In 1933 the heterodox astronomer Fritz Zwicky decided to “weigh” the cluster of galaxies located in the Coma Berenices constellation by measuring the motion of the galaxies. In doing so, he discovered that galaxies were moving at extremely high speeds, of the order of a thousand kilometers per second. You didn’t have to be very clever to see that this posed a serious difficulty. If the total mass of the cluster were only that contained in the galaxies, the one that is visible through our telescopes, it would have ceased to exist : the galaxies would have already dispersed through space because their total mass is not enough to keep them gravitationally bound. Now, since the cluster had not “evaporated”, there was only one solution left: there was more matter in the cluster than can be seen through telescopes . Zwicky concluded that the hidden mass of Coma must be on the order of two times greater than that visible in the form of galaxies. The always risky Zwicky fell short here: he is 10 times higher.
That “lost mass” – as Zwicky called it – slept the sleep of the just for almost half a century until in 1977 the astronomer Vera Rubin, observing the rotation of spiral galaxies, discovered that to explain it she needed more mass than what is found in the form of stars and nebulae. In our galaxy, the Milky Way, stars travel through space at about 240 kilometers per second, regardless of whether they are near the galactic center or on the outskirts. This is because there is a sphere of dark matter that surrounds the Galaxy . It is like the yolk of an egg: it is part of something greater. In our galactic region, astronomers infer that there is a mass ten times greater than that of all the visible mass in it and more than 90% of the total mass of galaxy clusters is in the form of non-visible, dark matter.
dark matter candidates
Now, what is that dark matter? From low-mass stars to black holes to exotic particles dreamed up by theoretical physicists, hunting down that missing matter is one of the most important problems facing astrophysics in the 21st century. Essentially dark matter candidates can be divided into three categories: hot dark matter, cold dark matter, and ordinary dark matter (also known as baryonic matter).
The easiest to imagine is the last of the three. And it is also the most difficult to justify, since the material we are made of does emit. We, for example, radiate in the infrared: hence the night vision goggles used by commandos and special forces. This ordinary matter that we have not yet been able to detect would be found in the form of Jupiter-type bodies, brown dwarf stars, dwarf galaxies…, or the sarcastically called MACHOs ( Massive Compact Halo Objects , objects of great mass of the galactic halo) -which include black holes, brown dwarf stars and Jupiter-like planets, and RAMBOs ( Robust Associations of Massive Baryonics Objects ), which would be nothing more than accumulations of MACHOs. Although they may exist, most astronomers doubt it because if numbers are made, frightening amounts come out: 5 billion “dark stars” compared to 200,000 million bright stars . It seems irrefutable that this type of dark matter cannot account for what happens in clusters and superclusters of galaxies. To explain the observations we must appeal to a completely different kind of matter.
It’s fascinating how cosmologists can talk about matter they not only haven’t seen but have no idea what it looks like: exotic particles from the overheated minds of theoretical physicists are worth as much here as copies of the Very Interesting magazine distributed around the world. space. The only drawback is justifying, in a reasonable way, what they do and how these specimens got to the cold intergalactic space. Despite this inconvenience, cosmologists classify this unknown exotic matter into two types: hot and cold. They are two cosmological terms that refer to the speed at which the particles that compose it move. If they have a mass of the order of a proton, then they will move slowly: it is cold matter. But if they are lighter, they will travel at speeds close to the speed of light: it is hot matter. The most popular candidate for hot dark matter is the neutrino , an extremely peculiar particle since it practically does not interact with anything: it can pass through a block of lead tens of light-years in size without noticing.
Hot or cold dark matter?
Why do cosmologists distinguish between the two types? Because the existence of one type or another of dark matter decides how the universe was built. From the different simulations of the distribution of large-scale galaxies taking into account hot and cold dark matter models, two strikingly different results are obtained. While the cold model ensures the formation of bottom-up structures – galaxies appear first and then clusters and superclusters -, the hot model does just the opposite: galaxies appear by fragmentation of larger structures.
Now, if we look at how they predict the distribution of galaxies on a large scale, both models have problems. Cold dark matter struggles to explain the large voids and filaments of galaxies and galaxy clusters seen in the universe, while hot dark matter “overshoots” and predicts too much structure. Solution? Totally Solomonic: Exotic dark matter is a mix of hot and cold .
However, the current consensus tends to favor the cold model as it is consistent with the fluctuations of the cosmic microwave background radiation. Now, the million dollar question is: What is that cold dark matter made of? Perhaps they are WIMPs ( Weakly Interacting Massive Particles ), massive particles that interact weakly . Under this name of detergent, a whole collection of particles are grouped that are born from the equations of the experts in theories that seek to unify the constituents of matter and the elemental forces under a single expression, in a single superparticle (or superforce).
Of course there are detractors that there is a type of matter that is neither seen nor interacts with ordinary matter -except gravitationally-. For the Nobel laureate in Physics, Abdus Salam, dark matter is like the invisible djinns (genies) from The Thousand and One Nights . Other physicists claim that there is no such dark matter but what we really need is a new physics. This implies modifying one of the most sacrosanct theories of physics of the 20th century, general relativity; They argue that just as Newtonian gravitation does not explain what happens when we have strong gravitational fields, maybe Einsteinian relativity does not apply to large distances. Whatever the solution, the only certainty is that dark matter has been with us for almost 90 years and we still don’t know what the hell it is.
References:
Panek, R. (2011) The 4% universe, HMH Books
