As our understanding of the Universe advances — and as we collect more comprehensive, better data about all that’s out there — we sometimes find that the actual Universe winds up defying our greatest expectations. Despite how impressive our model of reality has become, and how successfully we’ve put a wide variety of cosmic puzzle pieces together, every once in a while, new data comes along that winds up not only surprising us, but it forces us to rewrite the story we tell ourselves about what occurs, or even what’s possible, in our Universe.

Based on gravity, electromagnetism, and quantum and nuclear physics at a fundamental level, we have a theoretical foundation that underlies every object and phenomenon in the Universe. Given a set of initial conditions for a physical system, we can use these laws to predict what’s going to happen: at least under certain assumptions and with making some simplifying assumptions. But every once in a while, the Universe surprises us by giving us observations that show our current way of making sense of things can’t be right. Sometimes, even theorists surprise us: coming up with novel explanations or new ways to test the Universe that uncover aspects of reality we never realized were possible to know.

1.) Type Ia supernovae come from white dwarfs accreting enough matter, passing the Chandrasekhar mass limit, and exploding.

We know that stars come in all varieties of mass when they’re born, with their fate dependent on their initial mass. The heavier stars die in core-collapse supernovae, giving rise to neutron stars and black holes when they die. But lower-mass stars — including stars like the Sun — die a much gentler death: blowing off their outer layers in a planetary nebula while their cores contract down to form a white dwarf. Those white dwarfs, in theory, are made up of atomic nuclei and electrons, and those electrons provide the pressure (electron degeneracy pressure) that prevents them from collapsing further.

In theory, the Pauli exclusion principle prevents two electrons from occupying the identical quantum state, and so the sheer number and density of electrons prevents the white dwarf from collapsing, despite the influence of gravity. However, if you accumulate enough mass, you can force those electrons inside the atomic nuclei that are in these white dwarfs, triggering nuclear reactions like electron capture and creating a runaway set of fusion reactions that will destroy the white dwarf entirely. That mass threshold, in theory, is known as the Chandrasekhar mass limit — of about 1.4 solar masses — and since white dwarfs can exist in binary systems, it was thought that mass siphoning, as shown above, would be the mechanism for exploding these white dwarfs: creating Type Ia supernovae.

This illustration shows two merging white dwarfs, the preferred theoretical mechanism for the triggering of some, and perhaps most or even nearly all, type Ia supernovae. The double detonation scenario, where a “detonation” event on the surface propagates to the core and causes a detonation that leads to total destruction of the stellar remnant, is one very intriguing theoretical possibility for most type Ia events, although exceptional examples that don’t fit this scenario may also exist.

Only, that can’t be what happens at all. Observationally, we’ve now seen an enormous number of type Ia supernovae. They come from a variety of white dwarf masses, but all of them are significantly below the Chandrasekhar limit: ranging from around 1.0 to 1.2 solar masses. Moreover, there is no evidence for a “donor star” found near these white dwarfs that explode.

Instead, the preferred mechanism is that two white dwarfs collide and or merge, triggering a specific scenario known as double detonation: where a nuclear reaction first occurs on the white dwarf’s surface, triggering a detonation in the white dwarf’s interior that then leads to the complete destruction of the stellar remnant.

For a time, the accretion scenario and the crossing-over of the Chandrasekhar limit was the only scenario presented. Here in the 21st century. these two scenarios were held up side-by-side as the two ways that type Ia supernovae could be generated. Here in 2026, we now think that at least 80% and possibly up to 100% of all type Ia supernovae are caused by the double detonation scenario, with the original accretion-based scenario possibly never occurring at all in our Universe. The fusion of theory and observation overturned a longstanding “fact” about the second most common type of supernova in the Universe.

The lensed galaxies shown at the center of this image appear as though they’re being stretched and pulled apart, similar to the chromosome pairs that align and separate during cellular mitosis. However, this is only an optical illusion caused by the gravitational lensing effects of the massive foreground galaxy cluster: El Gordo. These are simply background galaxies whose light is distorted by the gravitational lens. Both spirals and ellipticals are richly represented within (and behind) most galaxy clusters, with ellipticals being most concentrated towards the centers of these clusters.

Credit: Jose M. Diego (IFCA), Brenda Frye (University of Arizona), Patrick Kamieneski (ASU), Tim Carleton (ASU), Rogier Windhorst (ASU); Processing: Alyssa Pagan (STScI), Jake Summers (ASU), Jordan C. J. D’Silva (UWA), Anton M. Koekemoer (STScI), Aaron Robotham (UWA), Rogier Windhorst (ASU)

2.) Elliptical galaxies arise when two comparably sized spiral galaxies experience a major merger.

