A century ago, Edwin Hubble first established that this so-called “spiral nebula” was actually very far outside our own Milky Way galaxy — at a distance of approximately 2.5 million light-years or roughly 25 Milky Way diameters. Prior to that, astronomers had long thought that the Milky way encompassed the entire universe. Overnight, Hubble’s discovery turned cosmology upside down by unveiling an infinitely grander universe.

Now, a century later, the space telescope named for Hubble has accomplished the most comprehensive survey of this enticing empire of stars. The Hubble telescope is yielding new clues to the evolutionary history of Andromeda, and it looks markedly different from the Milky Way’s history.

Without Andromeda as a proxy for spiral galaxies in the universe at large, astronomers would know much less about the structure and evolution of our own Milky Way. That’s because we are embedded inside the Milky Way. This is like trying to understand the layout of New York City by standing in the middle of Central Park.

“With Hubble we can get into enormous detail about what’s happening on a holistic scale across the entire disk of the galaxy. You can’t do that with any other large galaxy,” said principal investigator Ben Williams of the University of Washington. Hubble’s sharp imaging capabilities can resolve more than 200 million stars in the Andromeda galaxy, detecting only stars brighter than our Sun. They look like grains of sand across the beach. But that’s just the tip of the iceberg. Andromeda’s total population is estimated to be 1 trillion stars, with many less massive stars falling below Hubble’s sensitivity limit.

Photographing Andromeda was a herculean task because the galaxy is a much bigger target on the sky than the galaxies Hubble routinely observes, which are often billions of light-years away. The full mosaic was carried out under two Hubble programs. In total, it required over 1,000 Hubble orbits, spanning more than a decade.

This panorama started with the Panchromatic Hubble Andromeda Treasury (PHAT) program about a decade ago. Images were obtained at near-ultraviolet, visible, and near-infrared wavelengths using the Advanced Camera for Surveys and the Wide Field Camera 3 aboard Hubble to photograph the northern half of Andromeda.

This program was followed up by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), recently published in The Astrophysical Journal and led by Zhuo Chen at the University of Washington, which added images of approximately 100 million stars in the southern half of Andromeda. This region is structurally unique and more sensitive to the galaxy’s merger history than the northern disk mapped by the PHAT survey.

The combined programs collectively cover the entire disk of Andromeda, which is seen almost edge-on — tilted by 77 degrees relative to Earth’s view. The galaxy is so large that the mosaic is assembled from approximately 600 separate fields of view. The mosaic image is made up of at least 2.5 billion pixels.

The complementary Hubble survey programs provide information about the age, heavy-element abundance, and stellar masses inside Andromeda. This will allow astronomers to distinguish between competing scenarios where Andromeda merged with one or more galaxies. Hubble’s detailed measurements constrain models of Andromeda’s merger history and disk evolution.

A Galactic ‘Train Wreck’

Though the Milky Way and Andromeda formed presumably around the same time many billions of years ago, observational evidence shows that they have very different evolutionary histories, despite growing up in the same cosmological neighborhood. Andromeda seems to be more highly populated with younger stars and unusual features like coherent streams of stars, say researchers. This implies it has a more active recent star-formation and interaction history than the Milky Way.

“Andromeda’s a train wreck. It looks like it has been through some kind of event that caused it to form a lot of stars and then just shut down,” said Daniel Weisz at the University of California, Berkeley. “This was probably due to a collision with another galaxy in the neighborhood.”

A possible culprit is the compact satellite galaxy Messier 32, which resembles the stripped-down core of a once-spiral galaxy that may have interacted with Andromeda in the past. Computer simulations suggest that when a close encounter with another galaxy uses up all the available interstellar gas, star formation subsides.

“Andromeda looks like a transitional type of galaxy that’s between a star-forming spiral and a sort of elliptical galaxy dominated by aging red stars,” said Weisz. “We can tell it’s got this big central bulge of older stars and a star-forming disk that’s not as active as you might expect given the galaxy’s mass.”

