How does the actual average density of matter in the universe compare to the critical density?

The average density of matter in the universe is estimated to be only about a quarter of the critical density, which is the density required for the universe to achieve a flat geometry according to the Friedmann-Lemaître-Robertson-Walker (FLRW) model of cosmology. The critical density is dependent on the Hubble constant and plays a crucial role in determining the ultimate fate of the universe.

Current measurements indicate that the total matter density, which includes baryonic (visible) matter and dark matter, is around 30% of the critical density, with baryonic matter alone contributing approximately 4-5% of the total density. Therefore, when considering just visible (baryonic) matter, the fraction is significantly lower compared to the critical density. The insight that the total density of matter, including dark matter, is still less than the critical density implies that the universe is not yet fully meeting the conditions for a closed universe.

This understanding is vital for cosmological models and influences theories about cosmic expansion and the role of dark energy. In contrast, the other options misrepresent the relationship between average density and critical density by suggesting it is greater, equal, or that average density consists only of visible matter, overlooking the significant contribution of dark