Rainbows could eventually help us identify liquid clouds on planets orbiting other stars. Even when a distant world appears as a single point of light, its reflected starlight can carry information about its atmosphere and surface. A study published in 2023 explored how these optical signatures might be investigated with NASA’s future Habitable Worlds Observatory, or HWO. NASA is developing HWO to directly image planets around nearby stars and examine their potential habitability. Proposed telescope designs have mirrors between six and eight metres across. A coronagraph would suppress the overwhelming glare of the host star, allowing us to collect light from much fainter planets nearby. Measurements of that light’s brightness, spectrum and potentially its polarization could provide complementary ways to characterise these worlds. On Earth, rainbows form when sunlight enters liquid droplets, reflects inside them and emerges with different colours travelling in slightly different directions. This interaction also produces strongly polarized light, whose electric field oscillates preferentially in a particular direction. We could look for a characteristic polarization peak in the combined light of an exoplanet. Its properties could help distinguish liquid droplets from ice particles or dust and constrain what the droplets are made of. We’d be measuring the optical signature of those droplets across an unresolved planet. The viewing geometry matters. As a planet travels around its star, the illuminated fraction visible to us can change, much as the Moon goes through phases. Different optical effects become strongest at different phases. Unfortunately, some occur when the planet appears particularly close to its star, where the coronagraph can obscure it. Both the orbit’s orientation and the instrument’s ability to observe close to the star therefore determine which signals are accessible. Another useful signature is ocean glint: starlight reflecting from a liquid surface. At suitable viewing angles, this reflection could increase a planet’s brightness and alter its polarization. Such reflections already have a clear example elsewhere in our Solar System. Cassini detected sunlight glinting from a lake on Titan, where the surface liquid consists of hydrocarbons. Identifying a distant ocean as water would require interpreting the reflection alongside other evidence about the planet. Atmospheric gases offer another clue through Rayleigh scattering, the process responsible for Earth’s blue sky. Molecules scatter shorter wavelengths more strongly, and the resulting light can also be polarized. Measuring how that signal changes with viewing angle could help reveal an atmosphere. Using 164 candidate stars and one assumed telescope configuration, the researchers estimated that suitable viewing angles for water-cloud rainbows would be accessible in about 46 systems, and for ocean glint in about 16. These figures describe opportunities to observe hypothetical planets. Successful measurements would still depend on the planets’ actual properties and the instrument’s sensitivity. Liquid water clouds alone wouldn’t establish that a planet has oceans or supports life. They would, however, add valuable physical evidence to atmospheric spectroscopy. What I find particularly interesting is how much we might learn from light interacting with droplets, even when we can’t resolve the planet’s clouds or landscape. 👉 arxiv.org/html/2307.15137v1

Yes, you’re right. Light in a vacuum always travels at the same speed, c, when measured locally. “Local” doesn’t mean that speed varies; it means we’re measuring it in our immediate neighbourhood of space and time. Across enormous cosmic distances, the space between two regions can expand fast enough that their separation grows faster than c. Neither region is locally moving through space faster than light, and light still travels at c along its journey. That’s why we call it a local upper limit.

A disk is a broad, flattened collection of gas and dust rotating around a young star, or in this case a pair of stars. It’s the material from which planets form, and young planets can gather more of it as they grow. And yes, both planets orbit around the two stars together! Each planet has its own separate orbit surrounding the pair. The stars are very close together, while the planets are much farther out. We call these “circumbinary planets.”

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.

I see why it looks that way. In the simplest model, dark energy is indeed a constant multiplying the metric tensor. Dark matter has its own density, motion and clustering, described mathematically much like ordinary matter. Being represented by a scalar doesn’t make something a fudge factor; ordinary matter’s density is a scalar too. What matters is whether the model fits independent observations. But yes, we’re still missing an explanation of what these things actually are.

