Tag Archives: AMS-02

The Edge of the Sky, by Roberto Trotta: A book review

Last month I finished a first draft of an article about AMS-02 for the Yearbook of Astronomy 2016. (As I write this, 2016 seems a long way away – but the lead time for this volume is long and, besides, I have other writing commitments this year. I wanted to get a draft version out of the way while I had chance.) The target audience for the Yearbook consists of amateur astronomers – people who are deeply interested in astronomy and cosmology but who don’t necessarily have a scientific or mathematical training. Much of my article, therefore, is taken up by explaining the meaning of various technical words and phrases – “positron fraction”, “neutralino annihilation”, “primordial antimatter” and so on. Even the name of the experiment – Alpha Magnetic Spectrometer – requires explanation. It took me several thousand words to explain why Sam Ting and his AMS might, or might not, have seen something of great scientific interest.

When I heard that Roberto Trotta had written The Edge of the Sky – a book about cosmology using only the thousand most frequently used English words – I thought he must either be barking mad or else a member of the Oulipo (you know, those authors who decide to write novels while blindfolded and using only letters that appear on the left-half of a keyboard, or something equally arbitrary and constraining). I thought it would be a disaster. It turned out to be one of the most charming, fresh and inventive books of popular science that I’ve read.

In Trotta’s book, the Milky Way becomes the White Road, electrons are referred to as Very Small Drops and antimatter becomes Sister Drops. (So I guess the positron, which I write about in my Yearbook article, would be the Sister Drop of the Very Small Drop.) These and other word choices are wonderful and lead to a surprising clarity of expression.

But is it really possible to describe the complexity of modern science using this approach. Well, I tried to explain the importance of AMS-02 using polysyllabic words to replace other, more technical, polysyllabic words. In The Edge of the Sky a space-based detector such as AMS-02 becomes a flying Far-Seer in the sky. The heroine of Trotta’s book wonders whether dark matter will first show up in such a flying Far-Seer in the sky or in one of the big ears in the rock (in other words, one of the multitude of underground detectors such as LUX or DAMA), in the huge eye in the ice (the IceCube Neutrino Observatory) or in the Big Ring in the ground (the Large Hadron Collider). This approach is possible. It works, and it works beautifully.

The experience of reading The Edge of the Sky is strange and rather hypnotic. I think everyone can learn something from it. This book is a must-read. (A sentence that uses the 28th, 455th, 13th, 85th, 88th and 317th most-used English words according to Project Gutenberg.)

A hint of dark matter?

Yesterday Sam Ting, the 1976 Nobel prize winner, announced the first results from his Alpha Magnetic Spectrometer (AMS-02) experiment. The results hint at a possible dark matter signal. But it remains only a hint.

The story begins with the observation that the visible stuff in the universe – stars, galaxies, and so on – are all made of matter. We see no evidence for large amounts of antimatter. However, small amounts of antimatter are constantly being created when high-energy cosmic rays scatter off particles in the interstellar medium, or when the intense electromagnetic field surrounding pulsars produce electron-positron pairs. So Earth is certain to be hit by antimatter particles, and in particular by positrons, that have been created by events within our galaxy.

Now, astrophysicists are interested in measuring the so-called positron fraction that hits Earth. (The positron fraction is the ratio of positrons to the total number of electrons and positrons.) If the main production mechanism for positrons is the scattering of high-energy cosmic rays off particles in the galactic disk, an assumption that until recently seemed quite reasonable, then the positron fraction would decrease with energy since there are other processes that generate high-energy electrons without accompanying positrons. In 2008, however, the PAMELA experiment measured a rise in the positron fraction between 10 GeV and 100 GeV. The Fermi satellite later confirmed this excess number of positrons, and it showed that the rise extended up to 200 GeV. So what is going on? Well, one explanation for the rising positron fraction is that positron generation by a few nearby pulsars could be the cause. But there’s another possibility.

If dark matter exists then sometimes, simply because there’s so much of the stuff, dark matter particles and antiparticles will meet and annihilate. In some models, dark matter annihilation can give rise to excess numbers of positrons. Furthermore, the dark matter signal must take a particular form: it will be isotropic (in other words, the positrons will come equally from all directions in space) and the rising positron fraction will have a sharp cut-off after a certain energy is reached (an energy that is determined by the dark matter particle mass, since the particles can’t give rise to positrons that are more energetic than themselves). So after the PAMELA and Fermi results there was hope that we might be seeing hints of a dark matter signal, but the data were not clear enough to draw any conclusions. It wasn’t even certain that the positron excess was real.

Enter Sam Ting’s AMS-02 experiment.

AMS-02 is a cosmic ray detector on board the International Space Station. The detector was put in place by astronauts on 19 May 2011, and since then it has detected 30 billion cosmic rays. It has been able to measure the positron fraction to higher energies than any previous detector and with a precision that is much, much better than anything that has gone before. Yesterday, Ting announced the results of an analysis of the first 10% of data from AMS-02.

Artist view of AMS-02 installed on the attached S3 location on the main truss of the ISS

An artist’s view of AMS-02 installed on the attached S3 location on the main truss of the ISS (Credit: NASA/JSC)

The first result is that the excess seen by PAMELA and Fermi is real: the positron fraction increases from about 5% at 10 GeV to about 15% at 350 GeV. That in itself is significant, and requires an explanation. Second, the excess seems to be isotropic: if it turns out to be truly isotropic then that would tend to disfavour pulsars as being the source of the excess. Third, there are suggestions – and these are nothing more than tantalising hints – that AMS-02 might be seeing the start of a cut-off.

The positron fraction as a function of positron energy. Compare the AMS-02 error bars with previous experiments! (Credit: AMS collaboration)

The positron fraction as a function of positron energy. Compare the AMS-02 error bars with previous experiments! (Credit: AMS collaboration)

So are we seeing the effects of dark matter? As the years go by, and AMS-02 improves and extends the energy spectrum positron fraction yet further, astrophysicists might decide that the dark matter explanation is the only viable one. But at present it’s far too early to make any claims: the AMS-02 results announced yesterday are interesting, but nothing more.

The AMS paper has been published in Physical Review Letters.