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自然科學：普及篇 – 開欄文
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2023/12/23 15:40 瀏覽4,273 ｜回應26｜推薦3 |
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我是物理系畢業生，有了自然科學的基本常識；容易讀懂科學新知的報導，同時也有興趣接觸它們。
過去《中華雜誌》雖是政論性和人文學術性刊物，但有時會介紹一些自然科學的研究結果；每年也都會刊登有關諾貝爾獎得主的消息。我唸大學時就替《中華》翻譯過一篇報導天文學脈動星的文章。
同窗好友王家堂兄在1980前後，介紹我進入高能物理的「普及科學」世界；此後常常讀一些這方面的書籍。因此，我一直保持著對物理學的興趣，之後自然而然的進入宇宙學領域。
以上三點是這個部落格過去經常轉載自然科學方面報導/論文的背景。
本文於 2024/01/23 02:13 修改第 3 次
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又一個可能將生命賦予分子的情境 -- Mindy Weisberger
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2025/03/31 18:27 推薦1 |
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Scientists redid an experiment that showed how life on Earth could have started. They found a new possibility
Mindy Weisberger, CNN, 03/28/25
“Microlightning” exchanges among water droplets could have sparked the building blocks of life on ancient Earth, new research finds. Here, a wave breaks on White Sand Beach on the Thai island of Koh Chang. - Frank Bienewald/LightRocket/ Getty Images 請至原網頁觀看照片
“It’s alive! IT’S ALIVE!”
In the 1931 movie “Frankenstein,” Dr. Henry Frankenstein howling his triumph was an electrifying moment in more ways than one. As massive bolts of lightning and energy crackled, Frankenstein’s monster stirred on a laboratory table, its corpse brought to life by the power of electricity.
Electrical energy may also have sparked the beginnings of life on Earth billions of years ago, though with a bit less scenery-chewing than that classic film scene.
Earth is around 4.5 billion years old, and the oldest direct fossil evidence of ancient life — stromatolites, or microscopic organisms preserved in layers known as microbial mats — is about 3.5 billion years old. However, some scientists suspect life originated even earlier, emerging from accumulated organic molecules in primitive bodies of water, a mixture sometimes referred to as primordial soup.
But where did that organic material come from in the first place? Researchers decades ago proposed that lightning caused chemical reactions in ancient Earth’s oceans and spontaneously produced the organic molecules.
Now, new research published March 14 in the journal Science Advances suggests that fizzes of barely visible “microlightning,” generated between charged droplets of water mist, could have been potent enough to cook up amino acids from inorganic material. Amino acids — organic molecules that combine to form proteins — are life’s most basic building blocks and would have been the first step toward the evolution of life.
“It’s recognized that an energetic catalyst was almost certainly required to facilitate some of the reactions on early Earth that led to the origin of life,” said astrobiologist and geobiologist Dr. Amy J. Williams, an associate professor in the department of geosciences at the University of Florida. For animo acids to form, they need nitrogen atoms that can bond with carbon. Freeing up atoms from nitrogen gas requires severing powerful molecular bonds and takes an enormous amount of energy, according to Williams, who was not involved in the research.
“Lightning, or in this case, microlightning, has the energy to break molecular bonds and therefore facilitate the generation of new molecules that are critical to the origin of life on Earth,” Williams told CNN in an email.
Mist and microlightning
To recreate a scenario that may have produced Earth’s first organic molecules, researchers built upon experiments from 1953 when American chemists Stanley Miller and Harold Urey concocted a gas mixture mimicking the atmosphere of ancient Earth. Miller and Urey combined ammonia (NH3), methane (CH4), hydrogen (H2) and water, enclosed their “atmosphere” inside a glass sphere and jolted it with electricity, producing simple amino acids containing carbon and nitrogen. The Miller-Urey experiment, as it is now known, supported the scientific theory of abiogenesis: that life could emerge from nonliving molecules.
For the new study, scientists revisited the 1953 experiments but directed their attention toward electrical activity on a smaller scale, said senior study author Dr. Richard Zare, the Marguerite Blake Wilbur Professor of Natural Science and professor of chemistry at Stanford University in California. Zare and his colleagues looked at electricity exchange between charged water droplets measuring between 1 micron and 20 microns in diameter. (The width of a human hair is 100 microns.)
“The big droplets are positively charged. The little droplets are negatively charged,” Zare told CNN. “When droplets that have opposite charges are close together, electrons can jump from the negatively charged droplet to the positively charged droplet.”
American chemist Stanley Miller, using original laboratory equipment, recreates the Miller-Urey experiment, which supported the scientific theory that life could emerge from nonliving molecules. - Roger Ressmeyer/Corbis/Getty Images 請至原網頁觀看照片
The researchers mixed ammonia, carbon dioxide, methane and nitrogen in a glass bulb, then sprayed the gases with water mist, using a high-speed camera to capture faint flashes of microlightning in the vapor. When they examined the bulb’s contents, they found organic molecules with carbon-nitrogen bonds. These included the amino acid glycine and uracil, a nucleotide base in RNA.
“We discovered no new chemistry; we have actually reproduced all the chemistry that Miller and Urey did in 1953,” Zare said. Nor did the team discover new physics, he added — the experiments were based on known principles of electrostatics.
“What we have done, for the first time, is we have seen that little droplets, when they’re formed from water, actually emit light and get this spark,” Zare said. “That’s new. And that spark causes all types of chemical transformations.”
Water and life
Lightning was likely too infrequent to produce amino acids in quantities sufficient for life, researchers say. - Mariana Suarez/AFP/Getty Images 請至原網頁觀看照
Lightning is a dramatic display of electrical power, but it is also sporadic and unpredictable. Even on a volatile Earth billions of years ago, lightning may have been too infrequent to produce amino acids in quantities sufficient for life — a fact that has cast doubt on such theories in the past, Zare said.
Water spray, however, would have been more common than lightning. A more likely scenario is that mist-generated microlightning constantly zapped amino acids into existence from pools and puddles, where the molecules could accumulate and form more complex molecules, eventually leading to the evolution of life.
“Microdischarges between obviously charged water microdroplets make all the organic molecules observed previously in the Miller-Urey experiment,” Zare said. “We propose that this is a new mechanism for the prebiotic synthesis of molecules that constitute the building blocks of life.”
However, even with the new findings about microlightning, questions remain about life’s origins, he added. While some scientists support the notion of electrically charged beginnings for life’s earliest building blocks, an alternative abiogenesis hypothesis proposes that Earth’s first amino acids were cooked up around hydrothermal vents on the seafloor, produced by a combination of seawater, hydrogen-rich fluids and extreme pressure.
Yet another hypothesis suggests that organic molecules didn’t originate on Earth at all. Rather, they formed in space and were carried here by comets or fragments of asteroids, a process known as panspermia.
“We still don’t know the answer to this question,” Zare said. “But I think we’re closer to understanding something more about what could have happened.”
