Dark matter is a mysterious and unidentified form of matter that is thought to be composed of exotic, non-atomic particles that do not interact with light or any other form of electromagnetic radiation. Indeed, dark matter’s very name refers to the fact that, because it does not dance readily with light, it is transparent and invisible to the entire electromagnetic spectrum. However, even though the dark matter has not been directly observed, its ghostly presence and properties are inferred from its gravitational effects on the motions of visible, atomic matter–as well as on its ability to behave as a cosmic lens (gravitational lensing), and its weird phantom-like influence on the Universe’s large-scale structure. The good news is that in November 2016, a German-Hungarian team of astronomers announced that their new and ambitious supercomputer calculation has revealed that the axion–a hypothetical particle considered to be a leading candidate for the dark matter–if it exists, could be at least ten times heavier than previously thought. If true, this finding provides scientists with a precious tool that they can use to finally catch the elusive, invisible particle. The bad news is that the new research suggests that an experiment–that has been hunting for the axion for two decades–might be unlikely to find it. This is because the detector was designed to hunt for a lighter version of the axion.
The team of researchers, led by Dr. Zoltan Fodor of the University of Wuppertal in Germany, included scientists from the Eotvos University in Budapest, Hungary, and Forschungszentrum Julich. The important calculations were carried out on Julich’s supercomputer JUQUEEN (BlueGene/Q) in Germany. The scientists present their new results in the journal Nature.
“Dark matter is an invisible form of matter which until now has only revealed itself through its gravitational effects. What it consists of remains a complete mystery,” commented study co-author Dr. Andreas Ringwald in a November 2, 2016 Deutsches Elektronen-Synchrotron (DESY) Press Release. DESY is a Research Centre of the Helmholtz Association in Bonn and Berlin, Germany. Dr. Ringwald is based at DESY, and he is also the one responsible for proposing the research study.
The possible existence of the axion was first brought up back in 1977 as a possible solution to a nagging paradox arising from how the strong nuclear force–which holds particles in the nucleus of an atom together–influences matter and antimatter. This would explain an unexpected symmetry whereby the strong nuclear force–one of the four known forces of nature–has the same effect on matter as it does on antimatter. The other three known forces of nature are the weak nuclear force, the electromagnetic force, and gravity. Because many researchers also think that the axion might be one of the components of the dark matter, if the axion really does exist, it could solve two nagging mysteries at once.
Hints of the existence of the transparent dark matter come from–among other things–the astrophysical observation of galaxies, which rotate much too rapidly to be held together merely by the gravitational pull of their contents of visible, atomic matter. Scientists currently think that the observable Universe is composed of approximately 26.8 percent dark matter, 68.3 percent dark energy, and a mere 4.8 percent atomic (baryonic) matter. All of the stars, planets, clouds of gas and other objects inhabiting the Cosmos are composed of so-called “ordinary” atomic matter, which is clearly the runt of the cosmic litter. Dark energy, which is thought to account for most of the total mass-energy of the known Universe, is even more mysterious than the dark matter. Possibly a property of space itself, dark energy may be what is causing the Universe to speed up in its expansion. “Ordinary” atomic matter is really very extraordinary stuff–it is the form of matter that composes the world that we are familiar with.
According to the Standard Model for the formation of cosmic structure, invisible dark matter particles initially clump together gravitationally to create a crowded area, which is termed a dark matter halo. As time goes by, these transparent halos pull in–with the relentless grip of their powerful gravity–floating, billowing clouds of primordial, pristine, primarily hydrogen gas. Stars and galaxies are born as a result.
Physicists are desperately hunting for the exotic, non-atomic particles that are thought to compose the dark matter–which has only been detected indirectly, by the way it gravitationally influences the galaxies that inhabit the observable Universe. But so far, every experiment that has been devised to find dark matter particles has come up with nothing, nothing, nothing at all or produced results that are highly controversial. Most of these unsuccessful experiments have attempted to spot or produce what are called weakly interacting massive particles (WIMPs)
The mysterious dark matter can be composed of comparatively few, but extremely heavy particles, or a very large number of light ones. The direct hunts for the heavy dark matter particle candidates make use of large particle detectors situated in underground laboratories. Indirect searches for these invisible, heavy particles use large particle accelerators, and are still ongoing. However, these hunts have found nothing. This is the reason why a range of physical considerations render extremely light particles, such as axions, very likely candidates. Using very ingenious experimental devices, it might even be possible to spot direct signs of axions. “However, to find this kind of evidence it would be extremely helpful to know what kind of mass we are looking for. Otherwise the search could take decades because one would have to scan far too large a range,” Dr. Ringwald explained in the November 2, 2016 DESY Press Release.
The Axion Dark Matter eXperiment (ADMX) has been at the forefront of the quest to find a WIMP alternative–the axion. The experiment is composed of a metal cylinder that relies on powerful magnetic fields, which theoretically should cause some axions to experience a sea-change into photons (particles of light) that could be found in the radio-wave portion of the electromagnetic spectrum. The quest began at the Lawrence Livermore National Laboratory in California, back in 1996, and then moved to the University of Washington in Seattle in 2010.
Astrophysicists have been hypothesizing the existence of dark matter for decades because of observed important differences between the mass of large astronomical objects–determined from their gravitational influences–and the mass calculated from the matter that they contain, such as stars, gas, and dust.
The true nature and identity of the dark matter is one of the greatest mysteries in modern astrophysics.
