Latest news with #DarkEnergySpectroscopicInstrument
Time of India
19-05-2025
- Science
- Time of India
Will universe end far earlier than expected?
For most of the past generation, astronomy textbooks treated the cosmos as practically immortal. Students learned that after the last red dwarf flickered out, after the final black hole evaporated, darkness would stretch on for a number written with more than a hundred zeros. Tired of too many ads? go ad free now New research led by a group at Radboud University in the Netherlands asks us to erase most of those zeros. Their calculations show that the universe could finish its drawn-out fade after 'only' ten to the power of seventy-eight years. That still dwarfs every human timescale, yet within cosmology it represents a surprisingly quick goodbye. The revision starts with 's famous idea that are not completely black. According to , they emit tiny amounts of energy, lose mass, and eventually disappear. The Dutch team wondered whether any extremely dense, gravitationally bound object might share that fate. They applied the same mathematics to white dwarfs, which are the hot, Earth-sized cores that remain when sun-like stars exhaust their fuel. A white dwarf appears solid and inert, but the new paper argues that quantum fluctuations at its surface allow particles to leak away. Over unimaginableperiodse the entire star would evaporate, just as slowly and inevitably as a lake dries under the desert sun. Once white dwarfs are allowed to vanish, every late-stage forecast of must be compressed. Traditional models pictured those stellar remnants cooling into lightless 'black dwarfs' that wander the void forever. Take them out of the script, and the slowest actors exit much earlier, chopping hundreds of orders of magnitude from the final curtain call. Suddenly, the last sparks of matter are gon, not long after the last black hole, and the universe slides into an empty quantum haze with shocking speed—at least by cosmic accounting. Tired of too many ads? go ad free now While theorists digested that prospect, another group studying the large-scale expansion of space introduced a second, equally dramatic possibility. Data from the Dark Energy Spectroscopic Instrument hint that dark energy, the mysterious force pushing galaxies apart, may itself be fading. If future surveys confirm the trend, the outward rush could eventually stall, reverse, and race toward a catastrophic 'Big Crunch. ' Such a collapse would end everything far sooner than either the black-hole timetable or the newly shortened evaporation clock. The evidence is still thin, but the mere suggestion stirs debate and underscores how fragile our grandest predictions remain. None of these scenarios changes life on Earth. Our Sun will still swell into a red giant in about five billion years. Long before any deep-time physics matters, continents will shift, oceans will boil, and perhaps our descendants or their machines will have moved elsewhere. Yet cosmologists care deeply because the ultimate fate of the universe tests whether quantum theory and gravity truly mesh. A single adjustment in the equations can shrink eternity, proving that seemingly untouchable numbers are only as sturdy as the assumptions beneath them. So, will the cosmos end in a graceful fade after ten to the seventy-eighth years, or will dark energy flip the sign on gravity and pull everything back in a fiery finale? No one knows yet. What the new work makes clear is that our picture of 'forever' is still a draft, and every fresh observation has the power to shorten or lengthen the longest story ever told.
