Erika 

How Can a 13.8-Billion-Year-Old Universe Be 93 Billion Light-Years Across? The Universe is about 13.8 billion years old, yet the observable Universe is roughly 93 billion light-years across today. At first this seems impossible. If light has been travelling for only 13.8 billion years, how can we observe regions that are now more than 46 billion light-years away? The answer lies in one of the most important consequences of modern cosmology: light travels through space, but on the largest scales that space is itself expanding. In a static Universe, the reasoning would be straightforward. Light travels one light-year in one year, so after 13.8 billion years the most distant observable region should be no farther than 13.8 billion light-years away. But our Universe is not static. General relativity describes a dynamic spacetime whose large-scale geometry evolves, and observations show that the average distance between gravitationally unbound regions has increased throughout cosmic history. The distance separating us from a source therefore does not remain constant while its light is on the way. This distinction is why cosmologists use several different definitions of distance. A lookback time tells us how long light has been travelling, while a comoving distance describes the present separation between locations after accounting for cosmic expansion. Within the standard ΛCDM cosmological model, the present comoving radius of our particle horizon is approximately 46.5 billion light-years. Looking in every direction gives an observable diameter of roughly 93 billion light-years. The precise value depends slightly on the cosmological parameters used, so figures around 92–93 billion light-years are both commonly quoted. The mathematics behind this result comes from the cosmic scale factor, usually written as a(t), which describes how characteristic distances in the Universe change with time. The comoving distance travelled by light depends not simply on the elapsed time multiplied by the speed of light, but on the entire expansion history of the Universe. Because the scale factor was much smaller in the early cosmos, a relatively small physical separation then can correspond to an enormous present-day separation. This is why the age of the Universe expressed in years cannot simply be converted into its present radius by replacing years with light-years. The oldest electromagnetic radiation we can directly observe is the cosmic microwave background, or CMB. It was released about 380,000 years after the Big Bang, when the Universe cooled sufficiently for electrons and nuclei to combine into neutral atoms and space became transparent to radiation. Before this period, photons were continually scattered by the hot ionised plasma. The CMB therefore does not come from the exact beginning of the Universe; its surface of last scattering lies slightly inside the theoretical particle horizon, which represents the greatest distance from which any causal influence could have reached us since the beginning of cosmic expansion. ESA's Planck measurements place the age of the Universe at about 13.8 billion years and have provided some of the most precise constraints on this cosmological history. None of this requires light to travel faster than light. Every photon measured locally always propagates through vacuum at c. What can exceed c is the rate at which the distance between two extremely distant, gravitationally unbound regions changes because of cosmic expansion. This does not violate special relativity because its familiar speed limit applies to objects or information moving locally through spacetime. Cosmological recession is a statement about the evolution of distances across curved and expanding spacetime, which is governed by general relativity. This also explains why the Hubble sphere is not the edge of the observable Universe. There are regions whose recession rate can exceed the speed of light and whose ancient radiation can nevertheless reach us. Whether a photon eventually arrives depends on the complete history of cosmic expansion, not simply on the recession speed of its source at one particular moment. Most importantly, the 93-billion-light-year figure is not the size of the entire Universe. It describes only our observable region: the volume from which causal signals have had enough time to reach us given the expansion history of the cosmos. There is no physical wall 46.5 billion light-years away, and Earth does not occupy a special central position. Any observer elsewhere in the Universe would have their own observable Universe centred on them. What lies beyond our particle horizon remains inaccessible to direct observation. The full Universe could be vastly larger than the observable portion, or its spatial extent could be infinite. Current measurements cannot determine its total size. The apparent paradox therefore disappears once time, light-travel distance and present cosmological distance are treated as different quantities: the Universe is about 13.8 billion years old, while expansion has carried the regions defining our present observable horizon to distances of roughly 46.5 billion light-years in every direction.

Researchers at the University of Ottawa have been testing an alternative cosmological model that describes a universe approximately 26.7 billion years old and seeks to explain observations without invoking dark matter or dark energy. The framework is called CCC+TL. Its proposed age remains a model-dependent prediction; the standard cosmological estimate is about 13.8 billion years. CCC+TL stands for “covarying coupling constants plus tired light.” It allows certain physical constants to evolve together over cosmic time and attributes part of the redshift of distant light to photons losing energy during their journey. Cosmic expansion also contributes to the redshift. Combining these effects changes how observed redshifts translate into distances and cosmic ages, giving early galaxies considerably more time to develop within this framework. The study highlighted in the announcement, published in Universe, examines Big Bang nucleosynthesis: the formation of helium, deuterium and other light nuclei in the early universe. The authors find that CCC+TL can preserve the standard model’s predicted abundances if nuclear reaction rates and neutron decay rates scale together with the altered expansion rate. Their calculations then produce nearly identical abundances. This agreement depends on those coordinated changes. Adopting a lower baryon density can reduce the longstanding lithium discrepancy, but simultaneously produces too much deuterium. A separate paper examines galaxy rotation using the SPARC database, which contains 175 disc galaxies. A modified version of the CCC prescription reproduces rotation curves with performance comparable to MOND fitted individually to each galaxy. This extends the framework’s application to galactic dynamics, although fitting rotation curves alone cannot establish which explanation of the gravitational discrepancy is correct. The researchers have also explored gravitational lensing in the Bullet Cluster. Their preliminary model produces additional gravitational effects concentrated around the galaxies, resembling the observed separation between the lensing signal and the hot gas. The paper acknowledges that this resemblance partly follows from constructing the extra-gravity profiles around member galaxies. A full comparison using the observed positions of multiple lensed images remains a further step. Other tests remain at different stages of development. A supernova time-dilation analysis is still under review, while the cosmic microwave background analysis announced by the team has not yet been published. The cautious interpretation is that these studies provide conditional consistency checks and reasons for further investigation. They have yet to establish a revised age for the universe or a comprehensive replacement for ΛCDM. 🍁 share.google/dopYSTDmhYx2XBP…

