This cosmologist lectured via videoconference at the IAC Winter School on “Bayesian Astrophysics”
By ADELINA PASTOR
“We know that dark matter makes us some 25% of the universe, but we don´t know what it is. Theories suggest that it may be a new type of particles, different from all those we know, which could be up to a thousand times heavier than a proton”
“The big question is whether we can separate the gamma rays emitted by dark matter from those which come from visible matter”
“Bayesian methods do this very efficiently because they include more parameters about, for example, noise, and use them to obtain the final result”
Researcher at Imperial College, London, Roberto Trotta is also a passionate popularizer who works with several different communications media, and is author of a recent book The Edge of the Sky, a history of the discoveries and mysteries of cosmology written using only the thousand most commonly used words in English. “Translating this into other languages will be a challenge” he says good humorously. His doctorate is in astrophysics, and his curiosity has led him to statistics, which he considers basic to the interpretation of data. “Science is made up of theories, but also from observations. Only the data can tell you whether your theory is good, or just a story. My interest in distinguishing which theories are good and which are not led me to statistics, because statistics is the answer, the bridge between the theory and the data. As we propose more and more theories and they get more complicated, it is impossible to make sense of the observations without statistics.
Question: You have just published your first book of popularization –The Edge of the Sky-not yet translated into Spanish
Answer: It is an attempt to explain what we know about the universe briefly and using only the thousand most commonly used words in English. The story follows the adventures of a woman scientist who is researching into dark matter from one of the biggest ground-based telescopes. We follow her from sunset to dawn in her attempt to measure how dark matter has affected the light from a distant galaxy. While she does this she muses on how our knowledge of the universe has developed during the past three thousand years, and on the still unresolved mysteries such as dark matter, or the multiverse.
Q: Let’s talk about the mystery of dark matter, then…
A: Dark matter is one of the most important subjects in modern cosmology. We know that it makes up some 25% of the universe, but we don´t know what it is. The theories suggest that it is made of a new type of particles, different from all those we know, and which could be up to a thousand times heavier than protons, the basic particles of visible matter. Although we cannot see it, we know it is very abundant. We estimate that there is more than 5 times as much dark matter as visible matter, because we can see its gravitational effects on the visible part of the universe. To explain further, we can detect and measure the gravitational forces in action in the universe, but we do not detect the source of this gravity, so that we have to assume the existence of an invisible form of matter, which we have termed “dark matter”
Q: So is there any experimental proof of its existence?
For the time being we do not have proof, but there are very promising clues. When two dark matter particles collide and disintegrate, the prediction is that they should emit various types of particles, among them high energy photons we call “gamma-rays”. These rays cannot be observed from the ground because the atmosphere absorbs them, but fortunately we have satellites such as Fermi which, among other things, looks for signals due to dark matter. These signals should be stronger in the places where the dark matter is denser so that there are more collisions between the particles. For that reason among the regions most observed for this work are the centres of the galaxies, where we expect the dark matter density to be high. A number of teams have claimed to have observed an “excess” of gamma-rays coming from the centres of galaxies, more than would be expect from “normal” sources. Of course the centre of a galaxy is a zone with much activity, and there are many possible sources of these rays. In the end the big question is whether we can separate the gamma-rays emitted by the annihilation of pairs of dark matter particles from those which arise from visible matter sources. A number of groups claim that they can do this, and that indeed they have done it, but this is still not a proof, only an indicator, but it is compatible with theoretical models, and with a dark matter particle which has between 10 and 40 times the mass of a proton.
Q: The problem, then, is to distinguish between the signals. Can Bayesian inference help?
A: Definitely. The problem with the data is that you don´t know which part corresponds to the signal you are interested in, and which part is noise or background signal. What can you do to disentangle this? The simple way is not to take notice of that part of the signal which you don’t understand, and set this to zero, but by doing that you give the data more significance than you should. What you should really do is to build a model which includes both the signal and noise, and then use statistics to reduce the uncertainty in the part of the model which does not interest you. Bayesian methods do this very efficiently, because they allow you to include more parameters about, for example, the noise, and to use them to reach the final result, instead of assuming something and taking for granted that this assumption is correct. We can say that they let you make better use of the data.
Q: So the more we develop our statistics, the better we will understand the world
A: Exactly. We are at a point where our understanding of the universe does not depend on the quantity of data we may have, but on our ability to interpret this correctly. For that reason statistics is going to play an increasingly important role, because in future we will have ten or a hundred times more data than now. Imagine, for example, the Square Kilometer Array (an instrument which will be made up of three thousand telescopes on an area of around 1 km2) which will come into operation around 2020. This telescope will take images of all the galaxies in the visible universe, or almost all of them. From the SKA the quantity of data will be so enormous that it will be impossible to use them without treating them statistically.
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