Editorial 1 : With bad news from Cassini, is dark matter’s main rival theory dead?
Introduction
One of the biggest mysteries in astrophysics today is that the forces in galaxies do not seem to add up. Galaxies rotate much faster than predicted by applying Newton’s law of gravity to their visible matter, despite those laws working well everywhere in the Solar System.
The invisible substance
- To prevent galaxies from flying apart, some additional gravity is needed.
- This is why the idea of an invisible substance called dark matter was first proposed.
- But nobody has ever seen the stuff. And there are no particles in the hugely successful Standard Model of particle physics that could be the dark matter — it must be something quite exotic.
- This has led to the rival idea that the galactic discrepancies are caused instead by a breakdown of Newton’s laws.
- The most successful such idea is known as Milgromian dynamics or MOND, proposed by Israeli physicist Mordehai Milgrom in 1982.
The flaws in the model
- The main postulate of MOND is that gravity starts behaving differently to what Newton expected when it becomes very weak, as at the edges of galaxies.
- MOND is quite successful at predicting galaxy rotation without any dark matter, and it has a few other successes. But many of these can also be explained with dark matter, preserving Newton’s laws.
- The key is that MOND only changes the behaviour of gravity at low accelerations, not at a specific distance from an object.
- It is the amount of acceleration, rather than the distance, that predicts where gravity should be stronger.
- This means that, although MOND effects would typically kick in several thousand light years away from a galaxy, if we look at an individual star, the effects would become highly significant at a tenth of a light year.
- That is only a few thousand times larger than an astronomical unit (AU) – the distance between the Earth and the Sun.
- But weaker MOND effects should also be detectable at even smaller scales, such as in the outer Solar System.
The Cassini mission
- This brings us to the Cassini mission, which orbited Saturn between 2004 and its final fiery crash into the planet in 2017.
- Due to a quirk of MOND, the gravity from the rest of our galaxy should cause Saturn’s orbit to deviate from the Newtonian expectation in a subtle way.
- This can be tested by timing radio pulses between Earth and Cassini. Since Cassini was orbiting Saturn, this helped to measure the Earth-Saturn distance and allowed us to precisely track Saturn’s orbit.
- But Cassini did not find any anomaly of the kind expected in MOND. Newton still works well for Saturn.
More bad news for MOND
- MOND also fails to explain small bodies in the distant outer Solar System. Comets coming in from out there have a much narrower distribution in energy than MOND predicts.
- These bodies also have orbits that are usually only slightly inclined to the plane that all the planets orbit close to. MOND would cause the inclinations to be much larger.
- Newtonian gravity is strongly preferred over MOND on length scales below about a light year.
- MOND cannot provide enough gravity either, at least in the central regions of galaxy clusters.
- But in their outskirts, MOND provides too much gravity. Assuming instead Newtonian gravity, with five times as much dark matter as normal matter, seems to provide a good fit to the data.
Conclusion
The standard dark matter model of cosmology isn’t perfect, however. There are things it struggles to explain, from the universe’s expansion rate to giant cosmic structures. So we may not yet have the perfect model.
Editorial 2 : Global project ‘paints’ evidence of air pollution in India
Context
Researchers and artists have joined forces for an international project to make invisible air pollution in India visible, demonstrating the health risks posed to the population.
Visualizing the Invisible
- Invisible yet omnipresent, air pollution poses a significant threat to public health, particularly in densely populated regions like India.
- However, grasping the severity of this issue can be challenging due to its intangible nature.
- To bridge this gap between awareness and action, researchers and artists worldwide have embarked on a collaborative endeavor: making invisible air pollution visible.
- Combining digital light painting and low-cost air pollution sensors, the scientific team produced photographic evidence of pollution levels in cities across three countries — India, Ethiopia, and the U.K.
- By painting with light to create impactful images, we provide people with an easy-to-understand way of comparing air pollution in different contexts — making something that was largely invisible visible.
- Air of the Anthropocene creates spaces and places for discussions about air pollution, using art as a proxy to communicate and create dialogues about the issues associated with air pollution.
- Particulate matter, or PM, is the air pollutant most responsible for human morbidity and mortality. It has multiple impacts on physical health and is responsible for diseases, including heart disease, stroke, and cancers.
- The “painting with light” team used low-cost air pollution sensors to measure PM mass concentrations.
- The sensors’ real-time signal was used to control a moving LED array programmed to flash more rapidly as PM concentration increased.
- By providing a visual understanding of air pollution that is accessible to people who don’t necessarily have a scientific background, the light painting approach can demonstrate that managing air pollution levels can have a significant impact on people’s day-to-day lives.
The process
- In this innovative approach, the sensors are strategically placed in locations with high pollution levels, such as busy intersections or industrial areas.
- As the sensors detect pollutants in the air, the data is transmitted in real-time to a central database or computer system.
- Meanwhile, digital light painting equipment, consisting of light sources and cameras, is set up to capture long-exposure photographs of the surrounding environment.
- Using custom software or algorithms, the air pollution data is translated into visual patterns and colors that correspond to different pollutant concentrations.
- For example, higher concentrations of particulate matter might be represented by brighter or more intense colors, while lower concentrations appear fainter or less prominent.
- These visualizations are then projected onto buildings, walls, or other surfaces using digital light painting techniques.
- The result is a dynamic and immersive display that communicates the invisible presence of air pollution in a tangible and engaging manner.
- Passersby are drawn to the striking visuals, prompting them to pause and reflect on the environmental issues affecting their community.
Conclusion
By bringing awareness to the pervasive nature of air pollution, these installations spark conversations and inspire collective action to address the root causes of pollution.
