Introduced by British physicist T. W. B. Kibble in 1976 (1), cosmic strings are hypothetical topological defects that likely formed in the universe's earliest moments. Cosmologists study these defects to gain insights into the extreme physical conditions of the primordial cosmos.
Such structures are expected to be prime sources of gravitational waves. The rise of gravitational wave astrophysics—bolstered by LIGO and Virgo detections—now equips physicists with tools to potentially identify cosmic strings.
Note: Cosmic strings are distinct from the strings in string theory; they represent entirely different phenomena.
Topological defects are stable, hypothetical structures thought to have emerged in the universe's first instants. Theories predict their formation at the end of the inflationary epoch (3), at energy scales tied to grand unified theories—around 1015 GeV. These events belong to the realm of high-energy physics.
Specifically, they arose during phase transitions in the early universe. A phase transition occurs when a physical system's state or structure changes due to an external parameter shift, like water freezing into ice.
In the expanding, cooling universe, such transitions involved spontaneous symmetry breaking in the Standard Model. Certain symmetries underpinning physical laws shattered, allowing the four fundamental forces—unified post-Big Bang—to separate.
A classic example is the electroweak symmetry breaking, where W/Z bosons couple to the Higgs field, splitting weak and electromagnetic forces.
Quantum vacuum phase transitions also altered spacetime topology, creating stable matter configurations at boundaries via the Kibble-Zurek mechanism (akin to the Higgs mechanism). These are "topological defects," stemming from voids' structural irregularities.
Defect type depends on the broken symmetry: cylindrical yields cosmic strings; spherical, magnetic monopoles; planar, domain walls. Others include skyrmions, textures (unstable), or extra dimensions.
Cosmic strings formed post-inflation when cylindrical and axial symmetries broke. These one-dimensional, linear defects number roughly one per Hubble volume per Kibble's estimates (1)—about one every 1031 cubic light-years.
Theories distinguish two types by energy scale and field effects. Local cosmic strings lack long-range fields (e.g., electric, magnetic), exerting short-range attraction with energy density concentrated at their edges.
Global cosmic strings carry long-range fields, with amplitude E/(Hc)—E as string energy, H the Hubble constant, c light speed—extending across the observable universe (~14 billion light-years).
Though spanning cosmic scales, cosmic strings are incredibly thin (~1 femtometer, proton-sized) and modeled as zero-thickness for calculations using the Nambu-Goto action from string theory, ideal for energetic, string-like objects.
General relativity shows cosmic strings under tension, with mass proportional to it (4). A 1-km string matches Earth's mass despite its nanoscale width (4).
Quantum field theory in the Standard Model predicts them via the abelian Higgs mechanism: gauge symmetry breaking imparts mass to fields, birthing strings at breakpoints.
Superstring theory also forecasts them as stretched "F-strings" (fundamental strings) into galactic-scale "cosmic superstrings," per Joseph Polchinski building on Henry Tye's work (5).
Cosmic strings generate gravitational waves via network oscillations, where strings intersect, forming unstable kinks that burst waves—intense, detectable events.
They also shed unstable loops that decay into weaker waves, harder to detect amid stochastic backgrounds.
High density predicts gravitational lensing: duplicating distant galaxies' images. None confirmed yet (5). Cosmic microwave background duplicates expected, but Planck 2013 data showed none (6).
In 1979, the "double quasar" Q0957+561A,B puzzled astronomers with dual images and a 415-day lag, initially blamed on galaxy lensing. In 1994-1995, Harvard-Smithsonian observations noted luminosity shifts without lag, suggesting a 100-day oscillating cosmic string transit (7, Astronomy & Astrophysics, March 25, 2004).
Though unconfirmed, LIGO/Virgo data and other experiments constrain cosmic strings tightly, promising breakthroughs.
