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The 13 scales that define our physical Universe
The visible Universe extends 46.1 billion light-years from us, while we've probed scales down to as small as ~10^-19 meters.
KEY TAKEAWAYS:
On the smallest of physical scales, we have the fundamental, elementary particles, which build up to assemble nuclei, atoms, molecules, and even larger structures. On larger scales, we have planets, stars, stellar systems, galaxies, clusters of galaxies, and vast voids between them, all contributing to the enormous cosmic web. Overall, there are many different scales to view the Universe on. Here's the grand cosmic tour, from the extremely tiny to the unfathomably large.
Our Universe spans from subatomic to cosmic scales. All told, 13 different scales are presently known.
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1.) Fundamental, elementary particles. Down to 10-19 meters, these quanta have never been divided.
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2.) Nuclear scales. On femtometer (~10-15 m) scales, individual nucleons, composed of quarks and gluons, bind together.
When two protons, each one made of three quarks held together by gluons, overlap, it’s possible that they can fuse together into a composite state dependent on their properties. The most common, stable possibility is to produce a deuteron, made of a proton and a neutron, which requires the emission of a neutrino, a positron, and possibly a photon as well.
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3.) Atomic scales. Angstrom-sized (~10-10 m), atoms compose all matter on Earth.
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4.) Molecular scales. Nanometers (~10-9 m) and larger, molecules contain multiple atoms bound together.
Molecules, examples of particles of matter linked up into complex configurations, attain the shapes and structures that they do owing primarily to the electromagnetic forces that exist between their constituent atoms and electrons. The variety of structures that can be created is almost limitless.
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5.) Microscopic scales. Below 0.0001 meters (human hair width), tools beyond human eyes are required.
This tunneling electron microscope image shows a few specimens of the cyanobacterium species Prochlorococcus marinus. Each one of these organisms is only about half a micron in size, but all together, cyanobacteria are largely responsible for the creation of Earth’s oxygen: both initially and largely even during the present day. Like all bacteria, their lifetime is much, much shorter than the lifetime of a human.
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6.) Macroscopic scales. Our conventional perceptions extend from sub-millimeter to several kilometer scales.
In warm, shallow bodies of water, pink flamingos can often be found wading, preening, and searching for food. The lack of carotenoid pigments in their food supply, notable in some (but not all) of the flamingos shown here, causes many of these particular flamingos to be closer to a white color than a more stereotypical pink or red, but the behavior of standing on one foot instead of two does successfully cut their body heat loss nearly in half.
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7.) Sub-planetary scales. Where gravity cannot defeat electromagnetism, free-floating bodies can reach several hundred kilometers.
This selection of asteroids and comets visited by spacecraft spans many orders of magnitude in size, from sub-kilometer bodies to objects more than 100 km on a side. However, none of these objects have enough mass to be pulled into a round shape. Gravitation can hold them together, but electromagnetic forces are primarily responsible for their shapes.
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8.) Planetary scales. Spheroidal because of self-gravitation, planets are typically ~1000-200,000 kilometers across.
Now that Saturn has been imaged by JWST, the first “family portrait” of the gas giant worlds as seen by JWST’s eyes can be composed. Here, each planet is shown with an angular size that’s calibrated to how they would appear relative to one another as seen by JWST. Planets can be as large as about twice Jupiter’s size, but may be as small as 1000 km or even less.
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9.) Star-sized scales. From 0.08-to-2000 times the Sun’s size, these nuclear furnaces light up the Universe.
Brown dwarfs, between about 0.013-0.080 solar masses, will fuse deuterium+deuterium into helium-3 or tritium, remaining at the same approximate size as Jupiter but achieving much greater masses. Red dwarfs are only slightly larger, but even the Sun-like star shown here is not shown to scale here; it would have about 7 times the diameter of a low-mass star. Stars can be up to nearly 2000 times the diameter of our Sun within this Universe.
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10.) Stellar system scales. Extending up to ~2 light-years away, extended Oort-like clouds probe the limits of individual stellar systems.
An illustration of the inner and outer Oort Cloud surrounding our Sun. While the inner Oort Cloud is torus-shaped, the outer Oort Cloud is spherical. The true extent of the outer Oort Cloud may be under 1 light-year, or greater than 3 light-years; there is a tremendous uncertainty here. Any massive object that passes through the Oort cloud has a significant chance of perturbing the objects within its vicinity.
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11.) Galactic scales. From ~100-to-1,000,000 light-years, normal and dark matter hold galaxies together.
While there are many instances of numerous galaxies in the same region of space, they normally occur either between two galaxies only or in very dense regions of space, like at the centers of galaxy clusters. Seeing 5 galaxies interacting within a space of less than 1 million light-years is an extreme rarity, captured in gorgeous detail by Hubble here. As all of these galaxies are still forming new stars, they’re all classified as “alive” by astronomers.
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12.) Cluster-and-void scales. 10-to-100 million light-years wide, they’re the largest gravitationally bound structures.
In between the great clusters and filaments of the Universe are great cosmic voids, some of which can span hundreds of millions of light-years in diameter. While some voids are larger in extent than others, spanning a billion light-years or more, they all contain matter at some level. Even the void that houses MCG+01–02–015, the loneliest galaxy in the Universe, likely contains small, low surface brightness galaxies that are below the present detection limit of telescopes like Hubble.
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13.) Truly cosmic scales. The fully observable cosmic web extends ~92 billion light-years across.
On the largest scales, the way galaxies cluster together observationally (blue and purple) cannot be matched by simulations (red) unless dark matter is included. Although there are ways to reproduce this type of structure without specifically including dark matter, such as by adding a specific type of field, those alternatives either look suspiciously indistinguishable to dark matter or fail to reproduce one of the many other observations in support of dark matter.
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On even larger and smaller scales, new phenomena may still await discovery.
"As all of these galaxies are still forming new stars, they’re all classified as 'alive' by astronomers."
