The Evolving Enigma of Dark Energy and the Sun's Mysterious Past

0:00 Anna: Three decades of exploding stars pulled apart and rebuilt from the ground up into a single consistent picture, 2,884 of them. And when an Australian led team stepped back to look at what that picture was saying about dark energy, it wasn't saying what the textbook says. Avery: We've also got a study asking whether our, uh, own sun once swallowed a planet and whether the evidence for it is still sitting inside the star right now, waiting to be ready. Anna: Plus why the quietest stretch of the solar cycle is the one that matters most.

0:34 If you spend your working life at Avery: 35,000ft, and the James Webb Space Telescope has just run the most sensitive search for a moon around another planet that anyone has ever attempted, it found nothing at all. That is the good news. Anna: This is Astronomy AstroDailyPod. I'm Anna. Avery: And I'm avery. It's Monday 14th September 2026 and we're coming to you as always, from Syd. You, let's get into it. Anna: We're starting today with a result that has been quietly sitting in the open for about a week and I think it deserves a great deal more attention than it has had.

1:11 An international team led out of the University of Queensland has published what is now the largest and most internally consistent catalogue of type 1A supernovae ever assembled, 2,884 of them. And it adds fresh weight to a very awkward idea that dark energy might not be constant, which would be a Avery: problem because the word constant is doing enormous structural work. Anna: In modern cosmology, it is the load bearing wall. So let's build this up properly because the method here matters as much as the answer.

1:44 A, uh, type 1A supernova is what happens when a white dwarf star, the dense, burnt out core left behind by a star like our sun, gathers too much material and detonates. The crucial thing is that these explosions are close to identical. They go off at roughly the same intrinsic brightness every time. So if you measure how bright one looks from here, you can work out how far away it is. Astronomers call them standard candles. Avery: And that's the technique that won the Nobel Prize in 2011. Anna: It is.

2:16 And this is where Australia enters the storey early, because one of the three laureates, Brian Schmidt, was working at the Australian National University when that discovery was made. Two teams in 1998 independently found that distant supernovae were fainter than they should have been, which meant they were further away than expected, which meant the expansion of the universe is not slowing down under gravity, as everyone assumed, it's speeding up. And the thing doing the Speeding up, got the placeholder name dark Energy, because nobody knew, and to be completely honest, nobody still knows what it actually is.

2:53 Avery: So where does the new work come in? Anna: Here's the difficulty. In the 28 years since, we have collected supernovae from dozens of different surveys on dozens of different telescopes with different detectors, different filters, different calibrations observed across decades, in which our understanding of these explosions changed substantially. You cannot simply pour all of that into one bucket and start doing cosmology with it. The systematic errors will eat you alive. Avery: So somebody had to do the unglamorous work.

3:27 Anna: Somebody had to do the unglamorous work. And that somebody is ryan Camilleri, a PhD candidate at the University of Queensland's School of Mathematics and Physics, working with Professor Tamara M. Davis and a long list of collaborators across Australia, the United States, the United Kingdom, South Africa, Spain and France. What they've done is take Pantheon plus, which is the big historical compilation of supernovae going back roughly 30 years, and combine it with the Dark energy survey's full five year sample, which added around 1500 new high redshift supernovae of its own.

4:05 And rather than stapling the two together, they rebuilt both from the same starting assumptions in one framework with one set of calibrations. Avery: Camilleri's own description of it is the cleanest summary I've read. Quote, we've rebuilt three decades of astronomical observations into a single consistent framework. And he makes the point that this isn't just tidying up, it's reanalysis. Over the years, we've learned a lot more about how supernovae behave. So we've been able to go back and apply that improved understanding to older data.

4:42 Anna: That improved understanding is doing real work here. Two of the biggest headaches in supernova cosmology are, uh, dust, cosmic dust between us and the explosion reddens and dims the light in ways that mimic distance and the mass of the galaxy. The supernova went off in which turns out to correlate with the supernova's brightness in a way we still don't fully understand. Both had to be handled consistently across the whole sample. The team cared about that second problem enough that they published a companion paper on host galaxy masses alongside the main one.

5:16 Avery: Alright, so what does the rebuilt catalogue actually say? Anna: Two things, and the first one is reassuring. If you take the supernovae on their own and assume the standard model, a flat universe with a genuinely constant dark energy, you get a matter density of 0.310 that is bang in line with everything else we Know, the catalogue is not broken, it's behaving. Avery: And the second thing is the interesting one. Anna: The second thing is what happens when you fold in the other two great pillars of cosmology.

