James M. Overduin

Murthy et al. (2025) (hereafter Paper I) have recently reported the discovery of unexpectedly bright diffuse extreme-ultraviolet radiation at high latitudes in both the Northern and Southern Galactic Hemispheres. After correction for extinction by the total interstellar dust in the direction of each observation, the spectra are nearly identical, suggesting that the radiation has a unique source and likely originates in the halo of our galaxy. The observed spectrum extends down to 912 Å, the interstellar hydrogen absorption edge. Radiation even slightly short of that edge would, if ubiquitous, be sufficient to explain the high degree of ionization in our galaxy and throughout the universe. We hypothesize that this newly discovered radiation originates in the slow decay of dark matter. The intensity of the radiation implies that the decay cannot be via the weak interaction, suggesting the existence of a new, even weaker fundamental interaction, consistent with the exceedingly long decay lifetime required.

We use experimental limits on Lorentz violation within the framework of the Standard-Model Extension to derive quantitative constraints on Space-Time-Matter theory, a version of Kaluza–Klein theory in which the cylinder condition is relaxed so that four-dimensional physics can in principle depend on the extra coordinates. The extra dimensions are not necessarily compact or length-like. We find that the associated variation in fundamental quantities such as rest mass must occur slowly, on cosmological scales.

Riemann curvature invariants are important in general relativity because they encode the geometrical properties of spacetime in a manifestly coordinate-invariant way. Fourteen such invariants are required to characterize four-dimensional spacetime in general, and Zakhary and McIntosh showed that as many as seventeen can be required in certain degenerate cases. We calculate explicit expressions for all seventeen of these Zakhary–McIntosh curvature invariants for the Kerr–Newman metric that describes spacetime around black holes of the most general kind (those with mass, charge, and spin), and confirm that they are related by eight algebraic conditions (dubbed syzygies by Zakhary and McIntosh), which serve as a useful check on our results. Plots of these invariants show richer structure than is suggested by traditional (coordinate-dependent) textbook depictions, and may repay further investigation.

We have used data from the Galaxy Evolution Explorer to study the different components of the diffuse ultraviolet background in the region between the Galactic latitudes 70°–80°. We find an offset at zero dust column density (E(B − V) = 0) of 240 ± 18 photon units in the FUV (1539 Å) and 394 ± 37 photon units in the NUV (2316 Å). This is approximately half of the total observed radiation with the remainder divided between an extragalactic component of 114 ± 18 photon units in the FUV and 194 ± 37 photon units in the NUV and starlight scattered by Galactic dust at high latitudes. The optical constants of the dust grains were found to be a = 0.4 ± 0.1 and g = 0.8 ± 0.1 (FUV) and a = 0.4 ± 0.1 and g = 0.5 ± 0.1 (NUV). We cannot differentiate between a Galactic or extragalactic origin for the zero-offset but can affirm that it is not from any known source.

Popular demonstrations commonly use stretched spandex fabric to illustrate the way in which curved spacetime mimics the force of gravity in general relativity. There are significant potential conceptual pitfalls to such an approach. In particular, it obscures the fact that most of what we ordinarily feel as gravity is due to the warping of time rather than space, a concept that is admittedly harder to demonstrate. Nevertheless, with appropriate caveats simulations of this kind can convey some of the wonder of Einstein’s theory to non-specialists.

We have used GALEX observations of the north and south Galactic poles to study the diffuse ultraviolet background at locations where the Galactic light is expected to be at a minimum. We find offsets of 230–290 photon units in the far-UV (1531 Å) and 480–580 photon units in the near-UV (2361 Å). Of this, approximately 120 photon units can be ascribed to dust-scattered light and another 110 photon units (190 in the near-UV) to extragalactic radiation. The remaining radiation is, as yet, unidentified and amounts to 120–180 photon units in the far-UV and 300–400 photon units in the near-UV. We find that molecular hydrogen fluorescence contributes to the far-UV when the 100 μm surface brightness is greater than 1.08 MJy sr⁻¹.

The 1990 film “The Hunt for Red October” (based on Tom Clancy’s 1984 debut novel of the same name) featured actors like Sean Connery and Alec Baldwin, but the star of the movie for physicists was a revolutionary new magnetohydrodynamic (MHD) marine propulsion system. The so-called “caterpillar drive” worked with no moving parts, allowing a nuclear missile-armed Soviet submarine to approach the U.S. coast undetected. As the submarine captain (played by Connery) said, “Once the world trembled at the sound of our rockets … now they will tremble again—at the sound of our silence.