For nearly a century, the concept of an expanding universe has been foundational to astronomy. Edwin Hubble’s 1929 discovery showed that galaxies are receding from each other, a movement quantified by the Hubble constant. Today, with the advancement of telescope technology, a phenomenon known as the “Hubble Tension” has arisen. Observations of nearby galaxies’ light, analysed through redshift—which reveals the stretching of light to lower frequencies as galaxies move away, akin to the decreasing pitch (frequency) of an ambulance siren as it moves further away—suggest a rate of expansion around 74 km/s/Mpc. This rate, measured in kilometers per second per megaparsec, describes how fast galaxies are receding from us for every megaparsec of distance. This higher value involves comparing the expected and observed colours of this light emanating from these galaxies, and using the difference to calculate the galaxies’ recession speed, thereby inferring the universe’s expansion rate.
In contrast, light from the early universe, well before star formation, indicates a slower rate of about 67 km/s/Mpc. Despite increasing precision, the discrepancy between these measurements continues to grow. This lower value is derived from our most comprehensive cosmological model, often referred to as the “Big Bang Model.” This framework posits that the universe originated from a super dense soup of radiation and hot matter. Approximately 300,000 years after the big bang, the universe had expanded and cooled enough for electrons to bind with atomic nuclei. This pivotal moment allowed photons, the particles of light, to travel vast distances unimpeded, eventually evolving into the Cosmic Microwave Background (CMB) we detect today. The CMB represents the oldest light in the universe, a faint glow from this early epoch now stretched into low frequency microwave frequencies by the expanding universe. This radiation, crucially shifted into microwaves, supports the 67 km/s/Mpc figure. This model, explaining everything from star and galaxy formation to the emergence of life, stands as the most accurate representation of our universe to date.
One theory to reconcile these differing values revolves around our position in the universe. Initially, it was believed that the Milky Way resides in a sparsely populated region, where nearby galaxies are minimally influenced by gravitational forces and thus recede more rapidly. However, recent studies have revealed our galaxy’s membership in the Laniakea Supercluster, a colossal collection of over 100,000 galaxies bound together by gravity. This finding led to the hypothesis that the “peculiar velocities” of galaxies within Laniakea, which stem from mutual gravitational interactions, distort our local expansion rate measurements.
Leonardo Giani and colleagues at the University of Queensland embarked on an ambitious project to model Laniakea as an elliptical structure. This approach allowed them to meticulously map the galaxies within the supercluster, identifying those moving towards us and those receding. This comprehensive mapping was crucial in discerning the “peculiar velocities” – the individual motions of galaxies influenced by the supercluster’s gravitational dynamics.
Armed with this new understanding, the team isolated these peculiar velocities from the overall expansion of the universe. Their findings were surprising; the local expansion rate of the Universe actually increased to 74.5 km/s/Mpc. This adjustment, rather than bridging the gap, further distanced our local measurements from the CMB-based estimates, adding another layer of complexity to the Hubble Tension.
We are now confronted with two possibilities: either our observations of nearby galaxies are skewed by another unknown factor, or our foundational cosmological model requires revision.
With the advent of cutting-edge telescopes such as the James Webb Space Telescope, our understanding of the cosmos is on the brink of significant development. Recent discoveries such as “The Big Ring” and the observations of mature galaxies existing soon after the Big Bang challenge our current cosmological paradigms. Additionally, the enigmatic nature of dark matter and dark energy—which together comprise 95% of the known universe—continues to elude our understanding. These revelations remind us that our existing knowledge is merely a fraction of the universe’s vast mysteries. The ceaseless motion and boundless splendour of the universe invite us to constantly reassess and broaden our scientific horizons, urging us onward in our quest to unravel the cosmic mysteries that await.
References:
https://www.scientificamerican.com/article/hubble-tension-headache-clashing-measurements-make-the-universes-expansion-a-lingering-mystery/
Retrieved from https://arxiv.org/pdf/1409.0880.pdf
Retrieved from https://arxiv.org/pdf/2311.00215.pdf
Written by Nishwal Gora, a third year physics student at the University of Edinburgh, specifically interested in the foundational philosophical and scientific underpinnings of cosmology and quantum mechanics.
Edited by Anna Motylova, EUSci’s Online Editor.
