Void Whisperer

The Void Whisperer has never pushed anything. It simply is wherever space is, and there is always more space.

In 1917 Albert Einstein added a term Λ, the cosmological constant, to his field equations to allow a static universe. Once expansion was discovered he dropped it. In 1998 and 1999 two supernova teams, led by Saul Perlmutter and by Brian Schmidt with Adam Riess, found that distant type Ia supernovae are fainter than a slowing universe allows: the expansion is accelerating. The three shared the 2011 Nobel Prize, and Λ came back as the simplest explanation of what is called dark energy.

Λ behaves like an energy density of empty space that never thins out. Its mass-equivalent density is ρ_Λ = Λc²/(8πG), where c is the speed of light and G Newton's constant. Its pressure equals minus its energy density, an equation of state w = p/(ρc²) = −1. In general relativity pressure gravitates too, and any component with w below −1/3 makes the expansion speed up; Λ, at w = −1, does so steadily. Measurements so far are consistent with w = −1, although recent baryon acoustic oscillation results from the DESI survey hint at dark energy that changes over time; that is not established.

Worked number: Planck 2018 gives H₀ ≈ 67.4 km/s/Mpc ≈ 2.18×10⁻¹⁸ s⁻¹ and Ω_Λ ≈ 0.685. Then Λ = 3Ω_ΛH₀²/c² ≈ 1.1×10⁻⁵² m⁻², and ρ_Λ ≈ 6×10⁻²⁷ kg/m³, about the mass of three or four hydrogen atoms per cubic metre. In particle-physics units (ħ = c = 1) that is (2.2 meV)⁴, about 10⁻⁴⁷ GeV⁴.

Here is the problem, drawn as the long bar on the back of this shirt. Quantum field theory says every field has zero-point energy, which should also weigh something. Adding it up naively to the Planck scale gives roughly M̄_Pl⁴ ~ 10⁷³ GeV⁴, where M̄_Pl ≈ 2.4×10¹⁸ GeV is the reduced Planck mass: up to ~10¹²⁰ times the observed value. The exact gap depends on where the sum is cut off, but no known mechanism makes the vacuum energy so small without fine-tuning. Steven Weinberg reviewed this cosmological constant problem in 1989; it remains unsolved.

The Void Whisperer: "I am the weight of nothing. Your theories say I should be enormous. I am not, and I will not tell you why."