Final answer:
The critical mass density required to halt the expansion of the universe is rightly estimated at 10⁻²⁶ kg/m³, which sets a threshold for determining the universe's ultimate fate in terms of expansion or contraction.
Step-by-step explanation:
The statement that the critical mass density needed to just halt the expansion of the universe is approximately 10⁻²⁶ kg/m³ is true. This value is based on estimations of the critical density (ρc), which for a given rate of expansion, indicates the mass per unit volume that would cause the expansion of the universe to slow to zero at an infinite time in the future.
If the actual density is above this critical density, the universe would eventually stop expanding and begin to contract; if lower, the universe will continue to expand indefinitely. Neutrinos with mass, dark matter, and the rate of expansion are factors that could influence whether the actual density meets or exceeds this critical threshold.