Axions as a Dark Matter Candidate

The evidence for the existence of dark matter in the Universe, which is not baryonic (not made of the known stable particles of protons and neutrons making up atomic nuclei, and electrons) is overwhelming and impressive, based on the following observations:

1. Galaxies are surrounded by dark matter halos extending much further than their visible stars and gas clouds, as evidenced by rotation curves and other dynamical measurements. Clusters of galaxies contain dark matter halos distributed similarly to their galaxy members and hot, X-ray emitting gas. Dynamical measurements of the galaxies and the hot gas, as well as gravitational lensing observations, show that about 5/6 of all the mass is dark matter, or matter that is not visible as stars, gas or anything else and is noticed only through its gravitational effect. This dark matter has often an elliptical distribution in clusters that requires an anisotropic velocity dispersion to support an equilibrium configuration, implying collisionless matter.

2. The Cosmic Microwave Background brightness fluctuations have a small amplitude (about one part in 50000 on large angular scales), which can only be reconciled with the present fluctuations in the large-scale distribution of galaxies if dark matter exists. This is because dark matter starts the growth of gravitational evolution of fluctuations before the epoch of recombination, while baryonic matter cannot start this growth while it is coupled to the Cosmic Microwave Background radiation. Moreover, the Cold Dark Matter theory (making the simple assumption that dark matter is collisionless and has no initial velocity dispersion) predicts with astonishing accuracy the detailed shape of the peaks of the power spectrum of Cosmic Microwave Background fluctuations, caused by acoustic oscillations, as a function of the dark matter and baryon density, among other parameters. The required densities imply that dark matter accounts for about 5/6 of all the matter in the Universe, while the known baryonic matter accounts only for 1/6. In addition, the inferred baryon density coincides with the independent observational determination from light-element abundances predicted by the theory of primordial nucleosynthesis when the Universe was only a few minutes old.

3. The present large-scale structure distribution of matter in the Universe is also well fitted by predictions of the same Cold Dark Matter theory for the matter power spectrum, with the same parameters for the total matter density that fit the observations of the Cosmic Microwave Background.

4. The observations of Type Ia supernovae to determine the global evolution of the expansion of the Universe demand a total density of matter coincides with that inferred from the Cosmic Microwave Background and the large-scale structure power spectrum, and is inconsistent with the baryonic density alone.

These multiple observational tests show beyond reasonable doubt that most of the matter in the Universe is dark matter, which is not baryonic and behaves as collisionless and initially cold matter to within tight observational bounds. While a minority of cosmologists have continued to argue for possibilities of Modified Newtonian Dynamics as alternatives to dark matter, no such alternatives are compatible with all this observational evidence (these alternative generally focus only on explaining galaxy rotation curves and clusters of galaxies while ignoring the rest of the evidence).

The Standard Model of particle physics does not make a prediction of any dark matter candidate. However, this Standard Model is incomplete, so the existence of dark matter is not so surprising: there may simply exist a stable particle with mass and with very weak interactions, which has therefore not yet been discovered. The surprise is that the dark matter and baryon cosmic densities coincide within one order of magnitude, but it is also true that neutrinos, which are now known to have mass, are inferred to have a cosmic density not much lower than that of baryons, another unexplained coincidence.

Among all the hypothetical new particles proposed to explain dark matter, the axion stands out because, apart from satisfying observational constraints to account for dark matter, it is the only one that elegantly solves at the same time a well known problem in particle physics: the strong CP problem, or the fact that strong interactions do not violate CP symmetry. The absence of CP violation in the theory of Quantum Chromodynamics is manifested, among other things, in the lack of an electric dipole moment of the neutron. If there is no mechanism to cancel CP violation, the value of the neutron electric dipole moment is expected to lie between zero and one in some units, with roughly uniform probability, but the present experimental upper limit is already 10 billion times smaller than this typical value. To avoid atributing this to simple chance, we need a way to explain why the CP violation term is set to zero in the Standard Model of particle physics. An elegant mechanism for this is the Peccei-Quinn model, which implies the existence of an axion particle. After the model had been proposed for reasons having to do only with particle physics, it was realised the axion is an excellent candidate for the dark matter, and that its production in the early Universe might easily result, depending on the unknown axion mass, in an axion cosmic density similar to the observed dark matter density.

If axions were the dark matter they would behave today as a classical wave permeating all space. Although the axion mass is very small, this wavy dark matter is a perfect Cold Dark Matter candidate because axions would never thermalize with baryonic matter and photons, owing to their extremely feeble interactions. Among other effects, their very weak interactions imply a tiny modification of Maxwell's equations of electromagnetism. If we place a metallic resonant cavity, or waveguide for electromagnetic waves, in a strong magnetic field, this tiny modification results in what is known as the Primakoff effect: the axion wave energy can be converted to electromagnetic wave energy. Many experiments are attempting to detect the appearance of electromagnetic waves, apparently out of nothing (in reality out of the energy of axion dark matter), in resonant cavities in strong magnetic fields (several Teslas) at very low temperature to minimize thermal noise. The unknown axion mass determines the frequency at which electromagnetic waves are created, with a very narrow frequency range of only about one part in a million determined by the velocity dispersion of the dark matter of the Milky Way Galaxy in the solar vicinity. This means that one must search for the unknown frequency of axion conversion to photons over a broad frequency range, placing limits to the conversion rate of axions to photons at every possible frequency.