5:47 The cosmic microwave background, the leftover glow of the Big Bang, and baryon acoustic oscillations, which are frozen sound waves from the early universe that act as a cosmic ruler. Do that, and under the standard constant dark energy model, the three datasets start pulling against each other. There's a tension, and here's the pointed bit, that tension eases if you allow dark energy to change with time. The team find a preference for evolving dark energy over the standard model at between 2.5 and 3.1-sigma, depending on exactly which combination you use.

6:25 Avery: Let's be careful with sigma, because we've had this conversation before on this show. Anna: We have, and we should be careful. Back when we covered the LZ dark matter flash, we spent a while on this. Three Sigma is roughly a, uh, 1 in 700 chance of the data looking like this. If the standard model is right, that is interesting. That is worth chasing. It is emphatically not a discovery. Particle physics won't call anything discovered below five sigma. And cosmology has been burned by three sigma results before.

6:57 Nobody on this paper is claiming otherwise. Avery: But it's the second independent line of evidence pointing the same way, which is a different kind of argument. Anna: That's exactly the argument Professor Davis makes, and it's the heart of why this matters. Her words quote our Supernova data from DES in 2024 first showed hints that dark energy may be time varying. And this new compilation also sees a deviation from the standard model. And then. So two completely independent measurements have found hints of time variation in dark energy challenging the standard model.

7:34 That dark energy doesn't change. Avery: And this sits alongside what DESSI the, the Dark Energy Spectroscopic instrument has been reporting from an entirely different direction, using those baryon acoustic oscillations rather than supernovae. Anna: Right. Three separate methodologies with separate systematics, separate failure modes and separate teams. And they keep producing the same faint smell of something wrong. The other number worth flagging is precision. This compilation tightens the uncertainties on the dark energy parameters by about 30% compared with what came before.

8:12 Which means the next round of data won't just add noise. It'll actually be able to settle this. Avery: So spell out the stakes. What changes if dark energy really is Anna: evolving almost everything downstream? A constant dark energy is the simplest possible thing. Einstein's cosmological constant, a fixed energy density baked into empty space itself. If instead it's a field that changes strength over cosmic time, then it's not a constant. It's a dynamic thing with a history and possibly a future.

8:44 It changes how the universe ends. And Professor Davis takes it further than that. Her line is that all of this quote may also hold the clue to explain how gravity and quantum physics fit together. That's the biggest unfinished problem in physics. And dark energy is one of the very few places where the two are forced into the same room. Avery: Now, I want to note where this was published because it's not incidental. Anna: No, it isn't. This is in publications of the Astronomical Society of Australia, Australia's own journal, led from the University of Queensland with the Australian National University and Swinburne on the author list and South African colleagues alongside.

9:24 And the Dark energy survey data at the core of it came off the Blanco 4 metre telescope at Cerro Tololo in Chile. A southern telescope looking at a southern sky. From the Nobel winning work at Matt Stromlo through to this, the question of what dark energy is has been a southern hemisphere argument for a very long time. Avery: And there's a lovely thread back through our own recent episodes here. Anna: There really is. Back in episode 181, we covered a rebuttal from Nobel laureates, including Schmidt and Adam Reese, pushing back hard on a claim that the universe's acceleration was slowing and confirming the acceleration is real.

10:02 That still stands. This is a different question, not whether dark energy exists, but whether it holds still. On Thursday of last week, we talked about fast radio bursts being used to weigh the missing gas in the cosmic web, and how that bears on the S8 tension. And on Saturday, our Skywatch feature was SN2026AAIV, uh, a type 1A going off in NGC 7331. One single example of exactly the kind of explosion that fills this catalogue. Bright enough to chase with a backyard telescope. Avery: 2,883 more of those and you've got a cosmology that's the whole trick.

10:42 Anna: And what happens next is that the sample size goes up by an order of magnitude. The Vera Rubin Observatory in Chile is about to start finding these things in industrial quantities. And the Nancy Grace Roman Space Telescope, which we watched launch a fortnight ago and which powered up its coronagraph earlier this month, was designed in large part to nail exactly this measurement. If dark energy is changing, we are going to know within a few years. If it isn't, we'll know that too. And this will go down as A very well built catalogue that briefly made everybody nervous.

11:15 Avery: Either way, somebody had to do 30 years of homework first. Anna: Somebody did. His name's on the paper. Avery: Here's a question you don't often hear asked of our own star. Did the sun eat a planet? Anna: We ask that about other stars all the time, Constantly. Avery: It's one of the standard results in exoplanet science. You find a star with an oddly high abundance of the heavy rock forming elements. And the neat explanation is that it swallowed one of its own planets and and the debris is still floating in its outer layers.