“This detailed look at the resolved stars will help us to piece together the galaxy’s past merger and interaction history,” added Williams.

Hubble’s new findings will support future observations by NASA’s James Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope. Essentially a wide-angle version of Hubble (with the same sized mirror), Roman will capture the equivalent of at least 100 high-resolution Hubble images in a single exposure. These observations will complement and extend Hubble’s huge dataset.

One recent proposal seeks to infuse the atmosphere with hydrogen peroxide, insisting that it would both oxidize methane (CH₄), an extremely potent greenhouse gas while improving air quality.

Too good to be true?

University of Utah atmospheric scientists Alfred Mayhew and Jessica Haskins were skeptical, so they set out to test the claims behind this proposal. Their results, published on Jan. 3, confirm their doubts and offer a reality check to agencies considering such proposals as a way to stave off climate change.

“Our work showed that the efficiency of the proposed technology was quite low, meaning widespread adoption of the technology would be required to make any meaningful impact on atmospheric CH₄,” said Mayhew, a postdoctoral researcher with the university’s Wilkes Center for Climate Science & Policy. “Then, our results indicate that if this technology is adopted at scale, then we start to see some negative air-quality side effects, particularly for wintertime particulate matter air pollution.”

To conduct the study, the Utah scientists modeled what would happen if you deployed the technology patented by a Canadian company, which is proposing to spray aerosolized hydrogen peroxide, or H₂O₂, into the atmosphere during daylight hours from 600-meter towers. These towers would approach the height of the world’s tallest radio towers.

“When that hydrogen peroxide is in the presence of sunlight, it’s going to make a really powerful oxidant, the hydroxyl radical OH,” said Haskins, an assistant professor of atmospheric sciences. “That’s a natural scrubber in the atmosphere, and it’s going to help speed up the conversion of methane to CO₂.”

Methane is a single-bonded molecule combination of carbon and hydrogen, as opposed to the double-bonded compounds that are far more common in the atmosphere. Hydroxyls are more likely to oxidize those double-bonded molecules, such as the isoprene coming off trees or volatile organic compounds, so OH is just not that efficient for breaking down methane, according to Haskins.

“OH doesn’t react fast with methane,” Haskins said. “It’s reacting with so many other things.”

Methane’s outsized impact on the climate

While carbon dioxide from fossil fuels gets much of the blame for climate change, methane is also a big contributor. Eventually, methane breaks down into carbon dioxide and water.

The primary ingredient in the natural gas burned in home appliances and power plants, methane, or CH₄, packs 76 times more climate-warming punch than carbon dioxide over a 20-year timeframe. Methane persists in the atmosphere for only 12 years, but the gas is blamed for nearly a third of the rise in global temperatures since the Industrial Revolution, according to the International Energy Agency.

Anthropogenic sources, primarily oil, gas and coal operations and landfills, account for 60% of global methane emissions.

Artificially speeding up methane oxidation could slow climate change, but such geoengineering projects could carry adverse environmental impacts, which Haskins’s lab seeks to characterize. A recent report from the National Academy of Sciences concluded the unintended consequences of atmospheric methane removal technologies are likely significant but poorly understood. Haskins’ study is heeding the report’s call to scrutinize these technologies, such as the one that would release vast amounts of hydrogen peroxide.

“We could buy ourselves about 50 years and avoid some of the immediate impacts of climate change if we did this, but no one had actually previously done any side-effects studies to see what was going to happen,” Haskins said. “This is very first paper to assess any air quality side effects of such geoengineering solutions.”

Geoengineering’s potential side effects

Manipulating a system as complex as Earth’s atmosphere is an inherently dangerous action, potentially resulting in unforeseen problems.

“There’s so many feedbacks that can go on in the climate. Atmospheric chemistry is just one example. You change one thing and you think it’s going to do this, but it actually may do the opposite in one place versus the other,” Haskins said. “You have to be really careful and do these sorts of assessments. Is this a responsible thing to do? What’s the impact going to be?”

