universe. This percentage has remained relatively consistent as measurements have improved, but it hasn't significantly increased.