As scientists continue to acquire knowledge of the formation and structure of the universe, their discoveries unlock mysteries that test cosmic creation models. One such mystery is the particles that make up dark matter, which astronomers know accounts for 85% of the universe’s matter. However, thanks to sophisticated instrumentation, scientists may soon be able to identify particles that make up most of the universe’s dark matter and may help resolve cosmic mysteries.
Searching for Sterile Neutrinos It’s possible that a large fraction of the universe’s dark matter consists of sterile neutrinos. Thus, researchers spend considerable effort trying to detect them. In 2011, I wrote five articles about sterile neutrinos.1 Sterile neutrinos are distinguished from the active neutrinos that I described in last week’s article, “Neutrino Breakthroughs: More Evidence for Cosmic Creation and Design.”2 Active neutrinos interact very weakly with photons, protons, neutrons, and electrons through the weak nuclear force and the gravitational force. Active neutrinos come in three “flavors” or types: electron, muon, and tau. Sterile neutrinos are hypothetical particles that are believed to interact only through the gravitational force.
The standard particle creation model requires that there be exactly three different types of active neutrinos. However, if sterile neutrinos exist, there must be at least three different types of sterile neutrinos.3
So far, the only dark matter particles that astronomers and particle physicists have detected are the three active neutrinos. As I stated in my previous article, new measurements establish the sum of the individual electron, muon, and tau neutrino masses = 0.05841–0.087 electron volts (eV). This mass range implies that active neutrinos comprise just a small fraction of the universe’s dark matter.
Astronomers and physicists have proposed that either sterile neutrinos or axions (another hypothetical elementary particle) or both could make up the majority of the universe’s dark matter. These two particles hold the potential of explaining the following cosmic mysteries:
Why the first stars apparently form as early in cosmic history as they do
Why the universe produces slightly more baryons (protons and neutrons) than antibaryons
Why core-collapse supernovae produce unexpectedly high abundances of certain elements with atomic weight greater than 100
Why supernova shocks are so highly energetic
Why dark matter halos are relatively symmetrical and smooth
Why supermassive black holes form as early as they do in cosmic history
How to account for a small amount of warm dark matter to accompany the predominant cold dark matter that astronomers observe
Consequently, for the past two decades astronomers and physicists have sought to discover—both in the lab and in the sky—the existence of sterile neutrinos and/or axions. In the next few sections, I summarize the results of various lines of research. It’s technical, so skim if desired and get the overall picture as you proceed to “Philosophical Implications.”
Laboratory Sterile Neutrino Detections? In 2018, the MiniBooNE Collaboration announced that they had discovered an excess of electron neutrino oscillations in their MiniBooNE short-baseline neutrino experiment.4 They interpreted this excess as evidence for the existence of a fourth neutrino type at a significance level of 4.7 standard deviations (equivalent to 99.99% certainty). An excess of neutrino oscillation events was also detected by the Liquid Scintillator Neutrino Detector (LSND) with a similar level of certainty for the existence of a fourth neutrino type.5
Theoretical physicist Joachim Kopp, staff scientist at the CERN particle accelerator in Geneva, Switzerland, explained in a brief article why the signal detected by the MiniBooNE and LSND experiments is evidence for a sterile neutrino.6 Additional evidence for a fourth neutrino type came from an antineutrino anomaly observed in a French nuclear reactor that is best explained as an excess of electron neutrino oscillations7 and from measurements of antineutrinos in the Daya Bay Reactor Neutrino Experiment in China.8 The Daya Bay reactor produced 6% fewer antineutrinos than would be the case if only three neutrino types existed. However, combining the antineutrino flux and spectra of the Daya Bay results suggests that the antineutrinos might not be missing after all. It is possible that the predictions from nuclear theory could be incomplete.
Astronomical Sterile Neutrino Detections? In 2014, a team of astronomers led by Esra Bulbul detected a weak x-ray emission line in the stacked x-ray spectrum of 73 clusters of galaxies.9 Bulbul’s team demonstrated how the decay of sterile neutrinos with a mass of 7.1 keV best explains this spectral line. Also in 2014, a team of astronomers led by Alexey Boyarsky detected the same x-ray emission line in the core of the Andromeda Galaxy and in the Perseus Galaxy Cluster.10
Neutrinos suppress the growth of large-scale structure in the universe in proportion to the total mass of the neutrino types. Neutrinos also affect the expansion rate history of the universe. Therefore, observations of the clustering of galaxies and galaxy clusters plus maps of the cosmic microwave background radiation (the radiation remaining from the cosmic creation event) place constraints on the number of neutrino types and on the total mass of the different neutrino type particles.
