Centauri Dreams
Imagining and Planning Interstellar Exploration
New Uses for the Eschaton
One way to examine problems with huge unknowns – SETI is a classic example – is through the construction of a so-called ‘toy model.’ I linger a moment on the term because I want to purge the notion that it infers a lightweight conclusion. A toy model simplifies details to look for the big picture. It can be a useful analytical tool, a way of screening out some of the complexities in order to focus on core issues. And yes, it’s theoretical and idealized, not predictive.
But sometimes a toy model offers approaches we might otherwise miss. Consider how many variables we have to work with in SETI. What kind of signaling strategy would an extraterrestrial civilization choose? What sort of timeframe would it operate under? What cultural values determine its behavior? What is its intent? You can see how long this list can become. I’ll stop here.
The toy model I want to focus on today is one David Kipping uses in a new paper called “The Eschatian Hypothesis.” The term refers to what we might call ‘final things.’ Eschaton is a word that turns up in both cosmology and theology, in the former case talking about issues like the ultimate fate of the cosmos. So when Kipping (Columbia University) uses it in a SETI context, he’s going for the broadest possible approach, the ‘big picture’ of what a detection would look like.
I have to pause here for a moment to quote science fiction writer Charles Stross, who finds uses for ‘eschaton’ in his Singularity Sky (Ace, 2004), to wit:
I am the Eschaton. I am not your God.
I am descended from you, and exist in your future.
Thou shalt not violate causality within my historic light cone. Or else.
Love the ‘or else.’
Let’s now dig into the new paper. Published in Research Notes of the AAS, the paper homes in on a kind of bias that haunts our observations. Consider that the first exoplanets ever found were at the pulsar PSR 1257+12. Or the fact that the first main sequence star with a planet was found to host a ‘hot Jupiter,’ which back in 1995, when 51 Pegasi b was discovered, wasn’t even a category anyone ever thought existed. The point is that we see the atypical first precisely because such worlds are so extreme. While our early population of detections is packed with hot Jupiters, we have learned that these worlds are in fact rarities. We begin to get a feel for the distribution of discoveries.
Hot Jupiters, in other words, are ‘loud.’ They’re the easiest of all radial velocity planet signatures to find. And yet they make up less than one percent of the exoplanets we’ve thus far found. The issue is broad. From the paper:
…over-representation of unusual astronomical phenomena in our surveys is not limited to exoplanetary science. One merely needs to look up at the night sky to note that approximately a third of the naked-eye stars are evolved giants, despite the fact less than one percent of stars are in such a state—a classic observational effect known as Malmquist bias (K. G. Malmquist 1922). Or consider that a supernova is expected roughly twice per century in Milky Way-sized galaxies (G. A. Tammann et al. 1994)—an astoundingly rare event. And yet, despite being an inherently rare type of transient, astronomers routinely detect thousands of supernovae every year (M. Nicholl 2021), as a product of their enormous luminosities.
That’s quite a thought. Go for a walk on a clear winter evening and look up. So many of the stars you’re seeing are giants in the terminal stages of their lifetimes. Those we can see at great range, but our nearest star, Proxima Centauri, demands a serious telescope for us to be able to see it. So we can’t help the bias that sets in until we realize how much of what we are seeing is rare. Sometimes we have to step back and ask ourselves why we are seeing it.
In SETI terms, Kipping steps back from the question to ask whether the first signatures of ETI, assuming one day they appear, will not be equally ‘loud,’ in the same way that supernovae are loud but actually quite rare. We might imagine a galaxy populated by stable, quiescent populations that we are not likely to see, cultures whose signatures are perhaps already in our data and accepted as natural. These are not the civilizations we would expect to see. What we might detect are the outliers, unstable cultures breaking into violent disequilibrium at the end of their lifetimes. These, supernova style, would be the ones that light up our sky.
Kipping’s toy model works on variables of average lifetime and luminosity, examining the consequences on detectability. A loud civilization is one that becomes highly visible for a fraction of its lifetime before going quiet for the rest. The model’s math demonstrates that a civilization that is 100 times louder than its peers – through any kind of disequilibrium with its normal state, as for example nuclear war or drastic climate change – becomes 1000 times more detectable. A supernova is incredibly rare, but also incredibly detectable.
Image: The toy model at work. This is from Kipping’s Cool Worlds video on the Eschatian Hypothesis.
The Eschatian search strategy involves wide-field, high cadence surveys. In other words, observe at short intervals and keep observing with rapid revisit times to the same source. A search like this is optimized for transients, and the author points out that a number of observatories and observing programs are “moving toward a regime where the sky is effectively monitored as a time-domain data set.” The Vera Rubin Observatory moves in this direction, as does PANOPTES (Panoptic Astronomical Networked Observatories for a Public Transiting Exoplanet Survey). The latter is not a SETI program, but its emphasis on short-duration, repeatable events falls under the Eschatian umbrella.
Rather than targeting narrowly defined technosignatures, Eschatian search strategies would instead prioritize broad, anomalous transients—in flux, spectrum, or apparent motion—whose luminosities and timescales are difficult to reconcile with known astrophysical phenomena. Thus, agnostic anomaly detection efforts (e.g., D. Giles & L. Walkowicz 2019; A. Wheeler & D. Kipping 2019) would offer a suggested pathway forward.
I’ve often imagined the first SETI detection as marking a funeral beacon, though likely not an intentional one. The Eschatian Hypothesis fits that thought nicely, but it also leaves open the prospect of what we may not detect until we actually go into the galaxy, the existence of civilizations whose lifetimes are reckoned in millions of years if not more. The astronomer Charles Lineweaver has pointed out that most of our galaxy’s terrestrial-class worlds are two billion years older than Earth. Kipping quotes the brilliant science fiction writer Karl Schroeder when he tunes up an old Arthur Clarke notion: Any sufficiently advanced civilization will be indistinguishable from nature. Stability infers coming to terms with societal disintegration and mastering it.
Cultures like that are going to be hard to distinguish from background noise. We’re much more likely to see a hard-charging, shorter-lived civilization meeting its fate.
The paper is Kipping, “The Eschatian Hypothesis,” Research Notes of the AAS Vol. 9, No. 12 (December, 2025), 334. Full text.
A ‘Tatooine’ Planet Directly Imaged
I jump at the chance to see actual images – as opposed to light curves – of exoplanets. Thus recent news of a Tatooine-style planetary orbit around twin stars, and what is as far as I know the first actually imaged planet in this orbital configuration. I’m reminded not for the first time of the virtues of the Gemini Planet Imager, so deft at masking starlight to catch a few photons from a young planet. Youth is always a virtue when it comes to this kind of thing, because young planets are still hot and hence more visible in the infrared.