Sun vs. the Blue Supergiant. * Rigel, the supergiant blue star, dwarfs everything around it, including our Sun. * In this video, the focus of this visual is on their diameters - giving you a stunning scale comparison. But more than that, it's about what type of star each one is, and what that means: * The Sun is a stable main-sequence yellow dwarf, the kind of star that can host life-bearing planets like Earth. * Vega is a main-sequence white star, hotter, brighter, and spinning much faster than our Sun. * Arcturus is a cooler red giant, showing what stars like our Sun become in their later life stages. * Rigel is a massive, luminous blue supergiant, a short-lived giant destined to end in a spectacular supernova. * I've taken care to reflect not just size and color, but also realistic rotation speeds: * Vega rotates in just 0.7 days. * Rigel spins around in roughly 1.2 days. * While our Sun takes about 25 days to complete one rotation at the equator."

The brightest stars.

The Kirkwood gaps are among the most elegant fingerprints of celestial mechanics in our Solar System, silent voids carved into the asteroid belt by the persistent gravitational rhythm of Jupiter. When Daniel Kirkwood first noticed them in the 19th century, they appeared as strange absences: narrow regions where almost no asteroids lived, like missing notes in what should have been a continuous symphony of rocks orbiting the Sun. What he had stumbled upon, long before the language of chaos theory or modern orbital dynamics existed, was the subtle but relentless influence of orbital resonances. Each gap corresponds to an orbit where an asteroid would circle the Sun in a simple numerical ratio with Jupiter, the largest and most dramatic at the 3:1 resonance, where an asteroid completes three orbits for every one of Jupiter’s. The neighbouring 5:2 gap is another major sculptor, while the 4:1 and 7:3 resonances carve out their own distinct voids, adding further structure to the belt. These ratios create repeating alignments that allow Jupiter’s gravitational pull to strike at precisely the same point every cycle. One tug is nothing, but millions of years of rhythmic nudges amplify one another with extraordinary efficiency, slowly stretching an asteroid’s orbit into instability until it is pushed into a new region of the Solar System. This accumulation of tiny perturbations transforms the asteroid belt’s architecture over geological timescales, sending some bodies drifting inward toward Mars-crossing or Earth's vicinity, while others are ejected entirely. It is here that celestial mechanics becomes a story of fragile balance, where orbital stability dissolves into chaos and the long-term future of any given asteroid becomes sensitive to the smallest variations in initial conditions. Kirkwood gaps also reveal something profound about planetary system architecture. The same resonant sculpting Jupiter performs in our asteroid belt shapes Saturn’s rings, the Kuiper Belt, and even the debris disks around young stars. @almaobs has imaged vast resonant gaps in protoplanetary disks where emerging planets carve structures analogous to the ones Jupiter carved long ago. The physics scales effortlessly, from millimetre-sized grains in newborn systems to hundred-kilometre bodies circling a mature star. Despite their apparent emptiness, these gaps are not entirely devoid of life. A few asteroids survive temporarily inside resonances, trapped in delicate stability islands where they drift for millions of years before resonance-driven chaos eventually claims them. They hold valuable clues about planetary migration, early Solar System stirring, and the moment Jupiter and Saturn may have shifted orbits, reshaping the belt like a spoon through sand. The Kirkwood gaps transform the asteroid belt from a random scattering of rocks into a dynamic ecosystem governed by rhythm, resonance, and the subtle gravitational sculpting of a distant giant. They are not merely empty spaces but signatures of the deep mathematical heartbeat of the Solar System, the quiet fractures where Jupiter's influence has been at work for billions of years, rearranging the small worlds around it and revealing how profoundly interconnected every orbit truly is.

Astronomers using early data from the Vera C. @VRubinObs Observatory have identified 2026 ED138, a large object beyond Neptune that could qualify as a dwarf planet. It currently lies about 46 astronomical units from the Sun, roughly 6.9 billion kilometres. The discovery was announced on 27 October, although further observations are needed to establish its size and physical properties. Matthew Holman identified the object on 13 October using software developed with Kevin Napier to connect separate detections of moving objects. Mario Jurić then recovered additional observations in Rubin’s data, while searches of other surveys uncovered images dating back to 2016. That archival material gives astronomers more than a decade of observations with which to calculate its orbit. An object can appear in telescope images for years before anyone recognises that those detections belong to the same body. The estimated diameter is between approximately 600 and 1,500 kilometres. This considerable uncertainty comes from its unknown surface reflectivity, or albedo. At a given distance, a smaller object with a reflective surface can appear as bright as a larger object with a darker surface. The often-mentioned diameter of around 1,000 kilometres therefore depends on an assumed albedo. Follow-up measurements could narrow that range, including observations of a stellar occultation, when the object passes in front of a distant star. An icy body of this size could have enough gravity to become approximately rounded, one of the requirements for dwarf planet status. Under the International Astronomical Union’s definition, a dwarf planet orbits the Sun, has sufficient self-gravity to achieve a nearly round shape, has not cleared its orbital neighbourhood and is not a satellite. ED138 remains a candidate while its properties are investigated. ED138 was found against a crowded background of Milky Way stars, which probably helped it escape earlier searches. Rubin’s repeated observations and tracking algorithms allowed researchers to identify its movement through that field. Studying such objects helps reconstruct the early Solar System: their orbits and surface compositions preserve evidence of how the giant planets migrated and redistributed smaller bodies beyond Neptune. 🎃 rubinobservatory.org/es/news…

