巨型中微子或为远古黑洞的信使
内容总结：
三年前，一颗来自太空的高能中微子撞击地中海，触发了位于西西里海岸的立方公里中微子望远镜（KM3NET）探测器。这颗中微子的能量达到以往观测最高值的35倍，其来源引发了科学界的广泛猜测。
近期，麻省理工学院物理学家大卫·凯泽及其团队提出了一项大胆假设：这颗“巨无霸”中微子可能源自原初黑洞的爆炸。原初黑洞是理论上形成于宇宙极早期的高密度时空坍缩产物，其质量跨度极大，小至原子核，大如月球。该理论由苏联物理学家泽尔多维奇与诺维科夫于1966年率先提出，后经霍金与卡尔等人完善。
若原初黑洞质量处于小行星级别（约100万亿克），其寿命可与宇宙年龄相当，最终会通过霍金辐射机制爆炸消亡，释放包括中微子在内的多种高能粒子。凯泽团队计算发现，若一个质量约100万亿克的原初黑洞在距地球约2000天文单位处爆炸，有约8%的概率产生KM3NET探测到的中微子。相关研究成果已于2025年9月18日发表于《物理评论快报》。
原初黑洞的探索意义深远：若大量存在，可能构成暗物质的一部分，为这一宇宙谜题提供新解。尽管部分学者对此持怀疑态度——如威斯康星大学宇宙学家丹·胡珀认为该中微子与原初黑洞无关——但更多研究者保持开放态度。伦敦大学伯贝克学院教授伯纳德·卡尔表示：“我研究原初黑洞50年，此前只有限制性证据，而这可能是黑洞爆炸的首个迹象。”
当前，科学界正通过多种途径搜寻原初黑洞：包括监测黑洞合并产生的引力波、捕捉爆炸瞬间的高能光子爆发，以及分析火星轨道扰动等间接效应。马萨诸塞大学研究人员安德烈亚·塔姆团队甚至预测，若考虑暗光子等特殊物理机制，未来10年内探测到原初黑洞蒸发的概率可能超过90%。
尽管验证之路漫长，但此次高能中微子事件为探索宇宙极早期物理打开了新的窗口。正如耶鲁大学天体物理学家普里亚姆瓦达·纳塔拉詹所言：“原初黑洞迫使我们跳出传统暗物质模型的框架，以更开放的思维面对宇宙的奥秘。”
中文翻译：
巨型中微子或为远古黑洞的信使
引言
大约三年前，一颗来自太空的粒子猛烈撞击地中海，点亮了西西里岛海岸附近尚未完全建成的立方公里中微子望远镜（KM3NET）探测器。这颗粒子是中微子，一种物质的基本组成部分，以其能悄无声息地穿透其他物质而广为人知。
南极的冰立方中微子天文台，一个已运行十多年的类似探测器，已发现了数百个宇宙中微子——但没有一个与此次发现的完全相同。这颗粒子的能量比以往观测到的任何中微子高出约35倍，它可能源自一个高度活跃的星系——耀变体，或者科学家怀疑遍布宇宙的宇宙线高能粒子背景源。
但这并非唯一的可能性。就在KM3NET合作组织宣布探测到该中微子的第二天，物理学家大卫·凯泽走进麻省理工学院一间坐满同事的房间，提出了一个大胆的假设：如果这个巨型中微子来自一个正在爆炸的原初黑洞呢？
"这些黑洞'甚至可能在原子形成之前就已存在，更不用说恒星了，'"凯泽说，他一直深度参与寻找这些假想中的天体。
认为中微子来自原初黑洞的想法可能性不大；凯泽说他提出这个想法时是"半开玩笑的"。但在缺乏明确解释的情况下，这个想法依然引人入胜，尤其是因为原初黑洞的存在可能意味着它们在暗物质中扮演着某种角色。
那么问题来了：我们刚刚发现了一个吗？
转瞬之间
原初黑洞的概念最早由苏联物理学家雅科夫·泽尔多维奇和伊戈尔·诺维科夫于1966年提出，并由英国天体物理学家斯蒂芬·霍金在1971年巩固确立。随后，霍金与其学生、英国伦敦玛丽女王大学的伯纳德·卡尔在1974年详细阐述了原初黑洞的概念。
原初黑洞（PBH）大致定义为在宇宙诞生最初瞬间形成的黑洞。该假说认为，在空间的快速膨胀过程中，时空密度可能存在极高的峰值，以至于它们会坍缩成黑洞。这些黑洞的质量范围很广，取决于密度峰值的大小。有些可能小如原子核。
这个想法存在一些问题。纽约大学的理论物理学家温泽·秦表示，将原初黑洞纳入早期宇宙模型，需要物理学家非常精确地调整其模型的参数以符合观测结果。"你必须把旋钮调到恰到好处，"她说，"你需要密度峰值比标准宇宙学理论预测的高出10,000倍。"
就在霍金和卡尔阐述原初黑洞概念的同一年，霍金还提出了霍金辐射的概念。这是一个黑洞可能通过量子物理与引力在其边缘的相互作用，发射光子和其他基本粒子从而损失质量和能量的过程。
一个原本只有原子核大小的原初黑洞，通过释放霍金辐射，会在现代宇宙中走向终结，在缓慢衰变后，最终以一场突然、剧烈的粒子爆发告终。"在黑洞生命周期的大部分时间里，辐射掉的质量微乎其微，"麻省理工学院与凯泽合作研究原初黑洞的博士生亚历山德拉·克利普费尔说，"但在最后时刻，它会在一场非常迅速的爆炸中释放出大部分质量。它的温度升高得非常、非常快，这是一个失控的过程，最终以一场超高能粒子的大爆炸结束。"