Yahoo
07-05-2025
- Science
- Yahoo
Scientists Say That Something Very Weird Is Going on With the Universe
Yahoo is using AI to generate takeaways from this article. This means the info may not always match what's in the article. Reporting mistakes helps us improve the experience. Yahoo is using AI to generate takeaways from this article. This means the info may not always match what's in the article. Reporting mistakes helps us improve the experience. Yahoo is using AI to generate takeaways from this article. This means the info may not always match what's in the article. Reporting mistakes helps us improve the experience. Generate Key Takeaways Astronomers have made an intriguing discovery that could upend everything we know about the structure of the universe and its expansion. Scientists recently found that dark energy, the mysterious form driving the accelerating expansion of the universe, could be weakening over time. The findings could undermine the existing standard cosmological model of the universe called the lambda-cold dark matter (LCDM) model, which takes dark energy, ordinary matter, and cold dark matter — a hypothetical form of dark matter that moves slowly compared to the speed of light — into consideration. The symbol lambda in the model refers to Albert Einstein's cosmological constant, which assumes that the universe is accelerating at a fixed rate. Yet, last year, scientists concluded that dark energy isn't a constant after all, analyzing observations by the Dark Energy Spectroscopic Instrument (DESI) in Arizona, as New Scientist reports. They found that the mysterious force could be evolving and weakening over time. In March, scientists released a follow-up, strengthening the unusual findings. "This is exciting – it might actually be putting the standard model of cosmology in danger," Autonomous University of Madrid assistant research professor Yashar Akrami told New Scientist. Instead of making changes to the LCDM itself, Akrami and his colleagues suggested redefining dark energy as a "quintessence field," which has been used to explain observations of an accelerating rate of expansion of the universe. That could allow scientists to harmonize more advanced string theory with the standard cosmological model. "If you prove that quintessence is dark energy, it's very good for [string theorists]," Akrami told New Scientist. "That's why the string theory community is really excited now." An altered take on the quintessence model of dark energy suggests the mysterious force could be interacting with gravity itself. "We've always grown up thinking about the universe as having the gravitational force, and gravity fuels everything," University of Oxford astrophysicist Pedro Ferreira told the publication. "But now there's going to be an additional fifth force, which is due to the dark energy, which also fuels everything." But before we can add this fifth force, we'd have to reconcile the fact that we simply haven't seen any evidence for it, at least not when we're making precise measurements of our neighborhood of the universe. "Physics ends up being even more complicated than we thought it could have been, and that kind of makes you wonder, why do you want to go down that route?" Ferreira added. The researcher believes it's most likely that scientists will debate different models of dark energy and "never resolve it." Yet, there's still a chance researchers could observe gravity being influenced by dark energy in upcoming observations by the European Space Agency's Euclid satellite and DESI. More on dark energy: Scientists Say They've Built a "Black Hole Bomb"
Yahoo
02-05-2025
- Science
- Yahoo
Why ‘Evolving' Dark Energy Worries Some Physicists
In 2024 a shockwave rippled through the astronomical world, shaking it to the core. The disturbance didn't come from some astral disaster at the solar system's doorstep, however. Rather it arrived via the careful analysis of many far-distant galaxies, which revealed new details of the universe's evolution across eons of cosmic history. Against most experts' expectations, the result suggested that dark energy—the mysterious force driving the universe's accelerating expansion—was not an unwavering constant but rather a more fickle beast that was weakening over time. The shocking claim's source was the Dark Energy Spectroscopic Instrument (DESI), run by an international collaboration at Kitt Peak National Observatory in Arizona. And it was so surprising because cosmologists' best explanations for the universe's observed large-scale structure have long assumed that dark energy is a simple, steady thing. But as Joshua Frieman, a physicist at the University of Chicago, says: 'We tend to stick with the simplest theory that works—until it doesn't.' Heady with delight and confusion, theorists began scrambling to explain DESI's findings and resurfaced old, more complex ideas shelved decades ago. In March 2025 even more evidence accrued in favor of dark energy's dynamic nature in DESI's latest data release—this time from a much larger, multimillion-galaxy sample. Dark energy's implied fading, it seemed, was refusing to fade away. [Sign up for Today in Science, a free daily newsletter] Soon afterward, however, Daniel Green, a physicist at the University of California, San Diego, took to social media to argue over the DESI team's preferred interpretation of the data. 'I'm particularly skeptical of DESI's press release,' Green says. 