Starts With A Bang #125 - Large-scale structure

One of the most exciting developments in modern astrophysics isn't merely our standard "concordance cosmology" model, but rather the cracks that seem to be emerging in it. Sure, we've said for some 25 years now that our Universe is 13.8 billion years old, is made of mostly dark energy with a substantial amount of dark matter, and only 5% of all the normal stuff combined: stars, planets, black holes, plasmas, photons, and neutrinos. But more recently, a couple of cosmic conundrums have emerged, leading us to question whether this model is the best picture of reality that we can come up with.We don't merely have the Hubble tension to reckon with, or the fact that different methods yield different values for the expansion rate of the Universe today, but a puzzle over whether dark energy is truly a constant in our Universe, as most physicists have assumed since its discovery back in 1998. While "early relic" methods using CMB or baryon acoustic oscillation data favor a lower value of around 67 km/s/Mpc, "distance ladder" methods instead prefer a higher, incompatible value of around 73 km/s/Mpc. Now, on top of that, new large-scale structure data seems to throw another wrench into the works: supporting a picture of evolving dark energy, and specifically one where it weakens over cosmic time.Here to guide us through this is Dr. Kate Storey-Fisher, a cosmologist whose expertise is exactly on this topic, and who herself has recently become a member of the very collaboration, DESI, that provides the strongest evidence to date for evolving dark energy. The story, however, is only just beginning, and with current and future observatories poised to collect superior data, we take a look ahead as to what's in store for the Universe, and for those of us who are working oh so hard to try and understand it.(This image shows a "slice" through 3D space of the galaxies mapped out by the DESI survey, and color-coded by their distance/redshift from us. Features such as "great walls" can be seen even by eye within the data. Only 600,000 galaxies, or about 0.1% of DESI's total data, is displayed in this figure. Credit: DESI Collaboration/NOIRLab/NSF/AURA/R. Proctor)

All across the Universe, stars are dying through a variety of means. They can directly collapse to a black hole, they can become core-collapse supernovae, they can be torn apart by tidal cataclysms, they can be subsumed by other, larger stars, or they can die gently, as our Sun will, by blowing off their outer layers in a planetary nebula while their cores contract down to form a degenerate white dwarf. All of the forms of stellar death help enrich the Universe, adding new atoms, isotopes, and even molecules to the interstellar medium: ingredients that will participate in subsequent generations of star-formation.For a long time, however, we'd made assumptions about where certain species of particles will and won't form, and what types of environments they could and couldn't exist in. Those assumptions were way ahead of where the observations were, however, and as our telescopic and technological capabilities catch up, sometimes what we find surprises us. Sometimes, we find elements in places that we didn't anticipate, leading us to question our theoretical models for how those elements can be made. Other times, we find molecules in environments that we think shouldn't be able to support them, causing us to go back to the drawing board to account for their existence.Where our expectations and observations don't match is one of the most exciting places of all, and that's where astrochemist and PhD candidate Kate Gold takes us on this exciting episode of the Starts With A Bang podcast! Have a listen, and I hope you enjoy it as much as I enjoyed having this one-of-a-kind conversation!(This image shows the fullerene molecules C60 and C70 as detected in the young planetary nebula M1-11. This 2013 discovery was the first such detection of this molecule in this class of environment. Credit: NAOJ)

