The Silence Beyond: Cosmic Horizon / Observable Limit
Cosmology & the Sky arc, page 141 of 193. Back to The Pixelated Physics Universe
Who they are
The edge of sight is farther away than the universe is old, in light-years. The universe is 13.8 billion years old, yet the particle horizon, the edge of the observable universe, lies about 46 billion light-years away, because space kept expanding while the light travelled. The Silence Beyond on a pixel-art unisex tee; equation, symbols and sources in the panel below.
The Silence Beyond 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 observable universe ends where light emitted at the Big Bang has only just had time to reach us. Space kept expanding while that light was travelling, so the edge is now much farther away in light-years than the age of the universe.
$$ \begin{gathered} d_{p}(t_0) = a(t_0)\int_{0}^{t_0}\frac{c\,dt}{a(t)} \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
| $d_p$ | proper distance to the particle horizon | m |
| $t_0$ | age of the universe | s |
| $a(t)$ | scale factor | 1 |
| $c$ | speed of light | m/s |
Sources
- Rindler (1956) Visual Horizons in World Models, MNRAS 116, 662
- Encyclopaedia Britannica — Observable universe
- PDG 2024 review — Big-Bang cosmology
- Hubble (1929) A relation between distance and radial velocity among extra-galactic nebulae, PNAS 15, 168
- Davis & Lineweaver (2004) Expanding Confusion: Common Misconceptions of Cosmological Horizons…, PASA 21, 97
- Planck Collaboration (2020) Planck 2018 results VI. Cosmological parameters, A&A 641, A6
Crossovers
The Silence Beyond meets Null Beacon (Cosmic Event Horizon / Causal Disconnect). 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.
The Silence Beyond meets Nocturna (Olbers' Paradox). Nocturna asks why the night is dark. The Silence Beyond gives part of the answer: the universe has a finite age, so light from beyond the horizon has not reached us. Expansion and redshift dim the rest.
The Silence Beyond meets Omegoda (Oh-My-God Particle). The Silence Beyond is the edge of the observable universe, but Omegoda came from much nearer. At such extreme energies, cosmic-ray protons lose energy on the cosmic microwave background (the GZK effect), so they reach us only from within roughly a hundred megaparsecs.
Wear the lore
- The Silence Beyond – Cosmic Horizon / Observable Limit Physics T-Shirt
- Related infographic poster: Friedmann's Three Fates – Plate 071 Physics Poster
Concept keywords: Cosmic Horizon / Observable Limit, The Silence Beyond, Cosmology & the Sky, physics lore, Pixelated Physics