"这个过程的发生'是因为黑洞的温度与其质量成反比，'"克利普费尔说，"质量越轻，温度越高。"
由于霍金辐射释放的能量并不偏爱任何一种特定粒子，最终的爆发将包含标准模型（我们解释宇宙的基准模型）中的所有17种基本粒子。在那一刻，随着黑洞的湮灭，数以万亿计的粒子将爆炸进入太空，"包括中微子、夸克以及各种奇异物质，"凯泽说。
最微小的原初黑洞可能只存在了瞬间，但质量更大的可能至今尚存。"一个质量约为10^14克（100万亿克）的黑洞，其寿命与宇宙的年龄相当，"克利普费尔说。
科学家已经排除（或限定了）可能隐藏在当今宇宙中的原初黑洞的质量范围。质量太小，原初黑洞早已蒸发殆尽；质量太大，其引力效应会扭曲来自遥远恒星和星系的光线，从而被观测到。对于大多数原初黑洞来说，似乎最佳的质量窗口范围大约从10^17克（相当于一颗普通小行星的质量）到10^23克（相当于一颗卫星的质量）。而一小部分质量在10^14克或更低（相当于一颗小型小行星）的原初黑洞，则正处于蒸发的最后阶段。
如果原初黑洞确实存在于小行星质量窗口内，它们不仅能告诉我们宇宙诞生之初的条件，还可能回答天体物理学中的另一个悬而未决的问题。如果它们在当今宇宙中仍然存在，它们可能构成了我们在星系旋转和宇宙结构中推断出的部分或全部缺失质量，科学家通常将这些缺失质量与未被探测到的暗物质粒子联系起来。
"它们是解释暗物质可能是什么的少数几个可靠理论之一，"秦说，"因此继续寻找它们非常重要。"
巨型瞬间
令凯泽团队惊讶的是，认为撞击KM3NET探测器的中微子源自原初黑洞爆炸的想法在数学上是成立的。2025年9月18日，凯泽和克利普费尔在《物理评论快报》上发表了一篇论文，解释了这一机制。他们发现，如果一个初始质量相当于一颗小型小行星的原初黑洞，在距离地球大约2000个天文单位（即地球与太阳距离的2000倍）的地方爆炸，就可能产生这颗强大的中微子。
"我们发现这种情况发生的概率约为8%，"克利普费尔说，"这是一个低概率事件，但并非完全不可能。"
这个位于小行星质量窗口下限（100万亿克）的原初黑洞，在宇宙138亿年的寿命中，会持续稳定地释放霍金辐射，直至其爆炸性的终结。
如果这个原初黑洞距离我们近得多，我们很可能已经通过伽马射线或其他辐射看到了它最终爆炸的信号。如果它距离太远，中微子就会过于分散，以至于没有一颗能击中地球。
那些认为原初黑洞构成大部分或全部暗物质的模型也预测，会有足够多的小型原初黑洞具有合适的质量，恰好在它们爆炸时飞过我们的太阳系，产生一阵高能粒子爆发，其中包括那颗恰好击中地球、触发KM3NET探测器的中微子。
与霍金共同巩固了原初黑洞概念的卡尔，觉得这个想法很诱人，并希望KM3NET探测到的这颗壮观的中微子是一个信号，表明我们最终可能发现原初黑洞。"我研究原初黑洞已经50年了，一直没有任何声称的证据，只有各种限制条件，这真的很令人沮丧，"卡尔说，"这可能就是黑洞爆炸的证据。"
宇宙探测
并非所有人都信服。"我不知道这颗KM3NET中微子来自哪里，但我敢用一大笔钱打赌，它与原初黑洞毫无关系，"威斯康星大学麦迪逊分校的宇宙学家丹·胡珀说。他认为，如果这种爆炸的黑洞存在，它们应该很容易被观测到，"而我们并没有看到，"他说，"我认为完全可以排除这个假设。"
然而，胡珀并非完全反对原初黑洞本身的概念，他致力于研究它们如何在宇宙暴胀时期形成。"在早期宇宙中，它们有很多种形成方式，"他说。
其他人，如温尼伯大学的理论物理学家埃文·麦克多诺，则更确信原初黑洞今天仍然存在。"我个人的直觉是，宇宙中可能至少存在一个原初黑洞，"他说，"但它们是否以可观的数量存在，才是关键问题。"
耶鲁大学的理论天体物理学家普里亚姆瓦达·纳塔拉詹表示，尽管她认为原初黑洞最多只能构成暗物质的极小一部分，但她对它们作为一种探索暗物质概念的工具很感兴趣，特别是除了两种最流行的候选者——弱相互作用大质量粒子（WIMP）和更像波动的轴子之外。"原初黑洞确实迫使我们重新思考暗物质问题，摆脱对WIMP和轴子的执着，"她说，"从这个意义上说，它们在科学上非常重要。它们让我们打破了思维定式，变得更加开放。"
如果原初黑洞存在，我们有几种方法可能找到它们。例如，我们可能会在引力波信号中发现来自质量小于恒星但仍足够大、能在我们的仪器中产生可探测引力波的原初黑洞合并的信号。这种质量的黑洞需要通过原初方式形成。