The Gemini instrument is a multitasker, using adaptive optics as well as a coronagraph to work this magic. The new image comes out of an interesting exercise, which is to revisit older GPI data (2016-2019) at a time when the instrument is being upgraded and in the process of being moved to Hawaii from Chile, for installation on the Gemini North telescope on Mauna Kea. This reconsideration picked out something that had been missed. Cross-referencing data from the Keck Observatory, what was clearly a planet now emerged.
Image: By revisiting archival data, astronomers have discovered an exoplanet bound to binary stars, hugging them six times closer than other directly imaged planets orbiting binary systems. Above, the Gemini South telescope in Chile, where the data were collected with the help of the Gemini Planet Imager. Photo credit International Gemini Observatory/NOIRLab/NSF/AURA/T. Matsopoulos.
European astronomers at the University of Exeter independently report their own discovery of this planet in a paper in Astronomy & Astrophysics using GPI data in conjunction with ESO’s SPHERE instrument. So this appears to be a dual discovery and the planet is now obviously confirmed. More about the Exeter work in a moment.
Jason Wang is senior author of the Northwestern University study, which appears in The Astrophysical Journal Letters. There are obvious reasons why he and his collaborators are excited about this particular catch:
“Of the 6,000 exoplanets that we know of, only a very small fraction of them orbit binaries. Of those, we only have a direct image of a handful of them, meaning we can have an image of the binary and the planet itself. Imaging both the planet and the binary is interesting because it’s the only type of planetary system where we can trace both the orbit of the binary star and the planet in the sky at the same time. We’re excited to keep watching it in the future as they move, so we can see how the three bodies move across the sky.”
So what do we know about HD143811 AB b? No other directly imaged planet in a binary system presents us with such a tight orbit around its stars. This world is fully six times closer to its primaries than any other planets in a similar orbit, which says a good deal about GPI’s capabilities. The planet itself is mammoth, six times the mass of Jupiter, and evidently relatively cool compared to other directly imaged planets. Formation is thought to have taken place about 13 million years ago, so we’re dealing with a very youthful star but not an infant.
The stars are part of the Scorpius-Centaurus OB association of young stars. The HD 143811 system itself is some 446 light years away. And what a tight binary this appears to be, the twin stars taking a mere 18 Earth days to complete one orbit around their common barycenter. Their giant offspring (assuming this world formed from their circumstellar disk), orbits the twin stars in 300 years.
I suspect that the orbital period is going to get revised. As Wang says: “You have this really tight binary, where stars are dancing around each other really fast. Then there is this really slow planet, orbiting around them from far away. Exactly how it works is still uncertain. Because we have only detected a few dozen planets like this, we don’t have enough data yet to put the picture together.”
The European work, likewise consulting older GPI data, shows an orbital period with wide error margins, a “mostly face-on and low-eccentricity orbit” with a very loosely constrained period of roughly 320 years.” The COBREX (COupling data and techniques for BReakthroughs in EXoplanetary systems exploration) project specializes in re-examining data like that of the GPI, working with high-contrast imaging, spectroscopy, and radial velocity measurements. In this case, the work included a new observation from the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument installed at ESO’s Very Large Telescope in Chile.
Image: HD 143811 AB b, now confirmed by two independent teams of astronomers, using direct imaging data from both the Gemini Planet Imager and the Keck Observatory. The Exeter team confirmed the discovery using ESO’s SPHERE instrument. Credit: Squicciarini et. al.
I mentioned the issue of the new planet’s formation. That’s one of the reasons this discovery is going to make waves. The overview from Exeter:
This discovery adds an important data point for comparative exoplanetology, bridging the temperature range between AF Lep b (De Rosa et al. 2023; Mesa et al. 2023; Franson et al. 2023) and cooler planets with a slightly lower mass, such as 51 Eri b (Macintosh et al. 2015). Together, these directly imaged planets of varying ages and masses offer a unique testbed for models of giant planet cooling and evolution. Moreover, HD 143811 b joins the small sample of circumbinary planets detected in imaging. Monitoring the binary with radial velocities and interferometry, along with astrometric orbit follow-up, will constrain the dynamical history of the system. While a characterization of the atmosphere is currently limited by the spectral coverage, broader wavelength and high-resolution observations especially in the mid-infrared will be essential for probing temperature structure and composition. This characterization will clarify the differences between circumbinary and single-star planets and their relation to free-floating objects.
The discovery was announced in two papers. The Northwestern offering is Jones et al. “HD 143811 AB b: A directly imaged planet orbiting a spectroscopic binary in Sco-Cen,” (11 December 2025). Now available as a preprint, with publication in process at The Astrophysical Journal Letters. The University of Exeter discovery paper is Squicciarini et al., “GPI+SPHERE detection of a 6.1 MJup circumbinary planet around HD 143811,” Astronomy & Astrophysics Vol. 702 (2025), L10 (full text).
Catching Up with TRAPPIST-1
Let’s have a look at recent work on TRAPPIST-1. The system, tiny but rich in planets (seven transits!) continues to draw new work, and it’s easy to see why. Found in Aquarius some 40 light years from Earth, a star not much larger than Jupiter is close enough for the James Webb Space Telescope to probe the system for planetary atmospheres. Or so an international team working on the problem believes, with interesting but frustratingly inconclusive results.
As we’ll see, though, that’s the nature of this work, and in general of investigations of terrestrial-class planet atmospheres. I begin with news of TRAPPIST-1’s flare activity. One of the reasons to question the likelihood of life around small red stars is that they are prone to violent flares, particularly in their youth. Planets in the habitable zone, and there are three here, would be bathed in radiation early on, conceivably stripping their atmospheres entirely, and certainly raising doubts about potential life on the surface.
Image: Artist’s concept of the planet TRAPPIST-1d passing in front of the star TRAPPIST-1. Credit: NASA, ESA, CSA, Joseph Olmsted/STScI.
A just released paper digs into the question by applying JWST data on six flares recorded in 2022 and 2023 to a computer model created by Adam Kowalski (University of Colorado Boulder), who is a co-author on the work. The equations of Kowalski’s model allow the researchers to probe the stellar activity that created the flares, which the authors see as deriving from magnetic reconnection that heats stellar plasma through pulses of electron beaming.
The scientists are essentially reverse-engineering flare activity with an eye to understanding how it might affect an atmosphere, if one exists, on these planets. The extent of the activity came as something of a surprise. As lead author Ward Howard (also at University of Colorado Boulder) puts it: “When scientists had just started observing TRAPPIST-1, we hadn’t anticipated the majority of our transits would be obstructed by these large flares.”