The most sensitive maps of the cosmic microwave background radiation (CMBR) yield a measurement of the effective number of neutrino types. Since the three active neutrino types were not completely decoupled at the moment of electron-positron annihilation that occurred when the universe was only a few seconds old, these three types, by themselves, would give a measure for the effective number of neutrino types, Neff = 3.046.11 The best map of the CMBR, the Planck 2018 map, produced a measure of Neff = 2.99 ± 0.17.12 This measurement implies with 95% certainty that Neff must be less than 3.34. Furthermore, observational constraints on the primordial abundances of helium, deuterium, and lithium13 make a value of Neff = 4 highly unlikely.14 As the Planck Collaboration wrote in their paper, “The presence of a light thermalized sterile neutrino is in strong contradiction with cosmological data.”15 Even where the production of sterile neutrinos is suppressed by nonstandard interactions, the sterile neutrino mass cannot be any greater than 0.23 eV. Combining the Planck and Daya Bay data provides an upper limit of 0.2 eV for the sterile neutrino mass in all possible scenarios.15
Latest Constraints on Sterile Neutrinos Three physicists in Britain, Italy, and Spain combined the latest CMBR, baryon acoustic oscillation, type Ia supernovae, and cosmic structure growth rate observations to produce the tightest constraint on the total number of neutrino types. Their result was Neff = 3.05 ± 0.16, which means with 95% certainty that Neff must be less than 3.37.16 Meanwhile, the MiniBooNE Collaboration upgraded their experiment, dramatically improving its sensitivity. It is now called the MicroBooNE experiment.
In a preprint posted on October 29, 2021, the MicroBooNE Collaboration presented results from their initial observations of electron neutrino interactions from the Fermilab Booster Neutrino Beam using the MicroBooNE liquid argon time projection chamber.17 They achieved greater sensitivity than with MiniBooNE earlier, and found no excess of electron neutrino oscillation events. That is, they found no hint for the existence of sterile neutrinos.
Undeterred, researchers will reemploy MicroBooNE, which is set to deliver even more sensitive results. Another laboratory experiment, the STEREO experiment, is primed to achieve high-sensitivity output.18 Meanwhile, the X-ray sky is about to be probed by the eROSITA and Athena missions19 and the KM3NeT/ORCA telescope.20 If sterile neutrinos are lurking somewhere in the universe, they cannot remain hidden for long.
Constraints on Axions As I explained in previous articles, the existence of substantial numbers of axions would cause white dwarf stars to cool at more rapid rates.21 As far back as 1992, observations of white dwarf cooling had established that axions, if they exist, could not have a particle mass greater than 0.01 eV.22 About a decade ago, two different teams of astronomers demonstrated that the excess cooling of white dwarfs is well explained by axion emission where the axion particle mass is just a few milli-eV.23 While this excess cooling yielded the first positive indication that axions exist, it implied that axions provide only a small fraction of the universe’s dark matter.
The existence of axions was firmed up by the analysis of additional observations made by one of the two teams. The team led by Jordi Isern noted that the observed excess cooling of white dwarf stars could be an artifact introduced by the star formation rate. However, white dwarf populations in our galaxy’s thin disk, thick disk, and halo each have different star formation rates. The fact that astronomers observe the same excess cooling in all three white dwarf populations means that the excess cooling cannot be an artifact of the star formation rate. It is likely due to axion emission. Isern’s team derived an axion particle mass in the range of 4–10 milli-eV.24
The future of axion astronomy looks promising. More extensive observations of white dwarf cooling curves are underway and an axion telescope, the solar axioscope IAXO, is under development.25 If axions are part of the universe’s undetected dark matter, astronomers will likely know soon.
Philosophical Implications The constraints on the possible existence of sterile neutrinos have reached a point where, even if they do exist, they cannot make up a significant fraction of dark matter in the universe. Likewise, it is becoming increasingly evident that axions do not comprise a substantial fraction of the universe’s dark matter.
The universe’s dark matter is predominantly cold dark matter that’s comprised of particles traveling at much less than light’s velocity. However, a tiny fraction of the universe’s dark matter is warm dark matter that’s comprised of particles moving at a significant fraction of light’s velocity. Sterile neutrinos, if they exist, would be warm dark matter. It is possible, given current detection limits, that sterile neutrinos make up all, or most, of the universe’s warm dark matter. Axions, on the other hand, are cold dark matter particles.
That sterile neutrinos and/or axions do not comprise a substantial fraction of the universe’s dark matter does not mean that dark matter theories are in trouble. Astronomers and physicists have over thirty other candidate particles that could comprise the universe’s dark matter. However, sterile neutrinos and/or axions, if they do make up most of the universe’s dark matter, hold the greatest prospect for detection. The search for other dark matter candidate particles will be more challenging technologically. This is how science advances. It often takes many small steps to achieve breakthroughs. That’s why scientists test and retest.
