Astronomy’s Greatest Unsolved Mysteries

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Unraveling the Cosmos

Astronomy is a science of immense scales and significant questions. For millennia, humans have looked to the skies and wondered about our place in the universe. With each technological leap, from the first telescopes to space-faring observatories, we’ve peeled back layers of the cosmos, revealing wonders that have reshaped our understanding of reality. Yet, with every answer we find, new and more complex questions emerge. The universe, it seems, is a master at keeping secrets.

Today, we live in a golden age of astronomy. We can map the afterglow of the Big Bang, watch stars being born in distant nebulae, and detect planets orbiting other suns. We have a remarkably successful model of the universe’s history and composition, known as the Lambda-CDM model. This model tells a grand story of a universe that began 13.8 billion years ago, is expanding at an accelerating rate, and is filled with stars, galaxies, and a mysterious substance called dark matter, all driven by an even more enigmatic force called dark energy.

But this model is incomplete. It’s more of a detailed sketch than a finished portrait. Dotted across the landscape of modern astronomy are deep and persistent mysteries – observations that don’t quite fit, theories that crumble at the edges, and fundamental questions that we don’t even know how to properly ask yet. These unsolved problems aren’t signs of failure; they are the signposts of discovery, pointing the way toward a deeper, more complete understanding of the universe. They are the puzzles that drive scientific progress, inspiring new theories, technologies, and generations of astronomers. From the nature of the universe’s beginning and end to the composition of its unseen components and the potential for life beyond Earth, these are the greatest unsolved mysteries in astronomy.

Cosmology: The Grandest Questions

Cosmology deals with the origin, evolution, and ultimate fate of the universe. It’s the field that tackles the biggest questions of all. While we know the universe began with the Big Bang and has been expanding ever since, the details of its past, its present composition, and its ultimate destiny are shrouded in mystery.

The Ultimate Fate of the Universe

One of the most fundamental questions is: how will it all end? The fate of the universe is intimately tied to its shape, its density, and the nature of the mysterious dark energy. For a long time, the debate centered on two main possibilities. If the universe contained enough matter, the gravitational pull of everything would eventually halt the expansion and pull the universe back in on itself, ending in a fiery “Big Crunch,” a sort of reverse Big Bang. If there wasn’t enough matter, the expansion would continue forever, with galaxies growing ever more distant, stars burning out, and the universe becoming a cold, dark, and empty place known as the “Big Freeze” or “Heat Death.”

The discovery in the late 1990s that the universe’s expansion is accelerating threw a wrench into this picture. This acceleration, attributed to dark energy, suggests the Big Freeze is the most likely scenario. But what if dark energy isn’t constant? If its strength increases over time, it could lead to a far more dramatic end. This scenario, called the “Big Rip,” would see the repulsive force of dark energy grow so strong that it would eventually overcome gravity and all other forces. First, it would pull apart galaxy clusters, then galaxies themselves. In the final moments, solar systems, stars, planets, and even the atoms that make up everything we know would be torn asunder. We don’t know the true nature of dark energy, so we can’t be certain which of these fates, or perhaps another we haven’t even imagined, awaits our universe billions of years from now.

The Enigma of Dark Energy

The accelerating expansion of the universe is one of the most significant discoveries in modern science, and it points to the existence of dark energy. But what is it? The honest answer is that we don’t know. Dark energy seems to make up about 68% of the energy density of the entire universe, yet it remains a complete enigma.

The leading idea is that it’s a property of space itself, what was originally called the “cosmological constant.” In this view, every cubic centimeter of empty space contains a certain amount of this repulsive energy. As the universe expands, more space is created, and so more of this energy comes into being, driving the expansion even faster. Another possibility is a new kind of energy field, dubbed “quintessence,” that pervades the universe and can change over time. Distinguishing between these ideas requires incredibly precise measurements of the universe’s expansion history. Missions like the European Space Agency’s Euclidtelescope and NASA’s Nancy Grace Roman Space Telescope are designed to do just that, mapping the distribution of galaxies through cosmic time to chart the influence of dark energy with unprecedented accuracy. Uncovering its identity would revolutionize physics.

The Mystery of Dark Matter

Long before we knew about dark energy, astronomers were puzzled by another unseen component of the cosmos: dark matter. The mystery began in the 1930s with observations that galaxies in the Coma Clusterwere moving too fast. There wasn’t enough visible matter – stars and gas – to generate the gravity needed to hold the cluster together. A proposal was made for the existence of “dark matter” that provided the extra gravitational glue. These ideas were largely unexamined for decades.

The evidence became undeniable in the 1970s with detailed studies of spiral galaxies. Researchers studied the rotation of these galaxies and found something strange. According to our understanding of gravity, stars far from the galactic center should orbit more slowly than stars closer in, just as Pluto moves more slowly than Mercury. Instead, they found that the stars’ speeds remained constant or even increased at the outer edges. The only way to explain this is if galaxies are embedded in a massive, invisible halo of dark matter, extending far beyond the visible stars. We now believe that dark matter outweighs normal matter by a factor of more than five to one, making up about 27% of the universe.

We can see its effects through gravitational lensing, where its immense gravity bends the light from distant objects, and in the structure of the cosmic microwave background (CMB), the relic radiation from the Big Bang. But we still don’t know what it is. The leading candidates are hypothetical particles that don’t interact with light or normal matter, except through gravity. These include WIMPs (Weakly Interacting Massive Particles) and axions. Huge, sensitive detectors have been built deep underground to try and catch a fleeting glimpse of a WIMP bouncing off a nucleus, so far without success. Another idea is that dark matter is made of primordial black holes, ancient black holes formed in the very early universe. A more radical proposal suggests that dark matter doesn’t exist at all, and that our theory of gravity is wrong on cosmic scales, a concept known as Modified Newtonian Dynamics (MOND). Solving this mystery is a top priority in both astronomy and particle physics.

The Baryon Asymmetry Problem

The universe we see is made almost entirely of matter. We are made of matter. Earth, the Sun, and all the stars and galaxies are made of matter. But according to the Standard Model of particle physics, the Big Bang should have produced equal amounts of matter and its mirror-image counterpart, antimatter. When a particle of matter and a particle of antimatter meet, they annihilate each other in a flash of energy. If the early universe had perfect symmetry, all the matter and antimatter would have canceled out, leaving behind a universe filled with nothing but light.