Which would seem to be bad news for biology here, but we also learn from Kowalski’s equations that TRAPPIST-1 flares are considerably weaker than supposed. We can couple this result with two papers published earlier this year in the Astrophysical Journal Letters. Using transmission spectroscopy and working with JWST’s Near-Infrared Spectrograph and Near-Infrared Imager and Slitless Spectrograph, the researchers looked at TRAPPIST-1e as it passed in front of the host star. A third paper, released in November, examines these data and the possibility of methane in an atmosphere. Here we run into the obvious limitations of modeling.
The November paper is out of the University of Arizona, where Sukrit Ranjan and team have gone to work on methane in an M-dwarf planet atmosphere. With an eye toward TRAPPIST-1e, they note this (italics mine):
We have shown that models that include CH4 are viable fits to TRAPPIST-1e’s transmission spectrum through both our forward-model analysis and retrievals. However, we stress that the statistical evidence falls far below that required for a detection. While an atmosphere containing CH4 and a (relatively) spectrally quiet background gas (e.g., N2) provides a good fit to the data, these initial TRAPPIST-1 e transmission spectra remain consistent with a bare rock or cloudy atmosphere interpretations. Additionally, we note that our “best-fit” CH4 model does not explain all of the correlated features present in the data. Here we briefly examine the theoretical plausibility of a N2–CH4 atmosphere on TRAPPIST-1 e to contextualize our findings.
Should we be excited by even a faint hint of an atmosphere here? Probably not. The paper simulates methane-rich atmosphere scenarios, but also discusses alternative possibilities. Here we get a sense for how preliminary all our TRAPPIST-1 work really is (and remember that JWST is working at the outer edge of its limits in retrieving the data used here). A key point is that TRAPPIST-1 is significantly cooler than our G-class Sun. As Ranjan points out:
“While the sun is a bright, yellow dwarf star, TRAPPIST-1 is an ultracool red dwarf, meaning it is significantly smaller, cooler and dimmer than our sun. Cool enough, in fact, to allow for gas molecules in its atmosphere. We reported hints of methane, but the question is, ‘is the methane attributable to molecules in the atmosphere of the planet or in the host star?…[B]ased on our most recent work, we suggest that the previously reported tentative hint of an atmosphere is more likely to be ‘noise’ from the host star.”
The paper notes that any spectral feature from an exoplanet could have not just stellar origins but also instrumental causes. In any case, stellar contamination is an acute problem because it has not been fully integrated into existing models. The approach is Bayesian, given that the plausibility of any specific scenario for an atmosphere has an effect on the confidence with which it can be identified in an individual spectrum. Right now we are left with modeling and questions.
Ranjan believes that the way forward for this particular system is to use a ‘dual transit’ method, in which the star is observed when both TRAPPIST-1e and TRAPPIST-1b move in front of the star at the same time. The idea is to separate stellar activity from what may be happening in a planetary atmosphere. As always, we look to future instrumentation, in this case ESO’s Extremely Large Telescope, which is expected to become available by the end of this decade. And next year NASA will launch the Pandora mission, a small telescope but explicitly designed for characterizing exoplanet atmospheres.
More questions than answers? Of course. We’re pushing hard against the limits of detection, but all these models help us learn what to look for next. Nearby M-dwarf transiting planets, with their deep transit depths, higher transit probability in the habitable zone and frequent transit opportunities, are going to be commanding our attention for some time to come. As always, patience remains a virtue.
Here’s a list of the papers I’ve discussed here. The flare modeling paper is Howard et al., “Separating Flare and Secondary Atmospheric Signals with RADYN Modeling of Near-infrared JWST Transmission Spectroscopy Observations of TRAPPIST-1,” Astrophysical Journal Letters Vol. 994, No. 1 (20 November 2025) L31 (full text).
The paper on methane detection and stellar activity is Ranjan et al., “The Photochemical Plausibility of Warm Exo-Titans Orbiting M Dwarf Stars,” Astrophysical Journal Letters Vol. 993, No. 2 (3 November 2025), L39 (full text).
The earlier papers of interest are Glidden et al., “JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e,” Astrophysical Journal Letters Vol. 990, No. 2 (8 September 2025) L53 (full text); and Espinoza et al. “JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e,” Astrophysical Journal Letters Vol. 990, No. 2 (L52) (full text).
The Rest is Silence: Empirically Equivalent Hypotheses about the Universe
Because we so often talk about finding an Earth 2.0, I’m reminded that the discipline of astrobiology all too easily falls prey to an earthly assumption: Intelligent beings elsewhere must take forms compatible with our planet. Thus the recent post on SETI and fireflies, one I enjoyed writing because it explores how communications work amongst non-human species here on Earth. Learning about such methods may lessen whatever anthropomorphic bias SETI retains. But these thoughts also emphasize that we continue to search in the dark. It’s a natural question to ask just where SETI goes from here. What happens if in all our work, we continue to confront silence? I’ve been asked before what a null result in SETI means – how long do we have to keep doing this before we simply acknowledge that there is no one out there? But a better question is, how would we ever discover a definitive answer given the scale of the cosmos? If not in this galaxy, maybe in Andromeda? If not there, M87?
In today’s essay, Nick Nielsen returns to dig into how these questions relate to the way we do science, and ponders what we can learn by continuing to push out into a universe that remains stubbornly unyielding in its secrets. Nick is an independent scholar in Portland OR whose work has long graced these pages. Of late he has been producing videos on the philosophy of history. His most recent paper is “Human Presence in Extreme Environments as a Condition of Knowledge: An Epistemological Inquiry.” As Breakthrough Listen continues and we enter the era of the Extremely Large Telescopes, questions like these will continue to resonate.
by J. N. Nielsen
What would it mean for humanity to be truly alone in the universe? In an earlier Centauri Dreams post, SETI’s Charismatic Megafauna, I discussed the tendency to focus on the extraterrestrial equivalent of what ecologists sometimes call “charismatic megafauna”—which in the case of SETI consists of little green men, space aliens, bug-eyed monsters, Martians, and their kin—whereas life and intelligence might take very different forms from those with which we’re familiar. [1] We might not feel much of a connection to the discovery of an exoplanet covered in a microbial mats, which couldn’t respond to us, much less communicate with us, but it would be evidence that there is other life in the universe, which suggests there may be other life yet to be found, which also would mean that, as life, we aren’t utterly alone in the universe. This in turn suggests the alternative view that we might be utterly alone, without a trace of life beyond Earth, and this gets to some fundamental questions. One way to cast some light on these questions is through a thought experiment that would bring the method of isolation to bear on the problem. I will focus on a single, narrow, unlikely scenario as a way to think about what it would mean to be truly alone in the universe.