One of the great discoveries to be made out there in the grand scheme of things is alien life: the first detection of life that originated, survives, and continues to live beyond our own home planet of Earth. An even grander goal that many of us have, including scientists and laypersons alike, is to find not just life, but an example of intelligent extraterrestrials: aliens that are capable of interstellar communication, interstellar travel, or even of meeting us, physically, on our own planet. It's a fascinating dream that has been with humanity since we first began contemplating the stars and planets beyond our own world.Most of us, including me, personally, have assumed that this latter type of alien would not only be more technologically advanced than we are, but would also be far more scientifically advanced as well. That not only would they understand everything we presently do about the fundamental laws of physics, but far more: that they'd be a potential source of new knowledge for us, having equaled or exceeded everything we'd already gleaned from our investigative endeavors. And that assumption, as compelling as it might be, could be completely in error, argues physicist and author Dr. Daniel Whiteson.That's why I'm so pleased to bring you this latest episode of the Starts With A Bang podcast, where Daniel and I meet to discuss this very topic, with me taking the side of my own human-centered assumptions and Daniel taking a far more broad, philosophical, and cosmic approach: the same approach he takes in his new book, Do Aliens Speak Physics? And Other Questions About Science and the Nature of Reality. Have a listen to this fascinating conversation, see which set of arguments you find more compelling, and check out his book. You won't be disappointed!(This image shows the cover of Dr. Daniel Whiteson's and Andy Warner's newest book, Do Aliens Speak Physics? And Other Questions About Science and the Nature of Reality, which debuted on November 4, 2025! Credit: W.W. Norton & Company)

It's no secret that the Universe and the objects present within it, as we see them all today, have changed over time as the Universe has grown up over the past 13.8 billion years. Galaxies are larger, more massive, more evolved, and are richer in stars but fewer in number than they were back in the early stages of cosmic history. By looking farther and farther away, we can see the Universe as it was at earlier times, but we're going to be limited in many ways: by how deep our telescopes can see, by what wavelengths they're capable of seeing, and by what small fraction of the sky they're capable of observing.That's why an observing program like COSMOS-Web, the largest, widest-field JWST observing program to date, is so important. It isn't just revealing galaxies as they are nearby (at late times), at a variety of intermediate distances (and earlier times), and at ultra-large distances (and the earliest times of all), but due to its wide-field nature, is revealing galaxy types of varying abundances: the common-type galaxies, galaxies that are representative of more uncommon varieties, and even significant numbers of rare galaxies. And it's this aspect of galaxy evolution that makes me so proud and lucky to welcome Dr. Olivia Cooper to the podcast.Olivia is a recently-minted PhD who works as part of the COSMOS-Web team, specializing in galaxy evolution and using JWST data — along with data from other world-class observatories — to investigate how the galaxies in our Universe grew up, and what that can teach us about our own cosmic past. It truly is a banger of an episode that you'll want to listen to every minute of, so tune in and dive deep into the depths of the distant Universe on our latest adventure of the Starts With A Bang podcast!(This image shows a tiny sliver of the COSMOS-Web survey, with galaxies at a variety of distances along with a portion of a rich cluster of galaxies, at right, of this image. Credit: ESA/Webb, NASA & CSA, G. Gozaliasl, A. Koekemoer, M. Franco, and the COSMOS-Web team)

It's hard to believe, but it was only back in the early 1990s that we discovered the very first planet orbiting a star other than our own Sun. Fast forward to the present day, here in 2025, and we're closing in on 6000 confirmed exoplanets, found and measured through multiple techinques: the transit method, the stellar wobble method, and even direct imaging. That last one is so profoundly exciting because it gives us hope that, someday soon, we might be able to take direct images of Earth-like worlds, some of which may even be inhabited.Although it may be a long time before we can get an exoplanet image as high-resolution as even the ultra-distant "pale blue dot" photo that Voyager took of Earth so many decades ago, the fact remains that science is advancing rapidly, and things that seemed impossible mere decades ago now reflect today's reality. And the people driving this fascinating field forward the most are the mostly unheralded workhorses of the fields of physics and astronomy: the early-career researchers, like grad students and postdocs, who are just beginning to establish themselves as scientists.In this fascinating conversation with Dr. Kielan Hoch of Space Telescope Science Institute, we take a long walk at the current frontiers of science and peek over the horizon: looking at the good, the bad, and the ugly of what we're facing here in 2025. It's a conversation that might make you hopeful, angry, and optimistic all at the same time. After all, it's your Universe too; don't you want to know what comes next?(This composite image shows a brown dwarf star, center, with the first directly imaged exoplanet, 2M1207 b, in red alongside it. This image was acquired in 2004 by the Very Large Telescope in Chile, operated by the European Southern Observatory. In the years and decades since, dozens of more exoplanets have been directly imaged, with hundreds more expected in the next decade. Credit: ESO/VLT.)