Fair point, I focused too much on explaining the limit rather than your actual question. We haven’t directly measured a signal carried by dark matter or dark energy and established that it obeys that limit. There are indirect, model-dependent constraints, but that’s a different claim. I’d be cautious about calling them just fudge factors, because they account for several independent observations. But you’re right that their underlying nature remains unresolved and could reveal something fundamental missing from our physics. Applying the usual speed limit to them rests on our theoretical framework, rather than a direct experimental demonstration.

It’s natural to picture an explosion, but cosmic expansion means distances growing throughout space, so it doesn’t need a central point or an atmosphere to slow it down. In inflationary models, that rapid phase ended when the field driving it changed, transferring its energy into particles and radiation. The universe kept expanding, but the gravity of that matter and radiation slowed the expansion. Gravity can do this throughout a uniformly filled universe without pulling everything towards one centre. Much later, dark energy became dominant and expansion began accelerating again.

We haven’t directly tested that. The key distinction is that light speed is a local upper limit, not a speed everything must travel at. Massive dark matter particles would move slower, while dark energy may not carry travelling signals at all. Applying that limit to them comes from relativity; we’re still figuring out their underlying nature.

Cosmic inflation: what could have happened in the universe’s first moments? Look far enough in any direction and the universe has broadly similar properties. Individual galaxies are different, and matter gathers into clusters and filaments, but on sufficiently large scales the overall picture is remarkably uniform. Why should regions separated by such enormous distances have so much in common? Cosmic inflation offers a possible explanation. It proposes that the very early universe experienced an extraordinarily brief period of accelerated expansion, during which distances grew approximately exponentially. A small distance could double, then double again, repeatedly, in an exceptionally short time. To understand what that means, imagine two nearby points. As the space between them expands, they become farther apart even without moving locally through space. During inflation, that separation could have grown enormously. This is also why cosmologists can describe distant regions separating faster than light without breaking relativity: the local speed limit for matter and information still applies. One of the strongest motivations for inflation comes from the cosmic microwave background. This radiation has been travelling through the universe since around 380,000 years into its history, when conditions became cool enough for neutral atoms to form and light could travel freely over long distances. Today, we detect it across the sky at almost exactly the same temperature. That similarity needs explaining. If we trace cosmic expansion backwards using the hot Big Bang model without inflation, some of those regions wouldn’t have had time to exchange signals before releasing the light we now observe. Inflation allows them to have shared an earlier history, when they were close enough to interact, before expansion carried them far apart. It also helps explain why the geometry of space appears so close to flat. Think about how a small area of Earth’s curved surface looks almost flat to someone standing on it. Inflation would greatly increase the scale of any initial spatial curvature, making it difficult to detect within our observable region. Here, flatness refers to the geometry of three-dimensional space. We still can’t conclude that the entire universe is exactly flat or infinite. What could have driven such an expansion? Many models introduce a hypothetical field called the inflaton. A field is a physical quantity defined throughout space, and under the right conditions its energy could make cosmic expansion accelerate. We haven’t identified this field experimentally. Its properties remain one of the major unanswered questions. When inflation ended, the energy driving it would have been transferred into particles and radiation through a process called reheating. This would establish the hot, dense conditions described by the hot Big Bang model. The universe would continue expanding and cooling, eventually allowing atoms, stars and galaxies to form. The hot Big Bang describes that early thermal history extremely well, but it doesn’t establish whether space and time had an absolute beginning. There’s another reason inflation is so interesting: it offers an explanation for where cosmic structure came from. In inflationary models, tiny quantum fluctuations generated during that era are stretched to enormous scales. They provide the initial variations from which differences in density develop. Over time, gravity makes the slightly denser regions accumulate more matter. The first galaxies begin forming within the first few hundred million years, and structure continues growing over billions of years. According to this picture, the distribution of galaxies today ultimately traces back to processes operating at quantum scales in the very early universe. We can test that connection. The microwave background preserves information about the early variations, and observations such as those from Planck agree with several predictions of simple inflationary models. For example, the strength of the primordial fluctuations changes only slightly with scale, in a way those models can reproduce. Does that mean inflation is confirmed? We have meaningful indirect support, but we haven’t established a unique explanation for what happened. Different models predict different details, and some have already become difficult to reconcile with observations. An especially valuable clue would be primordial gravitational waves produced during inflation. They could leave a characteristic pattern in the polarisation of the microwave background. Finding it means separating an extremely faint signal from other effects, including Galactic dust and gravitational lensing. No confirmed inflationary gravitational-wave signal has been detected, and some models predict one too weak for present instruments. It makes ideas about an almost unimaginably early epoch accessible to observational tests. We can study ancient radiation and the distribution of galaxies, then check whether their properties fit the predictions. There’s still much we don’t understand, but we have concrete ways to investigate it.