Suppose, then, we find ourselves utterly alone in the universe—not only alone in the sense of there being no other intelligent species with whom we could communicate, and no evidence of any having existed in the universe’s past (from which we could experience unidirectional communication), but utterly alone in the sense that there’s not any sign of life in the universe, not even microbes. This scenario begins where we are today, inhabiting Earth, looking out into the cosmos to see what we can see, listening for SETI transmissions, trying to detect life elsewhere, and planning missions and designing spacecraft to extend this search further outward into the universe. This thought experiment, then, is consistent with what we know of the universe today; it is empirically equivalent to a universe positively brimming with other life and other civilizations that we just haven’t yet found; at our current level of technology and cosmological standing, we can’t distinguish between the two scenarios.
There is a cluster of related problems in the philosophy of science, including the underdetermination of theories, the possibility of empirically equivalent theories, theory choice, and holism in confirmation. I’m going to focus on the possibility of empirically equivalent theories, but what follows could be reformulated in terms of the others. What is it for a theory to be underdetermined? “To say that an outcome is underdetermined is to say that some information about initial conditions and rules or principles does not guarantee a unique solution.” (Lipton 1991: 6) If there’s no unique solution, there may be many possible solutions. Empirically equivalent theories are these many possible solutions. [2]
The discussion of empirically equivalent theories today has focused on the expansion of the consequence class of a theory, i.e., adopting auxiliary hypotheses so as to derive further testable consequences. We’re going to look at this through the other end of the telescope, however. Two theories can have radically different consequence classes while our ability conduct observations that would confirm or disconfirm these consequence classes is so limited that the available empirical evidence cannot distinguish between the two theories. That our ability to observe changes, and therefore the scope of the empirical consequence class changes, due to technologies and techniques of observation has been called “variability of the range of observation” (VRO) and the “inconstancy of the boundary of the observable.” (discussed in Laudan and Leplin 1991). Given VRO, there may be a time in the history of science when the observable consequence classes of two theories coincide, even while their unobservable consequence class ultimately diverges; at this time, the two theories are empirically equivalent in the sense that no current observation can confirm one while disconfirming the other. This is why we build larger telescopes and more powerful particle accelerators: to gain access to observations that can decide between theories that are empirically equivalent at present, but which have divergent consequence classes.
Returning to our thought experiment, where we began as we are today (unable to distinguish between a populous universe and terrestrial exceptionalism)—what do we do next? In our naïveté we make progress with our ongoing search. We build better telescopes, and we orbit larger and more sophisticated telescopes, with the intention of performing exoplanet atmospheric spectroscopy. We build spacecraft that allow us to explore our solar system. We go to Mars, but we don’t find anything there; no microbes in the permafrost or deep in subterranean bodies of water, and no sign of any life in the past. But we aren’t discouraged by this, because it’s always been possible that there was never life on Mars. There are many other places to explore in our solar system. Eventually we travel to interesting places like Titan, with its own thick atmosphere. We find this moon to be scientifically fascinating, but, again, no life of any kind is found. We send probes into subsurface liquid water oceans, first on Enceladus, then Europa, and we find nothing more complex in these waters than what we see in the astrochemistry of deep space: some simple organic molecules, but no macromolecules. Again, these worlds are scientifically fascinating, but we don’t find life and, again, we aren’t greatly bothered because we’ve only recently accustomed ourselves to the idea that there might be life in these oceans, and we can readily un-accustom ourselves as quickly. But it does raise questions, and so we seek out all the subsurface oceans in our solar system, even the brine pockets under the surface of Ceres, this time with a little more urgency. Again, we find many things of scientific interest, but no life, and no other unexpected forms of emergent complexity.
Suppose we exhaust every potential niche in our solar system, from the ice deep in craters on Mercury, to moons and comets in the outer solar system, and we find no life at all, and nothing like life either—no weird life (Toomey 2013), no life-as-we-do-not-know-it (Ward 2007), and no alternative forms of emergent complexity that are peers of life (Nielsen 2024). All the while as we’ve been exploring our solar system, our cosmological “backyard” as it were, we’ve continued to listen for SETI signals, and we’ve heard nothing. And we’ve continued to pursue exoplanet atmospheric spectroscopy, and we have a few false positives and a few mysteries—as always, scientifically interesting—but no life and no intelligence betrays itself. Now we’re several hundred years in the future, with better technology, better scientific understanding, and presumably a better chance of finding life, but still nothing.
If we had had some kind of a hint of possible life on another world, we could have had some definite target for the next stage of our exploration, but so far we’ve drawn a blank. We could choose our first interstellar objective by flipping a coin, but instead we choose to investigate the strangest planetary system we can find, with some mysterious and ambiguous observations that might be signs of biotic processes we don’t understand. And so we begin our interstellar exploration. Despite choosing a planetary system with ambiguous observations that might betray something more complex going on, once we arrive at the other planetary system and investigate it, we once again come up empty-handed. The investigation is scientifically interesting, as always, but it yields no life. Suppose we investigate this other planetary system as thoroughly as we’ve investigated our own solar system, and the whole thing, with all its potential niches for life, yields nothing but sterile, abiological processes, and nothing that on close inspection can’t be explained by chemistry, mineralogy, and geology.
Again we’re hundreds of years into the future, with interstellar exploration under our belt, and we still find ourselves alone in the cosmos. Not only are we alone in the cosmos, but the rest of the cosmos so far as we have studied it, is sterile. Nothing moves except that life that we brought with us from Earth. Still hundreds of years into the future and with all this additional exploration, and the scenario remains consistent with the scenario we know today: no life known beyond Earth. We can continue this process, exploring other scientifically interesting planetary systems, and trying our best to exhaustively explore our galaxy, but still finding nothing. At what threshold does this unlikelihood rise to the level of paradoxicality? Certainly at this point the strangeness of the situation in which we found ourselves would seem to require an explanation. So instead of merely searching for life, wherever we go we also seek to confirm that the laws of nature we’ve formulated to date remain consistent. That is to say, we test science for symmetry, because if we are able to find asymmetry, we will have found a limit to scientific knowledge.