Clearly, that’s not what happened. We exist. So, there must have been a slight imbalance, or asymmetry. For every billion antimatter particles, there must have been a billion and one matter particles. After all the annihilations were over, that tiny remnant of matter was left over to form everything we see today. This is the baryon asymmetry problem. Why was there an imbalance? Physicists have identified conditions necessary for this to happen, known as the Sakharov conditions, but the known physics of the Standard Model isn’t sufficient to explain the size of the asymmetry we observe. There must be some new, unknown physics, perhaps involving undiscovered particles or interactions in the extreme conditions of the early universe, that tipped the scales in favor of matter. Experiments at particle accelerators like the Large Hadron Collider at CERN are searching for clues by studying the subtle differences between matter and antimatter particles.

The Cosmological Constant Problem

While the cosmological constant is the leading explanation for dark energy, it comes with its own colossal problem. When physicists try to calculate the expected energy of empty space based on quantum mechanics, they get a number that is staggeringly, absurdly large. It’s about 120 orders of magnitude – that’s a 1 followed by 120 zeros – larger than the value we observe from the accelerating expansion of the universe. This discrepancy has been called “the worst theoretical prediction in the history of physics.”

It’s known as the cosmological constant problem. Why is the observed value so small compared to the theoretical one? It suggests that there must be some unknown physical mechanism that is canceling out almost all of the vacuum energy, leaving just a tiny, non-zero remnant that we detect as dark energy. Perhaps there’s a symmetry in nature we haven’t discovered yet. Or maybe the problem lies in our fundamental understanding of how gravity and quantum mechanics work together. Some theories, like supersymmetry or string theory, offer potential pathways to a solution, but none have been proven. It’s one of the deepest and most significant disconnects between theory and observation in all of science.

The Hubble Tension

The universe is expanding, but how fast? The rate of this expansion is described by the Hubble constant (H₀). Measuring this value precisely is essential for determining the age, size, and fate of the universe. The problem is that the two primary methods for measuring it give two different answers.

The first method looks at the “late” universe, the cosmos as it is relatively nearby. Astronomers measure the distances to objects with known intrinsic brightness, like Cepheid variable stars and Type Ia supernovae, which act as “standard candles.” By comparing their known brightness to how bright they appear, we can calculate their distance. Then, by measuring their redshift – how much their light has been stretched by the expansion of space – we can determine how fast they are moving away from us. This method consistently yields a value of around 73 kilometers per second per megaparsec.

The second method looks at the “early” universe. It uses the detailed patterns seen in the cosmic microwave background, the faint afterglow of the Big Bang. Satellites like the Wilkinson Microwave Anisotropy Probe(WMAP) and Planck have mapped these tiny temperature fluctuations. By feeding this data into our standard cosmological model (Lambda-CDM), scientists can predict what the expansion rate should be today. This method consistently gives a value of around 67 kilometers per second per megaparsec.

The discrepancy, known as the Hubble tension, is significant and has grown more statistically robust as measurements have improved. It’s not a matter of small observational errors. It could be a sign of systematic errors in one or both measurement techniques, but it could also be a crack in our standard model of cosmology. Perhaps there was a new type of “early dark energy” in the young universe that briefly accelerated the expansion before fading away, or some other unknown physics that our model doesn’t account for. Resolving this tension is a major focus of modern cosmology.

The Axis of Evil

The cosmological principle is a cornerstone of modern cosmology. It states that on large scales, the universe is homogeneous and isotropic, meaning it looks roughly the same everywhere and in every direction. This assumption is built into our standard model and seems to be supported by the large-scale distribution of galaxies. However, detailed maps of the cosmic microwave background from the WMAP and Planck satellites have revealed some strange, large-scale anomalies that appear to challenge this principle.

The most famous of these is the so-called “Axis of Evil.” It’s a peculiar alignment of several features in the CMB’s temperature fluctuations. The largest patterns in the sky, known as the quadrupole and octopole, seem to be aligned with each other, and this alignment points directly toward our solar system’s ecliptic plane and equinoxes. This is a very strange coincidence. Why should the largest structures in the entire observable universe be aligned with the plane of our local solar system?

It could be a massive statistical fluke, a one-in-a-thousand chance that we just happen to observe. Or it could be a sign of some contamination in the data, perhaps from our own galaxy, that we haven’t properly accounted for. But if the alignment is real, it could have significant implications. It might suggest that the universe is not truly isotropic, and that there is a preferred direction in the cosmos. Some unconventional theories propose that this could be related to the universe’s overall shape or topology. For now, it remains a tantalizing and controversial cosmic puzzle.

The Shape of the Universe

What is the overall geometry of space? According to the theory of general relativity, mass and energy curve spacetime. On a cosmic scale, the universe can have one of three possible shapes, depending on its total energy density. If the density is above a certain value, the universe would be “closed,” like the surface of a sphere. Parallel lines would eventually meet, and the universe would be finite in size. If the density is below that value, it would be “open,” like the surface of a saddle. Parallel lines would diverge, and the universe would be infinite. If the density is exactly at that value, the universe would be “flat,” like an infinite plane. Parallel lines would remain parallel forever.

Our best measurements, primarily from the CMB, indicate that the universe is remarkably close to being flat. This is a strange result. A universe that starts out even slightly curved will see that curvature grow dramatically over cosmic time. For it to be so flat today means it must have been unbelievably flat at the beginning. This is known as the flatness problem, and it’s one of the motivations for the theory of cosmic inflation. However, some recent data from the Planck satellite has hinted that the universe might actually be slightly closed. This finding is in tension with other datasets and is still hotly debated. Determining the true shape of the universe is key to understanding its origin and its ultimate fate.

The Problem of Cosmic Inflation

The theory of cosmic inflation was proposed in the 1980s to solve several major puzzles in Big Bang cosmology, including the horizon and flatness problems. The idea is that in the first fraction of a second after the Big Bang, the universe underwent a period of incredibly rapid, exponential expansion. It grew from a size smaller than a proton to something like the size of a grapefruit in an infinitesimal amount of time.

This period of inflation would have stretched any initial curvature of the universe into near-perfect flatness, explaining the flatness problem. It also explains the horizon problem: the entire observable universe was once in close contact before inflation, allowing it to reach a uniform temperature. Inflation also provides a mechanism for seeding the structures we see today; tiny quantum fluctuations during this period would have been stretched to cosmic scales, becoming the seeds for galaxies and galaxy clusters.

