Tag Archives: Fermi

The top-ten physicists

In a recent article Robin McKie presents his list of the 10 best physicists. Such lists are essentially meaningless – but it’s fun to argue and think about them! So here’s my own list of the 10 greatest physicists.

Isaac Newton – couldn’t not be on the list. Calculus, gravitation, laws of motion, optics, reflecting telescope and so on and so on and so on. I’m not ordering this list, but if I were Newton would be top.

Albert Einstein – special relativity, mass-energy equivalence, photoelectric effect, Brownian motion – all in one year! And general relativity on top of that. If I were ordering this list, which I’m not, Einstein would be second.

James Clerk Maxwell – electromagnetism, one of the great unifications in physics. Shame he’s not more widely known.

Galileo Galilei – Einstein called him the father of modern science. Galileo has to be on the list.

Archimedes – he doesn’t often appear on these “top-10” lists, but Archimedes was the leading scientist of antiquity.

Michael Faraday – discoveries include electrolysis, diamagnetism, electromagnetic induction; his experimental work formed the basis for Maxwell’s theories. One of the greatest experimentalists of all time.

Ernest Rutherford – the greatest experimentalist since Faraday. The father of nuclear physics.

Paul Dirac – early fundamental work in quantum mechanics and quantum electrodynamics; predicted the existence of antimatter; the Dirac equation describes the behaviour of fermions.

Enrico Fermi – one of my favourite physicists. He was the last of the great physicists who excelled in both experiment and theory.

John Bardeen – the only person to have one the Nobel prize for physics on two occasions. His work has had a huge impact on all our lives.

Three or four of the names can’t be argued with. After that, it gets more tricky. Cases can be made for Bohr, Schrödinger, Pauli, Heisenberg, Curie, Wigner… I could mention a dozen more.

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.

A dark matter bump?

The German astrophysicist Christoph Weniger has recently written an interesting paper (A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope) that adds to the general confusion surrounding dark matter.

In his paper Weniger analyses 3.5 years’ worth of publicly available data from the wonderful Fermi Gamma-ray Space Telescope. The Large Area Telescope (LAT) on Fermi, as I’m sure you know (and if you don’t you can read about it here), is an imaging gamma-ray telescope that covers the energy range from about 30 MeV to 300 GeV. The LAT has a very wide field of view (it can see about 20% of the sky at any time) and its continuous scans mean that it covers the entire sky once every three hours. It’s an incredible instrument.

The main purpose of the LAT is to perform an all-sky survey of high-energy phenomena such as active galactic nuclei and pulsars. But it can search for signs of dark matter, too.

If dark matter particles exist then, very occasionally, one of those particles might encounter its antiparticle and the pair will mutually annihilate (just as an electron and a positron, for example, can annihilate). The result would be something we can detect: a pair of high-energy photons, for example. We don’t know what energy those photons would possess because we don’t know the mass of the dark matter particle. But every dark matter particle will have the same mass, so all photons coming from annihilation will have the same energy. And that gives astrophysicist a signal they can look for!

Within our Galaxy, the density of dark matter particles is likely to be highest at the centre. Therefore the Galactic centre is the place where dark matter annihilation is most likely to take place. Thus the Galactic centre is the most likely place to observe high-energy photons coming from dark matter annihilation. Of course, the central regions of the Galaxy will be emitting high-energy photons from a variety of processes, but those photons will have a broad spread of energies. If astrophysicists detect a sharp spike at a particular energy then that would be excellent evidence for dark matter: no other astrophysical phenomenon that we know about could generate a narrow peak in its gamma ray spectrum.

I’m sure you are ahead of me. Weniger’s analysis of Fermi data of the Galactic centre show evidence for a line at 130 GeV. The statistical significance of this result is not strong enough to make any particular claims. Weniger’s analysis rests on data points from about 50 photons, so to reach a 5 sigma level of significance would probably require several more years’ worth of data. Furthermore, there may well be systematic errors of which Weniger is unaware (since his analysis, as mentioned above, is based on publicly available data). Nevertheless, it’s perhaps another piece of the jigsaw – but one that seems to contradict another recent piece of evidence. As I said, the picture is confusing!

One interesting aside: a dark matter particle at around 130 GeV wouldn’t be too far away from the 125 GeV peak seen by the LHC and taken to be hints of the Higgs. Could it be that the Large Hadron Collider is seeing signs of dark matter rather than the Higgs? Wouldn’t that be exciting!

The cosmic ray gun

It’s one of the most long-lasting questions in astrophysics: what’s the source of those really high-energy cosmic rays that sometimes hit Earth? What cosmic gun could possibly shoot such high-energy bullets towards us?

There are two obvious candidates: active galactic nuclei and gamma ray bursts.

There are seem to be two types of progenitors for gamma ray bursts, but the most luminous events probably come from the collapse of very massive, rapidly rotating stars. Models of such collapse events suggest that the fireball should, alongside the generation of extremely energetic gamma rays, generate high-energy cosmic rays and neutrinos. And scientists now have a detector that can hunt for neutrinos from gamma ray bursts: IceCube.