By way of example, Haskins raised the troubling history of humanmade gasses called chlorofluorocarbons, or CFCs, which ate into the protective layer of ozone that shields Earth from harmful ultraviolet radiation.

“We started using CFCs in industry as propellants and refrigerants, and suddenly we cause the ozone hole,” she said. “And we’ve been dealing with the consequences of that for 40 years. And we still won’t have a fully resolved no-ozone-hole year until probably 2060, so we have to be careful of what we’re doing.”

Mayhew and Haskins used a global chemical-transport model, called GEOS-Chem, to simulate the proposal to release hydrogen peroxide from towers. The goal was to estimate how much methane would be oxidized under three different emission scenarios, from light to extreme.

Their simulation envisioned the use of 50 towers spread around North America. Replicating the company’s proposal, the medium-release scenario called for each tower to spray 612 grams, or 1.35 pounds, per second for 10 hours a day for a year.

“This proposed solution just won’t remove any meaningful amount of methane from the atmosphere. It’s not going to solve global warming. At most, we found 50 towers could reduce 0.01% of annual anthropogenic methane emissions,” Haskins said. “You’d need about 352,000 of them to remove 50% of anthropogenic methane. It’s an insane number. And if you did 50 high-emission towers, you’d still need about 43,000.”

In the meantime, places with poor wintertime air quality could see particulate pollution get much worse.

“There’s potential that future research could show that the air quality impacts of placing these towers close to methane point sources is minimal if they’re activated at certain times of the year, and far from large population centers,” Mayhew said. “If that’s the case, then this technology (or similar approaches) could play a very small role in combating warming, but it’s clear from our work that the air-quality side effects should be placed as a central consideration for any proposed real-world implementation of technology like this.”

A study published in the Proceedings of the National Academy of Sciences outlines this novel approach. Presenting a mathematical model, SFI Complexity Postdoctoral Fellow Kaleda Denton with colleagues at Stanford University — former SFI Post-baccalaureate Fellow Elisa Heinrich Mora, SFI External Professor Marcus Feldman, and Michael Palmer — expand on previous research to offer a more realistic representation of how conformist and anti-conformist biases shape the transmission of cultural traits through a population.

“The idea behind this research was to come up with a better way to mathematically represent how individuals make decisions in the real world,” says Denton. “If we can do that, we can then scale things up to see what would happen in a population of 10,000 people over the long run.”

Traditional models of conformity often assume individuals gravitate toward the average or “mean” trait in a population. This concept works well if the most popular traits are near this mean, which may be the case for, say, working hours or food portion sizes. However, the mean is a poor indicator of popularity in other cases; for example, if most people fall on either the far left or far right of a political spectrum, but the mean lies in the center.

To address this gap, the authors designed a model that incorporates trait clustering. In this model, individuals conform by adopting traits that are more clustered together (e.g., variations of a far-left political belief) rather than the mean trait in the population (e.g., the centrist view). Anti-conformists, on the other hand, deliberately distance themselves from the traits of their peers, creating polarization.

Using computer simulations, the team analyzed how traits spread across populations over multiple generations. Conformity often led to groups clustering around specific traits, but not necessarily the average. Anti-conformity created a starkly different pattern: a U-shaped distribution, with individuals clustering at the extremes and leaving the middle sparsely populated.

One significant finding was that populations rarely converge to a single trait unless the unrealistic assumption of perfect behavioral copying is imposed. Instead, even small variations in how individuals interpret or adopt traits result in persistent diversity.

“These outcomes align with what we observe in the real world, where cultural practices and ideologies don’t simply average out but instead maintain significant variation,” Denton says.

The research also challenges the notion that conformity always leads to homogeneity. The model shows that under certain conditions, conformity can sustain diversity, while anti-conformity amplifies polarization.