The theory is wildly successful and its predictions match observations of the CMB with stunning precision. But we don’t know what caused it. What was the “inflaton,” the hypothetical field that drove this expansion? What turned it on, and more importantly, what turned it off? There are many different models of inflation, but we lack the data to distinguish between them. Physicists hope to find evidence in the form of primordial gravitational waves, ripples in spacetime that would have been generated during inflation. Detecting these faint signals in the polarization of the CMB is a primary goal for future experiments.

The Horizon Problem

The horizon problem is one of the key puzzles that inflation was designed to solve. When we look at the cosmic microwave background, we see that it has an almost perfectly uniform temperature in every direction, about 2.725 Kelvin. This means that regions of the universe that are on opposite sides of our sky, currently separated by more than 27 billion light-years, have the same temperature.

The problem is that, according to the standard Big Bang model (without inflation), these regions have never been in causal contact. There hasn’t been enough time since the beginning of the universe for a light signal, or any other kind of information, to travel between them. So how did they “know” to end up at the same temperature? It’s like finding two people on opposite sides of a vast desert who have never met or communicated, yet have their watches synchronized to the exact same second. Cosmic inflation provides a solution by postulating that these distant regions were once right next to each other, in thermal equilibrium, before being rapidly pushed far apart.

The Flatness Problem

The flatness problem, as mentioned earlier, is the puzzle of why the universe’s geometry is so close to being Euclidean, or flat. The density of the universe today is very close to the “critical density” required for a flat geometry. The problem is that this is an unstable equilibrium point. If the early universe had a density that was even slightly different from the critical density, that deviation would have been magnified enormously over 13.8 billion years of expansion.

For the universe to be as flat as we observe it today, its density in the first second after the Big Bang must have been tuned to the critical density to an accuracy of one part in 10⁶². That’s like balancing a pencil on its tip and having it stay there for billions of years. It’s a level of “fine-tuning” that seems unnatural and cries out for an explanation. Cosmic inflation provides a natural one: the incredible stretching of space would have flattened any initial curvature, just as inflating a tiny, crumpled balloon to the size of the Earth would make its surface appear very flat to any observer on it.

The Magnetic Monopole Problem

In our everyday experience, magnets always have two poles: a north and a south. If you cut a bar magnet in half, you don’t get a separate north and south pole; you get two smaller magnets, each with its own north and south pole. A particle with only one magnetic pole – a magnetic monopole – has never been observed.

The problem is that many compelling theories of particle physics, especially the Grand Unified Theories that attempt to unite the fundamental forces of nature, predict that magnetic monopoles should have been created in huge numbers in the extreme heat of the very early universe. If that’s true, where are they all? The universe should be teeming with them, but extensive searches have come up empty. This is the magnetic monopole problem. Cosmic inflation also provides a neat solution to this riddle. The rapid expansion would have diluted the density of any monopoles that were created, spreading them out so thinly across the vast expanse of the universe that the chance of finding one in our observable patch is practically zero.

The Eridanus Supervoid

The cosmic microwave background is incredibly uniform, but it does have tiny hot and cold spots. One of these spots is not so tiny. Located in the direction of the constellation Eridanus, the CMB Cold Spot is a vast region of the sky that is slightly colder than its surroundings. It’s much larger and colder than expected from the standard model of cosmology.

One possible explanation is that it’s simply a rare statistical fluctuation. In any random map, you’re bound to find some unusual patterns. But another, more exciting possibility is that the cold spot is the imprint of an enormous cosmic structure that lies between us and the CMB. Surveys of galaxies in that direction have revealed a massive underdensity of matter, a colossal void largely empty of galaxies, known as the Eridanus Supervoid. This void could be one of the largest structures in the universe, perhaps a billion light-years across. Light from the CMB loses energy as it enters such a void and then should regain it as it climbs out. Due to dark energy, this process isn’t perfectly symmetrical, resulting in a net cooling effect. However, it’s not clear if the observed void is large enough and empty enough to fully account for the coldness of the spot. Other, more exotic explanations have been proposed, such as the signature of another universe bumping into our own.

The Multiverse Hypothesis

The idea that our universe is not the only one, but is part of a much larger multiverse, sounds like science fiction, but it arises naturally from some of our leading scientific theories. The theory of eternal inflation, for example, suggests that while inflation stopped in our patch of the universe, it continues in other regions, constantly spawning new “bubble” universes with potentially different physical laws and constants. String theory also allows for a vast “landscape” of possible universes.

This is a tantalizing idea. It could potentially explain the fine-tuning problem – the observation that many of the fundamental constants of our universe seem to be perfectly tuned to allow for the existence of complex structures and life. If there are countless universes, each with different constants, it’s not surprising that we find ourselves in one that is hospitable to us. The problem is that the multiverse hypothesis is, at present, untestable and therefore unfalsifiable. How could we ever detect another universe? Some have suggested looking for the “bruises” of collisions between bubble universes in the cosmic microwave background, but these searches have been inconclusive. For now, the multiverse remains a fascinating but speculative idea on the very edge of science.

Galaxies and Beyond: Structures in the Cosmos

Galaxies are the great cities of the cosmos, magnificent islands of stars, gas, and dust. They come in a variety of shapes and sizes, from majestic spirals like our own Milky Way to giant, featureless ellipticals. But these cosmic structures present their own set of deep and enduring mysteries.

The Galaxy Rotation Problem

The galaxy rotation problem is the primary piece of evidence for the existence of dark matter. Observations show the outer parts of spiral galaxies rotate far too quickly. The visible matter simply doesn’t provide enough gravity to hold them together. Without the extra gravitational pull from a massive halo of dark matter, these galaxies should fly apart.

While the dark matter hypothesis is the most widely accepted explanation, it isn’t perfect. The alternative theory, Modified Newtonian Dynamics (MOND), proposes that gravity itself behaves differently on galactic scales, becoming stronger at very low accelerations. MOND is remarkably successful at predicting the rotation curves of many galaxies without invoking dark matter. However, MOND struggles to explain the behavior of galaxy clusters and the patterns in the CMB. The puzzle remains: is there an unseen form of matter governing the motions of galaxies, or is our understanding of gravity itself incomplete?

The AGN Feedback Puzzle

At the center of most, if not all, large galaxies lurks a supermassive black hole, with a mass millions or even billions of times that of our Sun. When these black holes are actively feeding on surrounding gas and dust, they become incredibly luminous and are known as Active Galactic Nuclei (AGN). The energy they release can be enormous, often outshining the entire host galaxy.

