According to current measurements, the size of the cosmos must be larger than a hundred billion light-years. This is the order of magnitude of the universe we have indirect access to, writes physicist Carlo Rovelli. “It is around 1060 times greater than the Planck length, a number of times that is given by a 1 followed by sixty zeroes. Between the Planck scale and the cosmological one, then, there is the mind-blowing separation of sixty orders of magnitude.”
In this space, adds Rovelli in Reality Is Not What It Seems-The Journey to Quantum Gravity—”between the size of the minute quanta of space, up to quarks, protons, atoms, chemical structures, mountains, stars, galaxies (each formed by one hundred billion stars), clusters of galaxies, and right up until the seemingly boundless visible universe of more than a hundred billion galaxies—unfolds the swarming complexity of our universe, a universe we know in only a few aspects. Immense. Huge. Extraordinarily huge. But finite.”
Astronomers are confident that the volume of space-time within range of our telescopes—‘the universe’—is only a tiny fraction of the aftermath of the big bang. “We’d expect far more galaxies located beyond the horizon, unobservable,” says the renowned astrophysicist, Martin Rees, “each of which (along with any civilizations it hosts) will evolve rather like our own.”
One of the most fundamental known unknowns in astronomy is just how many galaxies the universe contains. The Hubble Deep Field images, captured in the mid 1990s, revealed untold numbers of faint galaxies. It was estimated that the observable Universe contains between 100 to 200 billion galaxies.
“It boggles the mind that over 90% of the galaxies in the Universe have yet to be studied. Who knows what we will find when we observe these galaxies with the next generation of telescopes,” says astronomer and Google Scholar Christopher Conselice at the University of Nottingham, who led the team that discovered that there are ten times more galaxies in the universe than previously thought, and an even wider space to ultimately search for extraterrestrial life.
In 2016, astronomers using data from the NASA/ESA Hubble Space Telescopes and other telescopes performed an accurate census of the number of galaxies, and came to the surprising conclusion that there are at least 10 times as many galaxies in the observable universe as previously thought. The image itself was produced by the Frontier Fields Collaboration (a joint effort between NASA’s Hubble, Spitzer, and Chandra space telescopes) allowing scientists to detect galaxies that are as much as 100 times fainter than those independently captured before.
The international team, reports Nature, led by Conselice from the University of Nottingham, UK, have shown that this figure is at least ten times too low. Conselice and his team reached this conclusion using deep space images from Hubble, data from his team’s previous work, and other published data . They painstakingly converted the images into 3D, in order to make accurate measurements of the number of galaxies at different times in the Universe’s history.
In addition, they used new mathematical models which allowed them to infer the existence of galaxies which the current generation of telescopes cannot observe. This led to the surprising realization that in order for the numbers to add up, some 90% of the galaxies in the observable Universe are actually too faint and too far away to be seen — yet.
All that the human species will be able to view after a hundred billion years, will be the dead and dying stars of our Local Group. But these, says Martin Rees in On the Future, who was not part of Conselice’s team, “could continue for trillions of years—time enough, perhaps, for the long-term trend for living systems to gain complexity and ‘negative entropy’ to reach a culmination. All the atoms that were once in stars and gas could be transformed into structures as intricate as a living organism or a silicon chip—but on a cosmic scale. Against the darkening background, protons may decay, dark matter particles annihilate, occasional flashes when black holes evaporate—and then silence.”
We can only see a finite number of galaxies because there’s a horizon, a shell around us, delineating the greatest distance from which light can reach us. But that shell, observes Rees, “has no more physical significance than the circle that delineates your horizon if you’re in the middle of the ocean.”
In analyzing the data the team looked more than 13 billion years into the past. This showed them that galaxies are not evenly distributed throughout the Universe’s history. In fact, it appears that there were a factor of 10 more galaxies per unit volume when the Universe was only a few billion years old compared with today. Most of these galaxies were relatively small and faint, with masses similar to those of the satellite galaxies surrounding the Milky Way.
These results are powerful evidence that a significant evolution has taken place throughout the Universe’s history, an evolution during which galaxies merged together, dramatically reducing their total number. “This gives us a verification of the so-called top-down formation of structure in the Universe,” explains Conselice.
The decreasing number of galaxies as time progresses also contributes to the solution of Olbers’ Paradox — why the sky is dark at night. The astronomer Heinrich Olbers argued that the night sky should be permanently flooded by light, because in an unchanging Universe filled with an infinite number of stars, every single part of the sky should be occupied by a bright object. However, our modern understanding of the Universe is that it is both finite and dynamic — not infinite and static.
The team came to the conclusion that there is such an abundance of galaxies that, in principle, every point in the sky contains part of a galaxy. However, most of these galaxies are invisible to the human eye and even to modern telescopes, owing to a combination of factors: redshifting of light, the Universe’s dynamic nature and the absorption of light by intergalactic dust and gas, all combine to ensure that the night sky remains mostly dark.
Image at the top of the page shows Hubble image of Abell 2744. The light in the image comes from dead, ghost galaxies torn apart long ago by the cluster’s gravitational forces, and their stars were scattered into what is known as intracluster space — the space between the galaxies.
Image below shows clutter of both distant galaxies and foregrounded stars in the Milky Way. Photo: CFHT, Pierre-Alain Duc (Obs. De Strasbourg) & Jean-Charles Cuillandre (CEA Saclay/Obs. De Paris).
The Daily Galaxy, Sam Cabot, via Nature, Reality is Not What It Seems -The Journey to Quantum Gravity, Hubble Space Telescope, and Martin Rees On the Future