与此同时，马萨诸塞大学阿默斯特分校的安德烈亚·塔姆及其同事在9月份提出，如果我们引入一些奇异的物理理论，爆炸的黑洞可能比理论家想象的更常见，并可能在不久的将来被发现。
在他们的模型中，他们认为被称为暗光子和暗电子（正常物质的暗物质变体）的粒子，可能降低了宇宙中较低质量原初黑洞释放霍金辐射的速率。这可能意味着今天有更多的原初黑洞正处于蒸发的最后阶段。如果是这样，塔姆和同事们提出，在一定的限制条件下，未来10年内发现一个原初黑洞蒸发的概率超过90%。
"在（最终爆炸的）一个相当短的时间窗口内，大约1000秒，会释放出能量极高的光子，"塔姆说，她提到像墨西哥的高海拔水切伦科夫观测站这样的现有实验可以寻找此类事件。"然后，一旦原初黑洞爆炸，信号就会突然停止。"
凯泽提出的另一个新颖想法是，如果原初黑洞确实落入小行星质量范围，并且构成了大部分或全部暗物质，那么它们应该会偶尔以每秒数百公里的速度飞过我们的太阳系。在这种情况下，它们可能会产生明显的引力效应。
凯泽说，利用环绕火星运行的航天器，我们可以精确计算地球到这颗红色星球的距离，并寻找任何可能表明有原初黑洞飞过的微小摆动。他说，在任何特定时刻，太阳系内至少会有一个原初黑洞，可能产生可探测的霍金辐射。每三到十年，就会有一个原初黑洞足够接近火星，从而对火星的运动产生微小但可测量的变化。
"在几十厘米的尺度上，'火星将开始偏离其原本被精确追踪的轨道，'"凯泽说。他计划在未来几年与一组天文学家合作研究这个想法。
除此之外，凯泽希望继续留意其他高能中微子，以验证其中一些可能来自爆炸黑洞的想法。证明这一点很困难，因为来自原初黑洞的中微子看起来与来自其他来源的任何中微子一样，除非它们能与天空中来自爆炸原初黑洞的伽马射线闪光相关联。
要确定原初黑洞是否存在，还有很长的路要走，更不用说它们是否构成暗物质了。但就目前而言，科学家们不能排除这样一种可能性：大约三年前，一个原初黑洞曾掠过地球，向一个水下中微子探测器发送了一位孤独的信使。
"考虑到我当时是半开玩笑地走进房间的，这些数字的吻合程度简直惊人，"凯泽说。
编者注：普里亚姆瓦达·纳塔拉詹是《量子杂志》顾问委员会的成员。
英文来源：
Monster Neutrino Could Be a Messenger of Ancient Black Holes
Introduction
Nearly three years ago, a particle from space slammed into the Mediterranean Sea and lit up the partially complete Cubic Kilometer Neutrino Telescope (KM3NET) detector off the coast of Sicily. The particle was a neutrino, a fundamental component of matter commonly known for its ability to slip through other matter unnoticed.
The IceCube observatory in Antarctica, a comparable detector that has been running for more than a decade, has found hundreds of cosmic neutrinos — but none quite like this one. Some 35 times more energetic than any neutrino seen before, the particle might have shot out from a highly active galaxy — a blazar — or a background source of cosmogenic high-energy particles that scientists suspect pervade the cosmos.
But those aren’t the only possibilities. The day after the KM3NET collaboration announced the detection, the physicist David Kaiser walked into a room full of his colleagues at the Massachusetts Institute of Technology with a bold proposition: What if the monster neutrino came from an exploding primordial black hole?