Though the details of life’s origins on Earth may never be fully explained, “this study provides another avenue for the formation of molecules crucial to the origin of life,” Williams said. “Water is a ubiquitous aspect of our world, giving rise to the moniker ‘Blue Marble’ to describe the Earth from space. Perhaps the falling of water, the most crucial element that sustains us, also played a greater role in the origin of life on Earth than we previously recognized.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
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本文於 2025/03/31 18:32 修改第 2 次
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我們完全搞錯了「暗能量」？-Ben Turner
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2025/03/24 16:30 推薦1 |
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'The universe has thrown us a curveball': Largest-ever map of space reveals we might have gotten dark energy totally wrong
Ben Turner, 03/20/25
Findings from the Dark Energy Spectroscopic Instrument (DESI) suggest that dark energy could be evolving over time. If they're right, cosmology will need a new model.
Astronomers studying the largest-ever map of the cosmos have found hints that our best understanding of the universe is due a major rewrite.
The analysis, which looked at nearly 15 million galaxies and quasars spanning 11 billion years of cosmic time, found that dark energy — the presumed-to-be constant force driving the accelerating expansion of our universe — could be weakening.
Or at least this is what the data, collected by the Dark Energy Spectroscopic Instrument (DESI), suggest when combined with information taken from star explosions, the cosmic microwave background and weak gravitational lensing.
PLAY SOUND 請至原網頁觀看視頻
If the findings hold up, it means that one of the most mysterious forces controlling the fate of our universe is even weirder than first thought — and that something is very wrong with our current model of the cosmos. The researchers' findings were published in multiple papers on the preprint server arXiv and presented March 19 at the American Physical Society's Global Physics Summit in Anaheim, California, so they have not yet been peer-reviewed.
"It's true that the DESI results alone are consistent with the simplest explanation for dark energy, which would be an unchanging cosmological constant," co-author David Schlegel, a DESI project scientist at the Lawrence Berkeley National Laboratory in California, told Live Science. "But we can't ignore other data that extend to both the earlier and later universe. Combining [DESI's results] with those other data is when it gets truly weird, and it appears that this dark energy must be 'dynamic,' meaning that it changes with time."
The evolving cosmos
Dark energy and dark matter are two of the universe's most puzzling components. Together they make up roughly 95% of the cosmos, but because they do not interact with light, they can't be detected directly.
Yet these components are key ingredients in the reigning Lambda cold dark matter (Lambda-CDM) model of cosmology, which maps the growth of the cosmos and predicts its end. In this model, dark matter is responsible for holding galaxies together and accounts for their otherwise inexplicably powerful gravitational pulls, while dark energy explains why the universe's expansion is accelerating.
But despite countless observations of these hypothetical dark entities shaping our universe, scientists are still unsure where they came from, or what they even are. Currently, the best theoretical explanation for dark energy is made by quantum field theory, which describes the vacuum of space as filled with a sea of quantum fields that fluctuate, creating an intrinsic energy density in empty space.
In the aftermath of the Big Bang, this energy increases as space expands, creating more vacuum and more energy to push the universe apart faster. This suggestion helped scientists to tie dark energy to the cosmological constant — a hypothetical inflationary energy, growing with the fabric of space-time throughout the universe's life. Einstein named it Lambda in his theory of general relativity.
"The problem with that theory is that the numbers don't add up," said Catherine Heymans, a professor of astrophysics at the University of Edinburgh and the Astronomer Royal for Scotland who was not involved in the study. "If you say: 'Well, what sort of energy would I expect from this sort of vacuum?' It's very, very, very, very different from what we measure," she told Live Science.
"It's kind of exciting that the universe has thrown us a curveball here," she added.
An artist's illustration of the universe's evolution to the present day, with its expansion being driven by dark energy. 請至原網頁觀看示意圖
Scanning the dark universe
To figure out if dark energy is changing over time, the astronomers turned to three years' worth of data from DESI, which is mounted on the Nicholas U. Mayall 4-meter Telescope in Arizona. DESI pinpoints the monthly positions of millions of galaxies to study how the universe expanded up to the present day.
By compiling DESI's observations, which includes nearly 15 million of the best measured galaxies and quasars (ultra-bright objects powered by supermassive black holes), the researchers came up with a strange result.
Taken on their own, the telescope's observations are in "weak tension" with the Lambda-CDM model, suggesting dark energy may be losing strength as the universe ages, but without enough statistical significance to break with the model.
But when paired with other observations, such as the universe's leftover light from the cosmic microwave background, supernovas, and the gravitational warping of light from distant galaxies, the likelihood that dark energy is evolving grows.
In fact, it pushes the observations' disagreement with the standard model as far as 4.2 Sigma, a statistical measure on the cusp of the five-Sigma result physicists use as the "gold standard" for heralding a new discovery.
Whether this result will hold or fade over time with more data is unclear, but astrophysicists are growing confident that the discrepancy is less likely to disappear.
"These data seem to indicate that either dark energy is becoming less important today, or it was more important early in the universe," Schlegel said.
Astronomers say that further answers will come from a flotilla of new experiments investigating the nature of dark matter and dark energy in our universe. These include the Euclid space telescope, NASA's Nancy Grace Roman Space Telescope, and DESI itself, which is now in its fourth of five years scanning the sky and will measure 50 million galaxies and quasars by the time it's done.
"I think it's fair to say that this result, taken at face-value, appears to be the biggest hint we have about the nature of dark energy in the [rough] 25 years since we discovered it," Adam Riess, a professor of astronomy at Johns Hopkins University who won the 2011 Nobel Prize in physics for his team's 1998 discovery of dark energy, told Live Science. "If confirmed, it literally says dark energy is not what most everyone thought, a static source of energy, but perhaps something even more exotic."
另一則內容近似的相關報導：
Dark energy isn't what we thought – and that may transform the cosmos
Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.
Related：
After 2 years in space, the James Webb telescope has broken cosmology. Can it be fixed?
Cosmic voids may explain the universe's acceleration without dark energy
Could the universe ever stop expanding? New theory proposes a cosmic 'off switch'
'Heavy' dark matter would rip our understanding of the universe apart, new research suggests
Something invisible and 'fuzzy' may lurk at the Milky Way's center, new research suggests
本文於 2025/03/24 16:33 修改第 2 次
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生命起源新知 -- Darren Orf
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2025/03/16 12:47 推薦1 |
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地球形成：46億年以前
生命出現：42億年以前
如果有那麼丁點「宇宙感」，則「省籍情結」或「種族偏見」之類，都是從屁眼看世界得到的景觀。
All Life on Earth Comes From One Single Ancestor. And It's So Much Older Than We Thought.
Scientists have pushed back LUCA’s origin by hundreds of millions of years.
Darren Orf, 03/14/25
Life’s Common Ancestor Lived 4.2 Billion Years Ago blackred - Getty Images 請至原網頁觀看示意圖
* All life on Earth can be traced back to a Last Universal Common Ancestor, or LUCA.
* A new study suggests that this organism likely lived on Earth only 400 million years after its formation.
* Further analysis also shows that this life form likely sported an early immune system, which means it was probably fighting off viruses.
Life on Earth had to begin somewhere, and scientists think that “somewhere” is LUCA—or the Last Universal Common Ancestor. True to its name, this prokaryote-like organism represents the ancestor of every living thing, from the tiniest of bacteria to the grandest of blue whales.
