The most precise measurement of how fast the Universe is expanding

Most thorough-ever study of the cosmos expansion confirms that we still can't explain how it's expanding  

The most precise measurement yet of how fast the Universe is expanding shows we have a real and serious problem on our hands. The international H0DN Collaboration, a community consensus report on the Hubble constant, has remapped the markers we use to measure cosmic expansion, creating a framework which pins the rate at 73.5 km's/second/megaparsec for the local Universe, with a certainty of 7 sigma. The problem is that independent measurements still return a rate of 67.24 km's/second/megaparsec for the early Universe, and these new efforts have brought us no closer to resolving the discrepancy, known as the Hubble tension. A comprehensive new study combines decades of research to reveal that we're missing an essential component in our understanding of how the universe works. New research confirms, with the most thorough dataset ever, that something still doesn’t add up in our standard model of cosmology. There's a central crisis in cosmology: Different measurements yield different values for the expansion rate of the universe. Now, a comprehensive analysis combining decades of independent measurements suggests that this discrepancy is not due to error or uncertainty; instead, it's a potential pathway to new physics beyond the standard cosmological model.

Astronomers calculate the universe's expansion rate, or Hubble constant, in two ways. One method is to use measurements of the distance to the cosmic microwave background (CMB), the earliest light that spread out just 380,000 years after the Big Bang. The second method is to study the expansion of the local universe, using observations of "standard candles," nearby stars of a known brightness whose light gets stretched, or redshifted, as it reaches us. The first method's calculations yield a Hubble constant of around 67 or 68 km's/second/megaparsec, while the latter yield a value of approximately 73 km's /second/megaparsec. (One megaparsec is about 3.26 million light-years.). Although this seems like a diminutive discrepancy, it is far greater than statistical uncertainty can explain, presenting a puzzling disagreement known as the Hubble tension. So a large symposium of astronomers convened to vote on the best methods and data for constraining the Hubble constant and determining if the tension actually exists. Our Universe burst into existence some 13.8 billion years ago and has been expanding ever since. The rate at which it does so is known as the Hubble constant, or H0, and it's one of the fundamental measurements that we use to understand the cosmos around us. The Hubble constant helps calculate the age and size of the Universe. It helps us understand the influence of the mysterious dark energy which drives the Universe's expansion. It's one of the values required to calculate intergalactic distances.

In the resulting paper, published in the journal Astronomy & Astrophysics, the authors derived the most precise Hubble constant yet and found that the tension persists, suggesting that our current cosmological model is incomplete. "That's why the Hubble tension is so interesting," study co-author Richard Anderson, an astrophysicist at the University of Göttingen, said. "The comparison between the late and early-universe value of [the Hubble constant] tests basic physics on cosmological scales, and it tells us that something's missing." Astronomers have several very precise tools for determining the rate of H0, and this is where the problems start. Previous cosmological calculations relied on the creation of a cosmic distance ladder. Its rungs comprise increasingly distant celestial objects, including pulsating Cepheid variable stars within the Milky Way and more distant supernovas, whose distances can be calculated from the difference in their intrinsic brightness versus how bright they appear to us after their light has traveled through expanding space. Yet this recent community effort, launched at the International Space Science Institute Breakthrough Workshop in Bern, Switzerland, in March 2025, expanded the cosmic distance ladder into a comprehensive survey of the nearby universe called the Local Distance Network, achieving a lofty goal which was considered "potentially unreachable" a decade ago.

"This isn't just a new value of the Hubble constant," the researchers explained in a statement from the National Science Foundation's NOIRLab; "it's a community-built framework that brings decades of independent distance measurements together, transparently and accessibly." The unified framework combined decades of independent research using various techniques that may overlap in observations to achieve "redundancy", an invaluable technique to reduce systematic errors and statistical anomalies. For example, it allowed the researchers to perform a series of "leave me out" analyses: By excluding a specific technique, such as Cepheid-based calculations, they found a minimal change in the overall results of their newly constrained Hubble constant. However, these two epochs can't be brought into agreement with each other, suggesting we're missing something important. The H0DN collaboration approached the problem by focusing on the local Universe. To measure H0 in local space, astronomers rely on something known as the cosmic distance ladder, where each of the rungs on the ladder represents a different measurement technique. The first rung is parallax, which is the apparent shift in position of distant objects when viewed from different vantage points. As Earth moves around the Sun, the parallax of stars tells us how far away they are. The second rung is stars of known brightness, such as Cepheid variables. The third rung is Type Ia supernovae, which have a known brightness peak.

The Local Distance Network is founded on anchors, celestial objects whose distances have been determined geometrically through methods like parallax, an apparent change in an object's position which occurs with a change in perspective. Space telescope access may be limited, but you can reproduce parallax yourself by holding a finger at arm's length and seeing it seemingly shift positions by closing one eye and then the other. Accordingly, the researchers used multiple local-universe anchor points, including the galaxy NGC 4258, located more than 20 million light-years away; the Magellanic Clouds, which are a pair of dwarf galaxies about 200,000 light-years away; and numerous variable stars within the Milky Way. Then, they included a multitude of objects of measured distances, including dying old red giant stars and "megamasers," the intensely bright cosmic lasers generated in the accretion disks of supermassive black holes. The researchers also included more than 7,500 galaxies, observed by facilities such as the Hubble Space Telescope and the Dark Energy Spectroscopic Instrument, out to a distance of more than 1 billion light-years.