'The tendency should be to say, 'Hey, why don't we explore all the possible interpretations?' DESI didn't do that many analyses.' The situation, Green says, is akin to looking for a lost set of car keys in a dark parking lot—but only where the light is bright: 'When all you look under is one lamppost, you only see what you find there.' Other explanations exist for DESI's measurements, Green says, and not all of them require the cosmos-quaking prospect of an evolving dark energy. His preferred model instead invokes the putative decay of another mysterious aspect of cosmology, dark matter—thought to be a substance that gravitationally binds galaxies together but otherwise scarcely interacts with the rest of the universe at all. Yet his and other alternative proposals, too, have drawbacks, and the resulting scientific debate has only just begun. The standard cosmological model at the heart of all this is known as 'LCDM.' The 'CDM' component stands for 'cold dark matter,' and the 'L' stands for the Greek letter 'lambda,' which denotes a constant dark energy. CDM is the type of dark matter that best accounts for observations of how galaxies form and grow, and—until DESI's proclamation suggested otherwise, that is—a constant dark energy has been the best fit for explaining the distributions of galaxies and other patterns glimpsed in large-scale cosmic structures. 'Once they had this constant, everything snapped into place,' Green says. 'All of the issues that had been around for 20 years that we'd been hoping were just small mistakes were really resolved by this one thing.' But dark energy's constancy has always been more of a clever inference rather than an ironclad certainty. DESI is an effort to clarify exactly what dark energy really is by closely monitoring how it has influenced the universe's growth. Since 2021 the project has been meticulously measuring the motions and distributions of galaxies across some 11 billion years of cosmic time. DESI's data on galactic motions come from measurements of redshift, the stretching out of galaxies' emitted light to the red end of the spectrum by the universe's expansion. And its tracing of spatial distributions emerges from spying enormous bubblelike arrangements of galaxies thought to have formed from more primordial templates, called baryon acoustic oscillations (BAOs). BAOs are essentially ripples from giant sound waves that coursed through the hot plasma that filled the early universe, which astronomers can glimpse in the earliest light they can see, the big bang's all-sky afterglow known as the cosmic microwave background (CMB). The waves' matter-dense crests sowed the seeds of future galaxies and galaxy clusters, while galaxy-sparse voids emerged from the matter-poor troughs. Combined with CMB data as well as distance-pegging observations of supernovae, DESI's measurements offer a reckoning of the universe's historic growth rate—and thus the action of dark energy. DESI co-spokesperson Nathalie Palanque-Delabrouille, a physicist at Lawrence Berkeley National Laboratory, recalls the private December 2023 meeting where she and the rest of the DESI team first learned of the project's early results. Up until then, the researchers had worked on blinded data, meaning the true values were slightly but systematically altered so as to ensure that no one could deliberately or inadvertently bias the ongoing analysis to reach some artificially preordained result. These blinded data showed a huge divergence from LCDM. But when the real data were unveiled, 'we saw all the points came very close to LCDM, and that was initially a huge relief,' she recalls. That alignment suggested 'we did things right.' Those feelings quickly changed when the group noticed a small, persistent deviation in DESI's estimate for the value of lambda. Still, there was a considerable chance that the results were a statistical fluke. But in DESI's latest results, which were posted to the preprint server last March and incorporated much larger and richer data sets, the statistical robustness of the unexpected lambda value soared, and most talk of flukes dwindled. Theorists could scarcely contain their excitement—or their profound puzzlement. The results rekindled preexisting ideas about dynamic dark energy first formulated decades ago, not long after dark energy's discovery itself in 1998. One popular theory posits a fifth fundamental force in addition to the known four (electromagnetism, gravity, and the strong and weak nuclear forces), emerging from some as-yet-undiscovered dark matter particle that can influence dark energy. Frieman says the data from DESI is so precise that if this particle is the correct explanation, physicists already know its crucial parameters. Constrained by the DESI data, Frieman says, the best-fitting model that would support this 'fifth force' hypothesis 'tells us that this [hypothetical] particle has a mass of about 10–33 electron volts.' To put that into perspective, this means such a particle would be 38 orders of magnitude lighter than an electron—which, Frieman notes, is 'by far the lightest stable particle we know of that doesn't have zero mass.' But while some theorists used DESI's data to revive and sharpen intriguing theories of yesteryear, Green and others issued a warning. The problem: an evolving dark energy would seem to defy well-founded physical principles in other cosmic domains. The first major point of controversy involves something called the null energy condition, under which—among other things—energy can't propagate faster than light. If circumstances were otherwise, then perilous paradoxes could emerge: time machines could violate causality, matter could repel rather than attract, and even spacetime itself could be destabilized. Theorists have mathematically proven the condition's apparent necessity in numerous circumscribed scenarios within quantum and relativistic domains—but not for the universe at large. Appealing to this sort of theoretical incompleteness, however, 'is like a lawyer saying there's a loophole,' Green says. 