This energy can have a significant effect on the galaxy itself, in a process known as AGN feedback. Powerful jets and winds launched from the vicinity of the black hole can heat up and blow away the gas in the galaxy. This can quench star formation, effectively stopping the galaxy from growing. We see evidence for this process, and it’s a key ingredient in our models of galaxy evolution. Without it, simulations produce galaxies that are far more massive and have many more stars than we actually observe. The problem is that the physics of how this feedback works is incredibly complex and poorly understood. How is the energy from the tiny central region around the black hole coupled to the vast scales of the entire galaxy? How does it regulate star formation over billions of years? Answering these questions is key to understanding the life cycle of galaxies.

The Origin of Galactic Magnetic Fields

Magnetic fields are everywhere in the cosmos, and they play a vital role in many astronomical processes, from guiding the solar wind to shaping supernova remnants. Galaxies, too, are permeated by large-scale, organized magnetic fields. We can detect them by observing the polarized light from distant sources as it passes through a galaxy.

The mystery is how these large and coherent fields got there in the first place. We believe they are amplified by a “dynamo” process, similar to the one that generates Earth’s magnetic field. As a galaxy rotates, the motions of its plasma (ionized gas) can stretch, twist, and amplify weak, seed magnetic fields. But where did the seed fields come from? They could have been generated in the very early universe, during inflation or other phase transitions. Or they could have been ejected from the first stars and galaxies. We don’t have a definitive answer. Understanding the origin and evolution of galactic magnetic fields is essential for a complete picture of how galaxies form and evolve.

The Missing Satellite Problem

Our standard cosmological model, Lambda-CDM, has been incredibly successful at explaining the large-scale structure of the universe. Computer simulations based on this model predict that large galaxies like the Milky Way should have formed from the merger of many smaller “dwarf” galaxies. As a result, a galaxy like ours should be surrounded by hundreds, if not thousands, of smaller satellite galaxies orbiting it.

This is the missing satellite problem. When we look around the Milky Way, we see only a few dozen satellite galaxies, far fewer than the simulations predict. A similar discrepancy is found around our neighboring Andromeda Galaxy. Where are all the missing satellites? One possibility is that our simulations are too simple. Perhaps astrophysical processes like stellar feedback – energy from supernovae blowing gas out of small galaxies – prevented most of these dwarf galaxies from ever forming stars, so they remain as completely dark, unobservable halos of dark matter. Another idea is that the nature of dark matter itself is different from what we assume. If dark matter particles were “warm” instead of “cold,” they would move faster in the early universe, smoothing out small-scale structures and preventing the formation of so many small halos. Deep sky surveys with telescopes like the Vera C. Rubin Observatory are discovering more and more faint satellite galaxies, which is helping to ease the tension, but a discrepancy still remains.

The Tully-Fisher Relation Puzzle

The Tully-Fisher relation is a remarkably tight empirical relationship observed in spiral galaxies. It connects a galaxy’s intrinsic luminosity (the total amount of light it emits) to its rotation speed. Brighter galaxies rotate faster. This relationship is incredibly useful; by measuring a galaxy’s rotation speed, astronomers can determine its luminosity, and by comparing that to its apparent brightness, they can calculate its distance.

The puzzle is why this relationship is so tight and universal. In the standard dark matter paradigm, a galaxy’s luminosity is determined by its stars and gas (the baryonic matter), while its rotation speed is dominated by its dark matter halo. Why should these two completely different components be so closely linked? It suggests a deep connection between the formation of the visible galaxy and the properties of its dark matter halo. MOND proponents point to this relation as strong evidence for their theory, as it arises naturally from their proposed modification of gravity without needing to fine-tune the relationship between dark and normal matter. Explaining the origin of the Tully-Fisher relation is a key challenge for any model of galaxy formation.

Ultra-diffuse Galaxies

In recent years, astronomers have discovered a new class of bizarre galaxies known as ultra-diffuse galaxies(UDGs). These are galaxies that are as large as the Milky Way, but contain only about 1% as many stars. Their stars are spread out so thinly that the galaxies are incredibly faint and almost transparent, making them very difficult to detect.

The existence of these ghost-like galaxies poses a challenge to our theories of galaxy formation. How did they form? One idea is that they are “failed” Milky Way-type galaxies that lost all their gas early on, perhaps through interactions with a larger galaxy cluster, which quenched their star formation. Another puzzle is their dark matter content. Some UDGs appear to be almost entirely made of dark matter, with mass-to-light ratios hundreds of times that of normal galaxies. But in 2018, astronomers discovered a UDG named NGC 1052-DF2 that appears to have almost no dark matter at all. This was a shocking discovery, as it’s hard to explain how a galaxy could form without the gravitational scaffolding of a dark matter halo. The existence of UDGs, with their wide range of properties, presents a complex and fascinating puzzle for astronomers to solve.

The Puzzle of Ultra-luminous X-ray Sources

Ultra-luminous X-ray sources (ULXs) are point-like sources of X-rays in other galaxies that are far more luminous than any known stellar-mass black hole or neutron star. Their brightness seems to exceed a physical limit known as the Eddington limit, which is the maximum luminosity an object can have before the outward pressure of its own radiation blows away any infalling matter.

So what are they? For a long time, the leading hypothesis was that they are a missing link in black hole evolution: intermediate-mass black holes (IMBHs), with masses between a few hundred and a few hundred thousand times that of the Sun. Such objects would have a much higher Eddington limit and could easily explain the observed luminosities. However, recent discoveries have complicated the picture. Some ULXs have been found to be pulsating, which is a hallmark of a spinning neutron star, not a black hole. For a neutron star – an object only about 20 kilometers across – to be so bright is truly mind-boggling. It requires some form of “super-Eddington” accretion, perhaps involving powerful magnetic fields that channel the infalling matter, allowing the object to shine far brighter than we thought possible. The true nature of ULXs is likely a mixed bag, but they continue to challenge our understanding of accretion onto compact objects.

Fast Radio Bursts

One of the most exciting new mysteries in astronomy is the phenomenon of Fast Radio Bursts (FRBs). These are incredibly powerful but extremely short bursts of radio waves, lasting only a few milliseconds. They were first discovered in 2007, and since then, hundreds have been detected by radio telescopes around the world. Their high dispersion measure – a smearing of the signal as it travels through space – indicates that they originate from far outside our own galaxy, at cosmological distances.

