The cosmological constant problem: An accessible introduction

We present a pedagogical introduction to the cosmological constant problem that requires only basic knowledge of quantum field theory and general relativity. A massive real scalar field is used to illustrate how the quantum vacuum energy density and pressure can be calculated both in flat spacetime and in an expanding universe. Detailed computations are provided for dimensional, cutoff, and adiabatic regularizations. No attempt is made to address quantum gravitational effects, and the expanding universe background is treated classically. We point out that although the commonly cited discrepancy of 120 orders of magnitude between theory and observation is based on an estimate that does not account for regularization and renormalization, fundamental principles of quantum field theory nevertheless lead to a huge mismatch. In addition to this large discrepancy, we emphasize that there are also conceptual challenges related to cosmic expansion, such as the choice between comoving and physical scales in certain contexts and the non-uniqueness of vacuum.