The Universe’s “dark ages” were briefer than we imagined

Our star and galaxy-rich Universe wasn’t always this way.

This comparison image, showing the same region as imaged by Hubble’s eXtreme Deep Field (top) and JWST’s JADES survey (bottom) showcases a selection of many ultra-distant galaxies found in the young Universe. When we observe the Universe at great distances, we’re seeing it as it was in the distant past: smaller, denser, hotter, and less evolved. Back to the limits of JWST’s capabilities, we see evidence for stars and galaxies everywhere.

Over time, gravitation formed these cosmic structures from near-uniform beginnings.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, and richer in gas in general than the galaxies we see today. Over time, many smaller galaxies become gravitationally bound together, resulting in mergers but also in groups and clusters containing large numbers of galaxies overall.

But earlier on, we experienced what’s known as the “dark ages.”

More than 13 billion years ago, during the Era of Reionization, the Universe was a very different place. The gas between galaxies was largely opaque to energetic light, making it difficult to observe young galaxies. The James Webb Space Telescope (JWST) is peering deep into space to gather more information about objects that existed during the Era of Reionization to help us understand this major transition in the history of the Universe, and finding that bright, early galaxies were common, and the intergalactic neutral matter is insufficient to stop that light from arriving at our telescopes.

Early on, during the hot Big Bang, everything was brilliantly energetic.

In the earliest stages of the hot Big Bang, there were no bound structures that could form, only a “primordial soup” of matter particles, antimatter particles, and bosons like the photon. This hot, dense, and rapidly expanding state would quickly cool off.

As the Universe expanded, however, it cooled, stretching every photon’s wavelength.

As a balloon inflates, any coins glued to its surface will appear to recede away from one another, with “more distant” coins receding more rapidly than the less distant ones. Any light will redshift, as its wavelength ‘stretches’ to longer values as the balloon’s fabric expands. This visualization solidly explains cosmological redshift within the context of the expanding Universe. If the Universe is expanding today, that means it was smaller, hotter, and denser in the past: leading to the picture of the hot Big Bang. It also explains why all quanta lose kinetic energy as the Universe expands, and why photons have their wavelengths lengthen as the Universe expands.

When neutral atoms formed — 380,000 years onward — no stars yet existed.

In the hot, early Universe, prior to the formation of neutral atoms, photons scatter off of electrons (and to a lesser extent, protons) at a very high rate, transferring momentum when they do. After neutral atoms form, owing to the Universe cooling to below a certain, critical threshold, the photons simply travel in a straight line, affected only in wavelength by the expansion of space.

However, that background radiation was still hot enough to see, at ~3000 K.

At any epoch in our cosmic history, any observer will experience a uniform “bath” of omnidirectional radiation that originated back at the Big Bang. Note that the CMB isn’t just a surface that comes from one point, but rather is a bath of radiation that exists everywhere at once. As each new year passes, the CMB cools down further by about 0.2 nanokelvin, and in several billion years, will become so redshifted that it will possess radio, rather than microwave, frequencies.

Only after ~3.2 million years would temperatures cool below the human visibility threshold.

At the start of the hot Big Bang, the Universe was rapidly expanding and filled with high-energy, very densely packed, ultra-relativistic quanta. An early stage of radiation domination gave way to several later stages where radiation was sub-dominant, but never went away completely, while matter then clumped into gas clouds, stars, star clusters, galaxies, and even richer structures over time, all while the Universe continues expanding. The time after the relic radiation has faded away but before stars have ignited marks the cosmic dark ages.

That starts the clock on the pre-stellar era: the cosmic dark ages.

Initially, at left, the Universe is filled with neutral, light-blocking matter back before any stars have formed. When stars begin to form, however, they create ionizing ultraviolet photons, which lead to pockets that behave as though they’re transparent to visible light, as shown in red. Over time, as we move to the right, more and more of the Universe becomes reionized, until reionization completes around 550 million years after the Big Bang. This epoch of reionization, contrary to earlier thoughts, was mostly illuminated, rather than being a part of our true “dark age” past.

However, the first stars quickly arrived.

An illustration of the first stars turning on in the Universe. Without metals to cool down the clumps of gas that lead to the formation of the first stars, only the largest clumps within a large-mass cloud will wind up becoming stars: fewer in number but greater in mass than today’s stars. Although there’s plenty of light-blocking matter, some of this starlight can still escape into the Universe beyond.

JWST has shown us large, massive, evolved galaxies existed even early on.

This tiny fraction of the JADES survey area, taken with JWST’s NIRCam instrument, showcases relatively nearby galaxies in detail, galaxies at intermediate distances that appear grouped together, and even ultra-distant galaxies that may be interacting or forming stars, despite their faint nature and red appearance. Even though we’ve been performing JWST science for over two years, we are only beginning to probe the full richness of the cosmos with JWST.

The earliest, JADES-GS-z14-0, comes from just 290 million years after the Big Bang.

Shown within the context of the JWST JADES field, galaxy JADEs-GS-z14-0 is completely unremarkable, but nevertheless has just broken the cosmic distance record again, becoming the first galaxy ever found when the Universe was under 300 million years old: just 2.1% of its current age. From its vantage point within the expanding Universe, it would see our proto-Milky Way as it was some 13.52 billion years ago: when we were just 2.1% of our current age.

The surrounding matter — mostly neutral atoms — isn’t sufficient to completely block this starlight.

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.

Simulations support many large, massive galaxies even before the ~200 million year mark.

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. Note that even at the start of the scale on the x-axis, at ~150 million years, clumps of stellar matter with ~100,000 solar masses already exist.

Supermassive black holes likely formed, via direct collapse, even earlier.

This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions, and could lead to direct collapse black holes of an estimated ~40,000 solar masses. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the formation of stars and the growth of galactic structures.

This places the earliest stars just 50-100 million years into cosmic history.

An artist’s conception of what a region within the Universe might look like as it forms stars for the first time. As they shine and merge, radiation will be emitted, both electromagnetic and gravitational. But the conversion of matter into energy does something else: it causes an increase in radiation pressure, which fights against gravitation. Surrounding the star-forming region is darkness, as neutral atoms effectively absorb that emitted starlight, while the emitted ultraviolet starlight works to ionize that matter from the inside out.

Located at redshifts of 30 or more, JWST may be incapable of finding them.

The first stars and galaxies in the Universe will be surrounded by neutral atoms of (mostly) hydrogen gas, which absorbs the starlight. Without metals to cool them down or radiate energy away, only large-mass clumps in the heaviest-mass regions can form stars. The very first star will likely form at 50-to-100 million years of age, based on our best theories of structure formation and our best observations of the Universe to date, which corresponds to a redshift of between 30-and-50.

Our “dark ages” were incredibly brief.

This schematic diagram of the Universe’s history, highlighting reionization, was created before the JWST era began in 2022. Before stars or galaxies formed, the Universe was full of light-blocking, neutral atoms, and most of the Universe doesn’t become fully reionized until 550 million years afterward. However, we have since learned that the “dark ages” came to an end, as many sources of stellar and galactic lights came into existence and shone brightly, long before that mark: and likely within the first 100-200 million years, if not earlier.

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