The stars and gas in a galaxy are constantly moving. In galaxies such as the Milky Way, much of that motion is organised into rotation around the galactic centre. Understanding where this rotation comes from requires looking back to the formation of galaxies, when gravity began gathering gas and dark matter into growing structures. The early universe contained small differences in density, and the regions that would become galaxies had irregular shapes and uneven surroundings. Nearby concentrations of matter pulled more strongly on some parts than others, exerting gravitational torques that gave these regions angular momentum. Gas arriving along cosmic filaments and encounters between growing structures supplied additional angular momentum. When two systems merged, some of their orbital motion could become internal rotation in the resulting galaxy. Angular momentum describes motion around a point or axis and depends on mass, velocity and how matter is distributed. Without an external torque, a system’s total angular momentum remains constant. If its matter contracts towards the rotation axis, its rotation can therefore become faster. Conservation explains how existing rotation changes during contraction; the initial angular momentum comes from the system’s formation and interactions. Gas also behaves differently from stars and dark matter. It can collide, heat up and radiate energy, allowing it to settle into a thinner configuration. Its angular momentum limits how far it can collapse towards the centre and helps determine the size of the resulting disc. Stars born within that disc inherit much of the gas’s orbital motion, producing a population that largely travels around the galaxy in the same direction. Simulations show how cooling and the alignment of incoming gas contribute to the formation of thin stellar discs. Subsequent evolution can change this organisation. Mergers redistribute angular momentum between stars, gas, dark matter and material expelled into the surroundings. Their effects depend on the masses, gas content and orbital orientations of the galaxies involved. Some elliptical galaxies retain substantial ordered rotation, while others have very little overall rotation and contain stars moving on orbits with many different orientations. The fact that their stars are moving does not necessarily mean that the galaxy has a strong collective spin. Observations distinguish these different populations through detailed maps of stellar velocities. Spiral arms have their own dynamics. In density-wave models, stars and gas can move through regions of enhanced density rather than remaining permanently inside the same arm. Gas becomes compressed as it encounters these regions, encouraging the formation of young stars that make the arms particularly conspicuous. The visible spiral pattern and the orbital motion of individual stars therefore need to be distinguished. The central supermassive black hole contributes to the gravitational field, but its influence dominates mainly in its immediate neighbourhood. Across most of a galaxy, stellar and gas orbits respond to the combined distribution of stars, gas and dark matter. The black hole contains only a small fraction of the galaxy’s total mass. Measuring orbital speeds also reveals information about that mass distribution. In the outer regions of many spiral galaxies, speeds remain higher than the observed stars and gas can explain under standard gravity. Extended dark matter haloes provide the standard explanation for this discrepancy. However, approximately constant orbital speed does not mean constant angular speed: a star farther out travels around a larger orbit and takes longer to complete it. Our solar system, for example, takes roughly 230 million years to orbit the Milky Way’s centre.

