---
title: "Cosmic strings and the gravitational-wave background"
description: "Follow a cosmic-string loop as it radiates and shrinks, add up every generation of loops, then recompute a published plateau and read the pulsar-timing limits."
canonical_url: "https://sandboxphysics.com/stories/cosmic-strings/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/cosmic-strings/"
last_modified: "2026-11-14"
---

# Cosmic strings and the gravitational-wave background

Sandbox Physics · 2026-11-12

A cosmic string is a thin line of trapped energy left by the early universe. Start with one loop, see how it radiates and shrinks, add up every generation of loops into a background, then check a published number yourself and read what pulsar timing has found.

1. L1 · Concept

   ## [Why could a thin cosmic string shape a whole gravitational-wave spectrum?](https://sandboxphysics.com/stories/cosmic-strings/thin-strings-whole-spectrum/)

   A string network keeps cutting off loops. Each loop rings at a pitch set by its size, so loops from every era spread one signal over many decades of frequency.

2. L1 · Chapter

   ## [What the tension of a cosmic string measures](https://sandboxphysics.com/stories/cosmic-strings/what-string-tension-measures/)

   Work out how heavy a cosmic string is per metre, why one dimensionless number sets its gravity, and how a scaling network keeps producing loops.

3. L2 · Chapter

   ## [How a cosmic-string loop radiates and shrinks](https://sandboxphysics.com/stories/cosmic-strings/how-a-loop-radiates-and-shrinks/)

   Follow one Nambu–Goto loop: its period, the cusps that beam bursts, the harmonics it emits and the steady shrinking that ends its life.

4. L3 · Chapter

   ## [From a loop population to the background spectrum](https://sandboxphysics.com/stories/cosmic-strings/from-loop-population-to-background/)

   Build the loop number density, integrate its emission over redshift and derive why the radiation-era plateau grows as the square root of tension.

5. L3 · Method

   ## [Recompute the radiation-era plateau of a cosmic-string background](https://sandboxphysics.com/stories/cosmic-strings/recompute-the-radiation-plateau/)

   Rerun a fixed Nambu–Goto loop model, compare it with the published plateau formula and see which checks fail when one step is wrong.

6. L4 · Paper guide

   ## [Reading pulsar-timing and LISA results on cosmic strings](https://sandboxphysics.com/stories/cosmic-strings/reading-pta-and-lisa-string-limits/)

   Which NANOGrav 15-year figures constrain string tension, why a good spectral fit is not a detection, and what LISA forecasts can and cannot separate.