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

Astronomers have obtained an unusually detailed view of a young giant planet interacting with the gas around it. Using @almaobs in Chile, a team studied WISPIT 2, a system roughly 430 light-years away that contains two young giant planets within a disk of gas and dust. The observations connect a directly imaged planet with disturbances in the motion of its surrounding gas, giving researchers a particularly valuable opportunity to test how growing planets reshape their environment. The outer planet, WISPIT 2b, has approximately five times Jupiter’s mass and lies about 57 times the Earth–Sun distance from the centre of the system. Hydrogen emission detected in earlier observations indicates that it’s still gathering material. Its companion, WISPIT 2c, is closer in, at roughly 14 astronomical units, with an estimated mass of about ten Jupiters. Both were already known before these latest ALMA observations. ALMA traced emission from carbon monoxide to map the gas and measure its motion. Near WISPIT 2b, the researchers found a distinctive disturbance that matches predictions for a planet interacting with its disk. This is the first confirmed association between this kind of gas-motion signature and a directly imaged planet. The observations also reveal a gap at the outer planet’s orbit, a depleted inner cavity and a ring of gas and dust between the two planets. The interpretation still requires care. The emission around WISPIT 2b extends across a region larger than expected for a small disk bound to the planet alone. The researchers suggest that several effects contribute, including spiral disturbances driven by the planet’s gravity, local heating and possibly a circumplanetary disk. Observations with finer velocity resolution will be needed to separate those contributions. There’s another interesting detail: WISPIT 2 contains two central stars. A separate spectroscopic study found a close stellar companion, with the pair completing an orbit in about 4.8 days. That makes this the first known circumbinary system with directly imaged protoplanets, offering a way to investigate how planets develop in a disk surrounding a stellar pair. 👉 share.google/JYFVaIF1mxuDu2p…

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We’d need to detect its gravitational effects, then show that a black hole or neutron star couldn’t explain them. One promising route is gravitational waves: a boson star could deform and oscillate under a companion’s gravity, leaving distinctive features in the signal. Measuring those would require a sufficiently strong, clear signal and accurate models to compare it with. We could also look at how it bends light and how surrounding gas behaves. The difficulty is that some boson stars could look remarkably similar to black holes. A convincing identification would need evidence that consistently favours a boson star over the alternatives; one unusual feature alone wouldn’t be enough.

A black hole could separate from a surrounding bosonic field and leave a boson star behind, according to theoretical simulations by researchers at the University of Aveiro. The result concerns hypothetical black holes with “hair”, where an additional field exists outside the event horizon and contributes to the system’s properties. These configurations extend the familiar description of an isolated black hole through its mass, rotation and electric charge. Under suitable conditions, a rotating black hole can support a surrounding scalar field whose oscillation is synchronized with the horizon’s rotation. This connects two theoretical kinds of objects: black holes surrounded by bosonic fields and boson stars, configurations of those fields held together by gravity. The simulations examined systems where the surrounding field contains most of the energy. In the rotating model, the black hole moved outwards in a spiral, disrupted the field and absorbed most of it. A second model, involving electric charge and an interacting scalar field, produced a different outcome: the black hole was expelled, leaving an oscillating boson star moving in the opposite direction. What interests me most is the question this addresses: can a mathematically possible object survive disturbances? That’s essential when assessing whether such configurations could exist in the Universe. These simulations explore their theoretical behaviour; they don’t establish that nature actually produces them. 👉 share.google/fZeOdW12MRw42bs…

