Past exam of the relativistic Astrophysics and Cosmology course of the University of Cambridge / 2012 / Paper 1 / 3 / b / Solution

A positive cosmological constant contributes a constant vacuum energy density and negative pressure. In the time-unit convention of Question 2,It therefore causes accelerating expansion once its negative-pressure effect outweighs the decelerating components. Its density stays constant while matter dilutes, so a small early contribution can dominate late. For an isolated positive-vacuum component the scale factor grows exponentially. The cosmological constant problem is the mismatch between the small inferred vacuum density and generic quantum-field vacuum-energy expectations, rather than a failure of the background solution itself.

The key direct expansion evidence came from Type Ia supernova distance-redshift measurements. After light-curve and colour standardization, distant events were fainter than expected in simple decelerating matter-only models, indicating larger luminosity distances. Riess and collaborators' 1998 analysis and Perlmutter and collaborators' independent 42-supernova study favoured an accelerating low-matter-density cosmology with a positive vacuum contribution. Dust, source evolution, selection effects, photometric calibration and peculiar velocities must be controlled: faintness alone is not a model-independent measurement of $\Lambda$.^[ref][ref]

cosmic microwave background acoustic geometry and independent matter-density constraints help break the supernova parameter degeneracy: nearly flat geometry with substantially less than critical matter requires another component within the FLRW model. large-scale structure of the universe and distance measurements provide further consistency tests. The combined evidence supports a dark-energy contribution compatible with a constant; accelerating expansion by itself does not prove exactly constant vacuum energy or rule out every alternative gravity model.