SPACE/COSMOS
Universe's expansion 'is now slowing, not speeding up'
image:
Researchers used type Ia supernovae, similar to SN1994d pictured in its host galaxy NGC4526, to help establish that the universe’s expansion may actually have started to slow.
view moreCredit: NASA/ESA
Royal Astronomical Society press release
RAS PR 25/42
Embargoed until 00:01 GMT on Thursday 6 November
The universe's expansion may actually have started to slow rather than accelerating at an ever-increasing rate as previously thought, a new study suggests.
"Remarkable" findings published today in Monthly Notices of the Royal Astronomical Society cast doubt on the long-standing theory that a mysterious force known as 'dark energy' is driving distant galaxies away increasingly faster.
Instead, they show no evidence of an accelerating universe.
If the results are confirmed it could open an entirely new chapter in scientists' quest to uncover the true nature of dark energy, resolve the 'Hubble tension', and understand the past and future of the universe.
Lead researcher Professor Young-Wook Lee, of Yonsei University in South Korea, said: "Our study shows that the universe has already entered a phase of decelerated expansion at the present epoch and that dark energy evolves with time much more rapidly than previously thought.
"If these results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago."
For the past three decades, astronomers have widely believed that the universe is expanding at an ever-increasing rate, driven by an unseen phenomenon called dark energy that acts as a kind of anti-gravity.
This conclusion, based on distance measurements to faraway galaxies using type Ia supernovae, earned the 2011 Nobel Prize in Physics.
However, a team of astronomers at Yonsei University have now put forward new evidence that type Ia supernovae, long regarded as the universe’s "standard candles", are in fact strongly affected by the age of their progenitor stars.
Even after luminosity standardisation, supernovae from younger stellar populations appear systematically fainter, while those from older populations appear brighter.
Based on a much larger host-galaxy sample of 300 galaxies, the new study confirmed this effect at extremely high significance (99.999% confidence), suggesting that the dimming of distant supernovae arises not only from cosmological effects but also from stellar astrophysics effects.
When this systematic bias was corrected, the supernova data no longer matched the standard ΛCDM cosmological model with a cosmological constant, researchers said.
Instead, it aligned far better with a new model favoured by the Dark Energy Spectroscopic Instrument (DESI) project, derived from baryonic acoustic oscillations (BAO) – effectively the sound of the Big Bang – and cosmic microwave background (CMB) data.
The corrected supernova data and the BAO+CMB-only results both indicate that dark energy weakens and evolves significantly with time.
More importantly, when the corrected supernova data were combined with BAO and CMB results, the standard ΛCDM model was ruled out with overwhelming significance, the researchers said.
Most surprising of all, this combined analysis indicates that the universe is not accelerating today as previously thought, but has already transitioned into a state of decelerated expansion.
Professor Lee added: "In the DESI project, the key results were obtained by combining uncorrected supernova data with baryonic acoustic oscillations measurements, leading to the conclusion that while the universe will decelerate in the future, it is still accelerating at present.
"By contrast, our analysis — which applies the age-bias correction — shows that the universe has already entered a decelerating phase today. Remarkably, this agrees with what is independently predicted from BAO-only or BAO+CMB analyses, though this fact has received little attention so far."
To further confirm their results, the Yonsei team are now carrying out an "evolution-free test", which uses only supernovae from young, coeval host galaxies across the full redshift range. The first results already support their main conclusion.
"Within the next five years, with the Vera C. Rubin Observatory discovering more than 20,000 new supernova host galaxies, precise age measurements will allow for a far more robust and definitive test of supernova cosmology,: said research professor Chul Chung, a co-lead on the study along with PhD candidate Junhyuk Son.
The Vera C. Rubin Observatory, which sits on a mountain in the Chilean Andes, is home to the world's most powerful digital camera. It began scientific operations this year and could answer vital questions about our own solar system and the wider universe.
After the Big Bang and the rapid expansion of the universe some 13.8 billion years ago, gravity slowed it down. But in 1998, it was established that nine billion years after the universe began, its expansion had started to speed up again, driven by a mysterious force.
Astronomers dubbed this dark energy, but despite it making up about 70 per cent of the universe it is still considered to be one of the greatest mysteries in science.
Last year, data from DESI in Tucson, Arizona suggested that the force exerted by dark energy had changed over time, evidence for which has been growing ever since.
The hope is that with these new tools in their arsenal, astronomers will now be better equipped to find clues about what exactly dark energy is and how it influences the universe.
ENDS
Images & captions
Caption: Researchers used type Ia supernovae, similar to SN1994d pictured in its host galaxy NGC4526, to help establish that the universe's expansion may actually have started to slow.
