I ended the last article at the superactive galaxy M87. But Messier 87, at 53.49 million light-years from us, is in the Virgo Cluster, extending from 50 mly to at least 160 mly, and comprising over 100 galaxies and groups of galaxies. This is the nearest large cluster to us, and the beginning of a realm in which the structure of the Universe is quite unlike what I was brought up to believe in the 1960s. The chief contenders then were the Big Bang (a sarcastic term coiled by Fred Hoyle, as he then was), in which the Universe had been created in a huge explosion at a finite time in the past, and the Steady State (advocated by Hoyle, Bondi and others, including the Principal Teacher of Physics at my school), in which matter was continuously created between the galaxies. In the UK, the principal advocate of the Big Bang was the Astronomer Royal, Sir Martin Ryle, and it is probably fair to say that he was losing the debate on points until 1963.
Lecturing at Glasgow University in 1962, Sir Martin had begun with Olbers’ Paradox. If the Universe is infinite, and the stars are evenly spaced in all directions, why is the night sky dark? Wherever you look, you should be looking at a star, and the summed radiation from all the stars should make the sky not only bright, but hot – too hot for the Earth to be habitable. Demonstrably that’s not the case, and the first answer is that the stars aren’t evenly spaced in all directions to infinity: the Milky Way is a disc, within which we’re located, and it has a finite size of ‘only’ 100,000 light-years. That doesn’t get rid of the paradox, because there are galaxies in all directions. At first sight, they seem to obey the Cosmological Principle, which stated that our viewpoint is not especially privileged, and from all viewpoints in space, the Universe should look more or less the same, except for variations in the immediate surroundings. Sir Fred Hoyle was arguing for a Perfect Cosmological Principle, whereby the Universe should look the same from all viewpoints in space-time – and since it was expanding, new galaxies must be created to maintain the overall density. As we look out to ever-greater distances, we’re looking further back in time, and due to the expansion, the light of the galaxies is progressively more red-shifted with distance. Again that would solve Olbers’ Paradox for light, but not for heat, because the cumulative radiation would still be there. But there would be a Red Limit where the velocity of recession reached the speed of light, and nothing from beyond that could ever reach us. If the Big Bang had taken place, then further out towards the Red Limit we looked, the more closely packed the galaxies should be – and as radiotelescopes improved, the observational evidence increasingly suggested that was the case. Hoyle did not like that one bit.
(My late friend Chris Boyce half-humorously suggested what I called the Pluperfect Cosmological Principle – that from all viewpoints in spacetime, it looks as if it was created in a huge explosion billions of years ago. Cosmologists have been known to have a moment’s uneasy pause while they think of an answer to that one.)
1962 was a momentous year in space exploration, in which, among other things, John Glenn because the first American to orbit the Earth and Mariner 2’s flyby of Venus made it the first successful interplanetary probe. But the International Astronautical Federation rather unimaginatively voted that the most important mission of the year was Telstar. Apart from giving the Tornados a hit record and providing a somewhat shaky metaphor for the Queen’s Speech (“Jesus is my Telstar”, the satellite opened up the world of commercial satellite communications which had been predicted by Arthur C. Clarke and touched on by military projects like Project Score, Moonbounce and Courier. But to allow even brief communications slots Telstar was high up, in what nowadays is called Medium Earth Orbit, and as it wasn’t a large spacecraft, it didn’t have a lot of power. On both sides of the Atlantic, huge antennae were needed: the UK ones were at Goonhilly Down) and the big US one was built by Bell Labs at Andover, Maine. The new technology of ruby masers was used to ensure that the incoming signal would be minimally affected by receiver noise, and all this was documented in colour by the National Geographic Magazine.
And yet, there was a background hiss which the Bell technicians could not eliminate. They even laboriously scoured the Big Horn of pigeon droppings, and put up wire mesh to let no more in. Eventually, someone remembered a decades-old prediction that the background radiation from the Big Bang should still be here, all over the sky, cooled to 2.73 degrees above absolute zero and far below the limit of detection in those days. It took time to gain acceptance: to his dying day in 2021, Andy Nimmo of the Space Settlers Society clung to a still more obscure model in which the Universe would have turned inside-out at the moment of creation, so the Big bang radiation would be undetectable on the outside. Sir Fred Hoyle, as he now was, continued to put forward alternative explanations for it, e.g. in his book Astronomy Today (1975). But by that time new telescopes were producing optical evidence to back up the radio surveys: the Universe definitely was more crowded in the past.