What could produce such an intense flash of energy in such a short time? The list of proposed explanations is long and includes everything from colliding neutron stars to exotic cosmic strings. The discovery that some FRBs repeat has been a game-changer. This rules out any one-off cataclysmic event, like a supernova or a merger, as the source for these repeaters. The leading candidates for the repeating sources are magnetars, a type of young, highly magnetized neutron star. In 2020, a relatively weak FRB was detected from a known magnetar within our own Milky Way, providing a “smoking gun” connection. However, it’s still not clear if all FRBs, especially the non-repeating ones, come from magnetars. And the exact physical mechanism by which a magnetar could produce such a powerful burst is still a matter of intense theoretical debate.

The Quasar Conundrum

Quasars are the brightest objects in the universe. They are a type of Active Galactic Nucleus, powered by supermassive black holes feeding at a ferocious rate. We see them at extreme distances, which means we are seeing them as they were in the very early universe, less than a billion years after the Big Bang.

The conundrum is their existence. How did black holes grow to be so massive – billions of times the mass of the Sun – so quickly? Our standard models of black hole growth, where they grow by accreting gas and merging with other black holes, struggle to produce such monsters in the short time available in the early cosmos. It’s like trying to grow a giant redwood tree in a few years. It suggests that the “seed” black holes in the early universe must have been much more massive than we thought, perhaps forming from the direct collapse of massive primordial gas clouds. Or perhaps there are exotic, super-efficient accretion modes that allow black holes to grow much faster than the Eddington limit would suggest. Understanding the formation of the first quasars is a key piece of the puzzle of how the first large structures in the universe formed. The James Webb Space Telescope is playing a pivotal role in this field, peering deeper into the early universe than ever before to find and study these first cosmic beacons.

Lyman-alpha Blobs

Lyman-alpha blobs (LABs) are enormous clouds of hydrogen gas, some of the largest individual objects in the universe. They can stretch for hundreds of thousands of light-years and are found at high redshifts, in the young universe. They glow brightly in a specific ultraviolet wavelength of light emitted by hydrogen atoms, known as the Lyman-alpha line.

The mystery is what powers them. What is causing this vast amount of hydrogen gas to glow so intensely? The blobs are typically found in regions where galaxies are forming in dense clusters. One idea is that the glow is powered by the intense ultraviolet radiation from star-forming galaxies or quasars embedded within the blob, which ionizes the surrounding gas and causes it to fluoresce. Another possibility is that the gas is being shock-heated by powerful “superwinds” blowing out of these young galaxies. A third idea is that we are seeing the cooling radiation from primordial gas as it streams in from the cosmic web to form galaxies. It’s likely that a combination of these mechanisms is at play, but disentangling them is a major observational challenge. LABs offer a unique window into the environment in which the first galaxies were born.

Stars and Their Lives: Celestial Puzzles

Stars are the engines of the cosmos. They forge the elements, host planetary systems, and light up the universe. But even our own star, the Sun, holds deep secrets, and the lives and deaths of its stellar siblings are filled with puzzles that challenge our understanding of physics.

The Solar Cycle Mystery

Our Sun is not a static, unchanging ball of fire. It’s a dynamic, active star that goes through a regular cycle of activity, known as the solar cycle, which lasts about 11 years. During this cycle, the number of sunspots – dark, cool regions on the Sun’s surface associated with intense magnetic activity – waxes and wanes. This cycle also governs the frequency of solar flares and coronal mass ejections (CMEs), powerful eruptions that can send streams of charged particles hurtling through the solar system.

We know that the solar cycle is driven by the Sun’s magnetic field, which is generated by a dynamo process involving the rotation of the Sun and the convective motions of its plasma. But the details are frustratingly elusive. We can’t predict the strength of a future solar cycle with any great accuracy. Why was the most recent cycle so weak? Why do some cycles have two peaks of activity? And what caused the Maunder Minimum, a period in the 17th century when sunspots almost completely vanished for 70 years? A complete theory of the solar dynamo is one of the holy grails of solar physics.

The Coronal Heating Problem

One of the most enduring paradoxes about the Sun is the temperature of its outer atmosphere, the corona. The visible surface of the Sun, the photosphere, has a temperature of about 5,800 Kelvin. But the corona, which extends millions of kilometers into space, sizzles at a staggering one to two million Kelvin, hundreds of times hotter.

This is the coronal heating problem. It violates our everyday intuition. It’s like walking away from a campfire and finding that the air gets hotter, not cooler. There must be some mechanism that is pumping energy from the Sun’s surface into the corona and heating it to these extreme temperatures. The leading theories involve the Sun’s complex magnetic field. One idea is that countless small, constant explosive events called “nanoflares,” too small to be detected individually, are continuously releasing energy and heating the corona. Another idea involves plasma waves, like Alfvén waves, which can travel along magnetic field lines from the surface up into the corona, where they deposit their energy. NASA’s Parker Solar Probe and ESA’s Solar Orbiter are flying closer to the Sun than any spacecraft before, making direct measurements of the corona to try and finally solve this decades-old mystery.

The Space Weather Prediction Challenge

The Sun’s activity has a direct impact on Earth. Coronal mass ejections and solar flares can trigger intense geomagnetic storms, creating beautiful auroras but also posing a significant threat to our technology-dependent society. A powerful storm can disrupt radio communications, damage satellites, and even take down power grids. This is known as space weather.

The challenge is that we are not very good at predicting it. We can see a CME erupt from the Sun, and we know it’s heading our way, but it’s very difficult to predict its strength and its exact effects on Earth’s magnetosphere. This depends on the orientation of the magnetic field carried within the CME, which is almost impossible to measure until it’s just about to hit us. Improving our space weather forecasting capabilities is a matter of practical importance, requiring a better understanding of the fundamental physics of the Sun and how it interacts with the near-Earth environment.

The Supernova Mechanism

A supernova is one of the most violent events in the universe: the explosive death of a star. Core-collapse supernovae occur when a massive star runs out of fuel. Its core collapses under its own gravity, forming a super-dense neutron star or a black hole. This collapse releases a tremendous amount of energy in the form of neutrinos. The outer layers of the star come crashing down onto the core, bounce off, and are blasted out into space in a spectacular explosion.

