This column is a milestone. In 1983, while I was on a one-year sabbatical at the Hahn-Meitner Institute for Nuclear Physics in what was then West Berlin, I received a letter from Stan Schmidt informing me that Jerry Pournelle had decided that he no longer wished to be an Alternate View columnist for Analog and asking if I was interested in taking over as the AV columnist and “alternating” with G. Harry Stine.
This was a problem. At the time I had written about 80 papers for physics journals and a few science-fact pieces for Analog, but I was well aware that writing science-fact for a popular audience is harder and more time-consuming than it looks, and the idea of having to produce a sensible column for every-other issue of Analog on a regular basis was scary. I was not at all sure that I would have anything to write about when the deadlines came around. But I decided that the Analog soapbox was too tempting to pass up.
Fast forward to today. This is column number 150. Somehow, for over 25 years I have managed to meet each deadline with something (I hope) interesting to say about science in general and physics in particular. I think that popularizing science and making it accessible to interested readers is an important activity, and I hope you agree. With that said, let’s consider the subject of this column: possible indications of a new “dark” force in the universe.
It is now clear that our universe is a much stranger place than we had imagined only a decade ago. Its total mass-energy, according to our best cosmological models, divides up as 70% dark energy, 25% cold dark matter, 4% free hydrogen and helium, 0.5% stars (mostly hydrogen), 0.3% neutrinos, and only 0.03% atoms of elements heavier than helium, the stuff that we are mostly made of.
Dark energy, easily the most mysterious of these components, is an intrinsic energy of space spread uniformly through the universe and possessed by each otherwise completely empty volume of space. It creates a repulsive “pressure” that is accelerating the expansion of the universe. Dark matter, the next most mysterious, is some unknown form of mass that does not make or absorb light and interacts gravitationally with itself and with the normal mass of stars. Dark matter clusters around galaxies in a more-or-less spherical “halo”, accounts for most of the galactic mass, and causes stars in the outer reaches of a galaxy to orbit the galactic center much faster than they would in the absence of the dark matter halo.
But what is dark matter? It is definitely not ordinary matter (atoms, molecules, electrons) or any of the known fundamental particles including neutrinos. So what’s left? Nothing ordinary, so we are pushed into speculating that dark matter is made of a previously unknown family of particles. Some theories that attempt to extrapolate beyond the standard model of particle physics predict new particles: e.g., supersymmetric particles, WIMPs, axions, etc. For the purposes of this column, we’ll refer to them all as DMP (i.e., dark matter particles).
How do you look for DMPs? There are basically two techniques: (1) We assume that a speeding DMP can collide with a normal nucleus, giving it enough recoil energy and momentum to trigger a sensitive detector, and (2) We assume that DMPs come in matter and antimatter flavors that can annihilate with each other and produce radiation detectible with sensitive instruments in space. Both types of DMP searches have been going on for some time, and are getting results that are both interesting and confusing.
Gran Sasso, located about 130 km from Rome, is the highest mountain in the Apennines of Central Italy. In 1995, twin highway tunnels connecting Rome to Teramo were cut through the mountain, and at the same time an underground particle physics laboratory was created there, consisting of three large underground low-background chambers shielded from cosmic rays by 1,400 meters of rock. In one of these chambers is the DAMA/LIBRA experiment, operating a cluster of sodium iodide detectors with a total mass of 250 kg (¼ ton) designed to detect small signals arising from the collision of a DMP with a nucleus. When a nucleus recoils from a DMP hit, a small flash of light is made by the resulting ionization, and this light flash can be detected with photomultiplier tubes. The DAMA detectors can observe such signals over background with energies as small as 2 thousand electron volts or 2 keV (here keV means kilo electron volts, the quantity of energy needed to move one electron across a potential of 1000 volts). DAMA has a lower threshold than that of most competing DMP searches. DAMA/LIBRA and its previous incarnation as DAMA/NaI have been operating in the very low radiation Gran Sasso environment for a total of 11 years. During this long period of operation, some interesting and controversial data has been collected.
In its yearly orbit around the Sun, the Earth has a speed of about 30 km/s. Our Sun orbits the galactic center with a speed of about 220 km/s. If our galaxy is embedded in a cloud of DMPs more or less at rest with respect to the galactic center, the Earth passes through the DMP stream with a varying speed. At some times during its orbit, the Earth’s speed adds to the Sun’s speed, and at other times it subtracts. Thus, the DMPs streaming through the DAMA/LIBRA detector are expected to hit the detectors with more energy in June than in December.
Therefore, one of the signals examined by the DAMA/LIBRA collaboration is any annual variation in the counting rates in the detector, and indeed they have found such a variation at about the 2% level in counts with energies between 2 and 4 thousand electron volts (2 to 4 keV). As expected, the signal reaches a maximum around June 2, just when the Earth and Sun speeds add. The variation has period of exactly one year with a 0.2% uncertainty. They attribute their observations to the presence of DMPs in the galactic halo and ascribe a confidence level of 8.2 standard deviations to their result.
The problem is that other similar DMP detectors (Zeplin III, CDMS 2008) with a different detection medium and higher energy thresholds see no such effect. Also, the 2-4 keV signals seen by DAMA/LIBRA are lower in energy that is theoretically predicted for DMP-nucleus collisions. Therefore, the DAMA/LIBRA result has been the subject of debate and controversy at several recent international conferences.
