We live in an expanding universe. Although today this might seem almost obvious even to the layman, it was only about 75 years ago that Edwin Hubble realised, through a stunning amount of experimental work at the telescope, that the galaxies are receding from us at a velocity which is proportional to their distance – a cornerstone discovery for modern cosmology. This particular expansion law (a result that was found so counterintuitive that initially did not convince Albert Einstein) can only result from an equally peculiar fact: the universe has to be homogeneous. Again, this clashes with our everyday experience. If we look at the night sky, we see a large number of stars which are apparently organised in constellations and there are areas of the sky in which their number appears to be much larger than in others. This uneven distribution is what physicists define as an inhomogeneous distribution. It might seem that this evidence contradicts the physical basis of Hubble’s expansion law. However, this is not the case.

The stars we can see with bare eyes all belong to the same stellar system, our galaxy (or, more familiarly, the Milky Way), an immense agglomerate of about one hundred billion stars organised in a rotating, flat structure which resembles a disk. The radius of the disk is such that a light signal sent from the center of the galaxy has to travel for about 60,000 years to reach the disk edge. Galaxies such as the Milky Way are separated among them by a distance which is on average 250 times larger. Thus we might think of the matter distribution in the universe as made of knots of stars (the galaxies) separated by large distances. What is important is that if we select large enough volumes of the cosmos, on average the number of knots would be the same and it would be impossible to identify one volume from the other. In other words, the universe is indeed homogeneous, provided that we look at it on scales that are sufficiently large.

But how large is large? Observations of distant galaxies have been collected for many decades now and organised in large samples. From this type of data we have discovered that the distances over which the universe becomes homogeneous are about 20 times the mean separation between galaxies themselves (that is, 300 million light years).

Hence, the Hubble expansion law can be understood as a result of such homogeneous distribution of matter in space. A superb description of the expansion dynamics can be obtained through Einstein’s general relativity theory. But even such wonderful mathematical construction fails (or better, ceases to be applicable) at early enough epochs, because the expanding world picture extrapolates back to a singular state in which conventional physics becomes undefined. This singular state is commonly defined as the Big Bang. Immediately after this event, matter in the universe was in a physical state which is very different from the one we experience in our everyday life and it was broken in its most fundamental constituents, the elementary particles that we have discovered with powerful laboratory accelerators during the last several decades.

Temperatures during these initial phases were so high that the entire universe was in practice a nuclear reactor. During the first 100 seconds after the Big Bang, temperatures higher than 10 million degrees were reached, similar to those found at the centre of the sun today. In such extreme conditions nuclear reactions can take place and indeed some of the chemical elements (for example hydrogen, helium and lithium) were synthesised, the remaining elements being produced later on by nuclear reactions inside stars.

In spite of these violent events, this sea of particles was already very closed to be perfectly homogeneous. How do we know this? Immediately after the Big Bang, the hot, expanding cosmic fireball was emitting copious amounts of light. Even today, the space around us is permeated by this background radiation which, because of cosmic expansion, has shifted out of the human visible range but can still be observed at radio (i.e. lower) frequencies. For this reason, this background is usually referred to as the Cosmic Microwave Background, a sort of echo of the Big Bang. This radiation has travelled almost unaffected from the distant past, an epoch when the age of the universe was only 3% of the actual one (about 13.5 billion years). It brings us information on the state of the matter then. By studying in detail the properties of such background radiation, we have been able to establish that at that time the matter density (that is, the number of particles in a unit volume) was the same everywhere in the universe within less than 0.001%! Such astonishing degree of homogeneity is almost hard to believe. For example, if the Earth were to be considered spherical with the same degree of precision, it would have no mountains higher (or valleys deeper) than 60 meters. How nature managed to build up such a condition is still a mystery. The conclusion is that the universe, at least on the largest scales, has been almost perfectly homogeneous during its entire evolution.

Even more mysterious is the nature of the tiny density ripples, deviations from an otherwise perfectly homogeneous sea of matter. What is their origin? Scientists are convinced that they could have arisen from quantum mechanical effects. The energy that ultimately created the universe was a quantum field called inflaton; these physical entities are never at rest, and even the vacuum is roiling with fluctuations of “virtual particles” that continuously appear and disappear back into the vacuum. Thus the inflaton could never be perfectly homogeneous in space. The tremendous initial expansion which the universe underwent (named inflation after the inflaton), stretched these ripples to much larger scales. We could think of cosmic expansion as a hyper-powerful microscope that allows us to see the microscopic quantum structures that were present immediately after the Big Bang.

This connection between the biggest and the smallest things around us is fascinating and forces us to realize that the entire universe is a mesh of interacting scales. The probabilistic rules that are at the basis of quantum theory have influenced the apparently deterministic evolution of the universe. In some sense, everything that exists today was shaped by the processes that occurred as a result of microscopic quantum mechanical processes at the beginning of time. Like Darwin’s theory of evolution or Wegener’s theory of continental drift, the Big Bang theory is only a starting point to understand the subsequent evolution of the universe.

Now that we have understood that deviations from homogeneities were extremely small, but yet present, we can dare to ask what happened to them later on. Here another fundamental force of nature comes into play: gravity. The gravitational force is proportional to the mass (and hence to the density) of a given object. Therefore, quantum ripples that were (very slightly!) denser than the homogeneous background matter had a slightly larger than average gravitational potential and they were able to attract and accrete matter from the surroundings. During this process they became even more overdense. Similarly, the slightly underdense regions grew more underdense. The result of this process was the emergence of voids and density peaks in the matter distribution, a network that vaguely resembles a swiss cheese-like structure. A large fraction of cosmic matter tends to concentrate at the peaks of such distribution: these condensations will collapse further, start to form stars, and eventually become galaxies as those around us. The remaining (or intergalactic) part of the matter is located in filaments connecting the galaxies; the filaments delimiting the voids convey the intergalactic matter onto the galaxies, allowing them to grow. This complex network is what cosmologists refer to as the cosmic web. Today we can follow all the details of the build up of the cosmic web via computer simulations, which provide us with beautiful and detailed visual representations of the cosmic web as the one shown in the figure.

The role of this network of filaments and voids is not so much to connect the galaxies among them, but rather to regulate their growth and evolution by providing them with the necessary fuel (the gas) to form stars and shine. The filaments of the cosmic web contain most of the matter in the universe. Still, the gas that makes them is extremely rarefied, amounting at one gas particle per cubic centimetre. To better understand this, the air we breathe contains about 10 billions of billions of particles in the same volume! For this reason directly observing the cosmic web is very challenging and it is currently at the frontier of cosmological research. However, we can indirectly study the web by measuring how the light from distant sources is affected during its travel to reach us (a situation resembling a lighthouse shining through a foggy night). The haze of the cosmic web in fact modifies the intensity and colour of the background source light. By studying such an effect we have been able to get a first glance at the matter in these ancient times. But this is only the most recent accomplishment of the never-ending human need to explore the cosmic frontiers.