While the Cambrian Explosion kickstarted complex life in a major way some 530 million years, the true timeline of life on Earth is much longer. For years, scientists have estimated that LUCA likely arrived on the scene some 4 billion years, which is only 600 million years after the planet’s formation. But a new study from an international team of scientists pushes that timeline back even further to some 4.2 billion years ago, while also discovering some fascinating details about what life for LUCA might’ve been like. The results of the study were published in the journal Nature Ecology & Evolution.
The paper reads:
“The common ancestry of all extant cellular life is evidenced by the universal genetic code, machinery for protein synthesis, shared chirality of the almost-universal set of 20 amino acids and use of ATP as a common energy currency. As such, our understanding of LUCA impacts our understanding of the early evolution of life on Earth. Was LUCA a simple or complex organism? What kind of environment did it inhabit and when?”
To zero in on exactly when LUCA appeared on Earth, scientists had to work backward. First, the team compared genes in living species and counted the mutations that have occurred since sharing a common ancestor with LUCA. Using a genetic equation based on the time of separation between species, the team worked out that LUCA must’ve been mucking around on Earth as early as 400 million years after its creation, which puts this organism smack in the middle of the hellish geologic nightmare known as the Hadean Eon.
“The evolutionary history of genes is complicated by their exchange between lineages,” University of Bristol’s Edmund Moody, the lead author of the study, said in a press statement. “We have to use complex evolutionary models to reconcile the evolutionary history of genes with the genealogy of species.”
Not satisfied with just learning its age, the team took things a step further and retraced the physiological characteristics of living species to understand what LUCA must’ve been like 4.2 billion years ago—and the results gave some surprising answers. The scientists estimate that while LUCA was a simple prokaryote, it likely had an immune system, meaning it was already fighting off primordial viruses.
“It’s clear that LUCA was exploiting and changing its environment, but it is unlikely to have lived alone,” University of Exeter’s Tim Lenton, a co-author of the study, said in a press statement. “Its waste would have been food for other microbes, like methanogens, that would have helped to create a recycling ecosystem.”
While LUCA is the oldest common ancestor we know of, scientists still don’t understand how life evolved from its very origins to the early communities of which LUCA is a part. Further studies will need to dive deeper into this primordial history and uncover exactly how you, me, and every other living thing, came to be.
本文於 2025/03/16 12:54 修改第 1 次
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表徵遺傳學新進展 -- Jennifer Zieba
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2025/02/21 15:30 推薦1 |
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epigenetics：表徵遺傳學(擬遺傳學)
Scientists just rewrote our understanding of epigenetics
DNA and RNA epigenetics, once thought to be separate, have now been found to work together to fine-tune gene expression.
Jennifer Zieba, 02/13/25
Epigenetic modifications made to both DNA and its cousin, RNA, control gene activity. (Image credit: koto_feja/Getty Images) 請至原網頁查看照片
Scientists have uncovered a new way that cells control their genes — and it may rewrite our understanding of "epigenetics."
Epigenetics is a form of DNA modification that doesn't affect the DNA sequence itself. Instead, it describes when chemical groups attach to specific genes, thus switching those genes on or off, or else changing the 3D shape of chromosomes.
Now, in a study published Jan. 17 in the journal Cell, scientists have uncovered a whole new method of gene regulation that involves epigenetic tweaks made to both DNA and its molecular cousin RNA, at the same time.
請至原網頁查看視頻
Looking forward, the researchers want to unpack how this new type of gene control relates to cancer.
"It is truly exciting to uncover such a new mechanism, further expanding our understanding of gene regulation," Kathrin Plath, director of epigenomics, RNA and gene regulation at UCLA who was not involved in the study, told Live Science in an email.
A new layer of gene regulation
One common type of epigenetic modification is methylation, which describes the addition of a molecule called a methyl group to DNA or histones — proteins that DNA wraps around to become more compact and fit into the nucleus. A protein called DNMT1 adds these molecules to DNA, and its activity can turn gene expression up or down depending on where a given gene is methylated.
In recent years, researchers have also found that RNA — a molecule that shuttles instructions from DNA out into the cell to make proteins — can also be modified. This is mainly done by a protein complex called METTL3-METTL14. This methylation can destabilize the RNA molecule, reducing the amount of protein made.
Every cell in the body uses both RNA and DNA methylation to regulate gene expression. However, it was previously assumed that these processes operated independently. The new study puts that assumption into question.
In the study, the scientists looked at mouse embryonic stem cells and mapped the locations of DNA and RNA methylation as the cells developed. They found that thousands of genes and their complementary RNA molecules contained both methylation markers.
Through additional experiments, the team found that the METTL3-METTL14 complex that interacts with RNA also recruits and physically binds to DNMT1, the protein that tags DNA. This new, bigger complex can then methylate the same gene at the DNA or RNA level. This enables the cell to further fine-tune its gene regulation during cell differentiation — a process by which a stem cell assumes a specific identity, becoming a heart or lung cell, for example.
Previous studies have shown clear connections between DNA and histone modifications, as well as between histone and RNA modifications.
"So why would a cell not also connect an epigenetic modification of DNA and an epigenetic modification of RNA?" said study co-author François Fuks, director of the ULB Cancer Research Center in Belgium. "[Our study shows] the direct connection between DNA methylation and RNA modification that has not been seen before," he told Live Science.
According to Fuks, this study does have some limitations, namely, that it mostly focuses on embryonic stem cell differentiation. DNA and RNA modifications had separately been well characterized in stem cells in past studies, so it made sense for the researchers to start with them. But these same types of DNA and RNA modifications are present in all types of cells.
"Seeing this, it's very unlikely that [this mechanism] will be just in ES cells," Fuks said.
This discovery challenges the established view that these RNA- and DNA-modifying processes are completely separate, and it suggests that it may have broader implications in human biology and disease. To that end, Fuks and his team are trying to determine how this new mechanism relates to cancer.
If the coordination of DNA and RNA epigenetics gets thrown off, you may end up with too much or too little of a protein, Fuk suggested. "Now, a key protein will be expressed at a too high level," he said."This could be detrimental for a cell and contribute to tumorigenesis," or the formation of tumors.
There are already approved therapies that inhibit the methylation of DNA, and there's an early-phase clinical trial testing RNA methylation inhibition as a cancer treatment. Fuks and his team are testing the potential of combining these existing therapies to improve patients' outcomes. Preliminary data from their laboratory studies hint this strategy could be useful for patients with leukemia.
At least in petri dishes, "we can revert the cancer progression of leukemic cells by adding these two drugs together," Fuk said. "Eventually, down the line, why couldn't we combine these two drugs to treat patients?"
Jennifer Zieba earned her PhD in human genetics at the University of California, Los Angeles. She is currently a project scientist in the orthopedic surgery department at UCLA where she works on identifying mutations and possible treatments for rare genetic musculoskeletal disorders. Jen enjoys teaching and communicating complex scientific concepts to a wide audience and is a freelance writer for multiple online publications.
相關閱讀
Sperm cells carry traces of childhood stress, epigenetic study finds
Epigenetics linked to the maximum life spans of mammals — including us
Pregnancy may speed up 'biological aging,' study suggests
IVF may raise risk of certain disorders in babies — and epigenetic 'signatures' in the placenta could explain why
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本文於 2025/02/21 15:31 修改第 1 次
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對稱性生物結構的潛在風險 -- Michael Kay
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2025/01/21 18:00 推薦1 |
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Reflecting on the Risks of 'Mirror Life'
Scientists are considering the threats posed by creating bacteria that are chemical mirror images of natural ones.