At least, that’s the basic picture. The problem is that in our most sophisticated computer simulations, it’s very hard to get the star to actually explode. In many simulations, the shockwave from the core bounce stalls out and fizzles. The star fails to explode. There seems to be a missing ingredient that re-energizes the shockwave and drives the explosion. The leading theory is that the intense flood of neutrinos from the newly formed neutron star plays a key role, depositing some of their energy behind the shock and giving it the extra push it needs. But the physics is incredibly complex, involving hydrodynamics, nuclear physics, and general relativity. Cracking the supernova mechanism is a major computational and theoretical challenge, but it’s essential for understanding how heavy elements are forged and distributed throughout the universe.

The Puzzle of Blue Stragglers

In astronomy, the life cycle of stars in a cluster is usually very orderly. All the stars in a globular cluster, for example, are born at roughly the same time. The more massive stars burn through their fuel quickly and evolve into red giants, while the less massive stars remain on the “main sequence,” shining steadily. When you plot the color and brightness of the stars in a cluster, there should be a clear “turn-off point” where the massive stars have left the main sequence.

But in nearly every cluster, we find a strange population of stars called blue stragglers. These stars are hotter, bluer, and more massive than the stars at the cluster’s turn-off point. They appear to be younger than the cluster they live in, which shouldn’t be possible. They are straggling behind their peers in the aging process. The most likely explanation is that they are the result of stellar interactions in the dense environment of the cluster. They could be the product of a stellar merger, where two older, smaller stars collided and combined to form a single, more massive, rejuvenated star. Or they could be part of a binary system where one star has siphoned mass off its companion, adding fuel to its fire and making it look young again. Distinguishing between these scenarios is an active area of research.

Peculiar Stars

The universe is full of strange and wonderful stars that don’t fit neatly into our standard classification schemes. These peculiar stars exhibit unusual chemical compositions, magnetic fields, or rotational speeds. For example, some stars have atmospheres that are vastly overabundant in heavy elements like mercury or manganese. Others, like the Ap stars, have incredibly strong magnetic fields, millions of times stronger than the Sun’s.

The origin of these peculiarities is often a mystery. It’s thought that the chemical anomalies are caused by a slow sorting process within the star’s atmosphere, where some elements are pushed upwards by radiation pressure while others sink due to gravity. This process can only happen in stars that are rotating very slowly. The origin of the super-strong magnetic fields is also debated. They might be “fossil fields” left over from the star’s formation, or they could be generated by a dynamo, though a different kind from the one in the Sun. Each peculiar star is a cosmic puzzle that challenges our detailed models of stellar structure and evolution.

The Neutron Star Mass-Radius Relationship

Neutron stars are the incredibly dense remnants of core-collapse supernovae. They pack the mass of our Sun into a sphere only about 20-25 kilometers in diameter. A single teaspoon of neutron star material would weigh billions of tons. The physics of matter under these extreme densities is unlike anything we can create on Earth. The protons and electrons have been crushed together to form a sea of neutrons, governed by the laws of quantum mechanics.

A key open question is the equation of state for this ultra-dense matter. This equation relates the pressure of the material to its density, and it determines the relationship between a neutron star’s mass and its radius. Different models for the physics of the neutron star interior predict different equations of state, and therefore different mass-radius relationships. For example, do the neutrons remain as neutrons, or do they break down into their constituent quarks, forming an exotic “quark star”? By precisely measuring the masses and radii of many neutron stars, astronomers hope to constrain the equation of state and learn about the fundamental nature of matter at the highest achievable densities. The detection of gravitational waves from merging neutron stars by observatories like LIGO is providing a powerful new tool to probe this exotic physics.

The Oh-My-God Particle

Cosmic rays are high-energy particles, mostly protons and atomic nuclei, that constantly rain down on Earth from space. Most of them are relatively low-energy, but occasionally, we detect a particle with an absolutely astonishing amount of energy. In 1991, an experiment in Utah detected a cosmic ray with an energy of 3.2 x 10²⁰ electron volts. This was so surprising that it was dubbed the “Oh-My-God particle.”

This single subatomic particle was carrying the kinetic energy of a baseball traveling at 100 kilometers per hour. The mystery is what could possibly accelerate a particle to such an incredible energy. There are no known astrophysical objects in our galaxy that could do it. Furthermore, such high-energy particles should lose energy as they travel through intergalactic space by interacting with the cosmic microwave background. This means the source must be relatively close, within a few hundred million light-years. But when we look in the direction the particle came from, there are no obvious candidates, like a powerful Active Galactic Nucleus. Since the original discovery, a few dozen of these ultra-high-energy cosmic rays have been detected, but their origin remains one of the greatest unsolved mysteries in astrophysics.

Our Cosmic Neighborhood: Planetary Science Mysteries

Our own Solar System is our backyard laboratory for studying planets, moons, asteroids, and comets. Despite centuries of observation and decades of exploration with robotic probes, our local neighborhood is still full of surprises and unsolved puzzles.

The Formation of Planetary Systems

We have a broadly successful theory for how planets form, known as the nebular hypothesis. It posits that stars and their planets form from a collapsing cloud of gas and dust. As the cloud collapses, it forms a spinning disk around the central protostar. Within this disk, dust grains stick together to form pebbles, which then accrete to form planetesimals, which then collide and merge to form planets.

The problem is that the details are very messy and contain many unsolved steps. How do tiny, millimeter-sized dust grains overcome their tendency to bounce off each other and grow into kilometer-sized planetesimals? This is a major bottleneck in the theory. Another puzzle is how giant planets like Jupiter form so quickly. They need to grow massive enough to capture huge amounts of gas from the disk before the disk dissipates, which only takes a few million years. The standard “core accretion” model struggles to do this in time. An alternative, the “disk instability” model, suggests giant planets can form directly from the gravitational collapse of a dense clump in the disk, but it’s not clear if this happens in reality. The thousands of exoplanets discovered by missions like the Kepler space telescope have revealed a stunning diversity of planetary systems, many of which look nothing like our own, adding new layers to the puzzle of planet formation.

The Faint Young Sun Paradox

Our understanding of stellar evolution tells us that stars get brighter as they age. This means that 4 billion years ago, when life was first emerging on Earth, the Sun was about 25-30% dimmer than it is today. Under these conditions, the Earth should have been a frozen ball of ice. All of its surface water should have been solid.