The two most common types of large, massive galaxy in the Universe are spirals and ellipticals. Spirals are mostly found in relative isolation: away from the densest environments like the central regions of galaxy clusters. Their main hallmark is that they possess gas-rich disks with active star formation occurring within them. On the other hand, elliptical galaxies are generally larger and more massive than spirals are, and their stars primarily exist in an ellipsoidal bulge shape. They lack gas or star-forming material of any kind, and in most cases, haven’t formed new stars in billions of years.

Although Hubble’s original idea was that galaxies evolved from beginning as smooth, spherical structures and then evolved to become flattened — forming disks and then spiral arms — that idea was discredited many decades ago. Although the terminology of “early-type” and “late-type” galaxies persists, it has been generally recognized that gas-rich galaxies collide, leading to major episodes of star-formation, and that if the overall gas can be either used up, expelled via winds and radiation, or stripped out of galaxies entirely, that post-merger galaxy will evolve into an elliptical galaxy.

The galaxy NGC 7727 shows extended spiral arms: likely the aftermath of a recent major merger between two comparably massive galaxies. The presence of two supermassive black holes inside this galaxy, as well as the extended streams of gas and stars, show one possible outcome of a major merger of two similar-mass, initially gas-rich galaxies. Note that the end state is not going to be a giant elliptical galaxy, but another gas-rich disk galaxy instead.

Credit: ESO/VST ATLAS team. Acknowledgment: Durham University/CASU/WFAU

Since ellipticals are largely found at the centers of galaxy clusters, and simulations seemed to reproduce the idea that equally massed spirals merging together could create elliptical galaxies, it was reasonable to think that we had a formula for making them: spiral + spiral = elliptical. We even found some observational evidence of individual objects that seemed to support such a picture. By the turn of the century, this picture was standard.

But then something happened: we actually started observing a whole lot more of the Universe. Beginning with large-area, deep galaxy surveys like the Sloan Digital Sky Survey (SDSS), we started observing enormous numbers of galaxies in the process of merging together, however:

• pre-merger,
• during the merger process,
• and post-merger, as well.

It turns out that in the overwhelming majority of cases — something like 90-95% of the time — when two disk-rich galaxies merge together, they wind up producing a subsequent galaxy that also has a gas-rich disk. The formula is that two spirals can make an elliptical, but it usually takes a great many attempts before all of that star-forming material finally gets used up and expelled. Most of the time, another disk galaxy, with spiral arms emerging, is what arises instead.

Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. For practically all of human history, we thought our Solar System would be the template model for all other planetary systems around stars like our own. The data has shown otherwise.

Credit: NASA/Dana Berry

3.) Our Solar System serves as the template model for stellar and planetary systems throughout the Universe.

The defining feature of our Solar System is what orbits the Sun: four inner, rocky planets, an asteroid belt, four outer, gas giant planets, and a distant Kuiper belt beyond the final planet. Back at the turn of the century, we thought this would be standard, and when we hunted for exoplanetary systems, we recognized that there would be variety, but we presumed this picture would be standard.

Here in the 21st century, with the advent of planet-finding missions like Kepler and TESS, we’ve learned that:

• the most common stars, the low mass red dwarfs, rarely have giant planets at all,
• that the most common planetary mass is heavier than Earth but lighter than Neptune, a so-called mini-Neptune or super-Earth,
• that planets of any type can form at any radius from their parent star,
• and that stars can have between 0 and 3 belts, not just the “two” that we have here.

With over 6000 planets now under our belt around thousands of stellar systems, we’re more confident than ever that the configuration of planetary bodies in our own Solar System is just the result of random chance, rather than a cosmic norm in any way.

This view of the Fomalhaut system consists of mid-infrared JWST data (in yellow) overlaid with radio (red) ALMA data and blue (optical/UV) Hubble data. Only JWST is capable of revealing the inner structure of this system, including a large inner disk and a surprising intermediate belt: features only able to be seen at long wavelengths of light due to their incredibly cool temperatures.

4.) The cosmic neutrino background, predicted by the hot Big Bang, can’t ever be detected.

Here in our Universe, we owe our origins to the hot Big Bang. The discovery that spiral nebulae were actually galaxies beyond the Milky Way, and that the farther a galaxy was from us, the greater its redshift was, led to the picture of the expanding Universe. If space was expanding now, then in the past, things must have been denser, closer together, more uniform, less evolved, and hotter in the distant past. That leads to an early state known as the hot Big Bang.