Michael Kay, 01/16/25
Consider your hands. They are essentially identical in every respect bar one: They are mirror images of each other.
Many of the molecules of life, such as proteins and DNA, have the same property. They can exist in right- and left-handed forms that are otherwise chemically identical. However, due to a quirk we don’t fully understand, life evolved to use one set of molecules almost exclusively: Everything from the simplest bacteria and fungi to the most complex plants and animals uses left-handed proteins and right-handed DNA.
For over 30 years, I’ve been fascinated by the idea that "mirror" biological molecules — like right-handed proteins (or their miniature versions, peptides) — could be used as stealthy drugs for diseases lacking effective or practical treatments. In a breakthrough published during my first year of graduate school, Stephen Kent and co-authors demonstrated that proteases (digestive enzymes in the body) cannot break down mirror peptides — ones with the opposite handedness to the peptides your body produces naturally. Building on this principle, my lab has been creating mirror peptides that could help prevent or treat infectious diseases. These peptides last much longer in the body because they are not digested and also not recognized (or cleared) by the immune system.
Last spring, I was at a meeting of the Pandemic Antiviral Discovery initiative, which aims to help accelerate the development of medicines to prepare for the next viral pandemic. My lab has been looking at possible mirror peptide treatments for the Nipah and Hendra viruses — two closely related, highly lethal viruses that could cause deadly pandemics. A dinner discussion turned to the idea of engineering an entire mirror bacterium from scratch. Such an organism could be engineered to churn out larger versions of the kinds of molecules my lab builds at lower cost. But it would also present serious biosafety concerns. If mirror proteins are exciting as antivirals because they are largely invisible to digestive enzymes and the immune system, couldn’t that principle, scaled up, make a full mirror bacterium a uniquely dangerous pathogen?
For over 30 years, I’ve been fascinated by the idea that "mirror" biological molecules could be used as stealthy drugs for diseases lacking effective or practical treatments.
In December, 37 colleagues and I published a paper in Science arguing that mirror bacteria — self-replicating, synthetic cells whose every component exists in its mirror-image form — could indeed pose incredibly grave dangers if successfully created. First, they would likely evade most human, animal, and plant immune system responses because these have evolved to tackle natural bacterial threats, not mirrored ones. That intrinsic resistance could lead to widespread, lethal infections in many species (independent of any other factors that make pathogens dangerous, like toxins they can produce). Second, our world is not overrun with natural bacteria partly because they are kept in check by other organisms, such as viruses and amoebae, that prey on them. To the best of our current knowledge, reversed molecular structures would likely give mirror bacteria significant resistance to these predators, potentially enabling them to grow largely unchecked in a wide range of ecosystems.
We shouldn’t lose sleep, though. Nobody is currently close to creating a full mirror bacterium. No one has even achieved the much simpler feat of creating a natural bacterium from its individual components; doing so in mirror form would be an extraordinarily complex undertaking that could take decades. Our intention in publishing our paper was to kickstart the conversation about the potential risks long before they materialize.
But the idea of mirror bacteria raises an important question. We already know that mirror molecules are useful therapeutic agents with great potential for hard-to-treat diseases. We also now know that mirror bacteria could be incredibly dangerous, far outstripping any practical benefits they could provide. Where exactly should we draw the line?
Mirror bacteria and mirror molecules are a world apart. Mirror bacteria would be able to self-replicate: If they got into the wild, they could likely sustain and grow their populations on a range of simple foods that don’t have mirror-image forms (that is, foods that could be used equally well by natural and mirror life). What’s more, like any form of life, they could evolve to grow more efficiently on complex natural food sources as they spread. As we’ve seen from work like my co-author Richard Lenski’s long-term evolution experiment, bacterial populations can evolve very quickly.
Mirror molecules — chemically synthesized drugs — cannot replicate, and so cannot evolve. They do not present the dangers of mirror bacteria. And while mirror bacteria could be used to create mirror molecules, they're not the only option. Indeed, there’s a central irony here: The more progress we make on the chemical synthesis technologies that could help enable the creation of a mirror cell, the easier it will be to synthesize mirror proteins directly, without the help of bacteria — decreasing the relative benefits of building mirror bacteria even further.
But the question gets thornier as we consider systems more complex than a peptide. The ribosome is the factory of your body’s cells, churning out proteins and enzymes. Building a mirror ribosome is a much more achievable goal that could be realized in the next decade. A mirror ribosome could efficiently produce mirror protein therapeutics without the risks associated with full, living, self-replicating mirror bacteria. But it will be important to evaluate if the mirror ribosome could also be an enabling technology on the road to self-replicating mirror cells.
We published our paper, and its accompanying technical report, to spark a global conversation that considers these questions — and many others. This year, some of my coauthors will be hosting events at the University of Manchester, Institut Pasteur, and the National University of Singapore to convene some of the necessary discussions, and we hope to be joined by the global scientific community, policymakers, and other critical stakeholders as we chart the best path forward.
We’re in the fortunate position that we have many years before the threat of mirror bacteria arises. As a global scientific community, I hope we use this window of opportunity to discuss these questions in detail in the months and years to come.
Michael Kay is a professor of biochemistry in the Spencer Fox Eccles School of Medicine at University of Utah.
Tom Freeman of Milltown Partners provided feedback on a draft of this article
本文於 2025/01/21 18:01 修改第 1 次
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破解開天闢地之謎--IAI Panel
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2024/12/24 04:03 推薦1 |
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這是藝術與思想研究學會正在進行討論會系列之一。主持人外還有三位學者。我不習慣看視頻，所以沒有全程看完。由於過去和最近本部落格轉貼了很多關於宇宙起源的報導/論文，所以介紹它，分享給對這個議題有興趣的朋友。
本城市專載過多篇在該學會網誌上發表的專論。一般而言，水準都相當高。不過，需訂閱者才能閱讀該網誌。
The riddle of the beginning：Making sense of the beginning of the universe
Philosophy for our times：1000+ Debates from the world's leading thinkers
本文於 2024/12/24 04:08 修改第 3 次
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暗能量被反證？ ------ Royal Astronomical Society
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2024/12/21 21:20 推薦1 |
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這下子科學家們要吵翻天了。即使在自然科學領域，理論之爭從來不只是對、錯或名聲、地位之爭；它更是研究經費和教席地盤之爭。
Dark energy 'doesn't exist' so can't be pushing 'lumpy' universe apart, physicists say
Royal Astronomical Society, 12/20/24
This graphic offers a glimpse of the history of the universe, as we currently understand it. The cosmos began expanding with the Big Bang but then around 10 billion years later it strangely began to accelerate thanks to a theoretical phenomenon termed dark energy. Credit: NASA, Licence typeAttribution (CC BY 4.0) 請至原網頁觀看示意圖
One of the biggest mysteries in science—dark energy—doesn't actually exist, according to researchers looking to solve the riddle of how the universe is expanding.
Their analysis has been published in the journal Monthly Notices of the Royal Astronomical Society Letters.
For the past 100 years, physicists have generally assumed that the cosmos is growing equally in all directions. They employed the concept of dark energy as a placeholder to explain unknown physics they couldn't understand, but the contentious theory has always had its problems.
