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Why does gravity produce different structural shapes (spheres, disks, and filaments) depending on the cosmic scale?
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Asked<br>2 days ago
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At the scale of planets and stars, matter with enough mass is pulled inward equally from all sides until it reaches hydrostatic equilibrium, resulting in a sphere. However, as the scale increases to solar systems and spiral galaxies, the dominant shape flattens out into a disk. While I know other shapes like elliptical galaxies exist, my understanding is that they are usually the result of galactic mergers disrupting these original disk structures. And most galaxies were/are spirals or at least "flat".
Zooming out even further to the large-scale structure of the observable universe, matter organizes into the cosmic web. Instead of forming into even larger spheres or a single massive disk, galaxies cluster into massive filaments separated by vast, empty voids.
I don't understand why there is this difference. Why aren't the structures of the universe the same at all scales?
cosmology<br>gravity<br>structure-formation
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edited 2 days ago
PM 2Ring
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pie
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$\begingroup$<br>The Solar System started as a disk, and the end state of galaxies will eventually also be a "ball" of some fashion (OK, maybe a doughnut). Processes scale in space and time in parallel.<br>$\endgroup$
Peter - Reinstate Monica
Peter - Reinstate Monica
2026-07-06 16:10:24 +00:00
Commented<br>yesterday
$\begingroup$<br>You got good answers, but let me here just reverse the question: wouldn't it be surprising if scale had no incidence on the effects of gravity? When you scale length by 2, surface is scaled by 4 and volume by 8, while time is not scaled at all. So physics is generally not scale invariant<br>$\endgroup$
Stéphane Rollandin
Stéphane Rollandin
2026-07-07 20:19:13 +00:00
Commented<br>16 hours ago
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It is a tradeoff between density and angular momentum.
A planet or star is much denser than a solar system, galaxy, or filament. Even though the angular velocity of the planet/star is much higher, the radius is so small that there isn't much angular momentum and gravitational forces are strong. This favors a spherical shape.
At the surface of the Earth, the gravitational attraction of the Sun about $0.0006$ g at the distance of the Earth's orbit. Your weight changes is not noticeable as the Sun passes overhead. The Earth's orbital velocity is about $64$ times greater than its rotational velocity at the equator. And the distance to the Sun is more than $23,000$ times the distance to the center of the Earth. The orbital angular momentum of the Earth is $4.5$ million times its rotational angular momentum.
If you increased the Earth's rotational angular momentum by that factor, you would expect the Earth to flatten like a pancake and them fly apart.
At the largest scales, things do not rotate. See Is it possible something in the universe is not orbiting anything?
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mmesser314
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$\begingroup$<br>In your second paragraph, second sentence, is the first “angular momentum” supposed to be “angular velocity”<br>$\endgroup$
Dale
Dale
2026-07-06 03:08:08 +00:00
Commented<br>2 days ago
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About formation of large scale structures (galaxies, clusters of galaxies)
As we know, there is good reason to assume that prior to the first formation of galaxies the universe was near uniform in density.
There is good reason to assume that minute differences in density were sufficient to enable local gravitational contraction.
As we know: once contraction is under way any angular velocity vector (angular velocity averaged over all the contracting volume) will increase, as per conservation of angular momentum.
Interestingly, for a significant fraction of spiral galaxies the population of stars of the central bulge is characterized by random distribution of direction of orbital angular momentum.
In keeping with that: it seems highly plausible that the central-bulge-population-stars will tend to have a significantly non-circular orbit.
So: for a galaxy there is some degree of self-organization into a plane of galaxy rotation, but that doesn't necessarily extend to the central bulge.
For a solar...