Out there in the Universe, somewhere, a second example of an inhabit world or planet likely awaits us. It could be some other planet or moon within our own Solar System; it could be a spacefaring, interstellar civilization, or it could be an exoplanet around a different parent star. Although the search for life beyond Earth generally focuses on worlds that have similar conditions to Earth, like rocky planets with thin atmospheres and liquid water on their surfaces, that's not necessarily the only possibility. The truth is that we don't know what else is going to be out there, not until we look for ourselves and determine the answers.And yet, if you've been paying attention to the news, you might think that super-Earth or mini-Neptune type worlds, such as the now-famous exoplanet K2-18b, might be excellent candidate planets for life. Some have even gone as far as to claim that this planet has surefire biosignatures on it, and that the evidence overwhelmingly favors the presence of life within this planet's atmosphere. But the science backing up that claim has been challenged by many, including our two podcast guests for this episode: Dr. Luis Welbanks and Dr. Matthew Nixon.Beyond the breathless and sensational claims, what does the actual science concerning K2-18b in particular, and of biosignatures on exoplanets in general, actually teach us? What does the evidence indicate, and if we are going to find inhabited exoplanets, what will it take for us to actually announce a positive detection with confidence and less ambiguity? That's what this episode of the Starts With A Bang podcast is all about; I hope you enjoy it!(When an exoplanet passes in front of its parent star, a portion of that starlight will filter through the exoplanet’s atmosphere, allowing us to break up that light into its constituent wavelengths and to characterize the atomic and molecular composition of the atmosphere. If the planet is inhabited, we may reveal unique biosignatures, but if the planet has either a thick, gas-rich envelope of volatile material around it, or alternatively no atmosphere at all, the prospects for habitability will be very low. Credit: NASA Ames/JPL-Caltech)

Perhaps the strongest evidence we've ever acquired in support of the Big Bang has been the discovery of the leftover radiation from its early, hot, dense state: today's cosmic microwave background, or CMB. While there were many competing ideas for our cosmic origins, only the Big Bang predicted a uniform, omnidirectional bath of blackbody radiation: exactly what the CMB is.But it turns out the CMB encodes much more information than just our cosmic origins; it allows us to map the very early Universe from when it was just 380,000 years old, and gives us vital information about what has happened to light from that time over its 13.8 billion year journey to our eyes. It encodes information about our cosmic expansion history, about dark matter and dark energy, about intervening galaxy clusters, and about the material here in our own galaxy, along with much more. It is, arguably, the richest source of information from any one single observable in our entire Universe.Here to guide us through what CMB scientists are working on here in 2025, including what we've learned and what we're still trying to find out, I'm so pleased to welcome Dr. Patricio Gallardo to the show. We've got more than an hour and a half of quality science to go through, and by the end, I bet you'll be more excited about the upcoming Simons Observatory, designed to measure the CMB to higher precision than ever before, than you knew you should be. Enjoy!(This image shows the Large Aperture Telescope's colossal, 6-meter primary and secondary mirrors at the Simons Observatory in February of 2025. The telescope has already seen first light, and will soon begin delivering new CMB science as never before. Credit: M. Devlin/Simons Observatory)