'Most physicists would say that's totally crazy.' A discovery that something in the universe violates the null energy condition would be groundbreaking, to say the least: a more impolitic term would be 'nonsensical.' This astounding violation is exactly what Green and others say most of DESI's analyses are showing, however. On this point, several theorists push back. The controversy goes all the way down to the foundations of modern cosmology, centering on a parameter unceremoniously known as w(z). In 1917 Albert Einstein first introduced lambda as a way to ensure that a static universe would pop out of his equations. But after work led by Edwin Hubble proved the universe was expanding, Einstein abandoned his fudge factor (even calling it his 'greatest blunder'). It wasn't until the late 1990s, when astronomers found that the universe's expansion wasn't constant but in fact accelerating, that lambda once again returned to theoretical prominence. This time theorists interpreted it to represent the magnitude of the universe's dark energy density, a constant that doesn't change with time. But if there's one thing modern cosmology has shown, it's that little, if anything, about the universe is ever so neat and tidy. So, despite a lack of evidence, theorists of the time reimagined LCDM as w(z)CDM, where w(z) is a time-varying term representing the ratio of dark energy's pressure to its energy density. When w(z) has a value of exactly –1, w(z)CDM is equivalent to LCDM. For w(z) greater than –1, the universe's dark energy dilutes over time, consistent with DESI's findings. On the other hand, w(z) less than –1 leads to devastating consequences: dark energy's pressure overpowers its density, ultimately causing everything from galaxies all the way down to atoms to be ripped apart—a 'big rip' that violates the null energy condition and would seemingly doom the universe to a violent death. The DESI group collaboration's March preprint includes a graph that shows w(z) with values below –1 for later epochs in the universe's history, seemingly validating the criticisms of Green and others. But all is not as it seems. Such criticisms 'draw the wrong conclusions,' says Paul Steinhardt, a cosmologist at Princeton University. That's because in a second graph in the DESI paper, w(z) never crosses the critical –1 line. The difference: despite DESI's curved data, the first chart uses a simple line fit for w(z). Steinhardt and Frieman both say that because of the poor fit, the linear w(z) isn't physically meaningful. Researchers merely find it convenient for comparing different dark energy models and experiments. The second graph shows a curved fit for w(z) that more closely matches the data. It rolls down to, but never crosses, the critical –1 value, consistent with a weakening dark energy that would avoid the universe ending in a big rip. But Gabriel Lynch, a Ph.D. student at the University of California, Davis, who has an alternative explanation for the DESI data, says that even if any of DESI's w(z) estimates are physical, coaxing out a theory to support them leads to incredibly fraught circumstances. 'This is saying something weird,' Lynch says. 'It's not impossible, but maybe it would be good to look into some alternatives.' Whether or not DESI's results would violate the null energy condition, everyone agrees on another problem. Models that accommodate a changing dark energy inevitably conclude that a class of tiny fundamental particles known as neutrinos have a negative mass. Yet multiple generations of empirical experimentation have indisputably shown that neutrinos do have mass. Frieman suggests that something else, perhaps an unknown particle, might be mimicking a negative-mass neutrino. But a new approach by Lynch and his thesis advisor Lloyd Knox, detailed in a preprint that was posted to in March, sidesteps this 'negative neutrino' problem altogether. If some of the mass in the universe somehow disappeared over time, its influence on DESI's data would be the same as a weakening dark energy—without necessitating a negative mass for neutrinos. Although physicists have good reasons to believe that certain seemingly stable subatomic particles could contribute to this notional effect by decaying over time, this process is thought to be far too slow to account for DESI's observations. For instance, experiments have shown the proton to be so stable that its half-life must be at least a hundred trillion trillion times the age of the universe. But no one knows what the half-life of putative particles of dark matter would be. So, Lynch asks, what if dark matter has a half-life of roughly a billion years? Fast forward about 14 billion years to today, and some would have decayed into dark radiation, erasing the heavy matter signal. If the idea holds true, DESI's data might be a way to find the exact value for neutrino masses as well as for dark matter particles, which would be a big deal. 'That is a breakdown of LCDM that we totally expected,' Green says. 'And we were just waiting to detect it.' Owing to dynamic dark energy's paradoxes, 'you really need to explore every alternative explanation [for the results], because evolving dark energy is the absolute last one that I would be willing to believe,' Green says. Despite such strong words, all parties caution that this debate is still in its early days. 'This is only the first round of the fight,' Steinhardt says, and no model currently explains all of DESI's results. More data are needed, especially from even bigger and better cosmic surveys by planned next-generation telescopes. And, naturally, more analyses are needed, too, before the community can reach any consensus. Whether a resolution comes from dynamic dark energy, dark matter decay or something entirely different, the LCDM model has seemingly been stretched to its breaking point. Every reasonable explanation for DESI's data involves new, scarcely explored physics. 'They are all exotic models. We're beyond LCDM both ways,' Palanque-Delabrouille says. 'We just want to know the truth.'