Around the same time, study of the microwave background turned up another phenomenon which took time to be recognised as legitimate. To obtain a smooth plot of it, cosmologists had to subtract the Doppler shifts generated by the Earth’s movement around the Sun, the Sun’s motion within the Milky Way, etc.. But like the background itself, they were left with a residual motion of 60 kilometres per second, and it was at right-angles to the Virgo Cluster. The most obvious explanation was that we, the whole Local Group, were in orbit around it. In one of its less inspired moments, New Scientist published a comment that it couldn’t be true, because the Universe hadn’t been in existence long enough for us to complete a single revolution. So no artificial satellite would be in orbit until it had gone round the Earth more than once? And Yuri Gagarin’s place in history would have to be discarded, because his retrorockets were fired before he had gone a full 360 degrees around the planet. It wasn’t long before that objection was quietly dropped.
Also, the movement relative to the Virgo Cluster was only the beginning. Beyond the Virgo Cluster and in the same general direction, the Coma Cluster, 300 million light-years away and 10 million years across, contains over a thousand galaxies, and other superclusters around the sky are pulling too, moving all of the above in a huge, complex pattern which has been given the Hawaiian name ‘Laniakea’, hundreds of millions of light-years in extent. At its centre of mass, hidden from us behind the star clouds in Sagittarius, something called ‘the Great Attractor’ is pulling us all. Whatever it is, its mass is estimated to be millions of times the Milky Way’s. It may coincide with a recently discovered supercluster in the constellation Vela, or that may lie beyond the Great Attractor and be larger still.
As mapping of the surrounding galaxies, clusters and superclusters continued, more patterns emerged. One of the first to be noticed and cause a stir was a huge homunculus-like figure, a billion light-years from us and even larger across. It was quickly outdone by a still larger and more dense feature called the Sloan Great Wall, at about 500 million light-years. Already it was clear that the Cosmological Principle was as dead as the Steady State theory: the Universe must look very different from different locations within it. And as more and more data came to hand, increasingly it became clear that there was an overall pattern which we now called the Cosmic Web, filaments of Dark Matter around which the visible matter has congregated, clearly seen in the Dark Matter Survey by the Cerro Tololo telescopes in Chile. Some parts of it are even visible, lit by quasars, very violent galaxies in the early Universe. Last week we saw how the earliest galaxies are being found at the convergence of three inflowing streams of gas; now we see that’s because they formed at the nodes of the Web.
‘Monstrous baby galaxies’ have been found in high concentrations on the Great Wall, resolved by ALMA submillimetre array in northern Chile. Streams of more normal galaxies are seen along the Web in the Hubble Space Telescope’s Extreme Deep Field, with ‘orphan’ clouds of hydrogen and helium left over from the Big Bang, in between. The mysterious, very high-energy phenomena called Fast Radio Bursts are concentrated along the lines of the Web, and no doubt their sources will turn out to be a particular class of galaxies within it. The ring of gamma-ray bursts discovered in 2015 is the largest known structure in the Universe, 5 billion light-years across and 7 billion ly distant, but no doubt it too will soon be outclassed. Every day brings new discoveries, and it’s amazing to think how far we have come, and how much more complicated it has become, since I first discovered the existence of galaxies in an article on the Palomar Sky Survey in a 1952 National Geographic Magazine, ‘Our Universe Unfolds New Wonders’
And then there are the voids – huge dark patches separating the strands of the Web, with comparatively few galaxies within them. The Boötes Void, with a volume of 1 million cubic megaparsecs (where 1 parsec = 3.26 light-years) was one of the first to be discovered. The big question was, were the strands of the web linked, like the structure of a sponge, or were they isolated by the voids, like the structure of a Swiss cheese? The answer is, it’s like a sponge. The detective story of how that was established is told in detail by J. Richard Gott in his book, The Cosmic Web. Mysterious Architecture of the Universe, Princeton University Press, 2016.
Another fascinating detail, presumably related to the process of formation, is that galaxies and groups of galaxies are oriented to the Cosmic Web, not just grouped along it but at right-angles to it, and this is shown by the orientation of their supermassive black holes in clusters like ELAIS N-1, where the jets from then are all pointing the same way, over at least 100 megaparsecs. What that is telling us not yet understood, but a new discovery in February may point to the answer. Astronomers at Leiden University have made it using LOFAR (Low Frequency Array), groups of radio antennae at 52 locations across Europe, including Birr Castle in southern Ireland, home of the Parsonstown Giant telescope with which the 19th century Lord Rosse discovered the shapes, though not the natures, of objects like the Crab Nebula and the Andromeda Galaxy.
The Leiden team has discovered a single elliptical radio galaxy 5 megaparsecs (16.3 million light-years) long, 3 billion light-years away, 240 billion times the mass of the Sun and powered by a supermassive black hole of 400 million solar masses. It’s the jets issuing from that which make ‘Alcyoneus’ the largest galactic structure in the known Universe. What’s most exciting about it is that it’s embedded in a string of the Cosmic Web, and it’s big enough that we may hope to see how they interact. It will, definitely, be a task for the James Webb Space Telescope when it becomes operational round about July.
Use the search engine to find Duncan Lunan’s series of astronomy articles.