Artist's impression of the IceCube observatory

An artist's depiction of the IceCube observatory: 86 strings containing 5160 Digital Optical Modules are used to detect neutrino events.
(Danielle Vevea/NSF & Jamie Yang/NSF)

I don’t propose to discuss IceCube in detail in this post. I’ll surely do that in later posts, and you could always read the relevant chapter in New Eyes on the Universe. The exciting news yesterday is that IceCube has been used to look for neutrinos from 300 gamma ray bursts detected by the Swift and Fermi space telescopes. Neutrinos are of course notoriously difficult to spot, but IceCube should have seen several neutrinos from these exceptionally luminous events. It saw nothing.

This negative result suggests that our models of particle production in the fireball of a gamma ray burst might need some major tweaking – and perhaps that high-energy cosmic rays don’t come from burst after all. Perhaps active galactic nuclei are the guns that fire those cosmic ray bullets at us.

Fermi spies a superbubble

One of the mysteries I discuss in New Eyes on the Universe is the origin of ultra-high-energy cosmic rays. These are subatomic monsters, particles that smash into Earth’s atmosphere with macroscopic energy: the famous ‘Oh-my-God’ particle carried 3 x 1020 eV – the kinetic energy of a well-struck tennis ball. What mechanism can accelerate a subatomic particle to that sort of energy? No one knows. However, astronomers are closer to understanding the source of cosmic rays with slightly lower energies (up to about 1015 eV – so still far more energetic than anything the Large Hadron Collider can deliver!)

The Fermi space telescope (previously known as GLAST; as I mention elsewhere, thank heavens that not all astronomy missions are known by acronym) has found evidence for the source of at least some medium-to-high-energy cosmic rays. And thought the details are still to be determined it seems that these cosmic rays are accelerated by shock waves produced when supernovae eject material into space. This model of cosmic ray acceleration, appropriately enough, originated with Enrico Fermi.

Artist's impression of the Fermi space telescope

Artist's impression of the Fermi gamma-ray space telescope.
Credit: NASA

What the Fermi telescope actually found was a source of gamma-rays in the constellation of Cygnus. The source lay along a line between two clusters of stars, the clusters being separated by about 160 light years. One cluster contained over 500 massive stars (the sort of stars that form supernovae), the other cluster contained about 75 massive stars. So how does this relate to cosmic rays?

Well, the clusters contain dense gas clouds – that’s an environment in which massive stars are likely to form – but the stellar wind from a massive star pushes the gas away and creates a ‘bubble’ (When a star explodes as a supernova it also creates a ‘bubble’ around what’s left behind.) These bubbles grow and merge with bubbles around other stars and remnants to form ‘superbubbles’. What Fermi detected (the results are published in Science 334 1103-1107) was high-energy gamma-rays coming from a superbubble in Cygnus. (Since gamma-rays aren’t deflected by magnetic fields, they point straight back to their source; Fermi could thus determine the source of these gamma-rays. Cosmic-rays, being electrically charged, are deflected by the magnetic fields in our Galaxy and around Earth; the arrival direction of a cosmic ray does not necessarily point back to its source.)

The best interpretation of the Fermi data is that cosmic rays were being accelerated by shockwaves in the superbubble; whenever those cosmic rays collided with atoms or molecules inside the superbubble, gamma-rays were produced. The gamma-ray energy distribution was what one would expect from such collisions. Furthermore, the spatial distribution followed the shapes of the gas clouds and cavities. So this is good, strong evidence that some cosmic-rays originate from inside massive-star-forming regions of space.

But what precisely is the acceleration mechanism? An isolated shock wave from a single supernova remnant, or the combined effect of many different shocks? It’s not yet clear. As for the source of the ‘Oh-my-God’ particles – well, God alone knows at present.

Name the array

The main lesson I learned in writing New Eyes on the Universe is that physical scientists love acronyms, the more tortured the better. Anyone know what CANGAROO stands for? Or EDELWEISS? Or EURECA?

Perhaps it’s inevitable that scientists try to come up with ‘catchy’ acronyms for their grant proposals, but to my mind they’re acting like parents don’t give sufficient thought when naming their offspring. (If your surname is Dwyer, don’t call your daughter Barb. If your surname is King, don’t call your son Lee.) Occasionally an observatory is rescued from the blandness of an acronym. The Gamma-ray Large Area Space Telescope (GLAST) was renamed as the result of a public competition held by NASA; it’s now known as Fermi. Much better.

You now have the chance to do a similar service for another observatory. The Very Large Array (VLA) has been upgraded, and the NRAO are looking for a name for the vastly improved facility. Visit Name the Array, and you have the chance to give this wonderful observatory an imaginative and memorable name.

Very Large Array

The Very Large Array - it's about to be renamed
Credit: NRAO

My choice? Reber telescope. (And that’s in honour of Grote Reber. It doesn’t stand for Really Exceptionally Big and Extraordinary Radio telescope…)