Scientific American
01-05-2025
- Science
- Scientific American
Latest Dark Energy Study Suggests the Universe Is Even Weirder Than We Imagined
In 2024 a shockwave rippled through the astronomical world, shaking it to the core. The disturbance didn't come from some astral disaster at the solar system's doorstep, however. Rather it arrived via the careful analysis of many far-distant galaxies, which revealed new details of the universe's evolution across eons of cosmic history. Against most experts' expectations, the result suggested that dark energy —the mysterious force driving the universe's accelerating expansion—was not an unwavering constant but rather a more fickle beast that was weakening over time. The shocking claim's source was the Dark Energy Spectroscopic Instrument (DESI), run by an international collaboration at Kitt Peak National Observatory in Arizona. And it was so surprising because cosmologists' best explanations for the universe's observed large-scale structure have long assumed that dark energy is a simple, steady thing. But as Joshua Frieman, a physicist at the University of Chicago, says: 'We tend to stick with the simplest theory that works—until it doesn't.' Heady with delight and confusion, theorists began scrambling to explain DESI's findings and resurfaced old, more complex ideas shelved decades ago. In March 2025 even more evidence accrued in favor of dark energy's dynamic nature in DESI's latest data release—this time from a much larger, multimillion-galaxy sample. Dark energy's implied fading, it seemed, was refusing to fade away. On supporting science journalism If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. Soon afterward, however, Daniel Green, a physicist at the University of California, San Diego, took to social media to argue over the DESI team's preferred interpretation of the data. 'I'm particularly skeptical of DESI's press release,' Green says. 'The tendency should be to say, 'Hey, why don't we explore all the possible interpretations?' DESI didn't do that many analyses.' The situation, Green says, is akin to looking for a lost set of car keys in a dark parking lot—but only where the light is bright: 'When all you look under is one lamppost, you only see what you find there.' Other explanations exist for DESI's measurements, Green says, and not all of them require the cosmos-quaking prospect of an evolving dark energy. His preferred model instead invokes the putative decay of another mysterious aspect of cosmology, dark matter—thought to be a substance that gravitationally binds galaxies together but otherwise scarcely interacts with the rest of the universe at all. Yet his and other alternative proposals, too, have drawbacks, and the resulting scientific debate has only just begun. Constant Cosmology The standard cosmological model at the heart of all this is known as 'LCDM.' The 'CDM' component stands for 'cold dark matter,' and the 'L' stands for the Greek letter 'lambda,' which denotes a constant dark energy. CDM is the type of dark matter that best accounts for observations of how galaxies form and grow, and—until DESI's proclamation suggested otherwise, that is—a constant dark energy has been the best fit for explaining the distributions of galaxies and other patterns glimpsed in large-scale cosmic structures. 'Once they had this constant, everything snapped into place,' Green says. 'All of the issues that had been around for 20 years that we'd been hoping were just small mistakes were really resolved by this one thing.' But dark energy's constancy has always been more of a clever inference rather than an ironclad certainty. DESI is an effort to clarify exactly what dark energy really is by closely monitoring how it has influenced the universe's growth. Since 2021 the project has been meticulously measuring the motions and distributions of galaxies across some 11 billion years of cosmic time. DESI's data on galactic motions come from measurements of redshift, the stretching out of galaxies' emitted light to the red end of the spectrum by the universe's expansion. And its tracing of spatial distributions emerges from spying enormous bubblelike arrangements of galaxies thought to have formed from more primordial templates, called baryon acoustic oscillations (BAOs). BAOs are essentially ripples from giant sound waves that coursed through the hot plasma that filled the early universe, which astronomers can glimpse in the earliest light they can see, the big bang's all-sky afterglow known as the cosmic microwave background (CMB). The waves' matter-dense crests sowed the seeds of future galaxies and galaxy clusters, while galaxy-sparse voids emerged from the matter-poor troughs. Combined with