Now a team of physicists and astronomers at the university of Canterbury in Christchurch, New Zealand are challenging the status quo, using improved analysis of supernovae light curves to show that the universe is expanding in a more varied, "lumpier" way.
The new evidence supports the "timescape" model of cosmic expansion, which doesn't have a need for dark energy because the differences in stretching light aren't the result of an accelerating universe but instead a consequence of how we calibrate time and distance.
It takes into account that gravity slows time, so an ideal clock in empty space ticks faster than inside a galaxy.
The model suggests that a clock in the Milky Way would be about 35 percent slower than the same one at an average position in large cosmic voids, meaning billions more years would have passed in voids. This would in turn allow more expansion of space, making it seem like the expansion is getting faster when such vast empty voids grow to dominate the universe.
Professor David Wiltshire, who led the study, said, "Our findings show that we do not need dark energy to explain why the universe appears to expand at an accelerating rate.
"Dark energy is a misidentification of variations in the kinetic energy of expansion, which is not uniform in a universe as lumpy as the one we actually live in."
He added, "The research provides compelling evidence that may resolve some of the key questions around the quirks of our expanding cosmos.
"With new data, the universe's biggest mystery could be settled by the end of the decade."
Dark energy is commonly thought to be a weak anti-gravity force which acts independently of matter and makes up around two thirds of the mass-energy density of the universe.
The standard Lambda Cold Dark Matter (ΛCDM) model of the universe requires dark energy to explain the observed acceleration in the rate at which the cosmos is expanding.
Scientists base this conclusion on measurements of the distances to supernova explosions in distant galaxies, which appear to be farther away than they should be if the universe's expansion were not accelerating.
However, the present expansion rate of the universe is increasingly being challenged by new observations.
Firstly, evidence from the afterglow of the Big Bang—known as the Cosmic Microwave Background (CMB)—shows the expansion of the early universe is at odds with current expansion, an anomaly known as the "Hubble tension."
表單的底部
In addition, recent analysis of new high precision data by the Dark Energy Spectroscopic Instrument (DESI) has found that the ΛCDM model does not fit as well as models in which dark energy is "evolving" over time, rather than remaining constant.
Both the Hubble tension and the surprises revealed by DESI are difficult to resolve in models which use a simplified 100-year-old cosmic expansion law—Friedmann's equation.
This assumes that, on average, the universe expands uniformly—as if all cosmic structures could be put through a blender to make a featureless soup, with no complicating structure. However, the present universe actually contains a complex cosmic web of galaxy clusters in sheets and filaments that surround and thread vast empty voids.
Professor Wiltshire added, "We now have so much data that in the 21st century we can finally answer the question—how and why does a simple average expansion law emerge from complexity?
"A simple expansion law consistent with Einstein's general relativity does not have to obey Friedmann's equation."
The researchers say that the European Space Agency's Euclid satellite, which was launched in July 2023, has the power to test and distinguish the Friedmann equation from the timescape alternative. However, this will require at least 1,000 independent high quality supernovae observations.
When the proposed timescape model was last tested in 2017, the analysis suggested it was only a slightly better fit than the ΛCDM as an explanation for cosmic expansion, so the Christchurch team worked closely with the Pantheon+ collaboration team who had painstakingly produced a catalog of 1,535 distinct supernovae.
They say the new data now provides "very strong evidence" for timescape. It may also point to a compelling resolution of the Hubble tension and other anomalies related to the expansion of the universe.
Further observations from Euclid and the Nancy Grace Roman Space Telescope are needed to bolster support for the timescape model, the researchers say, with the race now on to use this wealth of new data to reveal the true nature of cosmic expansion and dark energy.
More information: Antonia Seifert et al, Supernovae evidence for foundational change to cosmological models, Monthly Notices of the Royal Astronomical Society: Letters (2024). DOI: 10.1093/mnrasl/slae112
Journal information: Monthly Notices of the Royal Astronomical Society Letters Provided by Royal Astronomical Society
本文於 2024/12/21 21:21 修改第 1 次
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「大爆炸」假說到底在講啥子 - Elisha Sauers
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2024/12/16 18:50 推薦1 |
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邵爾絲女士這篇對「大爆炸」假說的闡述相當簡潔；就普及科學工作而言，也掌握到該「假說」要點。下文可以跟本欄2024/12/12 與2024/05/13兩篇報導合看。
What most people think they know about the Big Bang is wrong
A rapid stretching of the universe.
Elisha Sauers, Mashable, 12/14/24
Cosmic inflation tries to describe one brief but crucial phase in the Big Bang that launched the universe onto its expansion course. Credit: Christine Daniloff / MIT / ESA / Hubble / NASA 請至原網頁觀看照片
Many textbooks and science educators have attempted to describe the Big Bang as the birth of the universe — an explosive start that happened at a specific point creating matter and flinging it into the void like shrapnel from a grenade.
But the Big Bang is not really the moment of creation — more like its aftermath. The Big Bang didn't emerge from a particular location in space, and it wasn't an explosion — at least not in the traditional sense.
Popular culture — and cosmologists, begrudgingly — made the unfortunate mistake of adopting a name for the theory that even evokes the sound of a gunpowder blast. So… bazinga?
"It’s often said that the whole universe we can now observe was once compressed into a volume the size of a golf ball," wrote John Mather, a Nobel Prize-winning astrophysicist and senior project scientist for NASA's James Webb Space Telescope, in an essay for Theedge.org. "But we should imagine that the golf ball is only a tiny piece of a universe that was infinite even then."
When the universe was still in its infancy, less than 1 billion years old, star formation fed on hydrogen that emerged from the Big Bang. Credit: NASA / ESA / A. Schaller (for STScI) illustration 請至原網頁觀看照片
The Big Bang Theory describes an event when existing space — much hotter, denser, and smaller at the time — suddenly and rapidly started stretching out. The primitive universe was a scalding goulash of tiny particles, light, and energy, but as it expanded, space cooled enough to allow important processes to occur, such as forming atoms and elements. The expansion continues today.
That's it. It doesn't suggest what the conditions were before expansion. It doesn't suppose what the universe is expanding into. It doesn't even explain what caused the expansion in the first place. And there are reasons why trying to imagine the event as an explosion can lead to some misinformed conclusions.
"No reputable scientist will claim that we understand in detail what happened at the exact moment when the universe began. We just don't," said Don Lincoln, senior scientist at Fermilab in Illinois, in a video. "In spite of the fact that we don't know everything about how the universe began, I'm constantly staggered by the fact that we know so much."
The Big Bang pertains to the visible universe
To understand the Big Bang — and Mather's previous comment — it's first important to clarify that this theory applies to the visible universe, not the universe as a whole. The visible universe is a bubble of the cosmos centered on our perspective from Earth, with a radius determined by the speed of light. The entire bubble is about 92 billion light-years wide.
The bubble's size is not determined by the range of telescopes, but the literal limitation of light. There is a maximum distance from which photons could have traveled to an observer in the age of the universe. This boundary is known as the cosmic light horizon: Any potential signals beyond it haven't had time to reach us — and they never will, not even billions of years into the future. That's because at a certain extreme distance, far-flung objects recede faster than the speed of light.