As mentioned above, the other way of searching for DMPs is to look for space radiation produced when somewhere a DMP and anti-DMP annihilate. One particle expected from such annihilations is the positron, the antimatter twin of the electron. The PAMELA detector, launched on a Russian satellite by the WIZARD collaboration, has recently reported an observation of the ratio of positrons to (electrons + positrons) in the energy range 1.5 to 90 GeV. (Here, 1 GeV is 109 eV; for reference a proton has a mass-energy of 0.938 GeV.)
The theoretical expectation is that such energetic positrons should be produced mainly in gas collisions during the propagation of cosmic ray protons in the galactic medium, and this leads to a positron ratio that should fall steeply with energy. The actual data, however, shows a strong increase in the ratio with energy, starting at a value of about 0.05 at 10 GeV and rising to above 0.15 at 90 GeV. There are similar reports of a positron excess in the 600-800 GeV range from the balloon-borne ATIC cosmic ray detector, (but these seem to be in conflict with recent GLAST/Fermi results). This excess of energetic positrons is not easily explained, and could be the result of DMP-anti-DMP annihilation.
The WMAP experiment, which has mapped the cosmic microwave background with great precision, has also reported a hard microwave “haze” coming from the center of our galaxy and not readily explained by known galactic emission mechanisms. This haze could be the synchrotron radiation from energetic electrons and positrons made in DMP-anti-DMP annihilation near the galactic center. Studies there with the EGRET gamma ray detector showed an excess of gamma rays at energies above those expected from pi0 meson decays. It is suggested that the observed gamma rays might arise from the inverse Compton scattering of energetic positrons and electrons colliding with starlight and cosmic background microwaves.
Thus, from several independent sources there is evidence of energetic electrons and positrons, possibly coming from DMP-anti-DMP annihilation. On the other hand, there is no corresponding evidence of any excess of strongly-interacting particles like pions and antiprotons, which would be expected from conventional matter-antimatter annihilations. In particular, the PAMELA measurements place tight limits the antiproton content of cosmic rays, and EGRET measurements place similar limits on gamma rays from pi0 meson decays. Neither provides any indication of strongly-interacting annihilation products.
Thus, these results present a paradox. If DMP-anti-DMP annihilations are producing electrons and positrons with energies of 90 GeV or more, why are these decays not making any excess of antiprotons or pi0 mesons? This is in direct conflict with expectations based on the standard model, so the observations may point to new physics beyond the standard model.
One version of such new physics has recently been suggested by Arkani-Hamed, Finkbeiner, Slatyer, and Weiner (AFSW). They propose a new “dark” force that acts only between the dark-matter particles. In this AFSW model, when there is a DMP-anti-DMP annihilation between dark matter particles that wander into each other, the particle momentarily produced in annihilation is the carrier of the new dark-matter force. And it is assumed that the force-carrier particle has a mass of around 0.1 GeV, so that its low mass constrains its subsequent decay into electron-positron pairs and/or pairs of gamma rays.
Also, because of this new force, as the particles approach each other before annihilation an attraction due to the force occurs that increases the probability that the annihilation will occur. This is called a Sommerfeld enhancement, and it can increase the chances of annihilation by several orders of magnitude. And AFSW model predicts that DMPs should interact primarily with heavy nuclei giving lower recoil energies, explaining why only detectors containing iodine (like DAMA) or lead nuclei show small but detectable recoil signals.
The AFSW theory can, at the expense of postulating a new and previously unknown force, explain all of the observations described above and make predictions that can be tested by new experiments and observations. In particular, the AFSW theory predicts that, as more data is collected, the positron enhancement observed by PAMELA should continue to increase up to the highest energies that the detector can resolve (~270 GeV). As the sources of positrons become better localized in position, the theory predicts that sources of the energetic positrons should be broadly distributed rather than localized at the galactic center (e.g, at the black hole there).
When the LHC begins operation in a few months (a year late because of a major cryogenics rupture) the AFSW theory predicts observation of a striking signature of highly energetic electrons and positrons among the produced particles in the proton-proton collisions. There are also predictions for new observations from the GLAST/FERMI and HESS detectors that have recently been launched.
So there may be a new force in the universe. What are the science-fiction implications of that? If the picture painted by the AFSW theory is correct, the matter content of the universe is mostly dark matter with its own particles, forces, and interactions. It clusters around galaxies in a somewhat different way than does the stars and planets of normal matter, and it may have complexities and structures that we have not imagined. Are there the equivalent of atoms and molecules made of dark matter that are invisible to us except through their gravitational interactions? There are almost no antiprotons in our galactic neighborhood, but apparently there is plenty of dark matter and antimatter. Can we find chunks of dark matter and dark antimatter and annihilate them for energy and propulsion?
So there may be another force in the universe, a fifth “dark” force that acts only between “dark” particles that ignore us as we ignore them, passing invisibly through our stars and planets as if they were not there.
The surface has just been scratched in this area. For science fiction this has the makings of new kinds of energy sources and propulsion, and perhaps even a new “dark’ territory to be explored by space travelers.
The AFSW Theory:
“A Theory of Dark Matter”, N. Arkani-Hamed, D. P. Finkbeiner, T. R. Slatyer, and N. Weiner, Physical Review D79, 015014 (2009), arXiv preprint 0810.0713v3 [hep-ph].
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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