We don’t have any non-arbitrary way to limit the scope of our scientific findings. If any given scientific findings could be shown to fail under translation in space or translation in time, then we would have reason to restrict their scope. Indeed, if we were to discover that our scientific findings fail beyond a given range in space and time, there would be an intense interest in exploring that boundary, mapping it, and understanding it. Eventually, we would want to explain this boundary. But without having discovered this boundary, we find ourselves in a quandary. Our science ought to apply to the universe entire. At least, this is the idealization of scientific knowledge that informs our practice. “On the one hand, there are truths founded on experiment, and verified approximately as far as almost isolated systems are concerned; on the other hand, there are postulates applicable to the whole of the universe and regarded as rigorously true.” (Poincaré 1952: 135-136) Earth and its biosphere are effectively an isolated system in Poincaré’s sense. We’ve constructed a science of biology based on experimentation within that isolated system (“verified approximately as far as almost isolated systems are concerned”), and the truths we’ve derived we project onto the universe (“applicable to the whole of the universe”). But our extrapolation of what we observe locally is an idealization, and our projecting a postulate onto the universe entire is equally an idealization. We can no more realize these idealizations in fact than we can construct a simple pendulum in fact. [3]
We need to distinguish between, on the one hand, that idealization used in science and without which science is impossible (e.g., the simple pendulum mentioned above), and, on the other hand, that idealization that is impossible for science to capture in any finite formalization, but which can be approximated (like the ideal isolation of experiment discussed by Poincaré). Holism in confirmation, to which I referred above (and which is especially associated with Duhem-Quine thesis), is an instance of this latter kind of idealization. Both forms of idealization force compromises upon science through approximation; we accept a result that is “good enough,” even if not perfect. Each form of idealization implies the other, as, for example, the impossibility of accounting for all factors in an experiment (idealized isolation) implies the use of a simplified (ideal) model employed in place of actual complexity. Thus one ideal, realizable in theory, is substituted for another ideal, unrealizable in theory.
Our science of life in the universe, i.e., astrobiology, involves these two forms of idealization. Our schematic view of life, embodied in contemporary biology (for example, the taxonomic hierarchy of kingdom, phylum, class, order, family, genus, and species, or the idealized individuation of species), is the idealization realizable in theory, while the actual complexity of life, the countless interactions of actual biological individuals within a population both of others of its own species and individuals of other species, not to mention the complexity of the environment, is the idealization unrealizable in theory. The compromises we have accepted up to now, which have been good enough for the description of life on Earth, may not be adequate in an astrobiological context. Thus the testing of science for symmetries in space and time ought to include the testing of biology for symmetries, but, since in this thought experiment there are no other instances of biology beyond Earth, we cannot test for symmetry in biology as we would like to.
Suppose that our research confirms that as much of our science as can be tested is tested, and this science is as correct as it can be, and so it should be predictive, even if it doesn’t seem to be doing a good job at predicting what we find on other worlds. We don’t have to stop there, however. If we don’t find other living worlds in the cosmos, we might be able to create them. Exploring the universe on a cosmological scale would involve cosmological scales of time. If we were to travel to the Andromeda galaxy and back, about four million years would elapse back in the Milky Way. If we were to travel to other galaxy clusters, tens of millions of years or hundreds of millions of years would elapse. These are biologically significant periods of time, by which I mean these are scales of time over which macroevolutionary processes could take place. Our cosmological exploration would give us an opportunity to test that. In the sterile universe that we’ve discovered in this thought experiment, we still have the life from Earth that we’ve brought to the universe, and over biological scales of time life from Earth could go on to its own cosmological destiny. In our exploration of a sterile universe, we could plant the seeds of life from Earth and seek to create the biological universe we expected to find. The adaptive radiation of Earth life, facilitated by technology, could supply to other worlds the origins of life, and if origins of life were the bottleneck that produced a sterile universe, then once we supply that life to other worlds, these other worlds should develop biospheres in a predictable way (within expected parameters).
It probably wouldn’t be as easy as leaving some microbes on another planet or moon; we would have to prepare the ground for them so they weren’t immediately killed by the sterile environment. In other words, we would have to practice terraforming, at least to the extent of facilitating the survival, growth, and evolution of rudimentary Earth life on other worlds. If every attempt at terraforming immediately failed, that would be as strange as finding the universe to be sterile, and perhaps more inexplicable. But that’s a rather artificial scenario. It’s much more realistic to imagine that we attempt the terraforming of many worlds, and, despite some initial hopeful signs, all of our attempts at terraforming eventually die off, all for apparently different reasons, but none of them “take.” This would be strange, but we could still seek some kind of scientific explanation for this that demonstrated truly unique forces to be at work on Earth that allowed the biosphere not only to originate but to survive over cosmological scales of time (the “rare Earth” hypothesis with a vengeance).
If the seeding of Earth life on other worlds didn’t end in this strange way (as strange as the strangeness of exploring a sterile universe, so it’s a continued strangeness), but rather some of these terraforming experiments were successful, what comes next could entail a number of possible outcomes of ongoing strangeness. Leaving our galaxy for a few billion years of exploration in other galaxies, upon our return we could study these Earth life transplantations. Transplanted Earth life on other worlds could very nearly reproduce the biosphere on Earth, which would suggest very tight constraints of convergent evolution. If origins of life are very rare, and conditions for the further evolution of life are tightly constrained by convergent evolution, that would partially explain why we found a sterile universe, but the conditions would be far stronger than we would expect, and that would be scientifically unaccountable.
Another strange outcome would be if our terraformed worlds with transplanted Earth life all branched out in radically different directions over our multi-billion year absence exploring other galaxies. We would expect some branching out, but there would be a threshold of branching out, with none of the biospheric outcomes even vaguely resembling any of the others, that would defy expectations, and, in defying expectations, we would once again find ourselves faced with conditions much stronger than we would expect. In all these cases of strangeness—the strangeness of all our engineered biospheres failing, the strangeness of our engineered biospheres reproducing Earth’s biosphere to an unexpected degree of fidelity, and the strangeness of our engineered biospheres all branching off in radically different directions—we would confront something scientifically unaccountable. Even though we have no experience of other biospheres, we still have expectations for them based on the kind of norms we’ve come to expect from hundreds of years of practicing science, and departure from the norms of naturalism is strange. All of these scenarios would be strange in the sense of defying scientific expectations, and that would make them all scientifically interesting.
These scenarios are entirely consistent with our current observations, so that a sterile universe with Earth as the sole exception where life is to be found is, at the present time, empirically equivalent with a living universe in which life is commonplace. However, the exploration of our own solar system could offer further confirmation of a sterile universe, or disconfirm it, or modify it. If, as in the preceding scenario, we find nothing at all beyond Earth in our solar system, this will increase the degree of confirmation for the sterile universe hypothesis (which we could also call terrestrial exceptionalism). If we were to find life elsewhere in our solar system, but molecular phylogeny shows that all life in our solar system derives from a single origins of life event, then we will have demonstrated that life as we know it can be exchanged among worlds, but the likelihood of independent origins of life events would be rendered somewhat less probable, especially if we were to determine that any of the over life-bearing niches in our solar system were not only habitable, but unambiguously urable. [4]
If we were to find life elsewhere in our solar system and molecular phylogeny shows that these other instances of life derive from independent origins of life events, then this would increase the degree of confirmation of the predictability of origins of life events on the basis of our present understanding of biology. The number of distinct origins of life events could serve as a metric to quantify this. [5] If we were to find life elsewhere in the solar system and this life consists of an eclectic admixture of life with the same origins event as life on Earth, and life derived from distinct origins events, then we would know both that the distribution of life among worlds and origins of life were common, and on this basis we would expect to find the same in the cosmos at large. An exacting analysis of this maximal life scenario would probably yield interesting details, such as particular forms of life that appear the most readily once boundary conditions have been met, and particular forms of life that are more finicky and don’t as readily appear. Similarly, among life distributed across many worlds we would likely find that some varieties are more readily distributed than others.
