The standard cosmological model of galaxy formation might be in trouble

The majestic spiral galaxy NGC 4414 as photographed by the Hubble Space Telescope. Image: NASA.

Scientists have a pretty good picture of how the universe formed and evolved – and how it is structured today. This knowledge all fits together nicely as a “standard cosmological model”, which has been able to successfully predict and describe many observational data in the universe. But now and then scientists discover something that threatens to tear down this valuable framework.

New research, published in Science, has done just that. The paper reports that a number of “satellite dwarf galaxies” – small galaxies orbiting around the much larger galaxy, Centaurus A – are rotating in synchrony in a single plane around their host, which is not at all what was expected.

This is a problem because the standard cosmological model predicts that galaxies form hierarchically, meaning they grow gradually larger by attracting smaller galaxies and tearing some of them apart. This happens as the force of gravity sucks them in, irrespective of which direction they are captured from. You would therefore expect these galaxies to be moving in all sorts of random positions and directions corresponding to however they were moving before being caught in orbit.

Despite being challenged at times, the hierarchical model still stands strong. It supports one of the most fundamental aspects of modern cosmology: isotropy. This is the expectation that the universe is uniform, looking the same in whatever direction you are viewing it. The same expectation for uniform behaviour holds true on the small scales of satellite galaxies. The new study challenges the hierarchical model and thereby also isotropy – it doesn’t make sense for some galaxies in some corner to behave in synchrony and others at random.

What’s more, we see plenty of evidence for the hierarchical model in the wreck caused by past satellite disruptions; also, smaller galaxies in the process of being accumulated gradually and others still surviving.

Estimating strangeness

But just how surprising is the new finding? A few other satellite galaxies moving in a single plane have been found before. Two such cases were discovered right in our cosmological backyard, one around our own Milky Way and one around the Andromeda galaxy. Three is not a big number, but we have not looked for these features much farther away yet. However there is tantalising evidence that about half of galaxies like the Milky Way may have satellites on ordered orbits.

So far, cosmologists have been writing off these planes as rare events, odd occurrences that don’t represent the wider universe. Using computer simulations of galaxy formation and sifting through all possible orientations of satellite galaxies in these models, scientists can estimate the number of such “outlier” galaxies we can expect to find in the universe. This shows that such planar distribution should not be freakishly rare – there is in fact a 10 per cent chance of it happening.

However, the chance of seeing a large number of satellite galaxies rotating in the same direction, such as those around Centaurus A, is less than 0.5 per cent. This means it is not impossible, but if we find too many such cases, the standard model would have to be rethought. In addition to the new study, we know of two other cases where that happens (also in Milky Way and Andromeda). So by now there are already three such examples in a sample of observations that is not yet very large.

Milky Way, taken from Australia. Image: Roanish/Wikipedia/creative commons.

But then there is the “look-elsewhere effect”. This is an effect well known to statisticians, which corrects for our propensity to reach all sorts of bizarre conclusions when a rare event happens in a small sample of observations. By looking elsewhere, that is, by increasing the number of observations and counting all the times when the event does not occur, the statistical significance of that event can be drastically reduced.

Alternative explanations

But what if we can’t find enough cases to show that these galaxies are an exception rather than a rule? Can the findings be explained at all by our current cosmological model? Possibly, but unusual observations call for unusual explanations. Potentially these dwarf galaxies could have been created in a single event – giving them coherent movement – rather than having been captured one at a time. A massive merger of two galaxies could potentially create this effect. As Centaurus A certainly shows signs of a violent past, this isn’t impossible.

Such a scenario has been proposed for explaining the plane of satellite galaxies in Andromeda, but it seems unlikely this would explain all such cases. The problem with this scenario is that the dwarf galaxies born in a single tidal event would have to share similar star formation histories. However, many of these dwarf galaxies don’t.

The environment of galaxies clearly plays a role. The distribution of galaxies on much larger scale can also have subtle effects on the motion of satellite galaxies. For example, an expanding empty region of space known as the Local Void is thought to “shepherd” galaxies near Centaurus A, steering them along preferred directions. This explanation is also fully compatible with the standard cosmological model. However, cosmological simulations cannot yet show exactly how this would match what’s actually seen.

A more drastic re-evaluation of the whole problem is to tweak the laws of gravity, for example by using something called modified newtonian dynamics. Computer simulations using such dynamics have been able to produce similar planes of satellite galaxies as seen in the Milky Way and Andromeda. However, this theory, although successful in many respects, is still some way from passing the same consistency checks as the standard model.

The ConversationSo it looks like our cherished cosmological model may just be able to survive, at least for a while.

Andreea Font, Senior Lecturer at Astrophysics Research Institute, Liverpool John Moores University.

This article was originally published on The Conversation. Read the original article.


To beat rising temperatures, Vienna launches a network of 'Cool Streets'

A Vienna resident cools off at one of the city's new Cool Streets installations. (Courtesy Christian Fürthner/Mobilitätsagentur Wien)

Over the past several months, Austria has recorded its highest unemployment rate since World War II, thanks to the economic aftermath of the Covid-19 pandemic. With no job or a suddenly smaller income – not to mention the continued threat of the virus – many Viennese will opt for a staycation this summer.  