But the geological evidence tells a different story. It shows that liquid water was abundant on the early Earth. This is the faint young Sun paradox. How did the Earth stay warm enough for liquid water when the Sun was so faint? The most likely solution is that the early Earth’s atmosphere was very different, containing much higher concentrations of greenhouse gases like carbon dioxide and methane. These gases would have trapped more of the Sun’s heat, keeping the planet warm. However, the exact composition of the early atmosphere is still a matter of debate, and it’s a challenge to create a model that keeps the Earth warm enough in the beginning without making it overheat as the Sun brightened.

The Kuiper Cliff Mystery

The Kuiper Belt is a vast, icy debris field beyond the orbit of Neptune, home to dwarf planets like Pluto and countless smaller icy bodies. Our models of Solar System formation predict that the density of objects in the Kuiper Belt should gradually decrease as you go further from the Sun.

Instead, observations show that at a distance of about 50 astronomical units (50 times the distance from the Earth to the Sun), the number of objects drops off suddenly and dramatically. This is known as the Kuiper Cliff. It’s as if the outer Solar System just ends. Why? One popular hypothesis is that the cliff was carved out by the gravitational influence of a yet-unseen planet, a massive object orbiting far beyond Neptune. This hypothetical “Planet Nine” could have swept up or ejected material from that region, creating the sharp edge we observe. Searches for this distant world are ongoing. Another possibility is that the material in the protoplanetary disk simply didn’t extend that far out, though it’s not clear why that would be the case.

The Enigma of Saturn’s Rotation

Measuring the rotation period of a gas giant planet is surprisingly difficult because they don’t have a solid surface to track. For Jupiter, astronomers use the regular radio pulses generated by its powerful magnetic field, which is tied to the rotation of its deep interior. But Saturn has proven to be a much harder problem.

The Voyager spacecraft measured a rotation period of 10 hours and 39 minutes in the 1980s, based on its radio emissions. But when the Cassini spacecraft arrived in the 2000s, it measured a period that was about 6 minutes longer. And the period seemed to vary over time. The problem is that Saturn’s magnetic field is almost perfectly aligned with its rotation axis. On Jupiter, the tilt of the magnetic field causes the radio signal to pulse like a lighthouse beam. On Saturn, this lack of tilt makes the radio signal much less reliable as a clock. The final solution came from an unexpected place: Saturn’s rings. Scientists were able to use wave patterns in the rings, which are affected by the planet’s gravitational field, to pin down the true rotation period of its interior: 10 hours, 33 minutes, and 38 seconds. But why the planet’s magnetic field is so perfectly aligned, unlike any other planet in our solar system, remains a mystery.

The Origin of Uranus’s Tilt

The planet Uranus is the oddball of the Solar System. While all the other planets spin more or less upright, like tops, Uranus is tilted on its side. Its axis of rotation is tilted by 98 degrees relative to its orbital plane, so it essentially rolls along on its side as it orbits the Sun. This means for long periods of its 84-year orbit, one pole is pointed directly at the Sun while the other is in total darkness.

The leading theory for this extreme tilt is that in the chaotic early days of the Solar System, a massive, Earth-sized protoplanet collided with the young Uranus and knocked it over. However, this simple impact scenario has some problems. It doesn’t easily explain why Uranus’s moons and ring system also orbit around its tilted equator. If the moons were there before the impact, they should have been left in their original orbits. If they formed after the impact from the resulting debris disk, that might work. Recent simulations suggest that a giant impact could indeed have knocked the planet over and created a debris disk from which the moons later formed, but the exact sequence of events is still not fully understood.

The Uniqueness of the Solar System

For centuries, our Solar System was the only one we knew, and we naturally assumed it was typical. But the discovery of thousands of exoplanetary systems has shown us that we might be the exception, not the rule. The most common type of planet in the galaxy seems to be something we don’t have: “Super-Earths” or “Mini-Neptunes,” planets with sizes between that of Earth and Neptune.

Our own system’s architecture is also somewhat peculiar. We have small, rocky planets close in, and giant gas and ice planets far out, with a large gap in between where the asteroid belt lies. Many other systems feature “Hot Jupiters,” gas giants orbiting incredibly close to their stars, and planets on highly eccentric or tilted orbits. Why is our Solar System so different? Did Jupiter’s gravitational influence prevent a super-Earth from forming in our inner Solar System? Is the current configuration of our system a rare outcome of the chaotic process of planet formation? Understanding why our home looks the way it does is a deep question that requires a statistical understanding of the full diversity of planets in the cosmos.

P-type Asteroids

P-type asteroids are a type of asteroid found predominantly in the outer asteroid belt and beyond. They are very dark, with a reddish hue, and are thought to be among the most primitive objects in the Solar System, rich in water ice and carbon compounds. One of the great mysteries of planetary science is why these asteroids are not more common among the meteorites that fall to Earth. We have thousands of meteorites in our collections, but very few, if any, seem to match the composition of P-type asteroids. If they are so common in the outer belt, why aren’t we seeing pieces of them on Earth? This discrepancy could be telling us something important about the dynamics of the asteroid belt and the processes that deliver meteorites to our planet.

The Search for Life: Astrobiology’s Great Unknowns

Are we alone in the universe? This is perhaps the most significant question that science can ask. Astrobiologyis the field that seeks to answer it, studying the origin, evolution, and distribution of life in the cosmos. It’s a field defined by a single data point – life on Earth – and a universe of possibilities.

The Habitability of Exoplanets

The search for life beyond Earth is currently focused on finding habitable exoplanets. The traditional definition of a habitable planet is one that orbits its star in the “habitable zone,” the region where temperatures are just right for liquid water to exist on the planet’s surface. With missions like Kepler and the Transiting Exoplanet Survey Satellite (TESS), we have now found dozens of Earth-sized planets in the habitable zones of their stars.

But being in the right place is not enough. A planet’s habitability depends on a whole suite of other factors. Does it have an atmosphere? What is the atmosphere made of? Does it have a magnetic field to protect it from the star’s radiation? Is the star itself stable, or is it a volatile flare star that would strip away the planet’s atmosphere and sterilize its surface? The next great challenge is to characterize the atmospheres of these potentially habitable worlds. The James Webb Space Telescope is a key player here, capable of analyzing the starlight that passes through an exoplanet’s atmosphere to look for “biosignatures,” gases like oxygen or methane that could be a sign of life.