If the solar system is brimming with life, we could still maintain that the rest of the cosmos is sterile, reproducing the same scenario as above, but the scenario would be less persuasive, or perhaps I should say less frightening, knowing that life had originated elsewhere and was not absolutely unique to Earth. Nevertheless, we could yet be faced with a scenario that is even more inexplicable than the above (call it the “augmented Fermi paradox” if you like). If we found our solar system to be brimming with life, with life easily originating and easily transferable among astronomical bodies, increasing our confidence that life is common in the universe and widely distributed, and then we went out to explore the wider universe and found it to be sterile, we would be faced with an even greater mystery than the mystery we face today. The dilemma imposed upon us by the Fermi paradox can yet take more severe forms than the form in which we know it today. The possibilities are all the more tantalizing given that at least some of these questions will be answered by evidence within our own solar system.
It seems likely that the Fermi paradox is an artifact of the contemporary state of science, and will persist as long as science and scientific knowledge retains its current state of conceptual development. Anglo-American philosophy of science has tended to focus on confirmation and disconfirmation of theories, while continental philosophy of science has developed the concept of idealization [6]; I have drawn on both of these traditions in the above thought experiment, and it will probably require resources from both of these traditions to resolve the impasse we find ourselves at present. Because science and scientific knowledge itself would be called into question in this scenario, there would be a need for human beings themselves to travel to the remotest parts of the universe to ensure the integrity of the scientific process and the data collected (Nielsen 2025b), and this will in turn demand heroic virtues (Nielsen 2025) on the part of those who undertake this scientific research program.
Thanks are due to Alex Tolley for suggesting this.
Notes
1. I have discussed different definitions of life in (Nielsen 2023), and I have formulated a common theoretical framework for discussing forms of life and intelligence not familiar to us in (Nielsen 2024b) and (Nielsen 2025a).
2. The discussion of empirically equivalent theories probably originates in (Van Fraassen 1980).
3. I am using “simple pendulum” here in the sense of an idealized mathematical model of a pendulum that assumes a frictionless fulcrum, a weightless string, a point mass weight bob, absence of air drag, short amplitude (small-angle approximation where sinθ≈θ), inelasticity of pendulum length, rigidity of the pendulum support, and a uniform field of gravity during operation of the pendulum. Actual pendulums can be made precise to an arbitrary degree, but they can never exhaustively converge on the properties of an ideal pendulum.
4. “Urable” planetary bodies are those that are, “conducive to the chemical reactions and molecular assembly processes required for the origin of life.” (Deamer, et al. 2022)
5. The degree of distribution of life from a single origins of life event, presumably a function of the particular form of life involved, the conditions of carriage (i.e., the mechanism of distribution), and the structure of the planetary system in question, would provide another metric relevant to assessing the ability of life to survive and reproduce on cosmological scales.
6. Brill has published fourteen volumes on idealization in the series Poznań Studies in the Philosophy of the Sciences and the Humanities.
References
Deamer, D., Cary, F., & Damer, B. (2022). Urability: A property of planetary bodies that can support an origin of life. Astrobiology, 22(7), 889-900.
Laudan, L. and Leplin, J. (1991). “Empirical Equivalence and Underdetermination.” Journal of Philosophy. 88: 449–472.
Lipton, Peter. (1991). Inference to the Best Explanation. Routledge.
Nielsen, J. N. (2023). “The Life and Death of Habitable Worlds.” Chapter in: Death And Anti-Death, Volume 21: One Year After James Lovelock (1919-2022). Edited by Charles Tandy. 2023. Ria University Press.
Nielsen, J. N. (2024a). Heroic virtues in space exploration: everydayness and supererogation on Earth and beyond,” Heroism Sci. doi:10.26736/hs.2024.01.12
Nielsen, J. N. (2024b). Peer Complexity in Big History. Journal of Big History, VIII(1); 83-98.
DOI | https://doi.org/10.22339/jbh.v8i1.8111 (An expanded version of this paper is to appear as “Humanity’s Place in the Universe: Peer Complexity, SETI, and the Fermi Paradox” in Complexity in Universal Evolution—A Big History Perspective.)
Nielsen, J.N. (2025a). An Approach to Constructing a Big History Complexity Ladder. In: LePoire, D.J., Grinin, L., Korotayev, A. (eds) Navigating Complexity in Big History. World-Systems Evolution and Global Futures. Springer, Cham. https://doi.org/10.1007/978-3-031-85410-1_12
Nielsen, J.N. (2025b). Human presence in extreme environments as a condition of knowledge: an Epistemological inquiry. Front. Virtual Real. 6:1653648. doi: 10.3389/frvir.2025.1653648
Poincaré, Henri. (1952). Science and Hypothesis. Dover.
Toomey, D. (2013). Weird life: The search for life that is very, very different from our own. WW Norton & Company.
Van Fraassen, B. C. (1980). The scientific image. Oxford University Press.
Ward, P. (2007). Life as we do not know it: the NASA search for (and synthesis of) alien life. Penguin.
The Firefly and the Pulsar
We’ve now had humans in space for 25 continuous years, a feat that made the news last week and one that must have caused a few toasts to be made aboard the International Space Station. This is a marker of sorts, and we’ll have to see how long it will continue, but the notion of a human presence in orbit will gradually seem to be as normal as a permanent presence in, say, Antarctica. But what a short time 25 years is when weighed against our larger ambitions, which now take in Mars and will continue to expand as our technologies evolve.
We’ve yet to claim even a century of space exploration, what with Gagarin’s flight occurring only 65 years ago, and all of this calls to mind how cautiously we should frame our assumptions about civilizations that may be far older than ourselves. We don’t know how such species would develop, but it’s chastening to realize that when SETI began, it was utterly natural to look for radio signals, given how fast they travel and how ubiquitous they were on Earth.