CMB data as well as distance-pegging observations of supernovae, DESI's measurements offer a reckoning of the universe's historic growth rate—and thus the action of dark energy. DESI co-spokesperson Nathalie Palanque-Delabrouille, a physicist at Lawrence Berkeley National Laboratory, recalls the private December 2023 meeting where she and the rest of the DESI team first learned of the project's early results. Up until then, the researchers had worked on blinded data, meaning the true values were slightly but systematically altered so as to ensure that no one could deliberately or inadvertently bias the ongoing analysis to reach some artificially preordained result. These blinded data showed a huge divergence from LCDM. But when the real data were unveiled, 'we saw all the points came very close to LCDM, and that was initially a huge relief,' she recalls. That alignment suggested 'we did things right.' Those feelings quickly changed when the group noticed a small, persistent deviation in DESI's estimate for the value of lambda. Still, there was a considerable chance that the results were a statistical fluke. But in DESI's latest results, which were posted to the preprint server last March and incorporated much larger and richer data sets, the statistical robustness of the unexpected lambda value soared, and most talk of flukes dwindled. Theorists could scarcely contain their excitement—or their profound puzzlement. The results rekindled preexisting ideas about dynamic dark energy first formulated decades ago, not long after dark energy's discovery itself in 1998. One popular theory posits a fifth fundamental force in addition to the known four (electromagnetism, gravity, and the strong and weak nuclear forces), emerging from some as-yet-undiscovered dark matter particle that can influence dark energy. Frieman says the data from DESI is so precise that if this particle is the correct explanation, physicists already know its crucial parameters. Constrained by the DESI data, Frieman says, the best-fitting model that would support this 'fifth force' hypothesis 'tells us that this [hypothetical] particle has a mass of about 10 –33 electron volts.' To put that into perspective, this means such a particle would be 38 orders of magnitude lighter than an electron—which, Frieman notes, is 'by far the lightest stable particle we know of that doesn't have zero mass.' But while some theorists used DESI's data to revive and sharpen intriguing theories of yesteryear, Green and others issued a warning. The problem: an evolving dark energy would seem to defy well-founded physical principles in other cosmic domains. Null and Void The first major point of controversy involves something called the null energy condition, under which—among other things—energy can't propagate faster than light. If circumstances were otherwise, then perilous paradoxes could emerge: time machines could violate causality, matter could repel rather than attract, and even spacetime itself could be destabilized. Theorists have mathematically proven the condition's apparent necessity in numerous circumscribed scenarios within quantum and relativistic domains—but not for the universe at large. Appealing to this sort of theoretical incompleteness, however, 'is like a lawyer saying there's a loophole,' Green says. 'Most physicists would say that's totally crazy.' A discovery that something in the universe violates the null energy condition would be groundbreaking, to say the least: a more impolitic term would be 'nonsensical.' This astounding violation is exactly what Green and others say most of DESI's analyses are showing, however. On this point, several theorists push back. The controversy goes all the way down to the foundations of modern cosmology, centering on a parameter unceremoniously known as w (z). In 1917 Albert Einstein first introduced lambda as a way to ensure that a static universe would pop out of his equations. But after work led by Edwin Hubble proved the universe was expanding, Einstein abandoned his fudge factor (even calling it his 'greatest blunder'). It wasn't until the late 1990s, when astronomers found that the universe's expansion wasn't constant but in fact accelerating, that lambda once again returned to theoretical prominence. This time theorists interpreted it to represent the magnitude of the universe's dark energy density, a constant that doesn't change with time. But if there's one thing modern cosmology has shown, it's that little, if anything, about the universe is ever so neat and tidy. So, despite a lack of evidence, theorists of the time reimagined LCDM as w (z)CDM, where w (z) is a time-varying term representing the ratio of dark energy's pressure to its