Wormholes are hypothetical solutions of general relativity in which spacetime itself forms a tunnel, or throat, connecting distant regions of the Universe, or, in some models, entirely separate universes. In their simplest realizations, such as the Einstein–Rosen bridge, these structures are not naturally stable: without support from exotic matter with negative energy density, they collapse almost instantly. Many well-known traversable wormhole solutions therefore rely on violations of classical energy conditions. Observational studies, including the one discussed here (linked below), typically set aside the question of internal stability and instead concentrate on the external spacetime metric, how the geometry of spacetime affects light propagation, because this is what astronomical observations are sensitive to. The recent theoretical work on electrically charged wormholes examines this external signature through gravitational lensing. Any compact spacetime curvature can bend and amplify light, but the detailed lensing pattern depends on the full metric, not simply on the presence of ordinary mass. This point is subtle but important: some famous wormhole solutions, such as the Ellis–Bronnikov wormhole, are mathematically “massless” in the usual sense, yet still produce measurable gravitational effects because spacetime curvature itself guides light along nontrivial paths. The study implicitly operates within this broader relativistic framework, focusing on how geometry, rather than mass alone, shapes observable signals. A central result concerns image multiplicity. Black holes, due to the presence of an event horizon and a photon sphere, can in principle generate an infinite sequence of increasingly faint relativistic images as light loops around the horizon multiple times. In contrast, the electrically charged wormholes considered here are restricted to producing either a single image or, at most, three. This sharp limitation reflects the different causal and geodesic structure of a wormhole spacetime and provides a potentially clean observational discriminator. The three-image configuration is particularly distinctive. In this case, the lens produces one extremely bright primary image and two much fainter secondary images with identical brightness. In the language of observational astronomy, the two secondary images have a flux ratio of exactly 1, a highly restrictive condition that is rare in conventional galaxy-scale lensing, where asymmetries in the lens mass distribution almost always break such equality. As expected from gravitational lensing theory, these twin images also have opposite parity: one is a mirror image of the other. This combination, unit flux ratio and opposite parity, constitutes a remarkably clean and recognizable lensing signature. The extreme brightness of the primary image arises only under very specific conditions. Magnifications approaching 100,000 occur when the background source lies extremely close to a caustic of the wormhole lens, a region in the source plane where light rays are strongly focused and amplification formally diverges. Such near-perfect caustic alignment is rare, but when it occurs, the wormhole would act as an extraordinarily powerful gravitational telescope, briefly elevating otherwise inaccessible distant objects into detectability. Gravitational lensing is not the only observational handle on exotic compact objects. Another major probe is the shadow produced by strongly curved spacetime. A true black hole possesses an event horizon that absorbs light completely, yielding a central region of near-total darkness. Wormholes lack an event horizon and instead feature a throat, which could allow some light to pass through or around the central region, potentially leading to residual brightness or a modified ring structure. While the present study focuses on lensing rather than imaging, these shadow properties offer a complementary route for distinguishing wormholes from black holes. Finally, the analysis concentrates on electrically charged, non-rotating wormholes, closely related to Reissner–Nordström–type solutions. Rotation, ubiquitous in astrophysical systems, would further modify both lensing and shadow signatures through frame-dragging and asymmetry. Extending these calculations to rotating, Kerr-like wormholes is a natural and necessary next step toward realism. The study does not claim that wormholes exist, nor that any observed lens must be one. Its significance lies in providing precise, falsifiable predictions. As lensing surveys become increasingly sensitive to image multiplicities, parities, and flux ratios, they offer a way to test whether spacetime ever arranges itself in forms more exotic than black holes, or whether wormholes remain elegant but unrealized solutions of Einstein’s equations. ☃️ arxiv.org/pdf/2207.05406 (Conceptual illustration of a wormhole showing the curvature of spacetime; this geometry is responsible for its potential gravitational-lensing effects.)

Point Nemo, also known as the "oceanic pole of inaccessibility," is the point in the ocean that is farthest from any land. It is named after Jules Verne's character Captain Nemo from "Twenty Thousand Leagues Under the Sea." Point Nemo is located in the ... 1/

.@ESA_Euclid was built to investigate dark matter and dark energy by mapping billions of distant galaxies. While carrying out that survey, ESA’s space telescope has also discovered something much closer to home: its first previously unknown comet, C/2026 S1 (Euclid), recorded in images taken between 16 and 24 September 2026. The discovery came through software developed by Mikko Pöntinen, a doctoral researcher at the University of Helsinki @helsinkiuni working with the Euclid Consortium’s Solar System Objects Science Working Group. His program searches for moving objects against the background of stars and galaxies. During a long exposure, a comet changes position enough to leave a small streak. Around this one, the images also revealed a diffuse cloud, or coma, indicating that it was releasing dust and gas. The streak itself records its movement during the exposure; it should not be confused with a comet’s physical tail. With no matching object in existing catalogues, Pöntinen reported it to the Minor Planet Center. Ground-based follow-up helped confirm the discovery on 8 October. The comet lies approximately 1.3 billion kilometres from Earth, high above the planets’ orbital plane. Its journey around the Sun takes many thousands of years, although its orbital period still needs refinement. Finding it required a fortunate alignment of circumstances. Euclid avoids regions close to the Solar System’s plane, where sunlight scattered by interplanetary dust interferes with its sensitive observations. This comet’s steeply inclined trajectory carried it into the telescope’s Deep Field North, where it appeared in 16 exposures across five fields of view. Those repeated sightings allowed the team to trace its motion and investigate its nature. Comets contain ice and dust inherited from the formation of the Solar System around 4.6 billion years ago. Studying them helps astronomers investigate the materials from which the planets formed. Euclid’s observations already show activity around this distant object, giving researchers an opportunity to follow how it develops as it continues along its orbit. The discovery also provides practical experience for ESA’s future Comet Interceptor mission, which will wait in space before travelling towards a suitable comet. C/2026 S1 is unsuitable for that mission, but finding and confirming it has tested the discovery and alert process that could help identify a future target.

