These Galaxies Appear Older Than the Universe. That's Not Possible.

The James Webb Space Telescope (JWST) has discovered galaxies that appear too ancient for the young universe, sparking a debate on whether our understanding of galaxy formation or the Big Bang model is incorrect. Telescopes act as time machines, allowing us to view distant objects as they were in the past. JWST, a powerful instrument, has observed galaxies whose light has traveled since the universe was only about 2% of its current age. At this early stage, galaxies were expected to be young and vigorously forming stars, like "hyperactive kids." However, JWST has also found much more developed, "adult" galaxies that seem too large and ancient for a universe only a few hundred million years old. This mystery has led to speculation, including the idea that the Big Bang model is flawed. While many have addressed this, the evolving nature of the conundrum, with new data sometimes contradicting previous findings, necessitates further discussion. Early universe galaxies are believed to have formed from density fluctuations in the cosmic microwave background (CMB), where dark matter's gravity pulled in hydrogen gas, leading to the birth of the first stars. These early galaxies started small, with intense star formation, and grew through collisions and mergers into the mature galaxies we see today. Our current models, based on CMB measurements and theoretical understanding, predict that early galaxies should have been forming stars rapidly and that very large dark matter halos—the giant pools of dark matter encompassing galaxies—should not have existed in the first 1.5 billion years. However, around 15-16 years ago, "high redshift galaxy surveys" began to reveal a few cases of what appeared to be giant halos and overly red galaxies at earlier times. Redder galaxies suggest older stellar populations with fewer active, short-lived blue stars. These initial findings, though not extreme, hinted at a tension with our models, articulated in the 2018 paper "the impossibly early galaxy problem." One proposed explanation for these "impossible" galaxies is that our methods for inferring dark matter halo mass from starlight rely on assumptions, particularly regarding the initial mass function (IMF)—the distribution of stellar masses formed during a starburst. If the early universe had a "top-heavy" IMF, meaning more massive stars formed, these galaxies would be overly bright for their actual halo mass, leading us to overestimate their size. A top-heavy IMF was considered a leading contender to solve the apparent giantness of dark matter halos. However, a new study complicates this by claiming to have identified modern counterparts of these early galaxies. By studying these closer galaxies, researchers could measure the IMF down to lower masses, finding it to be "bottom-heavy," with an excess of low-mass stars compared to the Milky Way. This bottom-heavy IMF actually worsens the problem, as it suggests we would underestimate stellar and halo mass when converting galaxy light, making it even harder to explain their rapid growth. It's important to note that the identification of these "likely descendants" is not definitive, and other factors could have muddled the IMF over 13 billion years. It's also plausible that an IMF could be both top-heavy and bottom-heavy compared to the Sun's mass. The apparent redness of early galaxies also presents a challenge, requiring a mechanism to shut down star formation much more quickly than expected. Early quasars, supermassive black holes blasting out radiation and winds, are a leading contender. This "feedback" from quasars could heat and expel gas, halting star formation and accelerating stellar population evolution. This would require very rapid growth of supermassive black holes, another intriguing problem. Ultimately, the most likely outcome is that we will gain significant new knowledge about structure growth, star formation, and black hole evolution. This will refine our understanding, making the "impossible" early galaxies an inevitable part of our updated view of early spacetime.