SPACE/COSMOS
Mainz researchers rule out “stupendously large” primordial black holes as significant part of dark matter
Cosmic microwave background measurements by Planck satellite show no evidence of stupendously large black holes formed shortly after Big Bang to contain significant amounts of dark matter
image:
The collapse of matter overdensities to primordial black holes in the early universe changes the spectrum of cosmic microwave background fluctuations.
view moreCredit: Christopher Gerlach
While physicists today know that dark matter must exist, there are still many unanswered questions surrounding it – such as what it is made of. For the latter question, a team of researchers at Johannes Gutenberg University Mainz (JGU) has now narrowed down the list of possible answers by analyzing measurements of the cosmic microwave background taken by the Planck satellite. In an article published in Physical Review Letters, they detail how their findings rule out one theory of how dark matter might have behaved after the Big Bang.
“A possible solution to the puzzle of dark matter was that it might have collected shortly after the Big Bang in giant black holes with masses ranging from 100 million to 10 billion billion times that of the sun”, said Christopher Gerlach, theoretical physicist at JGU and PRISMA++ Cluster of Excellence. However, this accumulation should have had a distinct impact on the young universe – so distinct that it would still be visible in the cosmic microwave background, or CMB, even to this day.
Looking for something invisible
The CMB is the light released when the universe was just 380,000 years old. To measure it, one must bring highly specialized equipment and leave the Earth’s atmosphere. ESA’s Planck space observatory did just that, conducting uniquely detailed measurements of the “echo of the Big Bang” in four and a half years of operation before its final shutdown in October 2013. “That we are still generating new insights into the universe’s evolution and structure 13 years after it was shut down only emphasizes how important the Planck satellite was and is to physicists”, explained Christopher Gerlach. Its measurements allow more precise calculations of the universe’s visible and dark matter contents. Analysis of primordial black holes, which were formed shortly after the Big Bang, has already shown that they are unlikely to have contained all dark matter in the universe. The JGU research team concentrated their efforts on the subset of so-called stupendously large black holes (SLABs), which were candidates if not for containing all dark matter, then at least a significant part of it.
Nicholas Leister, another JGU physicist in the research team: “If these SLABs had existed in large numbers and contained significant amounts of dark matter, they would have left a characteristic imprint on the CMB through their unique perturbations, or, in a word, making it grainy.” The physicists modeled how this imprint should look and compared it to the actual measurements. Not enough graininess was found, ruling out the possibility of SLABs as significant parts of dark matter.
Questions about the early universe remain
Stupendously large black holes were candidates for explaining several other phenomena in the early universe. One of them is the nano-Hertz gravitational-wave background, or “hum” detected in Pulsar Timing Arrays, another are the surprisingly large early galaxies observed by the James Webb telescope. “These questions are also unlikely to be answered by SLABs, since our findings show that they didn’t exist in large numbers”, said Nicholas Leister. “The early universe still has many puzzles waiting to be solved.”
Journal
Physical Review Letters
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Closing the Mass Window for Stupendously Large Black Holes
CORTEX: The brains behind autonomous close-range rendezvous for spacecraft servicing
There is a problem in low Earth orbit and it’s growing. According to the European Space Agency, there are about 19,440 satellites on orbit, 3,540 of which are non-functioning and/or not emitting a signal to warn other spacecraft of their location. The need to service these sleep-walking satellites in space will only increase as more are launched.
“There are also over 8,000 metric tons of debris, none of which is cooperative, in the sense that they cannot thrust or send a signal,” said Minduli Wijayatunga, a new aerospace engineering faculty member in The Grainger College of Engineering, University of Illinois Urbana-Champaign.
Although most of Wijayatunga's work can be applied to both debris and spacecraft, a recent study targets a spacecraft that needs repair but has lost the ability to communicate. Like an unconscious patient, the servicer must approach it with no help from the target.
Wijayatunga said the repairs need to be done autonomously because of the time- delay in communications between a repair spacecraft and Earth and the limited windows of time when communication is even possible.
“These spacecraft must be able to track the trajectories of uncooperative objects autonomously but also make go/no-go decisions without waiting for the ground to decide,” she said. “Suppose your thruster fails or you encounter high-order orbital perturbations that push you away from your nominal trajectory. In those instances, how do you make those decisions?”
In response to this question, Wijayatunga created a framework for close-range rendezvous trajectory design and guidance and demonstrated it in a lab. It’s called CORTEX, which stands for Convex Optimization for Rendezvous Trajectory Execution.
“CORTEX takes into account eclipses, timing the rendezvous when there is an optimal window of sunlight to see the target, complete your close-proximity operations and depart,” she said. “For my Ph.D., I did a project on trajectory design and guidance using convex optimization for the transfer phases. Eclipses matter there too; they can limit how much you can thrust. But they don’t compromise safety to the same extent, so you don’t need to replan or hold position because of them.
“I wanted to bring that same idea into developing CORTEX and expand it, taking into consideration sunlight windows so we don't have to operate in darkness. That, and the replanning versus abort architecture, are novel in CORTEX.”
She explained that CORTEX decides whether to replan a new trajectory and approach the target or abort the approach, retreat to a passively safe orbit and retry approach.
“CORTEX has trajectory design and guidance, so if you deviate significantly from your nominal mission trajectory, it replans the trajectory and re-approaches unless you are in severe violation of your keep-out zone, in which case it returns to a passively safe orbit. It's an algorithm to recover from the unexpected. We tried instances where you're supposed to use your thrusters, but you can't for whatever reason. And CORTEX was very robust to that.”
To stress test the hardware experiments in the lab, the servicer spacecraft was manually pushed to represent a perturbation that sends it off course from its target, thrusters were turned off during periods of time they were supposed to be on, and state and thrust errors were artificially increased to simulate unexpected situations like solar storms.
“In some of the hardware tests, if the servicer violates the approach corridor, it leaves the keep-out zone and retreats to a safe position and in others, the servicer is able to recover and carry on with its mission,” she said.
Missing the target and regrouping for another approach is also an autonomous decision.
Wijayatunga said she has a number of different projects that have stemmed from this work.
“One of the final projects in my class is using reinforcement learning to replan the phase that happens right before the close rendezvous. Far rendezvous happens when you see your target at a range of from 30 kilometers down to 3 kilometers.”
She said that although autonomous servicing has not yet been attempted on an uncooperative target, the approach isn’t theoretical. Close-range approach to an uncooperative object has already been experimentally validated in space by the ADRAS-J mission.
The study, “CORTEX: An autonomous, end-to-end, convex-based framework for close-range rendezvous trajectory design and guidance with hardware testbed validation,” by Minduli C. Wijayatunga, Julian Guinane, Nathan D. Wallace, Xiaofeng Wu and Salah Sukkarieh, is published in the journal Acta Astronautica. DOI: 10.1016/j.actaastro.2026.08.041
Journal
Acta Astronautica
Article Title
CORTEX: An autonomous, end-to-end, convex-based framework for close-range rendezvous trajectory design and guidance with hardware testbed validation
No comments:
Post a Comment