Our activity in axion detection experiments

The ICCUB is participating in the RADES experiment (Relic Axion Dark matter Exploratory Setup), in the context of the International AXion Observatory , or the IAXO Collaboration.

This experiment uses the idea of coherently connecting several resonant cavities to increase the volume for conversion of axions to photons, therefore increasing the signal without an increase in the noise. The search has been performed so far at a frequency near 8 GHz, using cavities constructed at the Universidad Politécnica de Cartagena and a 9 Tesla magned called CAST at CERN. With postdoctoral member Sergio Arguedas, who joined the ICCUB in 2021, we are contributing to RADES by analyzing the experimental data obtained at CERN and studying how to improve its sensitivity and tuning capacity. In the future we plan to use new magnets and resonant cavity structures.

We are also participating in the design of a new idea for an experiment to detect the electromagnetic wave generated in resonant cavities with Kinetic Inductance Detectors (a single-photon detector that is commonly used in observations of the Cosmic Microwave Background), at a higher frequency near 90 GHz, in an experiment proposed to the Canfranc Underground Laboratory in collaboration with several research institutions in Spain.

Consequences of axion dark matter in observational cosmology

A lot of my activity in axions as dark matter has been on their potential consequences in observational cosmology. An interesting consequence that is rather generic for the QCD axion is the formation of minihalos of dark matter on scales much smaller than that of galaxies: owing to the randomly fluctuating axion field when the axion mass is generated in the early Universe, dark matter density fluctuations on small scales may give rise to non-linear collapse and gravitationally bound objects, or minihalos, at a very early epoch, much before any galaxies were formed. These dark matter minihalos would be as light as the typical mass of an asteroid, but with a size similar to the orbit of the Earth around the Sun. Part of the dark matter would be today bound in these small minihalos, while some other part would still be in a diffuse form throughout the halos of galaxies. These minihalos may have some observational consequences in gravitational lensing, although with very small effects that are hard to detect. I have worked on one of these effects collaborating with Liang Dai (who is at present at the University of California at Berkeley): when a distant luminous star is observed under very high magnification due to gravitational lensing, the presence of small-scale granularity in the mass distribution of the lens due to these dark matter axion minihalos would cause a deviation of the microlensing lightcurve of the observed magnification with respect to our expectation for a smooth distribution of dark matter.

Axion dark matter in our Milky Way halo would also likely be distributed in a diffuse component and a component in bound minihalos. The axion minihalos would be tidally perturbed and heated as they pass near stars, with their outer parts frequently being tidally stripped. The mass lost in this way by minihalos would form tidal streams in the same way as galaxies in clusters interact with each other and lose their outer parts to stellar streams that are eventually dispersed into the intracluster light. This structure of dark matter streams may be present in our Solar System vicinity and may affect experiments to detect axions, such as resonant cavities in strong magnetic fields, and also alternative ones based on radio echos.

I am also investigating the possibility that axions can be detected in radio astronomy, through the decay of axions into two photons. This possibility may be enhanced by the radiation of a radio source, through the effect of stimulated emission, if we observe in the direction exactly opposite to a bright extragalactic radio source: in an effect we have designated as axion Gegenschein in our work with Oindrila Ghosh and Jordi Salvadó, the dark matter would emit radiation precisely in the opposite direction from where it receives it, by adding one photon to the same state of the incoming radiation and the other in the state with opposite momentum.

Selected Publications

  1. T. Venumadhav, L. Dai, J. Miralda-Escudé 2017, ''Gravitational Microlensing during Caustic Crossings'', ApJ, 850, 49 (arXiv:1707.00003).
  2. E. Armengaud, et al. 2019, ''Physics Potential of the International Axion Observatory (IAXO)'', JCAP, 6, 47 (arXiv:1904.09155).
  3. L. Dai & J. Miralda-Escudé 2020. ''Gravitational Lensing Signatures of Axion Dark Matter Minihalos in Highly Magnified Stars'', AJ, 159, 49 (arXiv:1908.01773).
  4. L. Álvarez-Melcón et al. 2021. ''Results of the CAST-RADES haloscope search for axions at 34.67 micro-eV'', JHEP, 10, 075 (arXiv:2104.13798).
  5. A. Abeln, et al.\ 2020. ''Conceptual Design of BabyIAXO, the intermediate stage towards the International Axion Observatory'', (arXiv:2010.12076).
  6. O. Ghosh, J. Salvado, J. Miralda-Escud\'e 2020. ''Axion Gegenschein: Probing Back-Scattering of Astrophysical Radio Sources Induced by Dark Matter'', submitted to PRD, arXiv:2008.02729.