Could meteorite discovery weaken dark energy's case?

日期:2019-02-26 02:04:09 作者:公冶徙 阅读:

By Stephen Battersby (Image: Science/AAAS) Whiskers of carbon found in ancient meteorites could hold clues to the earliest days of the solar system. More controversially, they might cast a shadow over the concept of dark energy, the unknown force that seems to be accelerating the expansion of the universe. Graphite whiskers are rolled-up sheets of carbon atoms. The needle-like structures have been created under high-temperature conditions in the lab but have never previously been detected in space. Still, astronomers had postulated that they form in the heat of supernova explosions and around young stars. Now, Marc Fries and Andrew Steele of the Carnegie Institution of Washington in the US, have found graphite whiskers in three carbonaceous chondrite meteorites, which contain some of the oldest material in the solar system. They studied dark patches in the meteorites using a technique called Raman spectroscopy, which shines a laser on a material to make it emit infrared light. The distinctive infrared spectral fingerprint produced is a signature of the molecular structure of a graphite whisker, which resembles a tiny, rolled-up poster, says Fries. In the meteorites, the whiskers occur in and around mineral fragments called calcium-aluminium-rich inclusions. CAIs, as they are called, are thought to have been among the first solid objects in the solar system, condensing about 4.5 billion years ago, so the whiskers were probably forged around the same time. Scientists still don’t know exactly what was going on in the solar system at the time, or what produced the high temperatures necessary to create these materials. “It’s a portion of the history of our solar system we don’t have a really good handle on,” Fries told New Scientist. That is where whiskers could help. Because they have such a distinctive spectrum, astronomers might be able to detect them around young stars in our galaxy, which would then give us a picture of what our solar system looked like at the time. That could give scientists some clues about how the first rocky fragments formed around the Sun, and how they eventually grew into planets. Fries and Steele also suggest that these whiskers might have been pumped out into deep space by the solar wind, and that the combined whisker output of many young stars might have filled interstellar space with whiskers. If so, they might have some relevance to dark energy. The unexpected dimness of distant supernova explosions at infrared wavelengths was what first led astronomers to the conclusion that the expansion of the universe is accelerating, and the proposal that some form of “dark energy” is to blame. Some astronomers, however, suggested that the size and special geometry of graphite whiskers might be the cause of this dimness, absorbing light from distant supernovae at the key infrared wavelengths (between 3 and 9 microns). Now that Fries and Steele have shown that these whiskers are indeed created in space, could they pose problems for the dark energy hypothesis? Not according to Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, US. “This is an immense extrapolation,” says Riess, one of the co-discoverers of the accelerated expansion. “Seeing a few whiskers in a meteor means they fill interstellar space blocking 25% of all the light we see? That’s quite a stretch.” He adds that the dimming goes away as astronomers look back to the most ancient supernovae. That fits the dark energy picture because in those early days it had much less effect on the universe. If dimming is caused by whiskers, it’s harder to explain. “Where did the whiskers go?” says Riess. Finally, he points out that there are now several independent lines of evidence that point to dark energy, all agreeing with the supernova data. “There are other indicators of dark energy, no doubt about that,” admits Steele. “But the supernova observations were the first. Now we’ve seen whiskers, there’s no harm in looking to see if they have an effect [on dark energy].” Journal reference: Sciencexpress (DOI: 10.1126/science.1153578) Cosmology – Keep up with the latest ideas in our special report. Comets and Asteroids – Learn more about the threat to human civilisation in our special report. More on these topics: