The 13 scales that define our physical Universe

Our Universe spans from subatomic to cosmic scales.

From macroscopic scales down to subatomic ones, the sizes of the fundamental particles play only a small role in determining the sizes of composite structures. Whether the building blocks are truly fundamental and/or point-like particles is still not known, but we do understand the Universe from large, cosmic scales down to tiny, subatomic ones. The scale of electrons, quarks, and gluons is the limit to how far we’ve ever probed nature: down to scales of ~10^-19 meters, where these structures remain point-like.

All told, 13 different scales are presently known.

On the right, the gauge bosons, which mediate the three fundamental quantum forces of our Universe, are illustrated. There is only one photon to mediate the electromagnetic force, there are three bosons mediating the weak force, and eight mediating the strong force. This suggests that the Standard Model is a combination of three groups: U(1), SU(2), and SU(3), whose interactions and particles combine to make up everything known in existence. Despite the success of this picture, many puzzles still remain.

1.) Fundamental, elementary particles. Down to 10^-19 meters, these quanta have never been divided.

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. This reaction is the first step of the proton-proton chain: the fusion reaction that primarily powers our Sun.

2.) Nuclear scales. On femtometer (~10^-15 m) scales, individual nucleons, composed of quarks and gluons, bind together.

Although you yourself are made of atoms, what you experience as “touch” doesn’t necessarily require another, external atom to come in actual overlapping contact with the atoms in your body. Simply getting close enough to exert a force is not only enough, it’s what most commonly occurs.

3.) Atomic scales. Angstrom-sized (~10^-10 m), atoms compose all matter on Earth.

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.

4.) Molecular scales. Nanometers (~10^-9 m) and larger, molecules contain multiple atoms bound together.

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, and while cyanobacteria are relatively primitive organisms, they “only” date back to no earlier than 2.7 billion years ago, whereas life on Earth goes back more than a billion years, at least, farther than this.

5.) Microscopic scales. Below 0.0001 meters (human hair width), tools beyond human eyes are required.

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.

6.) Macroscopic scales. Our conventional perceptions extend from sub-millimeter to several kilometer scales.

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.

7.) Sub-planetary scales. Where gravity cannot defeat electromagnetism, free-floating bodies can reach several hundred kilometers.

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.

8.) Planetary scales. Spheroidal because of self-gravitation, planets are typically ~1000-200,000 kilometers across.

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.

9.) Star-sized scales. From 0.08-to-2000 times the Sun’s size, these nuclear furnaces light up the Universe.

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.

10.) Stellar system scales. Extending up to ~2 light-years away, extended Oort-like clouds probe the limits of individual stellar systems.

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.

11.) Galactic scales. From ~100-to-1,000,000 light-years, normal and dark matter hold galaxies together.

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. The long-held idea that the Universe is held together by structures spanning many hundreds of millions of light-years, these ultra-large superclusters, has now been settled, and these enormous web-like features are destined to be torn apart by the Universe’s expansion.

12.) Cluster-and-void scales. 10-to-100 million light-years wide, they’re the largest gravitationally bound structures.

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.

13.) Truly cosmic scales. The fully observable cosmic web extends ~92 billion light-years across.

In modern cosmology, a large-scale web of dark matter and normal matter permeates the Universe. On the scales of individual galaxies and smaller, the structures formed by matter are highly non-linear, with densities that depart from the average density by enormous amounts. On very large scales, however, the density of any region of space is very close to the average density: to about 99.99% accuracy. On scales larger than a few billion light-years, no structures will ever form, owing to the presence and late-time domination of dark energy.

On even larger and smaller scales, new phenomena may still await discovery.

This vertically oriented logarithmic map of the Universe spans nearly 20 orders of magnitude, taking us from planet Earth to the edge of the visible Universe. Each large “mark” on the right side’s scale bar corresponds to an increase in distance scales by a factor of 10.

Mostly Mute Monday tells a cosmic story in images, visuals, and no more than 200 words.