As a result, the Local Distance Network developed in this study represents the most precise direct measurement of the Hubble constant in the local universe: 73.50 km's/second/megaparsec, with a relative uncertainty of 1.09%. The conclusion? The Hubble tension is real, similar to previously measured values, and not just an artifact. One possible explanation for the Hubble tension is that there may have been a miscalculation in one of the rungs of the distance ladder, which was carried through to the final measurement. To address this, the collaboration built, not a ladder, but a distance network built from many overlapping techniques for measuring distance, including Cepheid variables, stars at the tip of the red giant branch, Mira variables, megamasers, Type Ia and Type II supernovae, surface brightness fluctuations, the Tully-Fisher relation, and the Fundamental Plane. All these give accurate measurements to nearby stars and galaxies, some of which overlap with each other. Crucially, the researchers rigorously stress-tested their results. They tried removing, by turn, several of the methods and telescopes to see if taking one out changed the result, which would have indicated a flaw in that method. The fact that this discrepancy persists may hint that early-universe measurements need to be similarly reassessed on a deeper level.

"One interesting, relatively new, and perhaps more natural idea involves primordial magnetic fields, which could change the scale of the structure seen in the CMB," study co-author John Blakeslee, director of research and science services at NOIRLab, explained. Excitingly, this research further supports the idea that new physics are needed to illuminate dark energy and the other forces driving the expansion and ultimate fate of the universe. And because this framework is modular, upcoming methods and data from next-generation observatories may finally resolve the Hubble tension, but then again, that's what cosmologists have been hoping for more than a decade. They also tried using different datasets and changing the assumptions on which their analysis was based. The needle barely moved. This is the most stringent examination of H0 at the local level to date, and it survived everything the H0DN Collaboration could throw at it. But measurements of H0 in the distant Universe are also robust, and consistently hovering around the 67 km's/second/megaparsec mark. In recent years, some efforts have focused on overturning the Hubble tension on the basis that our measurements may be wrong. Generally, if our two options are human error and unknown physics, the culprit tends to end up being the former, so that's not an unreasonable expectation. However, this new research strongly indicates that the problem is indeed real, and may require new physics to resolve. "Rather than serving solely to constrain dark energy models, as envisioned a decade ago, the improved accuracy of H0 now exposes a broader inconsistency within the standard cosmological framework and strengthens the case for new physics or a deeper reassessment of early-Universe inferences," the H0DN Collaboration explains. "The evolving role of H0 has already reshaped our understanding of precision cosmology, and further surprises may lie ahead."

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Birth of Gold Hydride

 Scientists formed a new compound, accidentally proved that gold is a 'reactive metal'

In a high-pressure lab experiment, scientists accidentally created a new compound called gold hydride. Gold has always symbolised permanence and chemical calm. For centuries, scientists called it stubbornly unreactive. Now, a laboratory experiment has shattered that belief. Researchers have forced gold to react with hydrogen. The result is a never-before-seen compound called gold hydride. This discovery changes how scientists understand noble metals. This particular hydride formed when thin gold foil met dense hydrogen at pressures hundreds of thousands of times Earth’s atmosphere and blazing temperatures. The discovery challenges gold’s reputation as a nearly inert metal and shows how extreme conditions can push familiar materials into unfamiliar forms. By creating gold hydride in the lab, researchers opened a way to study dense hydrogen like that inside giant planets and fusing stars.

The experiment was led by Mungo Frost a staff scientist at Stanford Linear Accelerator Center(SLAC). They aimed to study hydrogen under extreme pressures. Gold was chosen as a stable, inert reference material. It usually acts as a passive X-ray absorber. Scientists expected gold to remain chemically silent throughout. No reaction was predicted or even considered possible. Then, unexpectedly, gold began interacting with hydrogen atoms. The “inert” metal refused to stay passive this time. A new compound formed, and scientists called it the first solid "gold hydride". What started as routine observation became groundbreaking discovery. Gold revealed a hidden chemical personality in extreme conditions. Gold was expected to remain inert during the experiment, since it is normally chemically unreactive. The accidental reaction produced the first confirmed solid compound made solely of gold and hydrogen atoms in any laboratory experiment. Under pressure and heat, hydrogen became superionic, a state where atoms move like a liquid inside a solid, making the gold hydride conductive. Hydrogen usually barely scatters X-rays, so the team watched changes in the gold lattice to deduce how the light atoms were moving. Simulations and measurements indicate that hydrogen diffuses rapidly through the hexagonal gold lattice at high temperature but separates when the sample cools. Interior models of Jupiter suggest a shell of metallic hydrogen surrounding a dense core, with pressures beyond anything on Earth’s surface. In those environments, hydrogen is compressed so tightly that it behaves more like a dense, electrically conducting fluid than a simple gas.

