Sadly, January 2024 saw the loss of one of the legendary figures in cosmology, Arno Penzias, who along with Robert Wilson discovered the cosmic microwave background. Their serendipitous discovery came whilst testing a large horn-shaped radio antenna, through which they detected a signal that seemed to be coming from every direction with no apparent source. So perplexed was the pair by this mysterious noise that they famously even scrubbed the horn free of pigeon droppings in case that was the source of the radiation.

Previously, as a result of Henrietta Swan Leavitt's period-luminosity relationship for Cepheid variable stars (the most important standard candles in astrophysics), Edwin Hubble was able to establish that we live in an expanding Universe. Extrapolation of the expansion backward in time led to the theory of a hot “big bang” (actually originally coined as a term of derision by Fred Hoyle). This idea led, in turn, to physicists Robert Dicke and George Gamow independently predicting that the thermal radiation from the early Universe should have been cosmologically redshifted to just a few degrees above absolute zero by the Universe's expansion, and this is exactly what Penzias and Wison had detected.

Popular media often characterises the CMB as the afterglow of the big bang, however, it is more accurately described as the last light to be scattered by the baryonic gas that existed before the formation of the first atoms - when the Universe was merely a few hundred thousand years old. At that time, having cooled to the point that electrons were able to combine with atomic nuclei, the previously opaque cosmic “soup” became transparent to photons, which as they propagated freely through space have had their wavelengths stretched by the expanding fabric of the Universe. The CMB, a snapshot from this somewhat confusingly dubbed recombination era, when matter and radiation were in thermal equilibrium, manifests itself as a remarkably uniform “fog”, displaying an almost perfect black body radiation profile with a peak of emission at 2.7 K.

Mainstream cosmology is based on an underlying cosmological principle, which states that the Universe is both isotropic and homogeneous. In other words, at a suitably large scale, everything is uniform in appearance in all directions. Obviously, on a small scale, there are cats, dogs, planets, stars and galaxies, but when you zoom out far enough, the Universe is reminiscent of a sponge made up of filaments of galaxy clusters separated by voids. The cosmological principle is backed up by the remarkable uniformity of the CMB, which is essentially a map of the structure of the very early Universe. But when examined closely it displays a range of anisotropies on varying angular scales, as evident in minute temperature differences, some of which correspond to pockets of varying density, seeded by quantum irregularities when the Universe was barely a fraction of a second old. 

So far so good, however, there are problems on the horizon for both the cosmological principle and the currently accepted model for the evolution of the Universe. A discovery was recently made by Alexia Lopez, a PhD student in the northwest of England, of a giant ring (actually a corkscrew) of galaxies, which at 1.3 billion light years across is inexplicably large. In fact, there is a growing list of enormous structures, which on paper shouldn't exist, that are challenging the underlying theory that the anatomy of the Universe was forged from a combination of attractive cold dark matter, shaping the galaxy clusters, and repulsive dark energy driving an accelerating expansion; the so-called lambda-CDM model. 

If this wasn't enough to contend with, there is an increasing tension between the two main methods of calculating the expansion rate of the Universe, pinning down an accurate value for the Hubble constant, the constant of proportionality in Hubble's law, which states that a galaxy’s recessional speed is proportional to its distance. One method relies on ascertaining the distances to supernovae in far-off galaxies, whilst the other method relies on matching computer model predictions of the appearance of the CMB to actual observations. Unfortunately, results from both the distance ladder method and the CMB method are increasingly at odds with each other.

Whatever the outcome of the so-called crisis in cosmology, if you are reading this then you exist at the most exciting and quickly evolving period in the history of this fascinating and all-encompassing discipline. Just watch this space(-time)!

Images

Horn Antenna by NASA, restored by Bammesk - Original version at Flickr: NASA on The Commons. This file was derived from: Horn Antenna-in Holmdel, New Jersey.jpeg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=89510549

CMB by NASA / WMAP Science Team - http://map.gsfc.nasa.gov/media/121238/ilc_9yr_moll4096.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=23285693

Alexia Lopez by University of Central Lancashire (2024). A big cosmological mystery. https://www.uclan.ac.uk/news/big-ring-in-the-sky (last accessed 11 February 2024)