11:49 New work published in Monthly Notices of the Royal Astronomical Society turns that telescope around and points it at us. It's by Professor Mutlu Yildiz at Egg University in Turkey. And the Royal Astronomical Society put it out on the 10th of September. Anna: And the argument is that the sun has form. Avery: The argument is that the sun has two long standing unexplained quirks. Anna: And. Avery: And one event could account for both. Quirk 1 is a genuine embarrassment in solar physics. We can measure the inside of the sun, not model it.

12:25 Measure it using helioseismology. Sound waves ring through the solar interior. We watch the surface vibrate and from that we can reconstruct the speed of sound at different depths with real precision. And the standard solar models don't quite match what we measure. They've never quite matched. Anna: That's been an open soar for a couple of decades. Avery: It has. Quirk two is lithium. The sun has far less lithium than it ought to. It's depleted by a factor of well over a hundred compared with the material it formed from.

13:00 And Yildiz's proposal is that if the young sun swallowed a super Earth, something in the range of 5 to 10 times the mass of our planet, the chemical rearrangement that follows can push the models toward the measurements and take the lithium down at the same time. Anna: How does eating a planet lower your lithium? Avery: Broadly by changing the structure and the mixing near the base of the convective zone, the boundary where the churning outer layers meet the still interior. That boundary is where lithium gets dragged down deep enough to be destroyed by nuclear reactions.

13:36 Adjust the composition and the temperature gradient there and you change how efficiently the sun burns its own lithium away. Yildiz's framing is careful. A planet several times more massive than Earth may have fallen into the young sun and left a uh, lasting chemical imprint deep inside it. And the ingestion of a super Earth could help explain long standing differences between standard solar models and observations. Anna: May could. Those are Load bearing words they are, Avery: and I want to keep them.

14:12 This is a modelling result offering a candidate solution to a modelling discrepancy. It is not a fossil. Nobody has dug up the planet. And there are other live explanations for the solar abundance problem. Revisions to the measured composition of the solar photosphere. For one that don't require eating anything. What makes this one attractive is economy. One event, two problems. Anna: And there's something faintly unsettling about it, given what it implies about the early inner solar system. Avery: That's the part that stays with me.

14:48 Super Earths are the single most common class of planet we find around other stars. And our solar system conspicuously lacks one. If the answer is that we had one and the sun ate it, that makes us a lot less unusual and makes the ground under Mercury's orbit feel a good deal less stable than it looks now. Anna: A storey about the sun from the opposite direction and one with a very practical edge, particularly for anyone listening from a crew rest seat. New research in the Journal of Geophysical Research Atmospheres finds that cosmic radiation at cruising altitude gets worse when the sun goes quiet.

15:29 Avery: Which is backwards from how most people would guess it. Anna: Completely backwards. And the logic is worth a minute. The work is by Dr. Roy Yaniv with Professor Yoav Yair and Professor Colin Price across the Hebrew University of Jerusalem, Reichman University and Tel Aviv University. They flew six instrumented balloons from southern Israel, each climbing to around 35 kilometres and measured the radiation environment the whole way up. Avery: So what's the profile look like? Anna: It peaks higher than you fly.

16:02 The maximum sits between 17 and 20 kilometres. That's a known feature called the Regener Fotzer maximum, where incoming cosmic rays have smashed into enough atmosphere to produce a full shower of secondary particles. But the shower hasn't yet been absorbed. Below that, it tails off at a typical airliner cruising altitude of around 10 kilometres. They measure roughly 0.9 to 1.3 microsieverts per hour. Avery: And um, the solar connection, the sun's Anna: magnetic field carried out on the solar wind, acts as a shield for the entire solar system.

16:38 It deflects galactic cosmic rays, the high energy particles arriving from supernovae and other violence out in the galaxy. When the sun is active, that shield is strong and fewer of those particles get through. When the sun goes quiet, the shield weakens and more of them arrive. The team measure that anti correlation directly at minus 0.71 and the size of the swing is the headline. At solar minimum, the dose rate runs something like 40 to 60% higher than Avery: at solar maximum 40 to 60% is not a rounding error.

