How “bound together” is our Universe?

Our Universe’s matter, fundamentally, is composed of elementary particles.

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).

But those interacting particles exist within spacetime.

At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. As the Universe expands and cools, an incredible amount of evolution happens, but the neutrinos created early on will remain virtually unchanged from 1 second after the Big Bang until today: the oldest particle signature we think we can hope to observe.

Quarks and gluons bind together, forming protons and neutrons.

Quarks and electrons come in slightly greater numbers than antiquarks and positrons. In a completely symmetric Universe, matter and antimatter annihilate away leaving trace and equal amounts of both. But in our Universe, matter dominates, indicating an early, fundamental asymmetry.

Protons and neutrons bind together, making atomic nuclei.

From beginning with just protons and neutrons, the Universe builds up helium-4 rapidly, with small but calculable amounts of deuterium, helium-3, and lithium-7 left over as well. Until the latest results from the LUNA collaboration, step 2a, where deuterium and a proton fuse into helium-3, had the largest uncertainty. That uncertainty has now dropped to just 1.6%, allowing for incredibly strong conclusions.

Electrons and nuclei form bound states, creating neutral atoms.

At early times (left), photons scatter off of electrons and are high-enough in energy to knock any atoms back into an ionized state. Once the Universe cools enough, and is devoid of such high-energy photons (right), they cannot interact with the neutral atoms, and instead simply free-stream, since they have the wrong wavelength to excite these atoms to a higher energy level. However, when you make a neutral atom in the ground state, you emit a high-energy photon from that process, and if a new atom then absorbs that photon, it gets excited and becomes easily ionized. This “bottleneck” must be passed, and cosmic expansion helps, but isn’t the only (or even the dominant) factor.

Those atoms can link together, creating molecules in limitless combinations.

The raw ingredients that we believe are necessary for life, including a wide variety of carbon-based molecules, are found not only on Earth and other rocky bodies in our Solar System, but in interstellar space, such as in the Orion Nebula: the nearest large star-forming region to Earth.

Molecular components can assemble to compose living, megafaunal organisms, including human beings.

Although human beings are made of cells, at a more fundamental level, we’re made of atoms. All told, there are close to ~10^28 atoms in a human body, mostly hydrogen by number but mostly oxygen and carbon by mass.

But an even greater force binds matter together on cosmic scales: gravitation.

The largest-scale observations in the Universe, from the cosmic microwave background to the cosmic web to galaxy clusters to individual galaxies, all require dark matter and dark energy to explain what we observe. While the equations that govern the evolution are well known, as are the magnitudes of the initially overdense regions in our Universe, obtaining the necessary small-scale resolution to tease out the masses and properties of the smallest, earliest galaxies remains difficult.

With no “negative” gravitational charges, only “positive” mass/energy, gravitation is always attractive.

There is a large suite of scientific evidence that supports the expanding Universe and the Big Bang. At every moment throughout our cosmic history for the first several billion years, the expansion rate and the total energy density balanced precisely, enabling our Universe to persist and form complex structures. This balance was essential if complex structures, like stars and galaxies, were to arise within the Universe.

However, the expanding Universe drives particles with large spatial separations farther apart.

This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them.

Over time, gravitation collects and collapses neutral gas clouds, forming stars: generation upon generation.

This multiwavelength view of the two largest, brightest galaxies in the M81 group shows stars, plasmas, and neutral hydrogen gas. The gas bridge connecting these two galaxies infalls onto both members, triggering the formation of new stars. Both galaxies are smaller and lower in mass than the Milky Way, but both house much more massive supermassive black holes than we do.

Star clusters grow and merge, forming galaxies, galaxy groups, and rich clusters of galaxies.

Here, galaxy cluster MACS J0416.1-2403 isn’t in the process of collision, but rather is a non-interacting, asymmetrical cluster. It also emits a soft glow of intracluster light, produced by stars that are not part of any individual galaxy, helping reveal normal matter’s locations and distribution. Gravitational lensing effects are co-located with the matter, showing that “non-local” options for modified gravity do not apply to objects like this. Clusters of galaxies contain all sorts of small-scale structures within them, from black holes to planets to star-forming gas and more.

Within them, black holes, stellar remnants, new stars, planets, and complex, organic ingredients continually accumulate.

This snippet from a medium-resolution structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter; normal matter plays only a minor role. The larger-scale your simulation is, however, the more that smaller-scale structure is intrinsically underestimated and “smoothed-out.”

On even grander cosmic scales, filamentary networks and superclusters begin to form.

The Sloan Great Wall is one of the largest apparent, though likely transient, structures in the Universe, at some 1.37 billion light-years across. It may just be a chance alignment of multiple superclusters, but it’s definitely not a single, gravitationally bound structure. The galaxies of the Sloan Great Wall are depicted at right.

But dark energy prevents them from remaining stable.

Possible fates of the expanding Universe. Notice the differences between models in the past; only a Universe with dark energy matches our observations, and the dark energy-dominated solution came from de Sitter all the way back in 1917. By observing the expansion rate today and measuring the components present in the Universe, we can determine both its future and past histories.

Over time, these pseudostructures are driven apart, breaking the cosmos into lonely, isolated clumps.

The Laniakea supercluster, containing the Milky Way (red dot), is home to our Local Group and so much more. Our location lies on the outskirts of the Virgo Cluster (large white collection near the Milky Way). Despite the deceptive looks of the image, this isn’t a real structure, as dark energy will drive most of these clumps apart, fragmenting them as time goes on.

Galaxy groups and clusters remain the Universe’s largest stable structures.

This illustrated map of our local supercluster, the Virgo supercluster, spans more than 100 million light-years and contains our Local Group, which has the Milky Way, Andromeda, Triangulum, and about ~60 smaller galaxies. The overdense regions gravitationally attract us, while the regions of below-average density effectively repel us relative to the average cosmic attraction. However, the individual groups-and-clusters are not gravitationally bound together and are receding from one another as dark energy dominates the cosmic expansion.

Beyond our Local Group, the unbound Universe forever recedes into oblivion.

The impressively huge galaxy cluster MACS J1149.5+223, whose light took over 5 billion years to reach us, is among the largest bound structures in all the Universe. On larger scales, nearby galaxies, groups, and clusters may appear to be associated with it, but are being driven apart from this cluster due to dark energy; superclusters are only apparent structures, but the largest galaxy clusters that are bound can still reach hundreds of millions, and perhaps even a billion, light-years in extent.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.