Cosmological constant problem

The cosmological constant problem concerns the unexpected discrepancy between theoretical predictions and observations of the universe’s energy. Quantum physics suggests empty space (vacuum) should have a huge energy density, which would cause the universe to expand incredibly rapidly. However, measurements show the actual expansion is much slower, indicating a much smaller energy. This huge mismatch—about 120 orders of magnitude—is a major puzzle in physics. It challenges scientists to understand why the vacuum energy doesn't have the enormous effects predicted, raising questions about the fundamental nature of space, energy, and gravity.