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Starts With A Bang

JWST’s early, massive galaxies agree with ΛCDM cosmology

These high-mass, rapidly star-forming galaxies have called modern cosmology into question. But hi-res simulations show no tension at all.
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
  • When the JWST first opened its novel eyes on the distant Universe, it surprised many by finding large numbers of early, massive, evolved, star-forming galaxies.
  • Observations showed a clumpier Universe with a richer set of structures than most theorists, and all cutting-edge simulations, had shown beforehand.
  • But with superior computational resolution, a new set of simulations actually reproduces these young, massive early galaxies, in total agreement with what's been observed.

Since JWST’s launch, astronomers have used it to investigate the young Universe.

JWST CEERS early release
The Cosmic Evolution Early Release Science Survey (CEERS Survey) broke the record for largest deep-field image taken by JWST, previously held by the first lensing cluster image released. This small patch of sky, near the handle of the Big Dipper, contains some ~200 luminous disk galaxy candidates found within the first ~3 billion years of the Universe’s history. The deepest views of the early Universe have given astronomers and astrophysicists much to ponder.
Credit: NASA, ESA, CSA, STScI; CEERS collaboration

With unprecedented technical capabilities, JWST has already broken Hubble’s cosmic distance record.

JADES deepest galaxies JWST
The viewing area of the JADES survey, along with the four most distant galaxies verified within this field-of-view. The three galaxies at z = 13.20, 12.63, and 11.58 are all more distant than the previous record-holder, GN-z11, which had been identified by Hubble and has now been spectroscopically confirmed by JWST to be at a redshift of z = 10.6. No doubt these records will themselves be broken, possibly with galaxy candidates that already exist within the same field-of-view.
Credit: NASA, ESA, CSA, M. Zamani (ESA/Webb), Leah Hustak (STScI); Science credits: Brant Robertson (UC Santa Cruz), S. Tacchella (Cambridge), E. Curtis-Lake (UOH), S. Carniani (Scuola Normale Superiore), JADES Collaboration

Numerous ultra-distant galaxies have been uncovered, revealing a rich, early Universe.

JWST deep field vs hubble
This animation switches points of view between the Hubble Ultra Deep Field and the JWST view of an overlapping region of space. Because of the difference in telescope size and resolution, the JWST views are downsampled by about a factor of 4 in resolution to make these two images match.
Credit: NASA, ESA, CSA, STScI, Christina Williams (NSF’s NOIRLab), Sandro Tacchella (Cambridge), Michael Maseda (UW-Madison); Processing: Joseph DePasquale (STScI); Animation: E. Siegel

These youthful galaxies appear massive, evolved, and are rapidly forming stars.

JWST most distant galaxy cluster
The galaxies that are members of the identified proto-cluster A2744z7p9OD are shown here, outlined atop their positions in the JWST view of galaxy cluster Abell 2744. At just 650 million years after the Big Bang, it’s the oldest proto-cluster of galaxies ever identified. This is early, but is consistent with simulations of when the earliest proto-clusters should emerge from the most initially overdense regions.
Credit: NASA, ESA, CSA, Takahiro Morishita (IPAC); Processing: Alyssa Pagan (STScI)

Although data is still incoming, many question whether these galaxies conflict with our consensus cosmology.

maisie's galaxy CEERS JWST
This collection of several different JWST “pointings” from the CEERS photometric survey contains Maisie’s Galaxy, a high-redshift galaxy candidate that was recently spectroscopically confirmed to be at z=11.4, placing it just 390 million years after the Big Bang. It also contains four separate, nearby galaxies at a confirmed redshift of 4.9, indicating a galaxy proto-cluster just 1.2 billion years after the Big Bang.
Credit: NASA/STScI/CEERS/TACC/S. Finkelstein/M. Bagley/R. Larson/Z. Levay

The initial conditions of our Universe are known: imprinted in the Big Bang’s leftover glow.

inflation CMB spectrum WMAP
The large, medium, and small-scale fluctuations from the inflationary period of the early Universe determine the hot and cold (underdense and overdense) spots in the Big Bang’s leftover glow. These fluctuations, which get stretched across the Universe in inflation, should be of a slightly different magnitude on small scales versus large ones: a prediction that was observationally borne out at approximately the ~3% level. By the time we observe the CMB, 380,000 years after the end of inflation, there’s a spectrum of peaks-and-valleys in the temperature/scale distribution of fluctuations, owing to interactions between normal/dark matter and radiation.
Credit: NASA/WMAP Science Team

The equations that govern gravitation and structure formation are also highly certain.