Denton sees broad implications for the study. “This framework could help explain voting behavior, social media trends, or even how people estimate values in group settings,” she says. “It offers a way to understand how individual decisions aggregate into societal patterns, whether that’s consensus-building or polarization.” This model can be tested on real-world data in future studies.

“We’re excited to see if this framework works in different scenarios,” Denton said. “The ultimate goal is to understand how individual choices influence entire populations over time.

A new measurement confirms what previous — and highly debated — results had shown: The Universe is expanding faster than predicted by theoretical models, and faster than can be explained by our current understanding of physics.

This discrepancy between model and data became known as the Hubble tension. Now, results published in the Astrophysical Journal Letters provide even stronger support to the faster rate of expansion.

“The tension now turns into a crisis,” said Dan Scolnic, who led the research team.

Determining the expansion rate of the Universe — known as the Hubble constant — has been a major scientific pursuit ever since 1929, when Edwin Hubble first discovered that the Universe was expanding.

Scolnic, an associate professor of physics at Duke University, explains it as trying to build the Universe’s growth chart: we know what size it had at the Big Bang, but how did it get to the size it is now? In his analogy, the Universe’s baby picture represents the distant Universe, the primordial seeds of galaxies. The Universe’s current headshot represents the local Universe, which contains the Milky Way and its neighbors. The standard model of cosmology is the growth curve connecting the two. The problem is: things don’t connect.

“This is saying, to some respect, that our model of cosmology might be broken,” said Scolnic.

Measuring the Universe requires a cosmic ladder, which is a succession of methods used to measure the distances to celestial objects, with each method, or “rung,” relying on the previous for calibration.

The ladder used by Scolnic was created by a separate team using data from the Dark Energy Spectroscopic Instrument (DESI), which is observing more than 100,000 galaxies every night from its vantage point at the Kitt Peak National Observatory.

Scolnic recognized that this ladder could be anchored closer to Earth with a more precise distance to the Coma Cluster, one of the galaxy clusters nearest to us.

“The DESI collaboration did the really hard part, their ladder was missing the first rung,” said Scolnic. “I knew how to get it, and I knew that that would give us one of the most precise measurements of the Hubble constant we could get, so when their paper came out, I dropped absolutely everything and worked on this non-stop.”

To get a precise distance to the Coma cluster, Scolnic and his collaborators, with funding from the Templeton foundation, used the light curves from 12 Type Ia supernovae within the cluster. Just like candles lighting a dark path, Type Ia supernovae have a predictable luminosity that correlates to their distance, making them reliable objects for distance calculations.

The team arrived at a distance of about 320 million light-years, nearly in the center of the range of distances reported across 40 years of previous studies — a reassuring sign of its accuracy.

“This measurement isn’t biased by how we think the Hubble tension story will end,” said Scolnic. “This cluster is in our backyard, it has been measured long before anyone knew how important it was going to be.”

Using this high-precision measurement as a first rung, the team calibrated the rest of the cosmic distance ladder. They arrived at a value for the Hubble constant of 76.5 kilometers per second per megaparsec, which essentially means that the local Universe is expanding 76.5 kilometers per second faster every 3.26 million light-years.

This value matches existing measurements of the expansion rate of the local Universe. However, like all of those measurements, it conflicts with measurements of the Hubble constant using predictions from the distant Universe. In other words: it matches the Universe’s expansion rate as other teams have recently measured it, but not as our current understanding of physics predicts it. The longstanding question is: is the flaw in the measurements or in the models?

Scolnic’s team’s new results adds tremendous support to the emerging picture that the root of the Hubble tension lies in the models.

“Over the last decade or so, there’s been a lot of re-analysis from the community to see if my team’s original results were correct,” said Scolnic, whose research has consistently challenged the Hubble constant predicted using the standard model of physics. “Ultimately, even though we’re swapping out so many of the pieces, we all still get a very similar number. So, for me, this is as good of a confirmation as it’s ever gotten.”