The Fermi Paradox

The Milky Way galaxy contains hundreds of billions of stars. We now know that planets are common, and many of them are likely to be in habitable zones. If even a tiny fraction of those planets developed life, and a tiny fraction of that life developed intelligence and technology, then the galaxy should be teeming with civilizations. Given the age of the galaxy, at least some of these civilizations should have had billions of years to develop technologies like interstellar travel. So why haven’t we seen any evidence of them? Where is everybody?

This is the Fermi paradox. The apparent contradiction between the high probability of extraterrestrial intelligence and the utter lack of evidence for it. There are many possible solutions. Perhaps life is incredibly rare. Or perhaps intelligent life is rare, or tends to self-destruct before it can colonize the galaxy. Maybe the distances are just too vast, and interstellar travel is too difficult. Perhaps advanced civilizations are out there, but they are deliberately hiding from us. Or maybe we are just not looking in the right way. Projects like SETI(Search for Extraterrestrial Intelligence) continue to scan the skies for signals, but for now, the great silence from the cosmos remains.

The Nature of ‘Oumuamua

In 2017, the Pan-STARRS telescope in Hawaii discovered the first known interstellar object to visit our Solar System. Named ‘Oumuamua, Hawaiian for “scout,” this object was unlike anything we had ever seen. It was small, dark, reddish, and highly elongated, perhaps ten times as long as it was wide. Most strangely, as it was leaving the Solar System, it accelerated away from the Sun in a way that could not be explained by gravity alone.

This “non-gravitational acceleration” is typical for a comet, as the Sun’s heat vaporizes its ice, creating jets that act like tiny rocket engines. But ‘Oumuamua showed no signs of a cometary coma or tail. So what was pushing it? Several natural explanations have been proposed, such as outgassing of unseen gases like hydrogen ice, or that it was an incredibly light and fluffy object being pushed by the pressure of sunlight. A more controversial hypothesis is that ‘Oumuamua was an artifact of extraterrestrial technology, perhaps a solar sail or a probe. Since the object has long since left our Solar System, we may never know for sure, but it gave us our first glimpse of the kind of material that drifts between the stars.

The Wow! Signal

On a summer night in 1977, the Big Ear radio telescope at The Ohio State University detected a powerful, narrow-band radio signal coming from the direction of the constellation Sagittarius. The signal lasted for 72 seconds, the maximum time the telescope could observe a single point in the sky as the Earth rotated. It was so strong and so close to a protected frequency where interstellar transmissions are forbidden that an astronomer reviewing the data scrawled “Wow!” on the printout.

This became known as the Wow! signal. It is the strongest candidate for an extraterrestrial radio transmission ever detected. The problem is that it has never been seen again. Numerous follow-up searches of the same region of sky have found nothing. Several natural explanations have been proposed, such as the signal being a reflection of an Earth-based transmission from a satellite, or a signal from an undiscovered comet, but none have been fully convincing. The signal’s characteristics were a close match for what we might expect from an alien beacon. Was it a fleeting message from another civilization? Or a one-in-a-billion cosmic or terrestrial anomaly? Without a repeat detection, the Wow! signal remains a tantalizing and unsolved mystery.

Other Cosmic Riddles

Beyond the grand questions of cosmology and astrobiology, the universe is filled with specific, baffling phenomena that defy easy explanation. These individual puzzles often point to gaps in our knowledge of fundamental physics or astrophysics.

The Nature of Tabby’s Star

KIC 8462852, better known as “Tabby’s Star” or “Boyajian’s Star,” is arguably the most mysterious star in the galaxy. Data from the Kepler space telescope showed that the star undergoes frequent, deep, and irregular dips in its brightness. While the dips caused by a transiting planet are typically small (around 1%) and perfectly regular, this star’s brightness has been seen to drop by as much as 22%.

What could be blocking so much of the star’s light? The dips are too irregular to be a planet. The leading natural explanation is a massive, orbiting swarm of dust, perhaps from a giant cometary cloud or the shattered remnants of a planet. But this “circulating dust” model has trouble explaining all the features of the light curve. A more outlandish, and highly speculative, hypothesis is that the dips are caused by a massive “alien megastructure,” like a Dyson swarm, built by an advanced civilization to harvest the star’s energy. While most astronomers favor a natural explanation, none of the proposed scenarios can fully account for the star’s bizarre behavior.

The Flyby Anomaly

The flyby anomaly is an unexpected and unexplained increase in the speed of several spacecraft as they perform an Earth-flyby maneuver. During these maneuvers, a spacecraft uses Earth’s gravity to gain speed and alter its trajectory, a technique known as a gravity assist. In at least six cases since 1990, spacecraft including Galileo, Rosetta, and Juno have been observed to gain a tiny amount of extra speed – on the order of a few millimeters per second – that cannot be accounted for by our best models of gravity and relativity.

The anomaly appears to depend on the geometry of the flyby, but no one has come up with a convincing explanation. The proposed causes range from the mundane, like unmodeled atmospheric drag or the push from the Earth’s reflected thermal energy, to the exotic, such as a modification of inertia, a halo of dark matter around the Earth, or a flaw in our understanding of general relativity. The effect is tiny, but the fact that it has appeared in multiple missions suggests it might be a real physical phenomenon that we don’t yet understand.

Summary

The universe is vast, and the list of what we don’t know is as expansive as the cosmos itself. The unsolved problems in astronomy are not just minor details to be ironed out; they are fundamental gaps in our knowledge that challenge the very foundations of our understanding. From the 95% of the universe made of dark matter and dark energy to the strange behavior of a single, distant star, these mysteries are the driving force of science.

Each of these puzzles represents an opportunity for discovery. Solving the Hubble tension might force us to rewrite the history of the early universe. Identifying dark matter would open up a whole new realm of particle physics. Understanding the supernova mechanism is key to knowing where the elements of life come from. And detecting a true biosignature in the atmosphere of an exoplanet would change humanity’s perspective on our place in the cosmos forever.

We are building ever more powerful tools to tackle these questions. New telescopes on the ground and in space are poised to collect data with unprecedented precision. Our computer simulations are becoming more sophisticated, allowing us to test our theories in greater detail. We don’t know when the next breakthrough will come or what it will be. But one thing is certain: the universe is not done with its secrets, and the quest to understand it is one of the grandest adventures of the human intellect. The night sky is a book of unsolved mysteries, and we have only just begun to read its pages.

Last update on 2025-12-20 / Affiliate links / Images from Amazon Product Advertising API