As for the biblically predicted big bang creation model,26 all these new dark matter particle findings and the prospects for future dark matter particle discoveries are consistent and anticipated by the big bang creation models. Big bang models that permit the possible existence of physical life predict a specified quantity of dark matter where the dark matter is comprised of particles, a quantity that is consistent with astronomers’ best measurements.27 These findings provide further scientific demonstration that the more we learn about the universe, the more evidence we discover for the intentional, supernatural handiwork of the Being beyond the universe who created and designed it.
Masahiro Ibe, Alexander Kusenko, and Tsutomu T. Yanagida, “Why Three Generations?” Physics Letters B 758 (July 10, 2016): 365–369, doi:10.1016/j.physletb.2016.05.025.
A. A. Aguilar-Arevalo et al. (MiniBooNE Collaboration), “Significant Excess of Electronlike Events in the MiniBooNE Short-Baseline Neutrino Experiment,” Physical Review Letters 121, no. 22 (November 30, 2018): id. 221801, doi:10.1103/PhysRevLett.121.221801.
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Joachim Kopp, “The Plot Thickens for a Fourth Neutrino,” Physics 11 (November 26, 2018): id. 122, doi:10.1103/Physics.11.122.
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Esra Bulbul et al., “Detection of an Unidentified Emission Line in the Stacked X-Ray Spectrum of Galaxy Clusters,” Astrophysical Journal 789, no. 1 (June 2014): id. 13, doi:10.1088/0004-637X/789/1/13.
A. Boyarsky et al., “Unidentified Line in X-Ray Spectra of the Andromeda Galaxy and Perseus Galaxy Cluster,” Physical Review Letters 113, no. 25 (December 19, 2014): id. 251301, doi:10.1103/PhysRevLett.113.251301; Kevork N. Abazajian, “X-Ray Line May Have Dark Matter Origin,” Physics 7 (December 15, 2014): id. 128, doi:10.1103/Physics.7.128.
Gianpiero Mangano et al., “Relic Neutrino Decoupling including Flavour Oscillations,” Nuclear Physics B 729, nos. 1–2 (November 21, 2005): 221–234, doi:10.1016/j.nuclpjysb.2005.09.041.
N. Aghanim et al. (Planck Collaboration), “Planck 2018 Results VI. Cosmological Parameters,” Astronomy & Astrophysics 641 (September 2020): id. A6, doi:10.1051/0004-6361/201833910.
Aghanim et al. (Planck Collaboration), “Planck 2018 Results.”
Matthew Adams et al., “Direct Comparison of Sterile Neutrino Constraints from Cosmological Data, νe Disappearance Data and νmu → νe Appearance Data in a 3 + 1 Model,” European Physical Journal C 80, no. 8 (August 19, 2020): id. 758, doi:10.1140/epjc/s10052-020-8197-y.
Eleonora Di Valentino, Stefano Gariazzo, and Olga Mena, “Most Constraining Cosmological Neutrino Mass Bounds,” Physical Review D 104, no. 8 (October 15, 2021): id. 083504, doi:10.1103/PhysRevD.104.083504.
P. Abratenko et al. (MicrorBooNE Collaboration), “Search for an Excess of Electron Neutrino Interactions in MicroBooNE Using Multiple Final State Topologies,” (October 29, 2021), arXiv:2110.14054.
H. Almazán et al. (STEREO Collaboration), “Improved Sterile Neutrino Constraints from the STEREO Experiment with 179 Days of Reactor-On Data,” Physical Review D 102, no. 5 (September 1, 2020): id. 052002, doi:10.1103/PhysRevD.102.052002.
Andrea Caputo, Marco Regis, and Marco Taoso, “Searching for Sterile Neutrino with X-Ray Intensity Mapping,” Journal of Cosmology and Astroparticle Physics 2020, no. 03 (March 2, 2020): id. 002, doi:10.1088/1475-7516/2020/03/001.
S. Aiello et al. (KM3NeT Collaboration), “Sensitivity to Light Sterile Neutrino Mixing Parameters with KLM3NeT/ORCA,” Journal of High Energy Physics 2021, no. 10 (October 21, 2021): id. 180, doi:10.1007/JHEP10(2021)180.
Ross, “Candidates Compete for Top Billing.”
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J. Isern et al., “Axions and the Luminosity Function of White Dwarfs: The Thin and Thick Discs, and the Halo,” Monthly Notices of the Royal Astronomical Society 478, no. 2 (August 2018): 2569–2575, doi:10.1093/mnras/sty1162.
Sebastian Hoof, Joerg Jaeckel, and Lennert J. Thormaehlen, “Quantifying Uncertainties in the Solar Axion Flux and Their Impact on Determining Axion Model Parameters,” Journal of Cosmology and Astroparticle Physics 2021, no. 9 (September 6, 2021): id. 006, doi:10.1088/1475-7516/2021/09/006.
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