Null Beacon: Cosmic Event Horizon / Causal Disconnect
Cosmology & the Sky arc, page 140 of 193. Back to The Pixelated Physics Universe
Who they are
Meet Null Beacon. Some galaxies we can see will never hear from us. In an accelerating universe there is a distance beyond which light sent today never arrives: the cosmological event horizon. Today it lies about 16.7 billion light-years away, and galaxies we now see at redshift about 1.9 are at that distance. We still see their old light, but anything they emit today is lost to us. Null Beacon on a pixel-art unisex tee; equation, symbols and sources in the panel below.
Null Beacon 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
In a universe whose expansion is speeding up, there is a largest distance from which light sent today can ever reach us: about 16.7 billion light-years. Galaxies we now see at redshift about 1.9 are at that distance today. We still see their old light; their new light is lost.
$$ \begin{gathered} d_{\text{EH}}(t) = a(t)\int_{t}^{\infty} \frac{c\,dt^{\prime}}{a(t^{\prime})} \\ d_{\text{EH}} \to \frac{c}{H_\Lambda} \quad (\text{pure de Sitter}) \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
| $d_{\text{EH}}$ | proper distance to the cosmological event horizon | m |
| $a(t)$ | scale factor | 1 |
| $c$ | speed of light | m/s |
| $H_\Lambda$ | Hubble rate of a Λ-dominated universe | s⁻¹ |
Sources
- Rindler (1956) Visual Horizons in World Models, MNRAS 116, 662
- 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 — Big-Bang cosmology
- Davis & Lineweaver (2004) Expanding Confusion: Common Misconceptions of Cosmological Horizons…, PASA 21, 97
- PDG 2024 review — Dark energy
- Planck Collaboration (2020) Planck 2018 results VI. Cosmological parameters, A&A 641, A6
Crossovers
Null Beacon meets Void Whisperer (Dark Energy Expansion). 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.
Null Beacon meets The Silence Beyond (Cosmic Horizon / Observable Limit). The Silence Beyond marks the particle horizon, the edge of what we can see now, about 46 billion light-years away. Null Beacon marks the event horizon, the edge of what we can ever reach with a signal sent today. They are two different horizons.
Wear the lore
- Null Beacon – Cosmic Event Horizon / Causal Disconnect Physics T-Shirt
- Related infographic poster: The Accelerating Universe – Plate 073 Physics Poster
Concept keywords: Cosmic Event Horizon / Causal Disconnect, Null Beacon, Cosmology & the Sky, physics lore, Pixelated Physics