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Starts With A Bang

How “bound together” is our Universe?

From quarks and gluons to giant galaxy clusters, everything that exists in our Universe is determined by what is (and isn’t) bound together.
jwst background galaxies
This extremely rich region of space was captured while viewing Stephan's Quintet with JWST's NIRCam instrument. Many of these galaxies are clustered together in real space, while others are simply serendipitous alignments along the same line-of-sight that appear to be clustered, but are actually not bound to one another. The deepest galaxies revealed by JWST may yet still be entirely explicable within modern cosmology's consensus picture.
Credit: NASA, ESA, CSA, and STScI
Key Takeaways
  • At a fundamental level, our Universe is composed of indivisible, elementary particles embedded in the backdrop of the spacetime in our expanding Universe.
  • But what we observe isn’t simply a collection of independent particles, but rather a series of bound structures: atomic nuclei, atoms, molecules, planets, stars, galaxies, and more.
  • When we add it all up, how bound together is our actual Universe? The answer might surprise you and being both more, and less, than you ever imagined.

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

standard model structure
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. With gravity thrown into the mix, there are a total of 26 fundamental constants required to explain our Universe, with four big questions still awaiting explanation.
Credit: Daniel Domingues/CERN

But those interacting particles exist within spacetime.

quark gluon plasma primordial soup
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. Yet even with these conditions, only a few specific states, or particles, can emerge, and by the time a few seconds have passed, the Universe is much larger than it was in the earliest stages. As the Universe begins expanding, the density, temperature, and expansion rate of the Universe all rapidly drop as well.
Credit: Brookhaven National Laboratory

Quarks and gluons bind together, forming protons and neutrons.

anitmatter annihilation
In the very early Universe, there were tremendous numbers of quarks, leptons, antiquarks, and antileptons of all species. After only a tiny fraction-of-a-second has elapsed since the hot Big Bang, most of these matter-antimatter pairs annihilate away, leaving a very tiny excess of matter over antimatter. How that excess came about is a puzzle known as baryogenesis, and is one of the greatest unsolved problems in modern physics.
Credit: E. Siegel/Beyond the Galaxy

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.
Credit: E. Siegel/Beyond the Galaxy (L); NASA/WMAP Science Team (R)

Electrons and nuclei form bound states, creating neutral atoms.

photon bath neutral CMB 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.
Credit: E. Siegel/Beyond the Galaxy

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

molecules organic ingredients life
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.
Credit: ESA, HEXOS and the HIFI consortium

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

atom composition human body
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.
Credit: Jim Marsh at

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

cosmic web formation growth
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.
Credit: Chris Blake and Sam Moorfield

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

big bang
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.
Credit: NASA / GSFC

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

expanding universe
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. The expanding Universe allows for galaxies up to 15 billion light-years beyond our present cosmic horizon to eventually become visible, even while fewer and fewer galaxies become reachable.
Credit: Rob Knop

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

m81 group
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. If each star were shrunk down to be a grain of sand, this group would be 36 million km away, but the two galaxies would be separated only by a little over 400,000 km: the Earth-Moon distance. The galaxies comprising the M81 group will likely be the very last ones to recede from our reach in our dark energy-dominated Universe.
Credit: R. Gendler, R. Croman, R. Colombari; Acknowledgement: R. Jay GaBany; VLA Data: E. de Block (ASTRON)

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

intracluster galaxy cluster starlight
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.
Credit: NASA, ESA and M. Montes (University of New South Wales)

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

dark matter
This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Over time, overdense clumps of matter grow richer and more massive, growing into galaxies, groups, and clusters of galaxies, while the less dense regions than average preferentially give up their matter to the denser surrounding areas.
Credit: Ralf Kaehler and Tom Abel (KIPAC)/Oliver Hahn

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

Sloan Great Wall
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, as dark energy is in the process of driving it apart. The galaxies of the Sloan Great Wall are depicted at right.
Credit: Willem Schaap (L); Pablo Carlos Budassi (R)/Wikimedia Commons

But dark energy prevents them from remaining stable.

dark energy
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.
Credit: NASA & ESA

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: approximately 100,000 to 150,000 known galaxies, at present. Our specific 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. Today, Laniakea spans 520 million light-years across, but will expand to many billions of light-years as cosmic time continues.
Credit: R.B. Tully et al., Nature, 2014

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

local supercluster
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.
Credit: Andrew Z. Colvin/Wikimedia Commons

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

unbound structures
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.
Credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley), and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI)

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


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