The Moon is not slowly falling toward Earth, as many people instinctively imagine. In reality, it is doing the opposite: it is gradually moving away from us at a rate of about 3.8 centimeters per year. The change is tiny on human timescales, but it reveals something deeply counterintuitive about gravity. Gravity does not always pull things inward. Sometimes, it redistributes energy and pushes orbits outward. The process begins with tides. The Moon’s gravity pulls on Earth’s oceans, raising tidal bulges on opposite sides of the planet. If Earth did not rotate, those bulges would remain aligned with the Moon. But Earth spins much faster than the Moon orbits, one rotation every 24 hours compared with about 27 days for a lunar orbit, so the oceans are carried slightly ahead of the Moon’s position. In short, Earth is dragging the tidal bulge forward. That small offset changes everything. The displaced ocean mass exerts a gravitational pull on the Moon, tugging it forward along its orbit. Moving forward in orbit means gaining energy. And gaining orbital energy does not make an object fall inward, it makes it climb outward, into a larger orbit. The Moon slowly moves farther away while Earth’s rotation gives up a tiny amount of its energy to make that happen. As Earth loses rotational energy, its spin gradually slows. Our days are becoming longer, by only milliseconds per century, but the effect is measurable. The long-term slowing of Earth’s rotation, driven mainly by tidal interaction with the Moon, along with smaller geophysical effects, is one reason we occasionally need leap seconds in our timekeeping systems to keep atomic clocks aligned with Earth’s actual rotation. This exchange follows conservation laws: angular momentum is not lost but transferred from Earth’s rotation to the Moon’s orbit. Gravity here acts less like a simple attraction and more like a coupling between two moving bodies. The Earth–Moon system is continuously evolving even though nothing external is driving the change. We know this process is real with remarkable precision thanks to experiments that began during the Apollo missions. Astronauts placed retroreflectors on the lunar surface, and scientists still fire laser pulses at them today. By measuring the return time of the light, researchers determine the Earth–Moon distance with millimeter accuracy, directly tracking the Moon’s steady outward drift. Over hundreds of millions of years the consequences become dramatic. The Moon was once much closer, tides were far stronger, and Earth rotated faster. Fossil coral growth patterns record this history, showing that a day once lasted only about 18 hours. Far in the future, if the system continues undisturbed, Earth and the Moon could eventually become tidally locked to each other, always showing the same face across the sky. What makes this story compelling is how it overturns intuition. We tend to imagine gravity as a force that only pulls objects together. Yet in rotating systems, gravity can transfer motion and energy in ways that slowly expand orbits. The Moon’s retreat is not a weakening of gravity but evidence of a long gravitational partnership, one that has been reshaping the Earth–Moon system for billions of years and is still unfolding, quietly, today.

For a long time, we’ve treated gravity as one of the fundamental forces of nature. Something built into the fabric of reality itself, alongside electromagnetism and the nuclear forces. It curves spacetime, governs the motion of planets, shapes galaxies, and dictates the large scale structure of the universe. But what if that picture is incomplete? What if gravity isn’t fundamental at all? In physics, we’ve seen this kind of shift before. Temperature feels like a basic property of matter, but it isn’t. It emerges from the collective motion of microscopic particles. Pressure, too, is not fundamental. It arises from countless interactions at smaller scales. In that sense, what we perceive as a smooth, continuous phenomenon is often the result of something deeper and more discrete. Some physicists have wondered whether gravity might be similar. This idea is known as emergent gravity. Instead of being a fundamental interaction, gravity could arise from underlying microscopic degrees of freedom. In some approaches, those degrees of freedom are tied to quantum information and entanglement, suggesting that spacetime itself may be built from how information is organized at the most fundamental level. One of the key clues comes from black holes. Black holes are not just gravitational objects. They also have temperature and entropy. Their entropy is proportional to the area of their event horizon, not their volume. That’s a surprising result. It suggests that the fundamental description of reality might live on lower dimensional boundaries, an idea closely related to the holographic principle. There are even deeper proposals. In some frameworks, connections between regions of spacetime may be related to quantum entanglement itself, an idea often summarized as ER = EPR. In that picture, spacetime is not just a stage where physics happens. It could be something that emerges from entanglement. Even more intriguingly, when you combine quantum theory, thermodynamics, and relativity, equations resembling Einstein’s field equations can be derived as emergent relations, not as fundamental laws. In this view, spacetime geometry, and therefore gravity, would be more like an equation of state than a basic ingredient. So gravity wouldn’t be “causing” motion in the traditional sense. It would be the macroscopic manifestation of deeper microscopic processes. There are also attempts to connect this idea to cosmology. Some versions of emergent gravity try to explain phenomena usually attributed to dark matter by modifying how gravity behaves on large scales, without introducing new particles. These models are still debated, and they don’t yet match all observations as well as the standard picture, but they highlight how much we still don’t know. The challenge is that we don’t yet have a complete underlying theory. If gravity is emergent, what is it emerging from? What are the fundamental degrees of freedom? How do they give rise to spacetime itself? Right now, those questions are open. And that’s what makes the idea so compelling. Because it shifts the question. Instead of asking how gravity works, we start asking why it exists at all, and whether what we call gravity is just the large scale shadow of something deeper. If that’s true, then spacetime itself may not be fundamental. And what we experience as the curvature of the universe might be closer to a thermodynamic illusion than a basic feature of reality. But like wind, which emerges from the motion of air molecules yet is undeniably real, an emergent phenomenon is not less real, only less fundamental.