When we search for life in the Universe, it makes sense to look for planets that are similar to Earth. To most of us, those signatures would look the same as the ones we'd see if we viewed our planet today: blue oceans, green-and-brown continents, polar icecaps, wispy white clouds, an atmosphere dominated by nitrogen and oxygen, and even the modern signs of human activity, such as increasing greenhouse gas emissions, planet modification, and electromagnetic signatures that belie our presence.But for most of our planet's history, Earth was just as "inhabited" as it is today, even though it looked very different. One fascinating period in Earth's history that lasted approximately 300 million years resulted in a planet that looked extremely different from modern Earth: a Snowball Earth period, where the entire surface, from the poles to the equator, was completely covered in snow and ice. This isn't just speculation, but is backed up by a remarkable, large suite of observational and geological evidence.So what was Earth like during this period? How did it fall into this phase, how did it remain trapped in that state for so long, and how did it finally thaw again? To help explore this topic, I'm so pleased to welcome PhD candidate Alia Wofford to the program, who conducts intricate climate models of early Earth to try to reproduce those early conditions. From that work, we're learning about what we should be looking for when it comes to potentially inhabited exoplanets, because Earth has been inhabited for around 4 billion years, and wow, has its appearance changed over all that time. Have a listen and see for yourself!(This illustration shows a frozen-over planet, but one that still possesses a significant liquid ocean beneath the surface ice. Many worlds in our Solar System may be described by this scenario at various points in cosmic history, including even planet Earth more than two billion years ago. Credit: Pablo Carlos Budassi/Wikimedia Commons)

It might seem hard to fathom, but it hasn't even been ten full years since advanced LIGO, the gravitational wave observatories that brought us our very first successful direct detection, turned on for the very first time. In the time since, it's been joined by the Virgo and KAGRA detectors, and humanity is currently closing in on 300 confirmed gravitational wave detection events. What was an unconfirmed prediction of Einstein's General Relativity for a full century has now become one of the fastest-growing fields in all of astronomy and astrophysics.Here in 2025, we're now looking forward to the LISA era: where we're going to build our first gravitational wave detectors in space. They'll have far longer baselines (i.e., separations between the various spacecrafts/stations) than any terrestrial gravitational wave detector, enabling us to detect fundamentally different classes (and masses) of objects that emit gravitational waves. At the same time, the rise of artificial intelligence and machine learning is enabling us to detect and characterize ever greater numbers of gravitational wave events, an incredibly exciting development.For this episode of the Starts With A Bang podcast, I'm so pleased to welcome Shaniya Jarrett to the program. She's here to guide us up to the frontiers and help us peer over the horizon, and is currently an astronomy PhD student at the University of Maryland after earning her Masters degree from the Fisk-Vanderbilt bridge program. Have a listen and learn all of the exciting science that's not only within our reach today, but that we all have to look forward to in the very near future!(The image above shows an illustration of the three future LISA, or Laser Interferometer Space Antennae, spacecrafts, in a trailing orbit behind the Earth. LISA will be our first space-based gravitational wave detector, sensitive to objects thousands of times as massive than the ones LIGO can detect. Credit: University of Florida/NASA)

Out there in the Universe, each star represents an opportunity: a chance for a stellar system to develop that just might possess something remarkable. While we normally think about life, and intelligent life at that, as the grand prize the Universe has to offer, there are a wide variety of fascinating phenomena that are out there to consider. Whereas Mercury, for example, is the closest world to our Sun in our own Solar System, it still takes 88 days to make a complete revolution. In other systems, however, exoplanets can be so hot that they orbit their parent star in less than a single Earth day.In fact, we've discovered a few systems that are so extreme, the planets that orbit them are in the process of disintegrating: where the heat, winds, and radiation from the parent star actually blows part of the planet itself away. This doesn't just include a planet's atmosphere, which is what we see for giant worlds, but even the surfaces and interiors of rocky planets in the most extreme cases. At temperatures of around 2000 degrees and upwards, these exoplanets can lose their crusts, mantles, and even their cores over long enough timescales.Believe it or not, we've actually caught a few exoplanets doing exactly this, and we've got the JWST spectra in hand for one of them now, teaching us, for the first time, what a planetary interior is made of outside of our own Solar System. I'm so pleased to have the first author from that 2025 study, soon-to-be Dr. Nick Tusay, as our guest on this edition of the Starts With A Bang podcast, as we take a look at the most extreme exoplanetary systems ever discovered!(This image shows an illustration of an evaporating, rocky exoplanet, with an enormous dust tail arising from the material blown off of the planet from its interaction with the nearby star. Credit: NASA/JPL-Caltech)