Critical Density Model

Definition

The critical density model is a cosmological model that describes the expansion of the universe based on the concept of a critical density. It suggests that the overall density of the universe determines the ultimate fate of its expansion, with the critical density acting as a threshold between a universe that will expand forever and one that will eventually collapse back on itself.

5 Must Know Facts For Your Next Test

1. The critical density is the average density of the universe required for the gravitational attraction between matter to eventually halt the expansion of the universe and cause it to collapse back on itself.
2. If the actual density of the universe is less than the critical density, the universe will expand forever, while if the density is greater than the critical density, the universe will eventually collapse.
3. The critical density is determined by the Hubble constant, which measures the rate of expansion of the universe.
4. The critical density model predicts that the universe is either 'open' (expanding forever) or 'closed' (eventually collapsing), depending on whether the actual density is less than or greater than the critical density.
5. Observations of the cosmic microwave background radiation and other cosmological measurements suggest that the actual density of the universe is very close to the critical density, implying a 'flat' or 'spatially flat' universe that will expand forever.

Review Questions

• Explain the concept of critical density and how it relates to the ultimate fate of the universe's expansion.
  • The critical density is the average density of matter and energy in the universe that would result in a gravitational force strong enough to eventually halt the expansion of the universe and cause it to collapse back in on itself. If the actual density of the universe is less than the critical density, the universe will continue to expand forever, but if the density is greater than the critical density, the universe will eventually recollapse. Observations suggest the actual density of the universe is very close to the critical density, implying a 'flat' universe that will expand forever.
• Describe how the critical density model is supported by evidence from the cosmic microwave background (CMB) and other cosmological measurements.
  • The critical density model is supported by observations of the cosmic microwave background (CMB), which provide crucial evidence for the Big Bang theory and the overall density of the universe. Measurements of the CMB, along with other cosmological data, suggest that the actual density of the universe is very close to the critical density required for a 'flat' universe that will expand forever. This alignment between the observed density and the critical density predicted by the model lends strong support to the critical density model and its implications for the ultimate fate of the universe.
• Analyze the role of dark matter in the critical density model and how it affects the predictions about the universe's expansion and ultimate fate.
  • Dark matter, the unobserved, non-luminous matter that makes up a significant portion of the universe's total mass, plays a crucial role in the critical density model. The amount of dark matter present in the universe affects the overall density and, consequently, whether the universe will expand forever or eventually collapse. If the density of dark matter is high enough to push the total density above the critical density, the model predicts that the universe will eventually collapse. Conversely, if the density of dark matter is low enough to keep the total density below the critical density, the model predicts that the universe will continue to expand forever. Understanding the distribution and properties of dark matter is therefore essential for accurately applying the critical density model and determining the ultimate fate of the universe.

Related terms

Cosmic Microwave Background (CMB): The oldest light in the universe, which is observed as a uniform glow of microwave radiation in all directions, providing crucial evidence for the Big Bang theory and the critical density model.

Deceleration Parameter: A measure of the rate of deceleration of the expansion of the universe, which is related to the critical density and the ultimate fate of the universe's expansion.

Dark Matter: The unobserved, non-luminous matter that makes up a significant portion of the universe's total mass, playing a crucial role in the critical density model's predictions about the universe's expansion and ultimate fate.