Void Whisperer: Dark Energy Expansion
Cosmology & the Sky arc, page 138 of 193. Back to The Pixelated Physics Universe
Who they are
Meet the Void Whisperer, keeper of the cosmological constant Λ, the energy of empty space itself. Since 1998 we have known that the expansion of the universe is speeding up. The simplest explanation is Einstein's Λ: a constant energy density of the vacuum with equation of state w = −1, making up about 68.5% of the universe today (Planck 2018). Its density is tiny, ρ_Λ ≈ 6×10⁻²⁷ kg/m³, while a naive quantum estimate of vacuum energy comes out up to ~10¹²⁰ times too big. Why Λ is so small is unsolved: the cosmological constant problem. The Void Whisperer on a pixel-art unisex tee; equations, symbols and sources in the physics panel below."Empty space is never empty. Every new metre of it brings me along."
Void Whisperer is a character of the Cosmology & the Sky arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
The physics
The universe's expansion is speeding up, as if empty space had its own constant energy, the cosmological constant Λ. It makes up about 68.5% of the universe today. Adding up quantum zero-point energy naively gives a value up to ~10¹²⁰ times too large, and nobody knows why the real one is so small.
$$ \begin{gathered} \rho_\Lambda = \frac{\Lambda c^{2}}{8\pi G} \approx 6\times10^{-27}\ \mathrm{kg\,m^{-3}} \approx (2.2\ \mathrm{meV})^{4}\quad(\hbar = c = 1) \\ \Lambda = \frac{3\,\Omega_\Lambda H_0^{2}}{c^{2}} \approx 1.1\times10^{-52}\ \mathrm{m^{-2}},\qquad w = \frac{p}{\rho c^{2}} = -1 \\ \rho_{\mathrm{vac}}^{\mathrm{naive}} \sim \bar M_{\mathrm{Pl}}^{4} \sim 10^{73}\ \mathrm{GeV^{4}}\quad\text{vs}\quad \rho_\Lambda \sim 10^{-47}\ \mathrm{GeV^{4}} \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
| $\Lambda$ | cosmological constant | m⁻² |
| $\rho_\Lambda$ | mass-equivalent density of the vacuum (dark) energy | kg/m³ |
| $c$ | speed of light | m/s |
| $G$ | Newtonian gravitational constant | m³ kg⁻¹ s⁻² |
| $\Omega_\Lambda$ | dark-energy fraction of the critical density today, ≈ 0.685 (Planck 2018) | 1 |
| $H_0$ | Hubble constant, ≈ 67.4 km/s/Mpc (Planck 2018) | s⁻¹ |
| $w$ | equation-of-state parameter, pressure over energy density | 1 |
| $p$ | pressure | Pa |
| $\bar M_{\mathrm{Pl}}$ | reduced Planck mass, ≈ 2.4×10¹⁸ GeV/c² | kg |
| $\hbar$ | reduced Planck constant | J·s |
Status: part of this topic is unsolved, unconfirmed or debated. The text above says which part; treat it as an open question, not settled fact.
Sources
- Planck Collaboration (2020) Planck 2018 results. VI. Cosmological parameters, A&A 641, A6
- Weinberg (1989) The cosmological constant problem, Rev. Mod. Phys. 61, 1
- Riess et al. (1998) Observational Evidence from Supernovae for an Accelerating Universe, AJ 116, 1009
- Perlmutter et al. (1999) Measurements of Ω and Λ from 42 High-Redshift Supernovae, ApJ 517, 565
- NobelPrize.org — The Nobel Prize in Physics 2011 (accelerating expansion)
- PDG 2024 review — Dark energy
- NIST CODATA 2022 — Newtonian constant of gravitation
Crossovers
Void Whisperer meets Null Beacon (Cosmic Event Horizon / Causal Disconnect). Void Whisperer's Λ makes expansion speed up. Null Beacon is the result: a cosmological event horizon beyond which light sent today will never arrive.
Void Whisperer meets Dark Matter Host (Gravitational Lensing / Invisible Mass). Void Whisperer and Dark Matter Host make up most of the cosmic budget, and neither has been seen directly. One pulls matter together; the other pushes the expansion faster. What each is made of remains unknown.
Wear the lore
- Void Whisperer – Dark Energy Expansion Physics T-Shirt
- Related infographic poster: The Accelerating Universe – Plate 073 Physics Poster
Concept keywords: Dark Energy Expansion, Void Whisperer, Cosmology & the Sky, physics lore, Pixelated Physics