Such black holes “could form before there were even atoms, let alone stars,” said Kaiser, who has been heavily involved in the hunt for these hypothetical objects.
The idea that the neutrino came from a primordial black hole is a long shot; Kaiser said he was “half-joking” when he suggested it. But in the absence of a definitive explanation, it remains intriguing, not least because the existence of primordial black holes could mean they play a role in dark matter.
So the question is, did we just spot one?
In a Split Second
The idea of primordial black holes was first proposed in 1966 by the Soviet physicists Yakov Zel’dovich and Igor Novikov, and it was cemented by the British astrophysicist Stephen Hawking in 1971. Hawking and his student Bernard Carr, of Queen Mary University of London in the U.K., then worked out the concept of primordial black holes in detail in 1974.
A primordial black hole, or PBH, is loosely defined as a black hole that formed in the first split second of the universe. The hypothesis goes that during the rapid expansion of space, there might have been spikes in the density of space-time that were so high that they would have collapsed into black holes. These black holes would have spanned a range of masses, depending on the size of the spikes. Some could have been as small as an atomic nucleus.
The idea has its problems. Incorporating PBHs into the early universe requires physicists to adjust the parameters of their models very precisely to fit with observations, said Wenzer Qin, a theoretical physicist at New York University. “You have to tune the knob just right,” she said. “You need the density spikes to be 10,000 times larger than you would have predicted in standard cosmological theory.”