Credit: NASA/ESA
Caption: The Hubble residual diagram before (top) and after (bottom) the age-bias correction. Corrections are applied to supernova data from the Dark Energy Survey project. After correction, the dataset no longer supports the ΛCDM model (red line) with a cosmological constant, but instead more closely fits with a time-varying dark energy model favoured by a combined analysis using only baryonic acoustic oscillations and cosmic microwave background data (blue line).
Credit: Son et al.
Caption: This diagram shows how the universe appears to be in a state of decelerated expansion (red line). The dotted vertical line marks the present epoch, while the black line shows the ΛCDM prediction. The green and red lines represent the new study’s model before (green) and after (red) age-bias correction, consistent with baryonic acoustic oscillations and cosmic microwave background data (blue line).
Credit: Son et al.
Dark Energy Spectroscopic Instrument
Caption: DESI is a state-of-the-art instrument which maps distant objects to study dark energy.
Credit: Marilyn Sargent/Berkeley Lab
Caption: The Vera C. Rubin Observatory began scientific operations this year and could answer vital questions about our own solar system and the wider universe.
Credit: RubinObs/NOIRLab/SLAC/NSF/DOE/AURA
Further information
The paper ‘Strong Progenitor Age-bias in Supernova Cosmology. II. Alignment with DESI BAO and Signs of a Non-Accelerating Universe’ by Junhyuk Son, Young-Wook Lee, Chul Chung, Seunghyun Park, and Hyejeon Cho has been published in Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/staf1685. For an advance copy of the paper, please email press@ras.ac.uk
Notes for editors
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Journal
Monthly Notices of the Royal Astronomical Society
Article Title
'Strong Progenitor Age-bias in Supernova Cosmology. II. Alignment with DESI BAO and Signs of a Non-Accelerating Universe'
Article Publication Date
6-Nov-2025
Euclid peers through a dark cloud’s dusty veil
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This shimmering view of interstellar gas and dust was captured by the European Space Agency’s Euclid space telescope. Part of a so-called dark cloud, named LDN 1641, the nebula sits at about 1300 light-years from Earth, within a sprawling complex of dusty gas clouds in the constellation of Orion.
In visible light this region of the sky appears mostly dark, with few stars dotting what seems to be a primarily empty background. But, by imaging the cloud with the infrared eyes of its NISP instrument, Euclid reveals a multitude of stars shining through a tapestry of dust and gas.view more
Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by M. Schirmer (MPIA, Heidelberg)
Ageing stars may be destroying their closest planets
University College London
image:
Artist’s impression of a dying Sun-like star engulfing an exoplanet. Here is a link to the original posting of the image: https://noirlab.edu/public/images/noirlab2311a/
view moreCredit: International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani
Ageing stars may be destroying the giant planets orbiting closest to them, according to a new study by astronomers at UCL (University College London) and the University of Warwick.
Once stars like the Sun run out of hydrogen fuel, they cool down and expand to become red giants. In the Sun’s case this will happen in about five billion years.
In the new study, published in the Monthly Notices of the Royal Astronomical Society, researchers looked at nearly half a million stars that had just entered this “post-main sequence” phase of their lives.
The team identified 130 planets and planet candidates (i.e., that still need to be confirmed), including 33 that were previously unknown, orbiting closely around these stars.
They found such planets were less likely to occur around stars that had expanded and cooled enough to be classed as red giants (i.e. that were further on in their post-main sequence evolution), suggesting many of these planets may already have been destroyed.
Lead author Dr Edward Bryant (Mullard Space Science Laboratory at UCL and the University of Warwick) said: “This is strong evidence that as stars evolve off their main sequence they can quickly cause planets to spiral into them and be destroyed. This has been the subject of debate and theory for some time but now we can see the impact of this directly and measure it at the level of a large population of stars.
“We expected to see this effect but we were still surprised by just how efficient these stars seem to be at engulfing their close planets.
“We think the destruction happens because of the gravitational tug-of-war between the planet and the star, called tidal interaction. As the star evolves and expands, this interaction becomes stronger. Just like the Moon pulls on Earth’s oceans to create tides, the planet pulls on the star. These interactions slow the planet down and causing its orbit to shrink, making it spiral inwards until it either breaks apart or falls into the star.”
Co-author Dr Vincent Van Eylen (Mullard Space Science Laboratory at UCL) said: “In a few billion years, our own Sun will enlarge and become a red giant. When this happens, will the solar system planets survive? We are finding that in some cases planets do not.
“Earth is certainly safer than the giant planets in our study, which are much closer to their star. But we only looked at the earliest part of the post-main sequence phase, the first one or two million years of it – the stars have a lot more evolution to go.