“We’re at a point where we’re pressing really hard against the models we’ve been using for two and a half decades, and we’re seeing that things aren’t matching up,” said Scolnic. “This may be reshaping how we think about the Universe, and it’s exciting! There are still surprises left in cosmology, and who knows what discoveries will come next?”

This work was conducted with funding from the Templeton Foundation, the Department of Energy, the David and Lucile Packard Foundation, the Sloan Foundation, the National Science Foundation and NASA.

A team of researchers from universities, health departments, and agricultural agencies from across the Northeast conducted a meta-analysis of data on how many blacklegged ticks there are and how many of them have the potential to pass pathogens responsible for Lyme disease and three other tick-borne diseases in the Northeast from 1989 to 2021, including in Connecticut, New York, New Hampshire, Vermont, and Maine.

Data was collected in Maine starting in 1989, while most of the other states began data collection in the mid- 2000s. Massachusetts and Rhode Island were not represented in the study due to either unavailable or insufficient data.

Lyme disease was first discovered in Lyme, Conn, in 1975. Its symptoms can vary depending on the stage and severity of the disease but can include a rash, fever, chills, fatigue, muscle or joint aches, and swollen lymph nodes. If left untreated, prolonged and more severe symptoms may develop.

Lyme disease is caused by a bacteria calledBorrelia burgdorferi. Some but not all, white-footed mice, chipmunks, birds, squirrels, and other small animals carry the bacteria in their blood, making them “competent” hosts. Blacklegged ticks are not born infected with the Lyme disease bacteria. But when a blacklegged tick feeds on an infected host, the tick can get the bacteria that causes Lyme disease and then potentially spread it to humans through its bite. Other animals, like white-tailed deer, are “incompetent hosts,” so while they are a food source for blacklegged ticks, they do not transmit the Lyme disease bacteria.

Blacklegged ticks typically consume three blood meals over the course of a two-year life cycle: after they hatch into larvae in the midsummer of their first year; as nymphs during the following late spring, often in May or June; and as adults that fall, most likely between September and November.

In general, ticks must be attached to a person for at least 24 hours to transmit the Lyme disease bacteria. So even though adult blacklegged ticks are more likely to carry the bacteria than younger ticks, because they are bigger, about the size of a sesame seed, there is particular concern about the younger ticks or nymphs, which are only about the size of a poppy seed, making them difficult to spot.

“While the bacteria responsible for Lyme disease has a complicated chain of transmission, our results show the relative abundance of blacklegged ticks, and just how many of them are carrying disease-causing pathogens throughout the Northeast,” says lead author Lucas Price, who was a postdoctoral fellow in geography at Dartmouth at the time of the study and is now a wildlife biologist at the Interior Department’s Bureau of Land Management.

The researchers analyzed the abundance of blacklegged ticks and the presence of Lyme disease bacteria and other pathogens so that they could determine how blacklegged ticks and the pathogens they carry are changing in time and space.

“Contrary to the well-documented spread of blacklegged ticks and Lyme disease over the past 30 years, we found very small changes in the abundance of blacklegged ticks, but think this is likely because we usually don’t start sampling a location for blacklegged ticks until they’re already established,” says senior author Jonathan Winter, an associate professor of geography and director of the Applied Hydroclimatology Group at Dartmouth. However, we did find an increase in the percentage of blacklegged ticks that carry the Lyme disease bacteria.”

These findings underscore advice from the Centers for Disease Control and Prevention and health professionals, who recommend a range of tick bite prevention measures, including conducting full-body tick checks after spending time outdoors in regions where pathogen-carrying ticks are present. While much of the data were already available publicly prior to the study, the team made the surveys consistent across states, creating one of the most comprehensive tick abundance and pathogen prevalence datasets in the United States, and establishing a baseline that can be used in the future.

The researchers have another study underway examining the relationship between climate change and the prevalence of blacklegged ticks and Lyme disease in the Northeast.

Joseph Savage, a graduate student in the Ecology, Evolution, Environment & Society program at Dartmouth, also contributed to the study.