Today, though, things have changed significantly since Frank Drake’s pioneering work at Green Bank. We’re putting out a lot less energy in the radio frequency bands, as technology gradually shifted toward cable television and Internet connectivity. The discovery paradigm needs to grow lest we become anthropocentric in our searches, and the hunt for technosignatures reflects the realization that we may not know what to expect from alien technologies, but if we see one in action, we may at least be able to realize that it is artificial.
And if we receive a message, what then? We’ve spent a lot of time working on how information in a SETI signal could be decoded, and have coded messages of our own, as for example the famous Hercules message of 1974. Sent from Arecibo, the message targeted the Hercules cluster some 25,000 light years away, and was obviously intended as a demonstration of what might later develop with nearby stars if we ever tried to communicate with them.
But whether we’re looking at data from radio telescopes, optical surveys of entire galaxies or even old photographic plates, that question of anthropocentrism still holds. Digging into it in a provocative way is a new paper from Cameron Brooks and Sara Walker (Arizona State) and colleagues. In a world awash with papers on SETI and Fermi and our failure to detect traces of ETI, it’s a bit of fresh air. Here the question becomes one of recognition, and whether or not we would identify a signal as alien if we saw it, putting aside the question of deciphering it. Interested in structure and syntax in non-human communication, the authors start here on Earth with the common firefly.
If that seems an odd choice, consider that this is a non-human entity that uses its own methods to communicate with its fellow creatures. The well studied firefly is known to produce its characteristic flashes in ways that depend upon its specific species. This turns out to be useful in mating season when there are two imperatives: 1) to find a mate of the same species in an environment containing other firefly species, and 2) to minimize the possibility of being identified by a predator. All this is necessary because according to one recent source, there are over 2600 species in the world, with more still being discovered. The need is to communicate against a very noisy background.
Image: Can the study of non-human communication help us design new SETI strategies? In this image, taken in the Great Smoky Mountains National Park, we see the flash pattern of Photinus carolinus, a sequence of five to eight distinct flashes, followed by an eight-second pause of darkness, before the cycle repeats. Initially, the flashing may appear random, but as more males join in, their rhythms align, creating a breathtaking display of pulsating light throughout the forest. Credit: National Park Service.
Fireflies use a form of signaling, one that is a recognized field of study within entomology, well analyzed and considered as a mode of communications between insects that enhances species reproduction as well as security. The evolution of these firefly flash sequences has been simulated over multiple generations. If fireflies can communicate against their local background using optical flashes, how would that communication be altered with an astrophysical background, and what can this tell us about structure and detectability?
Inspired by the example of the firefly, what Brooks and Walker are asking is whether we can identify structural properties within such signals without recourse to semantic content, mathematical symbols or other helpfully human triggers for comprehension. In the realm of optical SETI, for example, how much would an optical signal have to contrast with the background stars in its direction so that it becomes distinguishable as artificial?
This is a question for optical SETI, but the principles the authors probe are translatable to other contexts where discovery is made against various backgrounds. The paper constructs a model of an evolved signal that stands out against the background of the natural signals generated by pulsars. Pulsars are a useful baseline because they look so artifical. Their 1967 discovery was met with a flurry of interest because they resembled nothing we had seen in nature up to that time. Pulsars produce a bright signal that is easy to detect at interstellar distances.
If pulsars are known to be natural phenomena, what might have told us if they were not? Looking for the structure of communications is highly theoretical work, but no more so than the countless papers discussing the Fermi question or explaining why SETI has found no sign of ETI. The authors pose the issue this way:
…this evolutionary problem faced by fireflies in densely packed swarming environments provides an opportunity to study how an intelligent species might evolve signals to identify its presence against a visually noisy astrophysical environment, using a non-human species as the model system of interest.
The paper is put together using data from 3734 pulsars from the Australia National Telescope Facility (ATNF). The pulse profiles of these pulsars are the on-off states similar to the firefly flashes. The goal is to produce a series of optical flashes that is optimized to communicate against background sources, taking into account similarity to natural phenomena and trade-offs in energy cost.
Thus we have a thought experiment in ‘structure-driven’ principles. More from the paper:
Our aim is to motivate approaches that reduce anthropocentric bias by drawing on different communicative strategies observed within Earth’s biosphere. Such perspectives broaden the range of ETI forms we can consider and leverage a more comprehensive understanding of life on Earth to better conceptualize the possible modes of extraterrestrial communication… Broadening the foundations of our communication model, by drawing systematically from diverse taxa and modalities, would yield a more faithful representation of Earth’s biocommunication and increase the likelihood of success, with less anthropocentric searches, and more insights into deeper universalities of communication between species.
The authors filter the initial dataset down to a subset of pulsars within 5 kpc of Earth and compute mean period and duty cycle for each. In other words, they incorporate the rotation of the pulsar and the fraction in which each pulse is visible. They compute a ‘cost function’ analyzing similarity cost – how similar is the artificial signal to the background – and an energy cost, meaning the less frequent the pulses, the less energy expended. The terms are a bit confusing, but similarity cost refers to how much an artificial signal resembles a background pulsar signal, while energy cost refers to how long the signal is ‘on.’
So if you’re an ETI trying to stand out against a background field of pulsars, the calculations here produce a signal background period of 24.704 seconds and a duty cycle of ~0.004 (meaning that the signal is ‘on’ for 0.4 percent of the period). Such signals appear at the edge of the pulsar distribution – they would be signals that stand out by being relatively rare and also brief in contrast to the rest of the pulsar population. They would, in other words, serve as the optimal beacon for ETI attempting to communicate.
I spare you the math, which in any case is beyond my pay grade. But the point is this: A civilization trying to get our attention while broadcasting from a pulsar background could do so with a signal that has a long pulsar period (tens of seconds) and a low duty cycle. This would be sufficient to produce a signal that becomes conspicuous to observers. Now we can think about generalizing all this. The pulsar background is one of many out of which a possible signal could be detected, and the principles can be extended beyond the optical into other forms of SETI. The broad picture is identifying a signal against a background, proceeding by identifying the factors specific to each background studied.
Any time we are trying to distinguish an intentional signal, then, we need to optimize – in any signaling medium – the traits leading to detectability. Signals can be identified by their structural properties without any conception of their content as long as they rise above the noise of the background. Back to the fireflies: The paper is pointing out that non-human signaling can operate solely on a structure designed to stand out against background noise, with no semantic content. An effective signal need not resemble human thought.
Remember, this is more or less a thought experiment, but it is one that suggests that cross-disciplinary research may yield interesting ways of interpreting astrophysical data in search of signs of artificiality. On the broader level, the concept reminds us how to isolate a signal from whatever background we are studying and identify it as artificial through factors like duty cycle and period. The choice of background varies with the type of SETI being practiced. Ponder infrared searches for waste heat against various stellar backgrounds or more ‘traditional’ searches needing to distinguish various kinds of RF phenomena.