energy density. When w (z) has a value of exactly –1, w (z)CDM is equivalent to LCDM. For w (z) greater than –1, the universe's dark energy dilutes over time, consistent with DESI's findings. On the other hand, w (z) less than –1 leads to devastating consequences: dark energy's pressure overpowers its density, ultimately causing everything from galaxies all the way down to atoms to be ripped apart—a 'big rip' that violates the null energy condition and would seemingly doom the universe to a violent death. The DESI group collaboration's March preprint includes a graph that shows w (z) with values below –1 for later epochs in the universe's history, seemingly validating the criticisms of Green and others. But all is not as it seems. Such criticisms 'draw the wrong conclusions,' says Paul Steinhardt, a cosmologist at Princeton University. That's because in a second graph in the DESI paper, w (z) never crosses the critical –1 line. The difference: despite DESI's curved data, the first chart uses a simple line fit for w (z). Steinhardt and Frieman both say that because of the poor fit, the linear w (z) isn't physically meaningful. Researchers merely find it convenient for comparing different dark energy models and experiments. The second graph shows a curved fit for w (z) that more closely matches the data. It rolls down to, but never crosses, the critical –1 value, consistent with a weakening dark energy that would avoid the universe ending in a big rip. But Gabriel Lynch, a Ph.D. student at the University of California, Davis, who has an alternative explanation for the DESI data, says that even if any of DESI's w (z) estimates are physical, coaxing out a theory to support them leads to incredibly fraught circumstances. 'This is saying something weird,' Lynch says. 'It's not impossible, but maybe it would be good to look into some alternatives.' Negative Neutrinos? Whether or not DESI's results would violate the null energy condition, everyone agrees on another problem. Models that accommodate a changing dark energy inevitably conclude that a class of tiny fundamental particles known as neutrinos have a negative mass. Yet multiple generations of empirical experimentation have indisputably shown that neutrinos do have mass. Frieman suggests that something else, perhaps an unknown particle, might be mimicking a negative-mass neutrino. But a new approach by Lynch and his thesis advisor Lloyd Knox, detailed in a preprint that was posted to in March, sidesteps this 'negative neutrino' problem altogether. If some of the mass in the universe somehow disappeared over time, its influence on DESI's data would be the same as a weakening dark energy—without necessitating a negative mass for neutrinos. Although physicists have good reasons to believe that certain seemingly stable subatomic particles could contribute to this notional effect by decaying over time, this process is thought to be far too slow to account for DESI's observations. For instance, experiments have shown the proton to be so stable that its half-life must be at least a hundred trillion trillion times the age of the universe. But no one knows what the half-life of putative particles of dark matter would be. So, Lynch asks, what if dark matter has a half-life of roughly a billion years? Fast forward about 14 billion years to today, and some would have decayed into dark radiation, erasing the heavy matter signal. If the idea holds true, DESI's data might be a way to find the exact value for neutrino masses as well as for dark matter particles, which would be a big deal. 'That is a breakdown of LCDM that we totally expected,' Green says. 'And we were just waiting to detect it.' The Truth Is Out There Owing to dynamic dark energy's paradoxes, 'you really need to explore every alternative explanation [for the results], because evolving dark energy is the absolute last one that I would be willing to believe,' Green says. Despite such strong words, all parties caution that this debate is still in its early days. 'This is only the first round of the fight,' Steinhardt says, and no model currently explains all of DESI's results. More data are needed, especially from even bigger and better cosmic surveys by planned next-generation telescopes. And, naturally, more analyses are needed, too, before the community can reach any consensus. Whether a resolution comes from dynamic dark energy, dark matter decay or something entirely different, the LCDM model has seemingly been stretched to its breaking point. Every reasonable explanation for DESI's data involves new, scarcely explored physics. 'They are all exotic models. We're beyond LCDM both ways,' Palanque-Delabrouille says. 'We just want to know the truth.'