So what's beyond this bubble? No one knows because it's unseeable, but scientists could speculate there's more universe. After all, with the expansion of space, scientists are aware that, every second, thousands of stars are escaping our view, beyond that horizon.
Where exactly did the Big Bang happen?
The Big Bang should be thought of as a "point" in time but not happening at a particular place. Astronomers will often say that the Big Bang happened everywhere, which is a confounding idea if you've been thinking of the Big Bang like a detonating bomb.
Imagine instead a hypothetical scenario where space was condensed within a speck, like a pinhead-sized balloon. Then imagine that this tiny balloon somehow inflated into the size of an orange. In this analogy, you can begin to understand why there is no "origin point" for the Big Bang: Nothing left the pinhead where it began; the pinhead point got exponentially bigger.
This is one of the reasons why many astrophysicists say everywhere in the knowable universe could be considered part of the Big Bang's center. There was no particular site from which bits were blown away, according to the theory.
Astronomer Edwin Hubble used the 100-inch Hooker telescope in California to observe that galaxies were receding in space in all directions. Credit: NASA / Edwin P. Hubble Papers / Huntington Library請至原網頁觀看照片
The Big Bang wasn't really an explosion
Scientific observations support the idea of rapid universal expansion versus an explosion. If there had been a firecracker-type blast that scattered matter outward, for example, the laws of physics would dictate that debris farther from that place where it exploded would be moving faster than the stuff closer to that starting point.
"That's because objects far away from the firecracker have to be moving faster. That's how they got far away," Lincoln said.
But that is not what astronomers see. In the cosmos, the space between galaxies is increasing, in all directions — not just relative to a central spot. Astronomer Edwin Hubble, for whom the Hubble Space Telescope was later named, discovered this in 1929.
Using the 100-inch Hooker Telescope in California, Hubble noted that the farther a galaxy was from the Milky Way, Earth's home galaxy, the faster it seemed to be receding. He figured this out by plotting 24 nearby galaxies' velocities and distances. The plot showed that everything was drifting uniformly, at speeds proportional to distance, in all directions.
The rate of expansion has been dubbed the Hubble Constant. Two years after Hubble's observations, a Belgian astronomer and priest, Georges Lemaître, used this premise to publish the first Big Bang-like theory to explain the beginnings of the universe.
Cosmologists believe the universe has expanded over 13.8 billion years since the Big Bang. Credit: Britt Griswold (Maslow Media Group) / NASA illustration 請至原網頁觀看照片
How astronomers know the universe is expanding
With Hubble's finding that space itself is expanding, scientists have been able to estimate the age of the universe. The formula for velocity — which you might have learned in high school — is distance divided by time. Scientists already know the speeds of galaxies and their distances, so they can figure out the duration by dividing distance by speed.
If scientists rewind the clock from the present day to the time that everything in the knowable universe crumples back into that small deflated balloon, it occurred about 13.8 billion years ago.
So, if the universe is 13.8 billion years old, one might incorrectly assume that the visible bubble of the universe has a radius of 13.8 billion light-years, with an overall width of 27.6 billion light-years. But the universe isn't standing still, and the distance between objects isn't fixed. The expansion of space explains the discrepancy between 27.6 billion light-years and 92 billion light-years, the diameter of the visible universe.
Have scientists disproved the Big Bang?
Scientists have not disproved the Big Bang Theory, but they have discovered disagreements in the rate of expansion — the Hubble Constant — from different research teams' measurements. The disagreement is known as the Hubble tension.
In short, speed measurements based on telescope observations of the present universe are somewhat higher than projections based on known conditions of the universe during its infancy. For the past few years, astronomers have considered that something is causing the expansion rate to speed up. Studies using the Webb telescope have found that the small-but-significant divergence in the expansion rate is probably not the result of miscalculations but an aspect of the universe that is not yet understood.
As scientists work to solve this mystery, the Big Bang might need some tweaking, but so far this disparity has not upended the bottom line, which is that space was once smaller and hotter, then it suddenly stretched out, and it's still expanding.
A map of the Cosmic Microwave Background. U.S. physicists Arno Penzias and Robert Wilson unintentionally discovered the Cosmic Microwave Background, which fills the visible universe. Credit: ESA / Planck Collaboration 請至原網頁觀看照片
The expansion rate of the early universe
Researchers have calculated the expansion rate of the baby universe using data from the so-called Cosmic Microwave Background. U.S. physicists Arno Penzias and Robert Wilson accidentally discovered this phenomenon, a faint afterglow from 380,000 years after the Big Bang, using a radio telescope in 1965.
Around the same time, a separate team at Princeton University had predicted that such waves should exist. If astronomers were archaeologists, this discovery would be akin to finding the earliest fossil of light. It is the oldest thing in the universe anyone has seen.
This heat signature, radiating from atoms that are now more than 46 billion light-years away and stretched into microwaves, fills the sky. The European Space Agency’s Planck mission mapped the microwaves to measure teensy fluctuations in temperature. These slight variations allow scientists to infer the expansion rate at the time.
How 'cosmic inflation' theory fits into the Big Bang
Cosmic inflation tries to describe one brief but crucial phase in the Big Bang narrative that launched the universe onto its expansion timeline.
Alan Guth, a theoretical physicist at MIT, put forward the idea in 1980. It suggests that some repulsive form of gravity, something like dark energy, drove the universe's rapid expansion for an early instant. This phase would have lasted for a fraction of a trillionth of a second. Then, the energy that propelled inflation turned off.
"I usually describe inflation as a theory of the 'bang' of the Big Bang," Guth said in a 2014 Q&A by the university. "In its original form, the Big Bang theory never was a theory of the bang. It said nothing about what banged, why it banged, or what happened before it banged."
During the inflation phase, the tiny universe would have expanded at a rate faster than light. And get this: It wouldn't have broken any laws of physics.
"It's true that nothing can move through space faster than light, but there are no restrictions on how fast space can expand," Lincoln said.
How the 'Big Bang' got its name
Fred Hoyle, an astronomer and well-known science communicator in the United Kingdom, is largely credited with coining the "big bang" in 1949. He was in many ways the Neil deGrasse Tyson of his time. But today many astrophysicists and cosmologists lament that the misnomer stuck.
During a BBC broadcast, Hoyle described theories based on the idea that "all the matter in the universe was created in one big bang at a particular time in the remote past," according to a transcript published in a BBC magazine. He later mentioned the phrase again in his 1950 book "The Nature of the Universe."
Hoyle balked at the idea of a sudden origin of the universe, but he didn't use the words "big bang" disparagingly, according to a recent essay about it in the journal Nature. Instead, he meant to convey the hypothesis with descriptive metaphors to help get the point across over radio.
Bazinga, indeed.
SEE ALSO: Webb telescope spots proof of the first stars to light the universe
本文於 2024/12/16 18:55 修改第 2 次
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宇宙學需要新點子 -- R. Lea
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2024/12/12 17:26 推薦1 |
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'Our understanding of the universe may be incomplete': James Webb Space Telescope data suggests we need a 'new cosmic feature' to explain it all
"The discrepancy between the observed expansion rate of the universe and the predictions of the standard model suggests that our understanding of the universe may be incomplete. "
Robert Lea, 12/09/24
Credit: NASA, ESA, CSA, STScI, Jose M. Diego (IFCA), Jordan C. J. D’Silva (UWA), Anton M. Koekemoer (STScI), Jake Summers (ASU), Rogier Windhorst (ASU), Haojing Yan (University of Missouri) 請至原網頁查看照片
New observations from the James Webb Space Telescope (JWST) have corroborated data from its predecessor, the Hubble Space Telescope, to determine something is missing from our recipe of the cosmos.
