Dark Holes

Staring at what is missing from this view of the galaxy NGC 24 in the Sculptor Constellation: that’s what dark matter is about. It is finding the invisible. By now tens of thousands of papers have been written on the subject of dark matter. Wrapping our heads around them is unfeasible. Swamped by an array of questions, the birdlike mind watches from above. The possibility that a gravitational force, largely due to the mass of the Milky Way halo could play a role in the trajectory of celestial objects such as Oumuamua and the putative Planet Nine brings me back to a review of what I have gathered so far on dark matter.

If some are convinced of the existence of dark matter, others show skepticism. Theories intend to explain discrepancies in the distribution of dark matter between small and larger galaxies. It is suggested that the motion in retrograde manner of dark matter has important effects on the morphology and evolution of barred disc galaxies. If we can’t confront dark matter, dynamic mechanisms involved in the interaction between dark matter particles and baryons appear to be even more elusive. Sifting through data accumulated by the ATLAS experiment, it is concluded that no more than 13% of Higgs bosons produced in the Large Hadron Collider could be transformed into particles invisible to the detector. 

We travel through time building models and developing scenarios on how it began and what features were those of the early Universe that led to the current setting. A singularity out of sight lies beyond spacetime, at the beginning of the Universe. Hydrogen atoms gas fed the formation of the first entities: stars and black holes. The 21-cm spectral line produced by hydrogen atoms is an essential finding. The cooling effect or momentum transfer may have occurred between minicharged dark particles and baryons. On the edge of the infinitely small, it is suggested that only a fraction of those milli-charged dark matter particles interacted with ordinary matter.

Theorists looking for lost time at the birth of the Universe grapple with the ontological nature of reality.  We’re told that it all started with equal numbers of matter and antimatter but that we observe today a nonzero mass difference. At later times matter eventually will decay, with the baryon number reverting to zero. At CERN,  researchers are attempting to find any evidence of interaction. One of the Antiproton Decelerator experiments at CERN, the ALPHA collaboration, reported its first measurements of certain quantum effects in the energy structure of antihydrogen, the antimatter counterpart of hydrogen.

The line between the invisible and the visible, although swinging slightly bringing light into darkness, has not been crossed. While we do not know what dark particles are, we succeed at narrowing down the object of our search, placing constraints on their properties. Could dark matter be present right under our nose, gravitationally bound to celestial objects such as the Moon or Jupiter? 

Dark matter could be so many things that I would not know where to start. It may be a cold, fuzzy, collisionless fluid. It may be composed of a type of not yet discovered particles: right-handed neutrinos, baryons of mirror matter, or even fifth-dimensional dark fermions, to list a few. The cogenesis of dark matter and a process called leptogenesis — that produced the matter-antimatter asymmetry — may have occurred in the presence of primordial black holes. Those cosmic objects from the firsts moments of the Universe were in a microstate. Some became, in the course of their evolution, centers of galaxies while others still wander around without populations of stars orbiting them.

Romulus simulations can predict which supermassive black holes, following a galaxy merger, will make it to the center of their new host galaxy and how long that process takes. Many binaries are said to be formed after several billions of years of orbital evolution, while some never made it to the center. Milky Way-mass galaxies are found to host an average of 12 supermassive black holes, which typically lie throughout the dark matter halos.

Supermassive black holes are invisible and extremely heavy objects. Do the same invisibleness and heaviness apply to dark matter? Could dark matter accumulate near black holes, forming spike distribution on a black hole event horizon? Could it fall through the true horizon to a cosmic hidden sector? The introduction of extra dimensions, a fifth-dimension force or a mirror universe would make a serious dent in the concept of nothingness, diminishing its scope further. If there is no such thing as nothingness that precedes the generation of space from 'zero time’, then there may be no end to the shrinking entities of the Universe. Nothingness may stand as a threshold of a hidden dimension of spacetime. It would, though, require us to believe in something to exist beyond.

Space, like the sea with its currents, hosts tidal shocking and stripping causing gas clouds of baryonic matter and dark matter to jostle into each other,  pulling each other in and out. Tracks or wakes as if in the sea implicitly hint at density perturbations within spacetime and the existence of something in the makeup of the Universe instead of nothing. Satellite galaxies such as the Large Magellanic Cloud in its first infall may leave, through the Milky Way, dark matter debris no longer gravitationally bound to it. 

How to study dark matter footprints and stellar stream distortions is key to disentangling processes involving stellar streams and dark matter halos. Scenes of their encounters, impacting each other’s shape and velocity, are set in spacetime, etched in the memory of galaxies. Some see the Universe as if clusters of baryonic matter were embedded into halos of dark matter. Space acts as a screen on which scenes of universal life are projected. Scientists tackle both ends of the Universe: the extremely small and the infinitely large. As they try to identify the microscopic nature of dark matter, they attempt to describe at the same time the role of dark matter in the macroscopic Universe on the edges of galaxies. Spacetime becomes a scaffolding upon which large mergers alter the morphology of galaxies and dark matter halos.  Could a single field be responsible for both inflation and dark matter? Dark matter and dark energy may be two faces of the same coin.

In the end, dark matter may be heterogeneous, behaving differently depending on scale and location, whether it be in halos around galaxies and galaxy clusters or around galactic centers. Its heterogeneity would allow the possibility for collision and interaction between dark matter particles. Some would even imagine the self-interacting dark matter to be in many ways like what we observe with the baryonic matter. Could dark matter be at times warm or hot, linked to the thermalization history of the Universe? Could there be dark photons? Dark matter may not only be comprised of self-interacting dark matter but also made up of primordial black holes.

Radio surveys by EDGES, LOFAR, and HERA — which provided its first set of data — as well as in the future SKA, among others, are the first results available to us in our investigation into the topology of the early Universe. In a bouncing Universe, Penrose imagines dark matter particles as gravitational entities called erebons which decay completely at the end of each eon to then be created afresh at the beginning. Perhaps, he adds, we will need detectors of a different kind to see erebon decays in our eon. Adding to the detection of gravitational waves by observatories such as LIGO, Virgo, and KAGRA, we expect the James Webb telescope to match images to some of the early radio emissions and provide answers to lingering questions about black holes at the dawn of time. Furthermore, if dark matter particles could annihilate into gamma-ray radiation, it is suggested that the intensity of such events near a black hole could provide a good opportunity to detect the dark matter annihilation signal. 

The principle called Occam’s razor states that plurality should not be posited without necessity. Which of the above-mentioned hypotheses provides the simplest explanation? Instead of new particles that we have not discovered yet, could we do away with dark matter? A new Relativistic Theory for Modified Newtonian Dynamics is said to be best at reproducing key cosmological observables. But if its mass can be calculated and its increase and decrease recorded, with the advent of cosmological events such as the infall of the Large Magellanic Cloud, how can the concept of dark matter be dismissed by the introduction of a modified force of gravity? Could both represent fragments of reality?

Dancing around the center of the galaxy, we are lost in a land where a never-ending stream of questions flows. I still ponder why the length of time light travels from the galactic center to the Earth is within the same order as the time it takes for the Earth to go through one complete precession cycle.

Poets, I feel,  are not dying. They have fallen into a deep slumber, carried on a time ship hoping to return to where it all began.