Recent research has shown that superionic states in silica water and silica hydrogen mixtures could help explain magnetic fields in giant planets. Gold hydride offers a controlled environment where dense hydrogen’s structure and motion can be measured, giving theorists a clearer target for planetary calculations. The discovery took place at Stanford Linear Accelerator Center (SLAC), California. The experiments focuses on matter under extreme pressures and temperatures. SLAC’s advanced facilities allowed the team to recreate these extreme conditions. It was here that gold defied expectations and formed gold hydride. Gold hydride joins a catalog of exotic phases, including superionic water and silica compounds, that appear only when atoms are squeezed and heated. Many of these phases vanish once pressure or temperature drops, yet their existence helps explain how planets move heat and generate magnetic fields. Because hydrides of other metals already show properties like superconductivity, understanding gold hydride could one day help design new electronic materials. Gold hydride’s appearance under stress shows that even familiar elements in lab samples can behave unexpectedly when scientists push conditions beyond normal experience.

Stars like the Sun shine because gravity squeezes hydrogen until nuclei fuse, and fusion researchers try to recreate conditions in experiments on Earth. Accurate models of dense hydrogen, hydrogen compressed to extraordinary pressures and densities, are vital for understanding fusion fuel behavior. Simulations indicate that even small uncertainties in hydrogen’s behavior at high-density can significantly change fusion predictions. By pinning how hydrogen moves through gold at given pressures and temperatures, the measurements give fusion modelers a benchmark to test their calculations. Gold belongs to the noble metals group. These elements are famous for chemical stubbornness. They rarely bond with other elements. This stability made gold useful and predictable. It also made scientists stop questioning its behaviour. But this experiment reopened a closed chapter. Under extreme pressure, gold’s electrons rearranged. New bonding pathways suddenly became possible, and scientists discover that gold is a 'reactive metal' by accidentally creating this new compound.

The experiment was originally designed to clock how long simple hydrocarbons take to turn into diamond under crushing pressure and searing heat. Researchers squeezed tiny drops of hydrocarbon between the tips of a diamond anvil cell, a device that traps samples at immense static pressures. Laser heating inside such cells lets scientists study materials at extreme pressures, as shown in a recent review of diamond anvil work. At the European XFEL in Germany, X-ray pulses hit a thin gold foil in the sample, which then heated the surrounding hydrocarbons. The team cranked the pressure until it rivaled Earth’s lower mantle, then blasted the sample with trains of X-ray pulses. Under those conditions, the study reports gold hydride forming at temperatures above 3,500 degrees Fahrenheit and at pressures far beyond Earth’s mantle. X-ray scattering patterns confirmed that carbon atoms snapped into the tidy lattice of diamond, matching what the researchers expected from earlier work. Signals in the data revealed hydrogen atoms entering the gold lattice, forming gold hydride that altered how the metal scattered X-rays.

In everyday chemistry, gold is grouped with the noble metals that rarely form compounds, which is why jewelry stays bright for decades. In these experiments, gold formed a hydride that held more hydrogen as pressure climbed, yet separated into plain gold again when conditions eased. The findings indicate that extreme pressure and heat can enable forms of chemistry that do not occur under normal conditions. High pressure work has shown unreactive elements like xenon can form compounds, so gold hydride underscores how chemistry changes when matter is squeezed. The experiments relied on the European XFEL, a powerful X-ray laser facility that delivers thousands of pulses each second to targets. Those pulses deposit energy in the gold foil, allowing scientists to heat the sample rapidly while the diamond-anvil cell maintains the pressure. High-energy-density science, the study of matter under extreme pressures and temperatures, uses intense X-ray lasers together with diamond anvil cells. As these tools improve, from tougher diamond anvils to brighter X-ray sources, researchers can probe states of matter once considered purely theoretical.

The study is published in the National Library of Medicine. Understanding gold’s reactivity changes fundamental science. It affects models of planetary interiors. It informs high-pressure material research. The discovery could influence fusion energy studies. Dense hydrogen behaviour remains poorly understood. Gold hydride offers a rare experimental window. It also reminds scientists to question assumptions. Even trusted facts can bend under pressure. The simulation framework that captured superionic hydrogen in gold can predict how other elements behave when infused with hydrogen at different pressures and temperatures. Future experiments can swap gold for other metals or mixtures that resemble planetary materials more closely, letting researchers test whether strange hydrides emerge. Each compound uncovered at such extremes expands the periodic table of high-pressure phases and clarifies how ordinary elements behave when pushed hard. If gold can react, what else can? Are other noble metals hiding secret chemistry Platinum and silver may hold similar surprises. Future experiments will push limits further. More unexpected reactions may soon appear. Gold’s secret reaction sends a powerful message. In science, certainty is always temporary.

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