Macroscopic dark matter (macros) is a broad class of dark-matter candidates that represents an alternative to conventional particle dark matter with wide ranges of masses Mx and large cross sections σx that could still provide all of the dark matter.
Macros typically refer to a family of composite dark matter models arising from some early-universe phase transition, often composed of strange quark matter. Of particular interest would be macros of approximately nuclear density satisfying the geometric cross section:
as several models for macros describe potential candidates with approximately that density. The idea that macros could be formed entirely within the Standard Model was originally proposed by Witten in the context of a first-order QCD phase transition.
In the recent paper, Starkman and colleagues proposed that these candidates would transfer energy to matter primarily through elastic scattering. A sufficiently large macro passing through the atmosphere would produce a straight channel of ionized plasma. If the cross-section of the macro is σx ≿ 6 × 10¯9 cm², then under atmospheric conditions conducive to lightning (eg. a thunderstorm) the plasma channel would be sufficient to seed a lightning strike with a single leader. This is entirely unlike ordinary bolt lightning in which a long sequence of hundreds or thousands of few-meter-long leaders are strung together. This macro-induced lightning would be extremely straight, and thus highly distinctive. Neither wind shear nor magneto hydrodynamic instabilities would markedly spoil it’s straightness.
They also conducted a search on the only photographically documented case of a straight lightning bolt which was reported in Mutare, Zimbabwe on 15 February 2015 and recorded at 30 frames per second with a Panasonic Lumix DMC-TZ10 compact camera in scene mode and proposed that the observed lightning strike is a cloud-ground strike with no secondary strikes. The maximum projected deviations from perfect linearity are of order a few diameters. As the thickness of a beam of lightning is between 1m and 10m (and does not depend significantly on the considered macro parameter space), even this straight lightning strike is mostly likely not straight enough to have been induced by a macro.
The expected signature from a macro-induced lightning strike would be very unique. This presents, in theory, a straightforward way to search for macros by looking for macro-induced lightning strikes, and to place constraints on macros if no such strikes are observed.— told Starkman, lead author of the study
But the question is why didn’t we observed straight lightening yet? Well friends, Starkman and colleagues also calculated the macro-induced lightening rate on the Earth and found that macro induced lightening rate is much lower (ie. it is just 10¯6 s-¹) than the actual observed rate of lightening strikes on the earth (around 50–100 s-¹). This maybe due to sufficiently low mass macros passed through our atmosphere.
So, now the question arises where to find large macros? Well, maybe on Jupiter. Because the surface area of Jupiter is 125 times that of Earth, suggesting that it is a potentially valuable target to search for macro-induced fluorescence or macro-induced lightning. Lightning has been observed near the Jovian poles by every passing satellite.
But, hold on, I am not making concrete claims about the observability of macro-induced lightning on Jupiter, as it is difficult to observe. This is due to two factors: the physics of lightning in Jupiter’s atmosphere is even less well-understood than that on Earth, and the logistics of monitoring Jovian lightning is much more difficult given its distance. It is currently unclear why lightning does not form over the entire surface of Jupiter but only the poles.
But this will gonna be advantageous for us. Why? because this could reduce the region of parameter space that could be probed (in simple terms, we could be able to probe more lightening rates at particular place). There is one additional difference in the case of Jovian Lightning I forgot to tell you, we are only concerned with cloud-cloud lightning, as opposed to focusing on cloud-ground lightning for Earth. This is because the ‘ground’ for Jupiter is essentially unobservable, and thus it is more useful to look for intercloud lightning strikes in the upper layers of the Jovian atmosphere. In addition, observing the morphology of Jovian lightning presents obvious technical challenges, but could be overcome either by using high-resolution space telescopes from earth, or by using future Jovian weather satellites that will make precise measurements of Jupiter’s atmospheric phenomenon.
Given that many lightning strikes on Jupiter will be obscured by the cloud cover, it would be more advantageous to use detection methods that do not rely on visual morphology to differentiate macro-induced lightning from organic lightning. For example, one could potentially use the RF signal to differentiate straight lightning bolts without visual confirmation. This could be accomplished using RF instrumentation on existing probes such as JUNO, or proposed upcoming probes such as the Jupiter Ganymede Orbiter.
Despite these theoretical and observational challenges, Starkman and winch showed that the region of parameter space (in Fig 2) that could be probed assuming that lightning occurs only over 10% of the surface of Jupiter, which is likely an underestimate. They also assumed lightning physics is identical on Jupiter compared to on Earth, and that this lightning is detectable and distinguishable from non-macro induced lightning using some future technology.
We do not claim that our forecasts for constraints due to Jovian lightning are definitive, but instead present them as a potential future area of research, worthy of more in-depth investigation.— concluded authors of the study
1) Relationship between macro mass, cross-section and internal density
Macroscopic dark matter is much larger than the size of a proton or neutron, and therefore the cross section is both the geometric cross section and the cross section for elastic scattering.
Deriving the cross section with reference to nuclear density:
Taking ρnuclear = 3.6 × 10¹⁴ [gcm-³] and solving for the cross section in terms of the nuclear density.
2) All code and data is available at this https URL
Reference: Nathaniel Starkman, Jagjit Sidhu, Harrison Winch, Glenn Starkman, “Straight Lightning as a Signature of Macroscopic Dark Matter”, ArXiv, pp. 1-10, 2021. https://arxiv.org/abs/2006.16272
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