At the same time, last year, Austria’s capital experienced 39 days with temperatures of over 30°C (86°F), one of its hottest summers in history according to the Central Institute for Meteorology and Geodynamics.

Climate experts expect a similarly sizzling 2020 season, and city officials are now doubling down on efforts to combat the heat by launching a “Cool Streets” initiative as well as a new, state-of-the-art cooling park.

“As the city councilwoman in charge of climate, it is my job to ensure local cooling,” Vienna’s deputy mayor Birgit Hebein proclaimed at the opening of one of 22 new “Cool Streets” on 22 June.

“In Austria, there are already more heat deaths than traffic fatalities,” she added.

Hebein was referring to the 766 people the Austrian Agency for Health and Food Security included in its 2018 heat-associated mortality statistics. The number was up by 31% compared to 2017, and in contrast to the 409 people who died in traffic collisions the same year.

The project includes 18 temporary Cool Streets located across the city, plus four roads that will be redesigned permanently and designated as “Cool Streets Plus”.

“The Plus version includes the planting of trees. Brighter surfaces, which reflect less heat, replace asphalt in addition to the installation of shadow or water elements,” said Kathrin Ivancsits, spokeswoman for the city-owned bureau Mobilitätsagentur, which is coordinating the project.

Vienna's seasonal Cool Streets provide shady places to rest and are closed to cars. (Petra Loho for CityMetric)

In addition to mobile shade dispensers and seating possibilities amid more greenery provided by potted plants, each street features a steel column offering drinking water and spray cooling. The temporary Cool Streets will also remain car-free until 20 September.

A sensor in the granite base releases drinking water and pushes it through 34 nozzles whenever the outside temperature reaches 25°C (77°F) . As soon as the ambient temperature drops to 23°C (73°F), the sensor, which operates from 10 a.m. to 8 p.m., turns off the water supply.

The sensors were included in part to allay concerns about legionella, a pathogenic bacteria that can reproduce in water.  

“When the spray stops, the system drains, and therefore no microbial contamination can develop,” said Dr. Hans-Peter Hutter, deputy head of the Department of Environmental Health at the Center for Public Health at Medical University Vienna, in a televised interview.

Hutter also assured the public that there is no increased risk of a Covid-19 infection from the spray as long as people adhere to the one-meter social distance requirement.

But Samer Bagaeen of the University of Kent's School of Architecture and Planning notes that air cooling systems, like the ones used in Germany at abattoirs, have been found recently to be a risk factor for Covid-19 outbreaks.

“The same could be said for spay devices,” he warned.

Vienna’s district councils selected the 22 Cool Street locations with the help of the city’s Urban Heat Vulnerability Index. The map shows where most people suffer from heat by evaluating temperature data, green and water-related infrastructure, and demographic data.

“Urban heat islands can occur when cities replace the natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat,” as the US Environmental Protection Agency states.

A rendering of Vienna's planned park featuring a Coolspot, which is scheduled to open in August. Click to expand.
(Courtesy Carla Lo Landscape Architecture)

Vienna’s sixth district, Mariahilf, is such an area. The construction of the capital’s first “Cooling Park”, a €1 million project covering the 10,600 square-metre Esterházypark, is designed to provide relief. 

Green4Cities, a centre of excellence for green infrastructure in urban areas, designed the park’s main attraction, the “Coolspot”. The nearly 3.40-metre high steel trellis holds three rings equipped with spray nozzles. Textile shading slats, tensioned with steel cables, cover them.

The effects of evaporation and evapotranspiration create a cooler microclimate around the 30 square-metre seating area, alongside other spray spots selectively scattered across the park.

The high-pressure spray also deposits tiny droplets on plant and tree leaves, which stimulates them to sweat even more. All together, these collective measures help to cool their surroundings by up to six degrees.

The landscape architect Carla Lo and her team planned what she calls the “low-tech” park components. “Plants are an essential design element of the Cooling Park,” Lo says. “By unsealing the [soil], we can add new grass, herbaceous beds, and more climate-resistant trees to the existing cultivation”.

Light-coloured, natural stone punctuated by grass seams replaces the old concrete surfaces, and wooden benches meander throughout the park.

Living near the park and yearning for an urban escape close by, Lo says she’s motivated to ensure the park is completed by mid-August.

“If we don't do anything, Vienna will be another eight degrees Celsius hotter in 2050 than it already is,” Hebein said.

Vienna recently came in first in the World's 10 Greenest Cities Index by the consulting agency Resonance.

“There is no one size fits all on how cities respond to urban heat,” says the University of Kent’s Bagaeen, who points out that Vienna was one of the first European cities to set up an Urban Heat Islands Strategic Plan in 2015.

In the short term, prognoses on the city’s future development may be more difficult: Vienna votes this autumn.

Petra Loho is a journalist and photographer based in Austria.