Astronomers using @almaobs in Chile have detected emission from neutral oxygen in four galaxies seen as they were roughly 700 to 800 million years after the Big Bang. The observations provide a more direct way to investigate the neutral gas associated with early star formation, helping researchers understand the conditions that allowed young galaxies to grow. Stars form when gas cools and gathers into sufficiently dense regions for gravity to drive collapse. Studying that material in distant galaxies is difficult, and different kinds of gas can contribute to the same observed signal. Neutral gas contains atoms that retain their electrons, whereas ionized gas has had electrons removed, often by radiation from young stars. The team measured an emission line produced by neutral oxygen at a rest wavelength of 145 micrometres. This gives researchers a specific tracer of neutral regions. They also searched for emission from ionized nitrogen to assess how much of the previously detected carbon emission could come from ionized material. Their analysis indicates that most of the carbon signal in these galaxies originates in neutral gas, strengthening its usefulness for investigating their star-forming reservoirs. By comparing the oxygen and carbon measurements with infrared emission and physical models, the researchers inferred dense gas exposed to ultraviolet radiation. The estimated densities resemble those found in galaxies undergoing intense bursts of star formation, although the ultraviolet radiation fields are weaker than in those starburst systems. These young galaxies therefore appear to contain substantial concentrations of dense material without reproducing all the conditions of the most extreme star-forming environments. What I find particularly useful is that this result can improve the interpretation of observations we already have. Carbon emission has been measured in many distant galaxies, and establishing where that emission comes from makes it easier to connect those measurements to the material available for forming stars. The sample contains only four galaxies, so larger surveys will be needed to establish how widespread these conditions were during the Universe’s first billion years. 👉 share.google/fQAoLyS9tf9PZgH…

. @ESA_Euclid first public survey data are revealing how the structure of galactic discs changes over cosmic time. A new study examines how brightness falls from the inner regions of a galaxy towards its outskirts, and how changes in that pattern can help us understand the way galaxies grow. A galactic disc is the flattened component containing stars, gas and dust, where spiral arms are often found. Its brightness usually decreases outwards, but that decline doesn’t always follow a single smooth profile. In some galaxies, it becomes steeper beyond a particular radius. In others, it becomes shallower, leaving a more extended outer component. Astronomers call these transitions “disc breaks”. They aren’t physical cracks or gaps. The team developed an automated analysis of 8,748 disc galaxies from Euclid’s Quick Data Release 1, reaching a redshift of about one. They then used a subset of 4,385 galaxies with reliable classifications to investigate how the different profiles vary with cosmic epoch. Among nearby galaxies, profiles that steepen towards the outskirts account for around half the sample. Looking further back in time, profiles with a shallower outer decline and those combining different kinds of breaks become more common, each reaching approximately 30% at a redshift close to one. The authors interpret this evolution as evidence that the processes shaping discs change as galaxies mature. Environmental influences may play a greater role earlier on, while gradual internal evolution becomes increasingly important later. However, the brightness profiles don’t uniquely identify the mechanisms responsible, and comparing galaxies at different distances doesn’t mean following the same individual galaxies through their lives. IMO, the most interesting aspect is Euclid’s ability to turn detailed measurements of galactic outskirts into a large statistical study. The limitation is that automated classification remains imperfect, with an estimated accuracy of around 70%. The paper acceptance hasn’t been confirmed. 👉 arxiv.org/html/2609.26877v1

The fourth dimension. #HisVoice 😍