There is more than meets the eye

Few sights are as exciting as viewing a starry sky, far from sources of light pollution. When you look up at the sky, it feels like you are falling into the cosmos. However, astrophysics now assures us that what we see – planets, stars, nebulae, star clusters and galaxies – is only a small part of the whole. Thanks to a Swiss astronomer, we have known for almost a century that there is much more mass than we can observe. However, even today, no one knows what it is made of. On Friday 21 November at 6 p.m. at the Auditorium Campus Ovest in Lugano, Marco Lombardi, astrophysicist and professor at the University of Milan, will give a lecture organised by IRSOL on this topic at the frontier of fundamental physics research, where astrophysics and particle physics meet. Lombardi answered some of our preliminary questions so that we would not arrive at the lecture completely unprepared.

What were and still are the main clues that the matter that makes up the universe cannot be only that which we can observe directly?

The very first indications of the presence of dark matter were available to astronomers as early as the 1930s, when Swiss astronomer Fritz Zwicky studied the motion of galaxies that made up a galaxy cluster very close to us, the Coma Cluster, named after the constellation to which it belongs. By studying the spectra of the galaxies in the cluster, Zwicky was able to measure their radial motions using the Doppler effect. He realised that most of galaxies showed much higher motions than could be explained by luminous matter alone. In other words, if the mass present in the Coma Cluster were due solely to stars, all the galaxies in the cluster would have a velocity greater than the escape velocity. The cluster would therefore disappear in a relatively short period of time.

Zwicky's observations were largely ignored by contemporary astronomers until the 1970s, when the study of the rotation curves of spiral galaxies revealed a very similar problem: the stars that make up the arms of spiral galaxies rotate around the centre of the galaxies at a speed greater than the escape velocity, again assuming that the mass of the galaxies consists only of stars.

Today we know that galaxy clusters contain, in addition to stars, plasma (i.e. very hot and completely ionised gas), which emits X-rays (observed definitively in the early 1970s by the Uhuru satellite), and that this represents a considerable fraction of the "standard" matter present in clusters. However, even considering the presence of plasma, the velocities of the galaxies in clusters are still orders of magnitude higher than expected. Similarly, the gas and dust present in spiral galaxies are in no way capable of explaining the rotational motions of stars in the outer regions of the arms.

For these reasons, since the 1970s, astronomers have invoked the presence of a form of matter distinct from ordinary matter, already called "dark matter" by Zwicky.

Today, in addition to the classic observations mentioned above, we have many other measurements that indicate the presence of dark matter in galaxies and galaxy clusters, with the strongest evidence coming from observations of gravitational lensing and measurements of the anisotropy of the cosmic microwave background.

What are currently the leading hypotheses that best explain the observations?

For many years, astronomers and particle physicists have wondered about the nature of dark matter. Astrophysical data provide essential information about the basic characteristics of this form of matter. Obviously, it must have mass and exert gravity. It must not interact with electromagnetic radiation. It must be "cold", a term used to indicate that the typical velocities of dark matter particles must be low even in the early universe (a crucial fact for the formation of galaxies). It must interact at most through weak interaction with ordinary matter. The last point essentially guarantees that the Earth, in its motion around the Sun and around the centre of the Milky Way, must be able to pass through the dark matter present undisturbed without any obvious apparent effect.

For many years, the leading candidate for dark matter has been WIMPs, an acronym that stands for Weakly Interactive Massive Particles. These are thought to be elementary particles with a mass between 10 and 1,000 times that of a proton (i.e. between 10 and 1,000 GeV), which interact only through weak interaction. The reason for favouring this candidate is linked to the fact that, if such particles existed, then a "sea" of such particles would "automatically" be created in the moments following the Big Bang. Therefore, as with the photons of cosmic background radiation, we would be able to predict the abundance of WIMPs today. The "miracle" is that, assuming a mass of around 100 GeV and weak interaction, today there would be just the right amount of dark matter to justify the observations.

The problem is that, to date, there is no experimental evidence of the existence of WIMPs. Accelerators such as the Large Hadron Collider at CERN in Geneva should already have observed traces of WIMPs. Similarly, in astrophysics, WIMPs are expected to produce gamma rays through self-annihilation (WIMPs should be their own antiparticles), but the few signals observed can easily be explained as contamination from standard astrophysical sources. In 2016, the DAMA experiment, conducted at the INFN laboratories in Gran Sasso and aimed at detecting a direct interaction of WIMPs during the Earth's motion, showed a signal modulated on an annual basis: a fact that aroused considerable interest in the scientific community. Unfortunately, at present, DAMA's discovery has not been replicated by any other direct experiment, which has aroused a good deal of scepticism in the scientific community.

In conclusion, the "WIMPs miracle" is not yet completely exhausted, but the room for manoeuvre is shrinking as measurements become more sensitive. For this reason, many theoretical physicists have investigated possible alternatives, with the result that there is currently a "zoo" of dark matter candidates with various characteristics. At present, no candidate seems to be beyond the stage of theoretical speculation, but the situation could obviously change in the coming years.

Is it possible that dark matter does not exist and that observations can instead be explained by radically modifying the theories of fundamental physics?

In the early 1980s, Israeli astrophysicist Mordehai Milgrom proposed modifying Newton's second law, i.e. the law that relates the total force acting on a mass to the acceleration experienced by that mass, normally formulated as F = m ⋅ a. Milgrom noted that Newton's well-known law has been experimentally verified only in regimes where accelerations are relatively high, and not in regimes where accelerations are very small. However, very small accelerations are precisely those found in the stars of spiral galaxy arms or in galaxy clusters. Milgrom proposed replacing Newton's second law in these regimes with a quadratic law of acceleration, so that F = m ⋅ a² / a₀ , where a₀ is a parameter of the theory (called "MOdified Newtonian Dynamics" or more commonly "MOND"). The theory has since undergone various modifications (mainly to make it compatible with relativity), but it has been strongly criticised by many astrophysicists, mainly for its inability to justify the data from galaxy clusters.

More recently, several theories have been proposed that reproduce, at least from a phenomenological point of view, the results of MOND. Among these, perhaps the one that has attracted the most media attention is Erik Verlinde's "emergent" or "entropic" theory of gravity. The idea is that gravity does not exist as a force in its own right, but emerges from entropy and quantum information present in the structure of space-time, in a similar way to how temperature emerges from the random motions of atoms or molecules. The theory has been criticised, but it has also attracted a great deal of interest due to the novelty of its proposals. One positive aspect of this theory is that it provides predictions that are already (partially) verifiable with currently available data. The results are mixed, with some experiments appearing to agree with entropic gravity and others in stark contrast. At present, therefore, there is no unanimous opinion within the scientific community on Verlinde's theory.