Imagine that everything you see, the vastness of galaxies, the depth of oceans, the entire three-dimensional world around you, could, at a deeper level, be described by information encoded on a distant two-dimensional boundary. It sounds like an idea borrowed from science fiction, yet it emerged from one of the most surprising discoveries in modern theoretical physics. The possibility that the universe might be “holographic” did not begin as speculation about simulations or digital realities. It began with black holes. In the 1970s, physicists studying the thermodynamics of black holes encountered a puzzle that seemed almost impossible to reconcile with conventional intuition. Normally, the amount of information a physical system can contain, what physicists describe through entropy, grows with the system’s volume. A larger box can hold more particles, and therefore more possible microscopic configurations. But calculations by Jacob Bekenstein and Stephen Hawking showed something very different for black holes. The entropy of a black hole is not proportional to its volume. It is proportional to the area of its event horizon, the two-dimensional surface that surrounds it. This result immediately raised deep questions. If the maximum amount of information inside a region of space is determined by its surface area rather than its volume, perhaps the true degrees of freedom describing that region do not live throughout the volume at all. Perhaps they are encoded on the boundary itself. In other words, what we experience as a three-dimensional region might be fully described by information stored on a two-dimensional surface surrounding it. This idea eventually became known as the holographic principle. The analogy comes from optical holograms. In a holographic image, a flat surface encodes the information needed to reconstruct a three-dimensional picture when illuminated appropriately. The holographic principle suggests something conceptually similar in physics: the complete description of a volume of space may be equivalent to a theory defined on its boundary, which has one fewer spatial dimension. For years this idea remained intriguing but speculative. That changed dramatically in 1997, when Juan Maldacena proposed a remarkable theoretical relationship now known as AdS/CFT duality. In this framework, a universe containing gravity in a higher-dimensional curved spacetime can be mathematically equivalent to a quantum field theory without gravity defined on its lower-dimensional boundary. Two seemingly different descriptions, one with gravity and one without, can encode exactly the same physics. What makes this correspondence extraordinary is its precision. Calculations performed in one description produce results identical to those obtained in the other. Phenomena that appear geometric in one picture can emerge from purely quantum interactions in the other. The equivalence does not mean the universe literally exists on a physical surface somewhere. Instead, it shows that the same underlying reality can be described in radically different ways, and that spacetime itself might emerge from more fundamental informational relationships. The connection between geometry and information becomes especially striking when we look again at black holes. When matter falls into a black hole, the information describing that matter cannot simply vanish without violating quantum mechanics. Yet the event horizon seems to hide that information from the outside universe. One possible resolution is that the information is encoded on the horizon itself. In this sense, the surface of a black hole behaves like a storage layer containing the information about everything that has fallen inside. Over time, these insights have reshaped how physicists think about gravity. 1/2

When a nuclear reactor is switched on for the first time, an intense, almost hypnotic blue glow appears in the water surrounding the reactor core. This light is neither fire nor heat; it is Cherenkov radiation, a physical phenomenon that occurs when charged particles, such as high-energy electrons produced during nuclear fission, travel through a transparent medium faster than light can propagate within that same medium. While nothing can exceed the speed of light in a vacuum, light travels more slowly in materials like water. When a charged particle surpasses this reduced speed, it emits a coherent shock-like electromagnetic wave, often described as an optical analogue of a sonic boom. This radiation produces the distinctive blue glow. The colour arises because Cherenkov radiation is strongest at shorter wavelengths, which are dominated by blue and ultraviolet light. The phenomenon was first observed experimentally in 1934 and later explained theoretically, work that led to the Nobel Prize in Physics in 1958. Its explanation confirmed how relativity and electromagnetism operate in material media. Today, this deep blue light is both a warning and a scientific tool. It signals the presence of intense ionising radiation, while also being exploited in particle detectors, nuclear reactors, and neutrino observatories. It provides a rare, visible manifestation of subatomic processes that are otherwise hidden from direct human perception. #GottaLovePhysics #Physics

Astronomers have discovered an extraordinary star orbiting Sagittarius A*, the supermassive black hole at the centre of the Milky Way, on the most extreme stellar orbit observed there so far. The star, designated S301, was identified using the GRAVITY instrument and its upgraded GRAVITY+ system on ESO’s Very Large Telescope Interferometer in Chile. What makes S301 particularly important is not simply its enormous speed, but how deeply its orbit carries it into the strongly curved spacetime surrounding the black hole. S301 completes one orbit in only about 8.7 years, the shortest known period for a star around Sagittarius A*, and during its closest approach it passes roughly 1.78 billion kilometres from the black hole, only about 12 times the Earth–Sun distance and comparable to the distance between Saturn and the Sun. At that point it reaches around 25,000 km/s, more than 8% of the speed of light, making it the fastest known star in the Milky Way. Sagittarius A* contains approximately 4.3 million times the mass of the Sun, compressed into a region small enough to behave observationally as a black hole. Astronomers have been studying stars around it for decades because their trajectories provide exceptionally clean tests of gravity. The most famous example is S2, whose 16-year orbit has already allowed researchers to detect gravitational redshift and relativistic orbital precession exactly where general relativity predicts them. S301 takes this experiment much further. Its orbit is extremely elongated, with an eccentricity of about 0.98, and at pericentre it approaches Sagittarius A* roughly ten times more closely than S2 in terms of Schwarzschild radii. The resulting relativistic effects should therefore be considerably stronger. The most interesting consequence is that S301 may allow astronomers to directly measure the spin of Sagittarius A*. According to general relativity, a rotating black hole does not simply curve spacetime through its mass; its rotation also drags the surrounding spacetime with it. This phenomenon, known as frame dragging or the Lense–Thirring effect, produces an additional precession in the orbit of an object moving close to the black hole. The effect becomes rapidly weaker with distance, which is why it has been extremely difficult to detect using previously known stars around Sagittarius A*. S301 travels close enough that the change in its orbit caused by the black hole’s rotation may become measurable within roughly the next decade. Importantly, the researchers have not yet measured the spin of Sagittarius A* from S301. Rather, they have discovered a star whose orbit is sensitive enough to that spin that such a direct measurement may now be realistically achievable. The discovery was technically difficult because S301 is extraordinarily faint. In the infrared K band it has a magnitude of about 19.3, and ESO notes that it appears roughly two billion times fainter than Betelgeuse in the sky. GRAVITY achieves the necessary angular resolution by combining the light from four 8.2-metre Unit Telescopes of the VLT through interferometry, effectively producing a virtual telescope with far greater resolving power than any individual telescope. The team first clearly identified S301 in observations from 2023 and subsequently followed it during 2024 and 2025. Once its preliminary orbit was established, astronomers were able to trace it retrospectively in earlier data from 2021 and even find evidence for it in observations obtained in 2017. Altogether, 19 astrometric measurements were used to constrain its orbit. There is still an important limitation: S301 is currently too faint for researchers to obtain a reliable spectrum and radial velocity. Without that information, two possible three-dimensional orientations of its orbit remain compatible with the observations. Future instruments should resolve this problem. In particular, MICADO on ESO’s Extremely Large Telescope should be sensitive enough to obtain spectroscopy of S301 and determine its radial velocity, while continued observations with GRAVITY+ will refine its astrometry. Its next pericentre passage is expected in 2031, and observing at least two complete orbits should give researchers the precision needed to search for the subtle additional precession produced by the spin of Sagittarius A*. S301 may also provide clues about how stars end up so close to a supermassive black hole. Stars are unlikely to form normally at such small distances because the black hole’s tidal forces make the collapse of ordinary star-forming clouds extremely difficult. The researchers instead favour a scenario involving the Hills mechanism. S301 may originally have belonged to a tight binary system that approached Sagittarius A*. The black hole’s tidal gravity could have torn the binary apart, capturing S301 onto its present highly eccentric orbit while ejecting its companion at enormous velocity, potentially fast enough for that star to escape the Milky Way entirely. The observed orbit of S301 is consistent with this interpretation. The importance of the discovery therefore goes beyond setting a speed record. S301 effectively acts as a natural test particle moving through one of the strongest gravitational fields that astronomers can study using individual stars. Tracking its motion could provide the first direct stellar-dynamical measurement of the rotation of Sagittarius A*, improve tests of general relativity in the strong-field regime and, over longer timescales, potentially probe more subtle properties predicted for rotating Kerr black holes. Instead of observing the black hole itself, we can use the trajectory of S301 to map how Sagittarius A* deforms and twists the spacetime around it. 👉 nature.com/articles/s41586-0…

