The Universe, in its immensity and complexity, exhibits a structured organization that can be observed at different scales. From stars to the largest cosmic structures, three fundamental geometric shapes constantly emerge: the sphere, the disk, and the network of filaments. Each of these shapes reveals distinct physical processes and particular balances between the forces that govern the cosmos.
| Structure | Shape | Predominant force | Scale | Comment |
|---|---|---|---|---|
| Planet / star | Sphere | Gravity ⇄ Internal pressure | 106 – 109 m | gravity balances with internal pressure, forming stable spheres. |
| Spiral galaxy | Disk | Rotation ⇄ Gravity | 1020 – 1021 m | rotation and gravity produce flattened disks. |
| Cosmic filament | Filamentary network | Gravity → large scale | 1023 – 1025 m | dark matter sculpts the filamentary network and gravity amplifies anisotropies. |
Sources: Illustris Collaboration, and NASA WMAP.
In the cosmic immensity, one geometric shape seems to dominate all others: the sphere. From stars to planets, including interstellar gas bubbles, this perfectly symmetrical shape appears as a constant across the scales of the Universe. This predominance is not the result of chance, but the direct consequence of fundamental physical laws that govern our cosmos.
The main reason for this spherical omnipresence lies in gravity. This fundamental force acts isotropically, meaning it exerts equal attraction in all directions. When a sufficient amount of matter accumulates under the effect of its own gravity, it naturally tends to organize into a shape that minimizes its potential energy: the sphere. In this configuration, each particle is as close as possible to the collective center of mass, thus creating the most stable equilibrium.
The sphere represents the form of perfect equilibrium between internal pressure and gravitational force. For celestial objects of significant size, their own gravity is powerful enough to overcome the resistance of the materials that compose them, forcing them to adopt this quasi-spherical shape.
Another determining factor is rotation. Although gravity tends toward the perfect sphere, the rotation of celestial bodies introduces flattening at the poles and bulging at the equator. This is why some planets like Jupiter or Saturn are actually oblate spheroids rather than perfect spheres. However, the more massive an object is, the stronger its gravity and the more it resists this deformation, thus retaining a shape closer to the ideal sphere.
The same applies to gas bubbles in the interstellar medium, which take on this emblematic spherical shape when subjected to uniform internal pressure.
The predominance of the sphere in the Universe reminds us that the forms we observe are not arbitrary, but emerge directly from fundamental physical laws. From the birth of stars to the formation of planets, the sphere embodies equilibrium, stability, and energy efficiency, which seem to be organizing principles of our Universe.
If the sphere represents the shape of gravitational equilibrium, the disk embodies the rotational dynamics of the Universe. From spiral galaxies to forming planetary systems, this flat, circular shape systematically emerges where the conservation of angular momentum comes into play.
The collapse of a cloud of gas and dust into a disk is a direct consequence of the interaction between two fundamental physical principles: gravity and the conservation of angular momentum. Initially, the cloud has a slight rotation, imperceptible but present (nothing is fixed in the Universe). Under the effect of its own gravity, the cloud begins to contract. This contraction is necessarily accompanied by an acceleration of its rotation, according to the same principle as a skater who spins faster when bringing their arms closer to their body.
N.B.:
A skater spins faster when they bring their arms closer to their body; this physical phenomenon illustrates the conservation of angular momentum. When the skater brings their arms closer, their moment of inertia \( I \) decreases, and to conserve the product \( L = I \omega \) (constant angular momentum), their angular velocity \( \omega \) increases.
Gravitational force acts spherically, pulling all matter toward the center. However, rotation generates a centrifugal force that opposes this attraction along the equatorial plane. Perpendicular to this plane, where the centrifugal force is zero, the contraction continues freely. This difference in resistance to collapse according to direction creates progressive flattening.
Collisions between particles in the cloud dissipate energy but conserve the overall angular momentum. Particles with inclined orbits eventually collide and transfer their motion, gradually aligning in a common plane. This flattening process results in the formation of a thin disk-like structure, where matter orbits around the center of mass.
This universal mechanism explains why we observe disks at all cosmic scales, from planetary systems forming around young stars to spiral galaxies like our Milky Way. The disk thus represents the dynamic equilibrium between gravitational contraction and rotation, a fundamental signature of physics in the Universe.
The cosmic network of filaments, often described as the cosmic web, is the largest observable structure in the Universe. Its appearance results from the combined effect of gravity and the initial conditions left by the Big Bang. The tiny density fluctuations present in the primordial Universe, revealed by the cosmic microwave background, served as seeds for this colossal structure.
Under the effect of gravity, regions slightly denser than average began to attract surrounding matter. This attraction was exerted preferentially along the axes of highest density, creating bridges of matter between overdensities. Over billions of years, these bridges stretched and strengthened, forming the filaments we observe today.
Dark matter plays a fundamental role in this cosmic architecture. Representing about 85% of the total matter, it forms the gravitational scaffolding on which baryonic matter has accumulated. Its non-collisional nature allowed it to structure the Universe long before ordinary matter could do so.
At the intersection of several filaments, where the gravitational field is most intense, form galaxy clusters: the largest coherent structures in the Universe. Between these dense nodes, filaments extend over hundreds of millions of light-years, while void regions (called cosmic voids) separate this complex web.
This filamentary structure is not static but evolves continuously. Under the effect of the accelerated expansion of the Universe, the filaments stretch and thin, while new connections form between forming structures. The cosmic web thus appears as the large-scale manifestation of the action of gravity over billions of years, sculpting the Universe from the tiny initial irregularities.
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