17:13 It isn't. Anna: They also break down what's actually hitting you. Neutrons are about 40 to 45% of the dose. With electromagnetic radiation, another 35 to 40 neutrons matter because they're difficult to shield against and they're weighted heavily for biological damage. Avery: Now, proportion before anyone cancels a holiday. Anna: Yes, proportion. A, uh, micro sievert is a millionth of a sievert. A a long haul flight puts a few tens of micro sieverts on you, which is in the same broad territory as a chest X ray for a passenger.

17:47 This is not something to lose sleep over for aircrew who are occupationally exposed and monitored as radiation workers in many jurisdictions and for frequent flyers on the very long, very high, high latitude routes. And Australia runs some of the longest sectors on the planet. A, uh, 40 to 60% seasonal swing driven by where we sit in the solar cycle is a real input into how you calculate annual dose. Avery: And it dovetails with what we talked about on Saturday from the other end. Anna: It's the same dial on the weekend we covered the Max Planck work on the sun's capacity for a superflare.

18:25 The danger of the sun at its loudest, this is the danger of the sun at its quietest. An active sun can fire a particle storm at you. A quiet sun simply stops holding the galaxy's particles at the door. Two different risks, opposite ends of the same cycle. And both of them show up at altitude first. Avery: Our last storey before we look up is a, uh, null result. And I want to argue that it's one of the better pieces of news this week. The James Webb Space Telescope has just carried out the most sensitive search for a moon around another planet ever attempted.

18:59 It didn't find one. What matters is how thoroughly it didn't find one. Anna: Exomoons have been the great near miss Avery: of the field for 15 years. We have close to 6,000 confirmed planets around other stars and not one confirmed moon, which is faintly ridiculous given that our own solar system has hundreds. The new work is by David Kipping, who has effectively made exomoon hunting his life's work. And it went up on the preprint server on 4 September. Anna: Which planet did he point it at? Avery: LP899C, a rocky planet in the temperate zone of a very cool, very faint star.

19:40 Kipping used 12 separate JWST transits, 12 passes of the planet in front of its star and stacked them. And the sensitivity he gets out of that is the storey he can exclude Moons down to one tenth of Earth's radius and at 95% confidence across the planet's entire hill sphere. That's the whole region where a moon could gravitationally hang on. Anna: One tenth of Earth's Radius is about 650 kilometres which rules out the entire Avery: mid tier of our own solar system's moons. Europa's gone. Rhea's gone.

20:16 Umbriel's gone. If LP899C had anything like those or Webb would have seen it. Anna: So why is the absence good news? Avery: Two reasons. First, because it was always possible that we'd found no exomoons, simply because we can't see them. That the whole search was hopeless and we were wasting our time. This demonstrates the opposite. Webb can find astonishingly small moons and stacking transits buys you far more sensitivity than a single one. That's a capability result and it applies to every other target.

20:54 Second, this particular non detection is physically expected. The planet orbits at Ah, just 0.04 astronomical units, extremely close in and at that distance tidal forces would strip a large moon away over time anyway. So the theory predicted an empty hill sphere and the observation delivered an empty Anna: hill sphere Theory and observation agreeing is not the most thrilling headline but it's how you know the instrument is honest. Avery: Exactly right. And a small Southern footnote. The LP899 system was found by Speculus, a survey hunting planets around the coolest stars whose southern station sits at Paranal in Chile under the same skies as the telescopes that keep turning up in this programme.

21:45 Anna: Quick update before the skywatch on a storey we left open a couple of weeks back. NASA's Crew 13 mission to the International Space Station is now targeting no earlier than late September. Avery: That's the flight that was supposed to go on the 12th. Anna: It was on the 29th of August. NASA and SpaceX stood the mission down after teams found an oxidizer leak in the Dragon spacecraft's propulsion system during standard pre launch processing. At the time the statement was simply that a new target date would be announced once available and that joint teams would complete any necessary rework before flight.

22:21 The Canadian Space Agency has now confirmed the mission is aiming for no earlier than late September. Though as we record, NASA hasn't published Avery: a specific date and the crew is an interesting one. Anna: It is Commander Jessica Watkins and pilot Luke Delaney for NASA with mission specialist Joshua Kutryk of the Canadian Space Agency and and Sergey Teteryatnikov of Roscosmos. They'll fly on a Falcon 9 from Space Launch Complex 40 at Cape Canaveral. The same pad that quietly notched its 400th orbital flight yesterday.

22:54 We'll bring you the date the moment it's firm. Avery: Right, let's get you outside. And there's something live happening tonight. A large coronal hole on the sun has rotated around into a geo effective position and forecasters expect the fast solar wind streaming out of it to to reach Earth today. That means active geomagnetic conditions, which means Anna: aurora watching is on at both ends of the planet. Avery: It does for our southern hemisphere listeners. That's Aurora australis territory Tasmania first and best, with a genuine chance from southern Victoria and the far south of New Zealand.