Imagine a very ancient time, long before there were living creatures on our planet with eyes that could see, how a primordial, swirling sea composed of ancient, pristine gases and the phantom-like invisible, non-atomic dark matter, wandered throughout the very ancient Universe. Eventually, the two forms of matter combined together to form the familiar and distinct structures that we on Earth can observe today.
The existence of the dark matter was first proposed by the Dutch astronomer Jan Oort (1900-1992) in 1932 to attempt to explain orbital velocities of stars inhabiting our large, barred-spiral Milky Way Galaxy. Fritz Zwicky (1898-1974), a Swiss-American astrophysicist at the California Institute of Technology (Caltech) in Pasadena, California, in 1933 also proposed the existence of dark matter in order to account for evidence of “missing mass” in the orbital velocities of galaxies within galactic clusters. Evidence derived from galactic rotation curves was discovered by the Caltech astrophysicist Horace W. Babcock (1912-2003) in 1939, but he did not attribute his observations to dark matter.
Dr. Vera Rubin (b. 1928), in the 1960s and 1970s, was the first astrophysicist to propose the existence of the dark stuff based on strong evidence, using galaxy rotation curves. Dr. Rubin is currently a Senior Fellow in the Department of Terrestrial Magnetism at the Carnegie Institution in Washington.
Following Dr. Rubin’s findings, many important observations were made by other astronomers that suggested the presence of the invisible, ghostly dark matter in the Universe–including the gravitational lensing of background objects by foreground galaxy clusters such as the Bullet Cluster, the temperature and distribution of searing-hot gas in galaxies and galaxy clusters, and–more recently–the pattern of anisotropies discovered in the cosmic microwave background (CMB) radiation. Gravitational lensing is a phenomenon proposed by Albert Einstein in the General Theory of Relativity (1915) when he came to the realization that gravity could bend and warp the path of traveling light, providing it with lens-like attributes. The CMB is the lingering relic of the radiation of the Big Bang beginning of the Cosmos about 13.8 billion years ago. The anisotropies detected in the CMB were caused by temperature variations in the baby Universe.
In the primeval Universe, the powerful gravity of the invisible dark matter grabbed at clouds of mostly hydrogen gas and pulled them in. These ancient clouds of pristine hydrogen became the ancient cradles of the first generation of baby stars, and these first stars blasted the Universe with their brilliant fires, lighting up what was previously a featureless expanse of darkness. The powerful gravity of the invisible Cosmic Web snatched at its baryonic prey until the captured gas formed phantom-like clouds that swirled down and then softly fell into the bizarre dark matter halos.
The Quest For The Elusive Dark Matter
Early calculations indicated that the axion should sport a mass of approximately 5 microelectron volts –which would make it about 100 billion times lighter than the electron. As a result, ADMX was designed to be most sensitive to very light masses within that range.
However, in their research paper published in the November 2, 2016 issue of the journal Nature, Dr. Fodor and his team report the results of a complicated, large calculation showing that–under a particular set of circumstances–the axion’s mass is most likely to fall into the much more hefty range of between approximately 50 to 1,500 microelectron volts. This would mean that every cubic centimeter of the Universe would have to harbor on average ten million such extremely lightweight particles. Dark matter is not spread out evenly throughout the Universe, however, but forms clumps and branches of a weblike network called the Cosmic Web. Because of this, our local region of our Galaxy should contain approximately one trillion axions per cubic centimeter.
The axion, according to this research, is clearly beyond the reach of ADMX, which is currently sensitive to masses between approximately 0.5 and 40 microelectron volts.
“I think it is not very good news for ADMX,” Dr. Fodor commented in a November 2, 2016 Nature News Release.
Dr. Fodor and his colleagues used the supercomputer at the Julich Supercomputing Centre to simulate the formation of elementary particles immediately after the Big Bang. In order to do this they went back to a remote and very ancient era when temperatures skyrocketed to above a million billion degrees–ten times hotter than those reached in previous supercomputer simulations. This was the primeval era when axions—if they exist–would have been born in great abundance. In order to get to where they wanted to go, the team needed to create techniques to speed up their calculations, which otherwise might have taken millions of years to complete, Dr. Fodor added.
The new supercomputer simulation enabled the team of astrophysicists to calculate the axion’s mass. They did this under the assumption that axions were born after the era of inflation–a brief era that is thought to have occurred during the first instants of the Universe’s birth, when it expanded exponentially–from the size of an elementary particle to attain macroscopic size in a mere fraction of a second. In a model whereby the axion was born before the inflation, the scientists worked out the way the particle would have formed. However, they were not able to calculate its mass.
If the post-inflation scenario is correct, then the team’s results could suggest that ADMX will be unable to detect anything. However, in the pre-inflation model, what Dr. Fodor’s team found could actually render the theory of a lighter mass axion more plausible. This would put the mass of the axion exactly into ADMX’s “sweet spot”.
The calculations for the mass of the axion now provide physicists with a concrete range in which their hunt for axions is likely to be most promising–thanks to the Julich supercomputer. Dr. Fodor noted in the November 2, 2016 DESY Press Release that “The results we are presenting will probably lead to a race to discover these particles”.
The discovery of the axion would not only solve the dark matter mystery in the Universe, it would also answer the question about why the strong nuclear force is so surprisingly symmetrical with respect to time reversal. The astrophysicists expect that it will be possible within the next few years to either confirm or rule out the existence of axions experimentally.