“Unlike the missing giant planets in our study, Earth itself might survive the Sun’s red giant phase. But life on Earth probably would not.”
For their study, the researchers used data from NASA’s Transiting Exoplanet Survey Satellite (TESS). They used a computer algorithm to search for the repeated dips in brightness that indicate an orbiting planet is passing in front of the star, focusing on giant planets with short orbital periods (i.e., that took no more than 12 days to orbit their star).
The team began with more than 15,000 possible signals, and applied rigorous tests to rule out false signals, eventually whittling this number down to 130 planets and planet candidates. Of these, 48 were already known, 49 were already identified as planet candidates (i.e., they still need to be confirmed), and 33 were new candidates detected for the first time.
The team found that the more advanced a star’s evolution, the less likely it was to host a nearby giant planet. The overall occurrence rate of such planets was measured at just 0.28%, with the youngest post-main sequence stars showing a higher rate (0.35%) similar to that of main sequence stars, and the most evolved stars, which had cooled and swelled enough to be classed as red giants, dropping to 0.11%. (For this analysis, the researchers excluded the smallest 12 of the 130 identified planets.)
From the TESS data, researchers can estimate the size (radius) of these possible planets. To confirm them as planets rather than planet candidates, astronomers must rule out the possibility of these bodies being low-mass stars or brown dwarfs (“failed stars” whose core pressure is not high enough to start nuclear fusion) by calculating their mass.
This can be done by precisely measuring the movements of their host stars and inferring the gravitational tug of the planets (and therefore their mass) from wobbles in these movements.
Dr Bryant added: “Once we have these planets’ masses, that will help us understand exactly what is causing these planets to spiral in and be destroyed.”
The researchers received funding from the UK Science and Technology Facilities Council (STFC).
Journal
Monthly Notices of the Royal Astronomical Society
Sun: First glimpse of polar magnetic field in motion
Analysis of data from ESA's Solar Orbiter spacecraft from the solar south pole region reveals a surprise: The magnetic field is carried towards the pole faster than expected.
Max Planck Institute for Solar System Research
The Sun is governed by a strict rhythm. The magnetic activity of the Sun displays a cyclic variation, reaching a maximum approximately every eleven years. Two enormous plasma circulations, each in one solar hemisphere, set the pace for this rhythm thus defining the Sun’s eleven-year cycle: near the surface the plasma flows carry the magnetic field lines from the equator to the poles; in the solar interior, the plasma flows back to the equator in a huge cycle spanning the entire hemisphere.
Important details of this solar “magnetic field conveyor belt” are still poorly understood. The exact processes at the Sun's poles are likely to be crucial. From Earth, scientists have only a grazing view of this region making it impossible to determine the properties of the magnetic field. Most space probes have a similarly limited perspective.
Quote:
“To understand the Sun's magnetic cycle, we still lack knowledge of what happens at the Sun's poles. Solar Orbiter can now provide this missing piece of the puzzle.”
Sami Solanki, MPS Director and co-author of the new study
Since February 2020, ESA's Solar Orbiter spacecraft has been travelling in elongated ellipses around the Sun. In March of this year, it left for the first time the plane in which the planets – and almost all other space probes – orbit the Sun. From a trajectory tilted by 17 degrees, Solar Orbiter now for the first time has a better view of the Sun's poles.
In the new publication, which appears today in the journal Astrophysical Journal Letters, researchers led by MPS analyze data from Solar Orbiter's Polarimetric and Helioseismic Imager (PHI) and Extreme-Ultraviolet Imager (EUI). The PHI data are from March 21 of this year; the EUI data cover the period from March 16 to 24. The measurements provide information about the direction of plasma flows and the magnetic field on the solar surface.
The data reveal a refined picture of the supergranulation and magnetic network of the Sun at the south pole for the first time. Supergranules are cells of hot plasma, about two to three times the size of Earth, which densely cover the surface of the Sun. Their horizontal surface flows wash magnetic field lines to their edges, creating the Sun's magnetic network: a web of strong magnetic fields.
To the surprise of the researchers, the magnetic field is seen to drift toward the poles at approximately 10 to 20 meters per second, on average, almost as fast as their counterparts at lower latitudes. Previous studies based on the ecliptic-plane observations have seen much slower drifts of the magnetic field near the high polar latitudes. Their motion offers important clues about the Sun’s global plasma and magnetic field circulation.
Quote:
“The supergranules at the poles act as a kind of tracer. They make the polar component of the Sun's global, eleven-year circulation visible for the first time.”
Lakshmi Pradeep Chitta, research group leader at MPS and first author
It is still unclear whether the Sun's global “magnetic conveyor belt” does truly not slow down near the poles. The data now published only show a brief snapshot of the entire solar cycle. Further observational data, ideally covering longer time periods, are needed.