The JWST conducted its largest survey yet of the accelerating expansion of the cosmos as scientists attempt to discover why the universe is expanding faster today than our picture of its infancy, billions of years ago, says that it should. Currently, scientists theorize that the accelerating expansion is caused by a placeholder element, "dark energy," but they really need to know what dark energy actually is before a conclusive explanation can be found.
JWST's survey served to cross-check observations made by Hubble that suggested a discrepancy in measurements of the rate of cosmic expansion, known as the Hubble constant. This issue has been termed "Hubble tension," and these new findings show that errors in data from the long-serving space telescope of the same name are not responsible for it.
As the Hubble tension can't be accounted for by either our best models of the universe or errors in Hubble measurements, an extra ingredient still seems to be needed in our cosmic recipe.
"The discrepancy between the observed expansion rate of the universe and the predictions of the standard model suggests that our understanding of the universe may be incomplete," team leader Adam Reiss, an astrophysicist at Johns Hopkins University, said in a statement. "With two NASA flagship telescopes now confirming each other’s findings, we must take this [Hubble tension] problem very seriously — it's a challenge but also an incredible opportunity to learn more about our universe."
In 2011, Reiss won the Nobel Prize in Physics for the discovery of dark energy, a mysterious force that drives the acceleration of the expansion of the universe. This new research builds upon that Nobel Prize-winning work.
What is the Hubble tension?
Because the expansion of the universe works on very large scales, Hubble tension isn't something that affects us in our everyday life or even on scales of the solar system or even the Milky Way.
This discrepancy becomes really problematic when considering the distances between galaxies and the larger structure of the universe. That means cosmologists can't really understand the evolution of the universe until they know what the cause of the Hubble tension.
The Hubble tension arises from the fact that there are two ways to calculate the Hubble constant.
Scientists can use things like distances to Type Ia supernovas or variable stars, which they call "standard candles," to measure the distances from Earth to the galaxies that host them and then determine how rapidly these galaxies are moving away.
They can also use our models of cosmic evolution to "wind forward" the universe and calculate what the Hubble constant should be today.
However, when measurements of the Hubble constant are taken in the local universe, they are higher than the value predicted by working forward using the best model we have for cosmic evolution, the Lambda Cold Dark Matter (LCDM) model, also known as the Standard Model of Cosmology.
A diagram showing the evolution of the universe according to the prevailing cold dark matter model. Observations of El Gordo could throw this model into doubt 請至原網頁查看說明及圖示
The LCDM-based method gives a value for the Hubble constant of about 152,000 miles per hour per megaparsec (68 kilometers per second per megaparsec, or Mpc), while measurements based on telescope observations regularly give a higher value of between 157,000 mph per Mpc to 170,000 mph per Mpc (70 to 76 km/s/Mpc).
An Mpc is equivalent to 3.26 light-years or 5.8 trillion miles (9.4 trillion kilometers), so this is a huge discrepancy, one which scientists feared was too large to be explained by uncertainties in observations.
Looks like they were right!
Hubble was right!
To confirm the findings of Hubble, Reiss, and colleagues turned to the largest sample of data collected by the JWST during its first two years of operations, which came from two different projects.
To measure the Hubble constant, they used three independent methods to determine the distance to other galaxies. First, they used so-called "Cepheid variables," pulsating stars considered the gold standard for measuring cosmic distances. The team then cross-checked this with measurements based on carbon-rich stars and the brightest red giants across the same galaxies.
The team particularly honed in on galactic distances measured by Hubble.
The team's research with the JWST covered about a third of the full sample of galaxies as seen by Hubble using the galaxy Messier 106 (M106), also known as NGC 4258 and located around 23 million light-years away in the constellation Canes Venaticias, a reference point.
A dusty-looking section of space with orange and red streaks concentrated around a glowing greenish center. 請至原網頁查看照片
This not only helped them produce the most precise local measurements of the Hubble constant to date, but it also independently verified that Hubble's distance measurements were accurate.
The galaxies observed by the JWST yielded a Hubble constant of around 162,400 mph per Mpc (72.6 km/s/Mpc), nearly identical to the value of 162849 mph per Mpc (72.8 km/s/Mpc) found by Hubble for the same galaxies.
This eliminates the possibility that the Hubble tension is just an artifact arising from significant bias in the long-serving space telescope's measurements.
"The JWST data is like looking at the universe in high definition for the first time and really improves the signal-to-noise of the measurements,’’ team member and Johns Hopkins University graduate student Siyang Li said.
Of course, this means there is still a problem of Hubble tension that needs to be tackled. Because the expansion of the universe works on very large scales.
Johns Hopkins cosmologist Marc Kamionkowski, who was not involved with this study, thinks that solving the Hubble tension requires a new element to our models of the universe. He has an idea of what this element may be.
"One possible explanation for the Hubble tension would be if there was something missing in our understanding of the early universe, such as a new component of matter — early dark energy — that gave the universe an unexpected kick after the Big Bang," Kamionkowski said in the statement. "And there are other ideas, like funny dark matter properties, exotic particles, changing electron mass, or primordial magnetic fields that may do the trick.
"Theorists have license to get pretty creative.”
The team's research was published on Monday (Dec. 9) in the Astrophysical Journal.
Related Stories:
— James Webb Space Telescope spies never-before-seen star behavior in distant nebula (video, photo)
— Galactic penguin honors the 2nd anniversary of James Webb Space Telescope's 1st images
— James Webb Space Telescope directly images its coldest exoplanet target yet
本文於 2024/12/12 17:29 修改第 2 次
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黑洞影像佐證下的萬有引力理論 -- Robert Lea
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2024/11/28 18:53 推薦1 |
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轉載下文於此，略盡推廣科學普及教育之責。
索引：
anatomy：此處：(黑洞)結構分析；解剖學，解剖構造，(動植物)結構，身體，剖析
black hole, mimetic：「擬態引力理論」中黑洞的結構和性質
caveat：此處：提示；警告，注意，謹慎
curvature：彎曲率，彎曲度
ergosphere：動圈(在旋轉黑洞外面的區域)；黑洞能量/物質攝取區(胡卜凱的翻譯)
event horizon：事件穹界；(黑洞)阻隔區(胡卜凱的翻譯)
magnum opus：代表作，巨著，傑作
gravity, mimetic：擬態引力
mimic：模仿的，模擬的，非真的，假裝的
Schwartzchild solution：
singularity：「時-空」終結點(胡卜凱的翻譯)
singularity, naked：裸奇異點；在黑洞內，但是沒有阻隔區包圍的「『時-空』終結點」(胡卜凱的解讀)
spacetime：「時-空」
Black hole images deliver a deathblow to alternative theory of gravity
Robert Lea, 11/26/24
Images of the supermassive black holes wouldn’t have been possible if mimetic gravity was the right recipe for gravity. 請至原網頁觀看照片
Researchers have examined the historical images of supermassive black holes — Sagittarius A* (Sgr A*) at the heart of the Milky Way and the black hole at the center of the galaxy Messier 87 (M87) — to rule out an alternative to our current best theory of gravity.