New European
28-04-2025
- Science
- New European
Critical Mass: Unearthing the secrets of dark energy
'The physical mechanisms responsible for the accelerating expansion of the universe is [sic] one of the most important unsolved problems in physics and, arguably, all of science,' according to a team of researchers called the Dark Energy Spectroscopic Instrument (DESI) Collaboration, who have just announced a striking new discovery. Dark energy isn't what's currently emanating from the White House, but rather, the name astronomers and cosmologists give to whatever it is that is causing our expanding universe to accelerate: to expand ever faster. This cosmic acceleration was first seen in 1998 from observations of extremely distant supernovae, but there is no agreed explanation for it. Specifically: the amount of dark energy doesn't stay constant over time. To give you some orientation, if anyone were to make a claim like that for the ordinary matter or energy in the universe, the impact on physics would be seismic. It's one of the most fundamental tenets of physics that the amount of regular matter/energy (Einstein's E=mc2 shows that the two are related) in the universe is constant. But for dark energy we have no expectations because we don't know what it is. The discovery that dark energy seems to be dynamic – changeable – should help to narrow down the possibilities. This is no small matter for cosmologists. It is embarrassing enough that the amount of ordinary matter in the universe is apparently outweighed by about five times as much of the stuff called dark matter, which seems required to explain why galaxies don't fly apart and for other astrophysical reasons, but which remains totally unexplained because dark matter seems only to affect light and ordinary matter via its gravity. Efforts to detect dark-matter particles have produced nothing at all for decades. But then dark energy comes along, of which there is three times as much in the universe as the energy embodied by all the ordinary and dark matter put together. In other words, we don't actually know what about three-quarters of the universe is made of. Dark energy manifests itself as a kind of repulsive force that opposes gravity. The expansion of the entire cosmos is just what Einstein's theory of gravity – general relativity – predicts, and it means that all galaxies are receding from each other in all directions. (The usual analogy asks us to imagine them drawn on the surface of a balloon that is being blown up.) The gravitational attraction between galaxies pulls the other way, but not enough to curb the expansion. But if, as the 1998 discovery showed, the expansion is getting steadily faster, this implies the existence of some repulsive force that is speeding up the recession. One way to think about dark energy is in terms of the so-called cosmological constant. This was an idea Einstein introduced when he realised that his equations of general relativity predicted the universe was expanding – because at that point we didn't know that it was, and so Einstein figured his theory needed some added factor to prevent it. Once Edwin Hubble revealed the cosmic expansion in 1929, the cosmological constant seemed otiose. But adding it back into the equations can capture the accelerating influence of dark energy. But that doesn't help to explain where it comes from. One option is supplied by quantum physics, which predicts that empty space should be bubbling with 'virtual' particles that pop briefly in and out of existence, creating a kind of pervasive background of energy. The trouble is that this should produce way more energy than we need to explain dark energy, creating the problem of having to explain how most of that extra energy gets cancelled out but a tiny bit remains. There's no room to talk about the other ideas proposed to explain dark energy. But all are speculation without data. Now the DESI team has collected three years' worth of observations on distant galaxies and other astrophysical objects – the largest ever 3D map of the universe – to follow the cosmic expansion over time. (Because of the finite speed of light, the further we look out into the universe, the further back in time we're seeing.) Their first results were announced last year, but now they can conclude with more confidence that, when the findings are combined with other observations, they imply that dark energy has been growing weaker over time. If so, it's less likely that ultimately dark energy will rip all of spacetime apart. There's no good theoretical explanation for dynamical dark energy, and it conflicts with what is considered the best cosmological model so far, called the lambda cold-dark-matter model (which assumes a cosmological constant). For theoreticians, that's actually great news: there's concrete data to sink their teeth into.