The same year Hawking and Carr explained the concept of primordial black holes, Hawking also put forward his idea of Hawking radiation, a process in which black holes might lose mass and energy by emitting photons and other fundamental particles through the interaction of quantum physics and gravity at the black hole’s edge.
Giving off Hawking radiation, a primordial black hole that was originally the size of an atomic nucleus would meet its doom in the modern universe, slowly dwindling before ending in a sudden, extreme burst of particles. “Very little mass gets radiated over the majority of the black hole’s lifetime,” said Alexandra Klipfel, a doctoral student working with Kaiser on PBHs at MIT. “But then, right at the end, it emits a majority of its mass in a very rapid explosion. It heats up really, really quickly, a runaway process that ends in a big explosion of ultra-high-energy particles.”
That process occurs “because the temperature of the black hole is inversely proportional to the mass,” Klipfel said. “The lighter the mass, the hotter the temperature.”
Because the energy released in Hawking radiation doesn’t favor one type of particle over another, the final burst would include all 17 fundamental particles in the Standard Model, our benchmark model explaining the cosmos. In that moment, as the black hole was extinguished, trillions upon trillions of particles would explode into space, “including neutrinos and quarks and all kinds of exotic things,” Kaiser said.
The tiniest primordial black holes would have lasted only moments, but more massive ones could still be around. “A black hole with a mass of about 1014 [100 trillion] grams has a lifetime equal to the age of the universe,” Klipfel said.
Scientists have ruled out (or constrained) the possible masses of PBHs that could be hiding in the universe today. Too small and the PBHs would have evaporated already. Too big and their gravitational effects would have been spotted warping the light coming to Earth from distant stars and galaxies. The best window left for most PBHs, it seems, ranges from about 100 quadrillion (1017) grams — the mass of an average asteroid — to 100 sextillion (1023) grams, the mass of a moon. A smaller subpopulation of PBHs of 100 trillion (1014) grams or lower, the mass of a small asteroid, would be in their final stages of evaporation.
If primordial black holes do exist in the asteroid-mass window, not only can they tell us about the conditions at the dawn of the cosmos, but they might also answer another open question in astrophysics. If they still exist in the present universe, they might constitute some or all of the missing mass we can infer in the rotation of galaxies and the structure of the cosmos that scientists more commonly link to undetected particles of dark matter.
“They’re one of the few good theories for what dark matter could be,” Qin said. “So it’s important to keep looking for them.”
Monster Moment
To the surprise of Kaiser’s team, the idea that the neutrino that crashed into the KM3NET detector originated in the explosion of a primordial black hole worked mathematically, and on September 18, 2025, Kaiser and Klipfel published a paper in Physical Review Letters explaining the mechanism. They found that, if a primordial black hole with the initial mass of a small asteroid exploded about 2,000 astronomical units away — 2,000 times the distance between the Earth and the sun — it could have produced the powerful neutrino.
“We found about an 8% chance of this happening,” Klipfel said. “It’s a low-probability event. But it’s not a completely impossible event.”
The PBH, at the lower 100-trillion-gram end of the asteroid-mass window, would have steadily emitted Hawking radiation over the 13.8-billion-year lifespan of the universe until its explosive finale.
Had the PBH been much closer, we likely would have seen its flash in gamma rays or other radiation that would have signaled its final explosion. If it had been too far away, the neutrinos would have been too spread out for one to hit Earth.
Models in which PBHs account for most or all of the dark matter also predict that enough of the smaller PBHs would have the correct mass for one to fly past our solar system right as it exploded, producing a burst of energetic particles, including the neutrino that hit our planet in just the right spot to trigger the KM3NET detector.
Carr, who solidified the concept of PBHs with Hawking, finds the idea enticing and is hopeful that KM3NET’s spectacular neutrino is a sign that we might discover PBHs after all. “I’ve been working on primordial black holes for 50 years, and there was no purported evidence for them, just constraints, which is really sad,” Carr said. “This could be evidence for black hole explosions.”