The universe is filled with enigmatic structures, and among the most captivating are the Fermi Bubbles, massive, gamma-ray-emitting regions that extend from the center of our Milky Way galaxy. Discovered in 2010 by the Fermi Gamma-ray Space Telescope (@NASAFermi)(@NASAUniverse), these bubbles have puzzled astronomers ever since. Recent studies, including a comprehensive analysis published in the Proceedings of the National Academy of Sciences (@PNASNews), and insights from the field of astronomy, shed new light on these intriguing phenomena. Additionally, research from the University of Michigan (@UMichiganNews) provides further evidence supporting the theory that the bubbles are linked to our galaxy's supermassive black hole. The Fermi Bubbles are two colossal, bubble-shaped structures that emanate from the Milky Way's core, stretching approximately 25,000 light-years above and below the galactic plane. They are visible in gamma-ray light, which is the highest-energy form of electromagnetic radiation. These bubbles are not static; they exhibit intricate structures and variations in brightness, indicating dynamic processes at play. Several theories have been proposed to explain the origins of the Fermi Bubbles: Supermassive Black Hole Activity: One leading hypothesis, now supported by University of Michigan research, suggests that the bubbles were formed by past outbursts from the supermassive black hole at the center of the Milky Way. These outbursts could have expelled high-energy particles, creating the bubble-like structures. This theory is bolstered by recent findings indicating that the black hole's activity around 6 million years ago could have driven the formation of these massive structures. Star Formation and Supernovae: Another theory posits that intense periods of star formation, followed by supernova explosions, could have driven powerful winds that sculpted these bubbles. The energy released from numerous supernovae in the central regions of the galaxy could account for the observed features. Cosmic Ray Interactions: Interactions between cosmic rays and interstellar gas may also play a role in generating the gamma-ray emissions seen in the Fermi Bubbles. These interactions can produce gamma rays as cosmic rays collide with gas particles, creating the high-energy radiation detected by the Fermi telescope. The PNAS study, along with research from the University of Michigan, (linked bellow) delves into the structure and composition of the Fermi Bubbles, utilizing data from multiple observatories and advanced simulations. Key findings include: Complex Internal Structure: The bubbles are not uniform; they exhibit filamentary structures and regions of varying density. This complexity suggests a turbulent history, possibly influenced by multiple energetic events. Magnetic Fields and Shockwaves: The research indicates that magnetic fields and shockwaves play crucial roles in shaping the bubbles. These elements contribute to the containment and distribution of high-energy particles within the bubbles. Ongoing Processes: The Fermi Bubbles are still evolving, with evidence of ongoing particle acceleration and gamma-ray production. This suggests that whatever processes created them may still be active today. Black Hole Outburst Evidence: The University of Michigan study specifically ties the bubbles to outbursts from the supermassive black hole. It posits that a significant eruption around 6 million years ago could have supplied the necessary energy to inflate these massive structures, aligning with the observed characteristics of the bubbles. Understanding the Fermi Bubbles extends beyond our galaxy. These structures provide a laboratory for studying high-energy astrophysical processes, which are relevant to other galaxies and cosmic phenomena. Insights gained from the Fermi Bubbles can help refine models of galactic evolution, star formation, and the behavior of supermassive black holes. The Fermi Bubbles remain one of the most fascinating and mysterious features of our galaxy. As we continue to probe their secrets, we are likely to uncover more about the violent and dynamic processes that shape the cosmos. Each new discovery brings us closer to comprehending the vast and energetic universe we inhabit. For those eager to dive deeper into this cosmic enigma, the detailed studies and ongoing research provide a treasure trove of knowledge, propelling us on a journey through the high-energy frontier of astrophysics. References: 👉 pnas.org/doi/full/10.1073/pn… 👉 news.umich.edu/massive-bubbl…

