A newsletter for unflinching, ever-curious science-lovers. ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏ ͏
October 3, 2025—We've crossed another planetary boundary, some galaxies are moving faster than the speed of light, and mathematicians calculate the total number of humans that have ever lived. —Andrea Gawrylewski, Chief Newsletter Editor | | Dmytro Varavin/iStock/Getty Images | | In a new report, researchers examined nine geophysical limits that make up a sort of planetary life-support system; staying within these limits, they say, is the best hope for maintaining the climatic conditions humans and other organisms on Earth have adapted to. As of 2025, humans have pushed Earth past another of these planetary boundaries: Levels of ocean acidification have exceeded a critical threshold, becoming the seventh out of nine boundaries crossed. How it works: Carbon dioxide concentrations reached a record global high of 422.7 parts per million (ppm) last year. Much of that carbon dioxide gets absorbed by the ocean, increasing its acidity, which can have profound impacts on marine ecosystems. At low enough pH levels, corals and shells can begin to dissolve. These effects could destabilize entire ecosystems and devastate many commercially valuable species, such as oysters. What the experts say: "The movement we're seeing is absolutely headed in the wrong direction. The ocean is becoming more acidic, oxygen levels are dropping, and marine heatwaves are increasing. This is ramping up pressure on a system vital to stabilize conditions on planet Earth," Levke Caesar, co-lead of PIK's Planetary Boundaries Science Lab, said in the new evaluation's press statement. | | Amanda Montañez; Source: "Planetary Health Check 2025: A Scientific Assessment of the State of the Planet," Planetary Boundaries Science, Potsdam Institute for Climate Impact Research (data). | | The most distant galaxies in this deep field image from the James Webb Space Telescope appear as small, faint dots. ESA/Webb, NASA & CSA, G. Östlin, P. G. Perez-Gonzalez, J. Melinder, the JADES Collaboration, M. Zamani (ESA/Webb), CC BY 4.0 INT | | | Faster-Than-Light Galaxies
| Nothing can move quicker through space than the speed of light. The distinciton is important. The universe—space itself— is constantly expanding, but the rate of expansion grows the farther away from us you measure. A galaxy one megaparsec from us (about 3.26 million light-years) will be receding at 70 km/sec. A galaxy two megaparsecs away will be moving twice as fast, or at 140 km/sec, and so on. So at a certain point, a galaxy will be moving away from us at the speed of light. Calculations show that this distance, which is called the Hubble sphere, is about 14 billion light years away. Anything farther away would be moving faster than light from our perspective. How it works: Though these galaxies are moving quicker from us than the speed of light, they are not moving through space faster than light. They are moving with it, writes Phil Plait in his weekly column. He gives an analogy: "Imagine a boat on the ocean that can move across the water at 20 km/hour. If the boat is headed away from you, that's how fast you'll see it moving. But now imagine the boat's in a current moving at 30 km/hour away from you. You'd now measure the boat moving at 50 km/hour, even though the speed of the boat relative to the water is only 20. To be clear, this is only an analogy and shouldn't be taken too far. But it helps to picture how this works." What this means: The cosmos was born approximately 13.8 billion years ago. Hundreds of millions of years later, galaxies formed. Light from distant galaxies has taken about 12 billion years to reach us, but over that time, the universe has expanded. Technically, the light has traveled much farther than 12-billion-light-years to reach us. By the time it reaches us, the galaxy is more like 23-billion light-years away. —Andrea Tamayo, Newsletter Writer | | | | |
- See how well you read Scientific American this week by taking today's quiz! Also, try to solve Spellements and our killer version of Sudoku. This week, Bruce F. and Amir C. both found the word modally in the Spellements puzzle. It's an adverb relating to modality. Excellent word.
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| | Thanks for reading Today in Science this week. We're barreling past troubling markers of the planet's health, and the stakes can feel overwhelming. But it gives me real comfort to be part of this community of science-minded readers—curious, informed, and unwilling to look away. If change is going to come, I suspect it will begin with people like you. | | —Andrea Gawrylewski, Chief Newsletter Editor
With contributions by Andrea Tamayo | | | | |
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