Cosmic Detection
Not everyone is convinced. “I don’t know where this KM3NET neutrino comes from, but I would bet an awful lot of money that it has nothing to do with primordial black holes,” said Dan Hooper, a cosmologist at the University of Wisconsin, Madison. He argues that if such exploding black holes existed, they would be easy to see, “and we don’t see those,” he said. “I think you can completely rule out that hypothesis.”
Hooper is not completely opposed to the idea of PBHs themselves, however, and he has worked to understand how they might have formed during the period of cosmic inflation. “There are lots of ways they could have been made in the early universe,” he said.
Others, like Evan McDonough, a theoretical physicist from the University of Winnipeg, are more confident that PBHs exist today. “My personal hunch is that there is probably at least one PBH out there,” he said. “Whether or not they exist in some sizable amount is the big question.”
Priyamvada Natarajan, a theoretical astrophysicist from Yale University, says that although she thinks PBHs cannot account for more than a tiny fraction of dark matter, she is interested in them as a tool to explore ideas for dark matter outside its two most popular candidates, weakly interacting massive particles (WIMPs) and the more wavelike axions. “PBHs are really forcing us to rethink the dark matter problem and move away from our fixation with WIMPs and axions,” she said. “In that sense they’re scientifically really important. They’ve allowed us to break out of a mindset and be more open.”
If primordial black holes exist, there are several ways we might find them. For example, we might spot a signal in gravitational waves from the merger of PBHs smaller than the mass of a star but still huge enough to produce detectable gravitational waves in our instruments. Black holes of such mass would have needed to form through primordial means.
Andrea Thamm of the University of Massachusetts, Amherst and her colleagues, meanwhile, proposed in September that exploding black holes might be more common than theorists thought and could be spotted in the near future, if we invoke some exotic physics.
In their model, they suggest that particles known as dark photons and dark electrons — dark matter variants of normal matter — could have reduced the rate at which lower-mass PBHs in the universe emitted Hawking radiation. That could mean many more PBHs are in the final stages of evaporation today. If that’s so, Thamm and colleagues suggest that within certain constraints, there is a more than 90% chance of spotting the evaporation of a primordial black hole in the next 10 years.
“Within a fairly short time window [of the final explosion], around 1,000 seconds, there would be very highly energetic photons [emitted],” said Thamm, who said existing experiments like the High-Altitude Water Cherenkov observatory in Mexico could look for such events. “And then it would very suddenly stop once the primordial black hole has exploded.”
Another novel idea, proposed by Kaiser, is that if PBHs do indeed fall into the asteroid-mass range and constitute most or all of dark matter, they should occasionally fly through our solar system at hundreds of kilometers a second. In this scenario, they might produce noticeable gravitational effects.
Using spacecraft orbiting Mars, Kaiser said, we could precisely calculate the distance from Earth to the red planet and look for any wobbles that might indicate a PBH flying past. At any given time, he said, there would be at least one PBH in the solar system, possibly producing detectable Hawking radiation. Every three to 10 years, one would get close enough to Mars to produce a tiny but measurable change in the planet’s motion.
In the tens of centimeters, “Mars will begin rocking away from its otherwise well-tracked orbit,” Kaiser said. He is planning to work on the idea with a team of astronomers over the next couple of years.
Alongside that, Kaiser wants to keep an eye out for other high-energy neutrinos to test the idea that some might come from exploding black holes. Proving that this is true is difficult because PBH neutrinos would look just like any other neutrinos produced from another source, unless they could be associated with a flash of gamma rays in the sky from an exploding PBH.
There is still a long way to go before we can say whether primordial black holes exist, let alone whether they make up dark matter. But for now, scientists cannot rule out the possibility that one slipped past Earth nearly three years ago, sending a lone emissary into an underwater neutrino detector.
“The numbers are outrageously congruent, considering I walked in half-joking,” Kaiser said.
Editor’s note: Priyamvada Natarajan is a member of Quanta Magazine’s advisory board.