It will be interesting to see how the study of non-human species on Earth contributes to future detectability methods. Are there characteristics of dolphin communication that can be mined for insights? Examples in the song of birds?
The paper is Brooks et al., “A Firefly-inspired Model for Deciphering the Alien,” available as a preprint.
A Reversal of Cosmic Expansion?
We all relate to the awe that views of distant galaxies inspire. It’s first of all the sheer size of things that leaves us speechless, the vast numbers of stars involved, the fact that galaxies themselves exist in their hundreds of billions. But there is an even greater awe that envelops everything from our Solar System to the most distant quasar. That’s the question of the ultimate fate of things.
Nobody writes about this better than Fred Adams and Greg Laughlin in their seminal The Five Ages of the Universe (Free Press, 2000), whose publication came just after the 1998 findings of Saul Perlmutter, Brian Schmidt and Adam Riess (working in two separate teams) that the expansion of the universe not only persists but is accelerating. The subtitle of the book by Adams and Laughlin captures the essence of this awe: “Inside the Physics of Eternity.”
I read The Five Ages of the Universe just after it came out and was both spellbound and horrified. If we live in what the authors call the ‘Stelliferous era,’ imagine what happens as the stars begin to die, even the fantastically long-lived red dwarfs. Here time extends beyond our comprehension, for this era is assumed to last perhaps 100 trillion years, leaving only neutron stars, white dwarfs and black holes. A ‘Degenerate Era’ follows, and now we can only think in terms of math, with this era concluding after 1040 years. By the end of this, galactic structure has fallen apart in a cosmos littered with black holes.
Eventual proton decay, assuming this occurs, would spell the end of matter, with only black holes remaining in what the authors call ‘The Black Hole Era.’ Black hole evaporation should see the end of the last of these ‘objects’ in 10100 years. What follows is the ‘Dark Era’ as the cosmos moves toward thermal equilibrium and no sources of energy exist. This is the kind of abyss the very notion of which drove 19th Century philosophers mad. Schopenhauer’s ‘negation of the Will’ is a kind of heat death of all things.
But even Nietzsche, ever prey to despair, could talk about ‘eternal recurrence,’ and envision a future that cycles back from dissolution into renewed existence. You can see the kind of value judgments that float through all such discussions. Despair is a human response to an Adams/McLaughlin cosmos, or it can be. Even recurrence couldn’t save Nietzsche, who went quite mad at the end (precisely why remains a subject of debate). I have little resonance with 19th Century philosophical pessimism, so determinedly bleak. My own value judgment says I vastly prefer a universe in which expansion reverses.
Image: Friedrich Nietzsche (1844-1900). Contemplating an empty cosmos and searching for rebirth.
These thoughts come about because of a just released paper that casts doubt on cosmic expansion. In fact, “Strong progenitor age bias in supernova cosmology – II. Alignment with DESI BAO and signs of a non-accelerating universe” makes an even bolder claim: The expansion of the universe may be slowing. Again in terms of human preference, I would far rather live in a universe that may one day contract because it raises the possibility of cyclical and perhaps eternal universes. My limited lifespan obviously means that neither of the alternatives affects me personally, but I do love the idea of eternity.
An eternity, that is, with renewed possibilities for cosmic growth and endless experimentation with physical structure and renewed awakening of life. The paper, with lead researcher Young-Wook Lee (Yonsei University, South Korea) has obvious implications for dark energy and the so-called ‘Hubble tension,’ which has raised questions about exactly what the cosmos is doing. In this scenario, deceleration is fed by a much faster evolution of dark energy than we’ve imagined, so that its impact on universal expansion is greatly altered.
What is at stake here is the evidence drawn from Type 1a supernovae, which the Nobel-winning teams used as distance markers in their groundbreaking dark energy work. Young-Wook Lee’s team finds that these ‘standard candles’ are deeply affected by the ages of the stars involved. In this work, younger star populations produce supernovae that appear fainter, while older populations are brighter. Using a sample of 300 galaxies, the South Korean astronomers believe they can confirm this effect with a confidence of 99.999%. That’s a detection at the five sigma level, corresponding to a probability of less than one in three million that the finding is simply noise in the data.
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. Credit: NASA/ESA.
If this is the case, then the dimming of supernovae has to take into account not just cosmological effects but the somewhat more mundane astrophysics of the progenitor stars. Put that finding into the supernovae data showing universal expansion and a new model emerges, diverging from the widely accepted ΛCDM (Lambda Cold Dark Matter) cosmology, which offers a structure of dark energy, dark matter and normal matter. This work forces attention on a model derived from baryonic acoustic oscillations (BAO) and Cosmic Microwave Background data, which shows dark energy weakening significantly with time. From the paper:
…when the progenitor age-bias correction is applied to the SN data, not only does the future universe transition to a state of decelerated expansion, but the present universe also already shifts toward a state closer to deceleration rather than acceleration. Interestingly, this result is consistent with the prediction obtained when only the DESI BAO and CMB data are combined… Together with the DESI BAO result, which suggests that dark energy may no longer be a cosmological constant, our analysis raises the possibility that the present universe is no longer in a state of accelerated expansion. This provides a fundamentally new perspective that challenges the two central pillars of the CDM standard cosmological model proposed 27 yr ago.
Let’s pause a moment. DESI stands for the Dark Energy Spectroscopic Instrument, which is installed on the 4-meter telescope at Kitt Peak (Arizona). Here the effort is to measure the effects of dark energy by collecting, as the DESI site says, “optical spectra for tens of millions of galaxies and quasars, constructing a 3D map spanning the nearby universe to 11 billion light years.” Baryon acoustic oscillations are the ‘standard ruler’ that reflect early density fluctuations in the cosmos and hence chart the expansion at issue.
Here’s a comment from Young-Wook Lee:
“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.”
Presumably it will receive more scrutiny now, with the team continuing its research through supernovae data from galaxies at various levels of redshift. That dark energy work is moving rapidly is reflected in the fact that the Vera Rubin Observatory is projected to discover on the order of 20,000 supernova host galaxies within the next five years, which will allow ever more precise measurements. Meanwhile, the evidence for dark energy as an evolving force continues to grow. Time will tell how robust the Korean team’s correction to what it calls ‘age bias’ in individual supernova readings really is.
The paper is Junhyuk Son et al., “Strong progenitor age bias in supernova cosmology – II. Alignment with DESI BAO and signs of a non-accelerating universe,” Monthly Notices of the Royal Astronomical Society, Volume 544, Issue 1, November 2025, pages 975–987 (full text).
Follow with RSS or E-Mail