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

Therefore, we should successfully predict how massive structures can be all throughout cosmic history.

large scale structure growth
Over time, gravitational interactions will turn a mostly uniform, equal-density Universe into one with large concentrations of matter and huge voids separating them. For as long as radiation is still important, exerting an outward pressure even once the Universe becomes matter-dominated, the growth of matter imperfections is very small.
Credit: Volker Springel/MPE

One underappreciated limitation of modern simulations, however, is resolution.

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. The entropy of the Universe, at every step along the way, is always increasing, even though the entropy density (with expansion included) may drop.
Credit: Ralf Kaehler and Tom Abel (KIPAC)/Oliver Hahn

Prior simulations had difficulty reproducing massive, evolved galaxies so early.

how much dark matter
While the web of dark matter (purple, left) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red, at right) can severely impact the formation of structure on galactic and smaller scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Structure formation is hierarchical within the Universe, with small star clusters forming first, early protogalaxies and galaxies forming next, followed by galaxy groups and clusters, and lastly by the large-scale cosmic web.
Credit: Illustris Collaboraiton/Illustris Simulation

With both greater mass resolution and spatial resolution, however, the Renaissance simulations offer a different perspective.

simulation resolution cosmology
Although this table might appear to be just a jumble of numbers, the key to high resolution is the final two columns: lower masses and smaller spatial separations lead to higher-resolution simulations, which means more reliable predictions for structures at lower masses, on smaller scales, and at earlier times. Note how superior Renaissance is to all others in these regards.
Credit: J. McCaffrey et al., Open Journal of Astrophysics (submitted), 2023

The unprecedented resolution highlights just how much mass the most initially overdense regions accumulate.

regions of various density renaissance dark matter simulations
Regions born with a typical, or “normal” overdensity, will grow to have rich structures in them, while underdense “void” regions will have less structure. However, early, small-scale structure is dominated by the most highly peaked regions in density (labeled “rarepeak” here), which grow the largest the fastest, and are only visible in detail to the highest resolution simulations.
Credit: J. McCaffrey et al., Open Journal of Astrophysics (submitted), 2023

Rare but strongly overdense regions grow to contain the earliest, most massive galaxies of all.

likelihood of early galaxies vs age for jwst
The three simulated regions highlighted earlier, using the Renaissance suite, lead to predictions for how massive galaxies should be in those three regions (orange, blue, and green lines). The 5 earliest galaxies revealed so far with JWST, with error bars shown, have about a probability of “1” of occurring within the observed regions. If they were truly rare, they’d be brighter and more massive, as shown by the ~10^-3 and ~10^-6 likelihood curves.
Credit: J. McCaffrey et al., Open Journal of Astrophysics (submitted), 2023

Even with modest, completely realistic star-formation rates, JWST’s most distant galaxies are perfectly typical within our standard ΛCDM cosmology.

star formation growth rate cosmology
Three different but still realistic estimates for sustained star-formation rates are given by the three lines, with model galaxies as revealed by simulations shown with shadings and the actual, JWST observed galaxies shown atop them. Note the 100% overlap.
Credit: J. McCaffrey et al., Open Journal of Astrophysics (submitted), 2023

Surprises and new records still await JWST, but claims like “JWST broke cosmology” were all premature.

This composite image from 7 of JWST’s NIRCam filters shows the central part of galaxy cluster Abell 2744: Pandora’s Cluster. The three main cluster components are highlighted with insets, with foreground and background objects, totaling some ~50,000 of them, present in this 0.007 square degree patch of sky.
Credit: R. Bezanson et al., ApJ submitted, JWST UNCOVER Treasury Survey, 2023

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


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