23:33 If it holds up, look south, get away from town lights and be patient. Keep cameras will pick up colour your eyes won't. For, uh, our North American listeners, you're looking north and the northern tier states and Canada are in play. This is a coronal hole stream rather than a big flare event. So think a steady moderate glow rather than a spectacular overhead display. But it's free and it's tonight and Anna: the moon is out of the way for it. Avery: Beautifully out of the way. New Moon was last Friday, so we're in a young crescent evening sky and the deep sky is still yours.

24:14 Over the next week, the Moon climbs back into the evening and walks past two landmarks worth Antares, the Red Heart of Scorpius and the Teapot of Sagittarius. Use the Moon to find them between now and the 20th. And from Sydney, the centre of the Milky Way is still riding high overhead after dark. That dense textured band through the teapot is the galactic core and it is one of the genuine privileges of southern observing. From mid northern latitudes, it's much lower in the south, so northern listeners should hunt for a clear southern horizon while the season lasts.

24:56 Anna: Planets. Venus is the headline. Avery: Venus is the headline and Thursday is the date. On the 18th of September, Venus reaches greatest brilliancy for this evening apparition. At magnitude -4.8. That is spectacularly bright. Bright enough to cast a shadow from a dark sight. Bright enough that you'll field phone calls about it. Look west shortly after sunset. You'll notice some listings give a different date for this. We're going with the 18th, which is the correct one for the standard definition.

25:31 And this apparition favours the south. From Sydney, Venus sits higher and lingers longer in a darker sky than it does from most of the United States, where it's a lower, briefer object in the twilight. Anna: Take the win m. Anything else worth chasing? Avery: Three things Mercury is low in the western twilight, tricky but doable with a clear horizon. Mars is in the morning sky and passes about 6 degrees south of Pollux, the brighter of the Gemini twins, also on the 18th. That one's a northern favourite view.

26:09 And Saturn is building towards opposition on the 4th of October, with the rings about 7 degree open. So it is well placed all night and getting better. The Harvest Moon rides past it on the 26th with Neptune nearby. For anyone with binoculars or a scope, Anna: there's also a date for the diary. Next Saturday there is. Avery: 19th September is international observe the Moon night, which is one of the few global astronomy events that works equally well from either hemisphere. With no equipment at all, the Moon will be a fat crescent in the evening sky, which is genuinely the best phase for it because the shadows along the terminator throw the craters and mountains into relief.

26:54 If you've got a pair of binoculars, that's all you need. If you've got a telescope and a neighbour, that's even better. Anna: And the equinox is coming, the 22nd Avery: spring here, autumn for our northern listeners. And it brings the zodiacal light with it. That's sunlight scattering off dust in the plane of the solar system and around the equinox. It's an evening object low in the west from the southern hemisphere, a false dusk and a pre dawn object in the east from the northern hemisphere. A faint tapering cone of light, dark skies, no moon and patience.

27:33 Anna: And the safety note, which is not optional on this programme. Avery: Never optional. With Venus at its most brilliant, some of you will be tempted to hunt it in daylight and it is genuinely findable. But that means aiming optics near the sun and that is how people permanently damage their eyes. If you are going to look anywhere near the sun, use a filter Certified to the ISO 123122 standard fitted over the front of the instrument, never at the eyepiece. Cheque it for scratches and pinholes before every use.

28:07 Sunglasses, exposed film, smoked glass and welding glass below shade 14 are not safe and never were. And supervise children the entire time. Anna: And that's Astronomy daily for Monday 14th September 2884. Supernovae rebuilt from three decades of observations by an Australian led team quietly making the case that dark energy might not hold still. Avery: A sun that may have eaten a super earth and still carries the receipt. A reminder that the sun's quiet years are the ones that let the galaxy's particles through.

28:45 And the most sensitive exomoon search ever attempted, finding precisely nothing in the most useful possible way. Anna: All of Today's storeys with links to the papers and the press releases are are at astronomydaily IO you'll find the full back catalogue there too, along with our news feed and the newsletter. If you'd like this in your inbox Avery: and we do read the contact form. Questions, corrections and storey tips all land with us, and they have shaped more than one segment lately. You can also find us on socials.

29:19 Anna: Astrodaily pod astronomy daily is part of the bytes.com podcast network. I'm Anna. Avery: And I'm Avery. Clear skies wherever you're standing.