Black holes are often described as objects that simply swallow anything that approaches them, but the reality of accretion is considerably more complicated. When a star passes close enough to a supermassive black hole, tidal forces can tear it apart in what we call a tidal disruption event, or TDE. Some of the stellar material forms an accretion flow around the black hole, while part of the matter and energy can ultimately be driven outward in powerful outflows or jets. These ejections, sometimes informally described as cosmic “burps”, do not come from inside the event horizon; they are produced by the extremely energetic plasma and magnetic fields in the region surrounding the black hole. A new study has now found evidence that the timing of these outflows follows a remarkably consistent physical rule. Astronomers studied twenty tidal disruption events using observations across radio, optical, ultraviolet and X-ray wavelengths, with ten systems providing sufficiently detailed data to model both the evolution of the accretion flow and the launch of the outflows. They found two distinct phases in which jets or outflows can appear. One occurs early, while the black hole is accreting matter at extremely high, sometimes super-Eddington rates. The second occurs much later, hundreds to thousands of days after the original disruption, when the accretion luminosity has declined to roughly 2% of the Eddington luminosity. That 2% value is particularly important because approximately the same transition has already been observed in stellar-mass black holes, systems only around several to tens of times the mass of the Sun. Supermassive black holes can contain millions or billions of solar masses, yet the transition between accretion states and jet production appears to occur at essentially the same fraction of the Eddington limit. This suggests that the coupling between the accretion flow and the production of jets may be scale-invariant: the same basic physics could operate around black holes separated by many orders of magnitude in mass. Tidal disruption events make this comparison possible because they effectively compress the evolution of a supermassive black hole accretion episode into a period that humans can observe. Normal active galactic nuclei may evolve through comparable changes over thousands of years, making it almost impossible to watch an individual supermassive black hole move through different accretion states. After a star is disrupted, however, its supply of material gradually decreases over months and years, allowing astronomers to watch the accretion flow evolve and determine when an outflow appears. The result could also explain why some tidal disruption events produce radio emission almost immediately while others remain quiet for years before suddenly brightening. Rather than being completely unpredictable, delayed jets may simply be waiting for the accretion rate to cross this critical threshold. Because radio observations allow astronomers to follow material moving outward from these systems, knowing approximately when this transition should occur could make future observing campaigns much more efficient. More importantly, the study provides observational evidence that black holes may follow a common accretion and jet-launching mechanism regardless of their mass. If larger samples confirm the relationship, tidal disruption events could become particularly useful laboratories for understanding not only how black holes grow, but also how the energy and matter they return to their surroundings influence the evolution of their host galaxies. 👉 share.google/N0QqYiaxkYWB3wx…

Two studies offer encouraging results for the search for life on Enceladus. One examines how material from its buried ocean reaches space; the other tests whether a terrestrial microorganism can remain active under some of the conditions expected there. Together, they strengthen the case for investigating this moon’s habitability and analysing its ejected ice particles for possible biological material. Enceladus releases ocean material through fractures near its south pole. Cassini sampled these plumes and detected salts and organic compounds. The new research suggests that droplets freeze gradually during their passage through the icy crust, allowing their dissolved constituents to accumulate in different regions. Subsequent fragmentation can produce grains with very different compositions, some containing particularly concentrated substances. This could help future instruments identify compounds that would be harder to detect in a more diluted mixture. The biological experiment explored another question: whether the ocean’s alkaline chemistry could prevent certain microbes from obtaining enough carbon dioxide for their metabolism. Researchers tested Methanothermococcus okinawensis, an archaeon from terrestrial hydrothermal environments that uses hydrogen and carbon dioxide to produce methane without requiring oxygen. In the simulated Enceladus environment, it continued growing and producing methane despite the scarcity of dissolved carbon dioxide, using hydrogen supplied by reactions between water and rock. These findings don’t establish that Enceladus is inhabited. A terrestrial organism’s response to selected laboratory conditions cannot tell us whether life originated in an extraterrestrial ocean. They do provide a stronger experimental basis for asking what could survive there. The sampling research also suggests that any microbial material might become concentrated in a small proportion of the ejected grains. A future spacecraft would therefore need to examine many particles individually to improve its chances of encountering such material. 👉 share.google/siWAtz3J4D4SSh3… 👉 science.org/doi/10.1126/scia…

New images captured by SMILE, a joint mission between ESA and the Chinese Academy of Sciences, show the entire auroral oval surrounding Earth’s North Pole in ultraviolet light, a global view that has not been seen in this way for around 18 years. The video also shows the ring becoming brighter during a geomagnetic substorm, when stored energy in the magnetosphere is suddenly released and particles are accelerated toward the polar regions. As these particles enter the upper atmosphere, they excite atmospheric gases and produce the auroral glow. Observing the aurora from space makes it possible to study it as a complete, dynamic structure rather than only from individual locations on the ground.

The true size of Black Holes. © YT morn1415. #Science #Cosmos #BlackHole