In doing so, the team behind this research also help to confirm the existence of dark energy and dark matter, the two most mysterious and difficult-to-explain aspects of the universe.
Despite being arguably the most ubiquitous “force” experienced by humanity, gravity hasn’t necessarily been easy to explain. Newton’s theory of gravity was a good early attempt and still works perfectly well for relatively small-scale calculations, but it starts to fail when considering massive objects, even struggling to explain the wobbly orbit of Mercury.
In 1915, Einstein put forward the theory of general relativity, suggesting that gravity is not a force in the traditional sense but instead arises from the curvature of space and time, united as a four-dimensional entity called “spacetime,” caused by the presence of mass. The more mass an object has, the greater the curvature of spacetime and, thus, the greater the gravitational influence of that object.
One of the most remarkable aspects of general relativity is the number of concepts that it predicted, including black holes and gravitational waves, that later came to be evidentially verified. General relativity is one of the most tested theories in science, and it has survived every experimental challenge thrown at it. It has thus supplanted Newton’s theory of gravity.
General relativity isn’t perfect, however. One of the major problems with Einstein’s magnum opus theory is the fact cosmological theories that tell the story of the universe based upon it can’t account for the so-called “dark universe.” That is dark energy, the mysterious force that drives the acceleration of the universe’s expansion, and dark matter, the strange “stuff” that out-populates ordinary matter by five to one but remains effectively invisible.
The dark universe problem for Einstein
Dark energy accounts for an estimated 70% of the universe’s matter/energy budget, while dark matter accounts for a further 25% of that budget. This means that everything we see in the universe around us, all the stars, planets, moons, asteroids, animals, etc… account for just 5% of the contents of the universe. No wonder most scientists are desperate to discover what dark energy and dark matter are.
Why the caveat most?
That’s because other scientists propose that dark matter and dark energy don’t exist. Instead, they suggest that the effects we attribute to them are a consequence of the fact that general relativity isn’t the “right recipe” for gravity. These researchers posit theories of “modified gravity” that do away with the need for the dark universe to exist. Some modify Newton’s theory of gravity; others attempt to extend on general relativity.
One of the most credible modified gravity theories is mimetic gravity, suggested in 2013 by researchers Slava Mukhanov and Ali Chamseddine. Mimetic gravity extends general relativity, leading to the appearance of a dust-like perfect fluid that can mimic cold dark matter at a cosmological level and can explain the late-time acceleration of the cosmic expansion attributed to dark energy.
To surpass and supplant general relativity, one thing any modified gravity theory must do is also (to explain) the phenomenon in the universe that conforms to Einstein’s 1915 theory. That is where the images of black holes come in. 註：(to explain)為原文所無，我加上來補足文法和文意；可能是鍵誤或漏植。
In April 2019, when scientists from the Event Horizon Telescope (EHT) revealed the first-ever image of a black hole to the public, the supermassive black hole M87*, they expressed how surprised they were that it almost exactly conformed to the appearance of a black hole and its surroundings predicted by general relativity. This was compounded in May 2022, when the first image of “our black hole” Sgr A* also tightly conformed to expectations and closely resembled M87* despite the fact that the latter is much more massive than the Milky Way’s supermassive black hole.
Thus, it is only natural to put theories of modified gravity up against observations of the supermassive black holes M87* and Sgr A* collected by the EHT, a global network of instruments that effectively creates a single Earth-sized telescope. That is exactly what the authors of a new paper set out to do.
“In a sense, the mere fact that we can see these images rules out mimetic gravity!” said University of Trento researcher Sunny Vagnozzi. “In short, our findings completely rule out baseline mimetic gravity, which was previously one of the least unlikely modified gravity-based models for dark matter and dark energy.
“In some sense, it empirically gives even more support to the fact that dark matter and dark energy may be ‘real’ and not the effect of modifications of gravity.”
The anatomy of black holes: General relativity vs. mimetic gravity
To understand why the team’s research and the EHT images are bad news for supporters of mimetic gravity, it is necessary to delve into the anatomy of black holes a little bit.
All black holes are considered to be composed of a central singularity, an inestimably small region of space with infinite mass where the laws of physics fail, and an outer boundary called an “event horizon.” The event horizon is the point at which the gravitational influence of the black hole becomes so great that not even light is fast enough to escape. Thus, anything that passes the event horizon of a black hole is on a one-way trip to the central singularity.
Around the event horizon is a region of space that is constantly dragged along with the rotational of the black hole due to its immense gravity. It is impossible for matter to sit still in this region, called the “ergosphere.” Further out is matter whipping around the black hole at near-light speeds, causing it to glow. This matter appears as a striking golden ring in the images of M87* and Sgr A*, with the shadow of the black holes appearing in the center of these rings.
That is, if general relativity is the correct recipe for gravity and if a solution to its equations called the Schwartzchild solution accurately describes the anatomy of black holes.
Mimetic gravity has two different ideas about black holes. Vagnozzi explained that one of the two natural classes of objects in mimetic gravity is a naked singularity. This is a central singularity that is not bounded by a light-trapping event horizon. No event horizon would have meant no EHT image.
The second possible object predicted in mimetic gravity is a so-called “mimetic black hole.” If the EHT had imaged one of these objects when it snapped M87* or Sgr A*, what researchers would seen is an image with a much smaller dark region at its heart than the dark region that was seen in these black hole images.
“We demonstrated that the naked singularity does not cast a shadow. It should not lead to an image in EHT observations,” Vagnozzi said. “To use an everyday life analogy, say EHT images are actually the reflection of ourselves we see in the mirror. If I were a mimetic naked singularity, I would look in the mirror and see no reflection. If I were a mimetic black hole, my image in the mirror would be much smaller than it actually is.
“This analogy is stretching it a lot, but it should give an idea of what is happening.”
Vagnozzi explained that although the interpretation of EHT data to create the images of M87* and Sgr A* is a complex process with some margin of error, this possible uncertainty simply isn’t significant enough for the team’s conclusion to be incorrect.
The researcher stresses that the research conducted by the team rules out only a “baseline” version of mimetic gravity, adding that more complex mimetic gravity theories with more adjustments to general relativity could still be possible.
“This is absolutely a demonstration of the importance of the EHT and its observations. It demonstrates that EHT has the potential to rule out candidate theories of dark matter and dark energy, which were previously completely viable,” Vagnozzi said.
“The takeaway message is very important: any theory that claims to explain dark matter and dark energy needs not only be consistent with cosmological observations but also with observations of black holes, and this provides a highly non-trivial test of many such models, which may be inconsistent with EHT images.
“We believe this idea deserves to be explored in much more detail.”
Reference:
Mohsen Khodadi, Sunny Vagnozzi, and Javad T. Firouzjaee Event Horizon Telescope observations exclude compact objects in baseline mimetic gravity, Scientific Reports (2024). DOI: 10.1038/s41598-024-78264-y
本文於 2024/11/28 18:54 修改第 1 次
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