Journal
The Astrophysical Journal Letters
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
Supergranulation and Poleward Migration of the Magnetic Field at High Latitudes of the Sun
Article Publication Date
5-Nov-2025
Are there different types of black holes? New method puts Einstein to the test
Physicists from Frankfurt and Shanghai compare shadow images of black holes with alternative theories of gravity
Goethe University Frankfurt
image:
At the current resolution of telescopes, black holes predicted by different theories of gravity still look very similar. Future telescopes will make the differences more visible, making it possible to distinguish Einstein's black holes from others.
view moreCredit: Luciano Rezzolla/Goethe University
Frankfurt. Black holes are considered cosmic gluttons, from which not even light can escape. That is also why the images of black holes at the center of the galaxy M87 and our Milky Way, published a few years ago by the Event Horizon Telescope (EHT) collaboration, broke new ground. “What you see on these images is not the black hole itself, but rather the hot matter in its immediate vicinity,” explains Prof. Luciano Rezzolla, who, along with his team at Goethe University Frankfurt, played a key role in the findings. “As long as the matter is still rotating outside the event horizon – before being inevitably pulled in – it can emit final signals of light that we can, in principle, detect.”
The images essentially show the shadow of the black hole. This finding now opens up the opportunity to closely examine the theories behind these extreme cosmic objects. So far, Einstein’s general theory of relativity is considered the gold standard in physics when it comes to the description of space and time. It predicts the existence of black holes as special solutions, along with all their peculiarities. This includes the event horizon, beyond which everything – including light – disappears. “There are, however, also other, still hypothetical theories that likewise predict the existence of black holes. Some of these approaches require the presence of matter with very specific properties or even the violation of the physical laws we currently know,” Rezzolla says.
Together with colleagues from Tsung-Dao Lee Institute Shanghai (China), the Frankfurt-based physicist introduced a new possibility to check such alternative theories in the journal “Nature Astronomy”. Until now, there has been no solid data to enable either the refutation or confirmation of these theories – something the researchers plan to change in the future by using shadow images of supermassive black holes.
“This requires two things,” Rezzolla explains. “On the one hand, high-resolution shadow images of black holes to determine their radius as accurately as possible, and on the other hand, a theoretical description of how strongly the various approaches deviate from Einstein’s theory of relativity.” The scientists have now presented a comprehensive description of how different types of hypothetical black holes diverge from relativity theory and how this is reflected in the shadow images. To investigate this, the team conducted highly complex three-dimensional computer simulations that replicate the behavior of matter and magnetic fields in the curved spacetime surrounding black holes. From these simulations, the researchers then generated synthetic images of the glowing plasma.
“The central question was: How significantly do images of black holes differ across various theories?” explains lead author Akhil Uniyal of the Tsung-Dao Lee Institute. From this, they were able to derive clear criteria that, with future high-resolution measurements, could often allow a decision to be made in favor of a specific theory. While the differences in images are still too small with the current resolution of the EHT, they systematically increase with improved resolution. To address this, the physicists developed a universal characterization of black holes that integrates very different theoretical approaches.
“One of the EHT collaboration’s most important contributions to astrophysics is turning black holes into testable objects,” Rezzolla emphasizes. “Our expectation is that relativity theory will continue to prove itself, just as it has time and again up to now.” So far, the results align with Einstein’s theory. However, the measurement uncertainty is still so high that only a few very exotic possibilities have been ruled out. For instance, the two black holes at the center of M87 and our Milky Way are unlikely to be so-called naked singularities (without an event horizon) or wormholes – just two of the many other theoretical possibilities that need to be checked. “Even the established theory must be continuously tested, especially with extreme objects like black holes,” the physicist adds. It would be groundbreaking if Einstein’s theory were ever proven invalid.
The EHT offers outstanding opportunities for such measurements. This collaboration of several large radio telescopes across the globe achieves a resolution equivalent to a telescope the size of Earth, for the first time enabling a sharp view into the immediate surroundings of black holes. In the future, additional telescopes on Earth are planned to be integrated into the EHT. Scientists are also hoping for a radio telescope in space, which would significantly improve the overall resolution. With such a high-resolution view, it would be possible to subject various theories about black holes to a rigorous test. As the newly presented study shows, this requires angular resolutions of less than one millionth of an arcsecond – comparable to viewing a coin on the Moon from Earth. While this exceeds today’s capabilities, it is expected to be achievable in a few years.
Journal
Nature Astronomy
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
The future ability to test theories of gravity with black-hole shadows
Article Publication Date
5-Nov-2025