There are four conventional “cornerstone observations” that come out of the hot Big Bang framework. Working backwards in time and space, we have:

• the expanding Universe,
• the growth and formation of cosmic structure,
• the relic cosmic microwave background radiation from when neutral atoms are formed,
• and the relic abundance of the light elements from when the Universe was hot enough to fuse atomic nuclei, even before any stars were formed.

However, there’s a fifth theoretical cornerstone — long predicted, but long thought to be forever unobservable — that should emerge from an even earlier period in cosmic history: a leftover background of neutrinos and antineutrinos, remaining from the first one second of post-Big Bang cosmic history.

There are peaks and valleys that appear, as a function of angular scale (x-axis), in various temperature and polarization spectra in the cosmic microwave background. This particular graph, shown here, is extremely sensitive to the number of neutrinos present in the early Universe, and corresponds to the standard Big Bang picture of three light neutrino species. If there were non-standard neutrinos present, this data would be noticeably different.

But after the turn of the century, astrophysicists realized that an incredibly small shift would be potentially detectable in two potential places: the pattern of peaks-and-valleys in the cosmic microwave background, and the oscillatory features imprinted in galaxy clustering within the large-scale structure of the Universe. Because the neutrinos behave as radiation early on, but have a different temperature than the temperature of the cosmic microwave background (owing to the freeze-out of neutrinos before electron-positron annihilation occurs in the early Universe), they leave a distinctive signature in both the cosmic microwave background and the cosmic large-scale structure.

The CMB signature came first, in 2015, and the galaxy clustering signal followed a few years later in 2019. While the idea of directly detecting the neutrinos still remains elusive, these two methods for indirect detection prove that the cosmic neutrino background exists, and is consistent with — to within a 2% observational error — the predicted temperature from the theory of the hot Big Bang. It’s a remarkable example of overcoming the limits we thought would forever render a prediction beyond the reach of actual data from the Universe.

If there were no oscillations due to matter interacting with radiation in the Universe, there would be no scale-dependent wiggles seen in galaxy clustering. The wiggles themselves, shown with the non-wiggly part subtracted out (bottom), are dependent on the impact of the cosmic neutrinos theorized to be present by the Big Bang. Standard Big Bang cosmology corresponds to β=1. Note that if there is a dark matter/neutrino interaction present, the acoustic scale could further be altered.

5.) Black holes are only formed when stars die in core-collapse supernovae events.

Finally, we come to the idea of how to realistically make black holes within our Universe. The notion that core-collapse supernovae could do the job has been around for a very long time, and for most of the 20th century, that was thought to be pretty much the only way that the Universe actually did it. Sure, it might be possible for a stellar remnant, like a neutron star, to accumulate enough matter or merge with another object that would lead to it crossing a mass threshold to form a black hole, but in general it was assumed that you needed a star — and the supernova-driven death of that star — to collapse a stellar core and produce a black hole.

No more is that the case! Sure, we received direct evidence in 2017 that two merging neutron stars can indeed create a black hole, but we found a number of other surprises along the way.

In 2007, the Hubble space telescope observed a massive star: one determined to have about 25 times the mass of the Sun. In 2015, when it returned to that same star, it found that the star was gone entirely! No supernova, no remnant, no nothing. As theorists had suggested, and consistent with contemporaneous indirect evidence, it looks like the star simply collapsed directly to a black hole: an all-new way to create a black hole in the Universe.

The visible/near-IR photos from Hubble show a massive star, at least 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. The direct collapse of this particular object, while still under investigation, may have been triggered by a stellar companion.

More recently, we’ve used a combination of JWST with other observatories to discover the existence of overmassive black holes in the early Universe: supermassive black holes that are just as massive as all the stars in the galaxy that host them combined. If that’s the case, then there’s no way that stars could have provided the seed origins of those black holes; direct collapse must have been at play. Apparently, the Universe might make black holes most frequently from the remnants of massive stars, but that can’t be the only way, after all, to explain the full suite of what we’re observing happens out there in the cosmos.

It’s worth remembering that even what’s taken to be an incontrovertible fact about the Universe might later be shown to be incomplete, incorrect, or just a crude approximation of a deeper, more comprehensive picture of physical reality. Even though we had good reason to believe all of these purported “facts” about the Universe just a quarter-century ago, we now have a new picture of each of these aspects of reality that’s superseded what we thought previously. The way we’ve done it, in each case, is by investigating the Universe with new, more powerful tools than ever before, and giving the cosmos the opportunity to reveal secrets to us that we couldn’t uncover previously. After all, it’s by giving the Universe the chance to surprise us that greatest revolutions in our understanding can occur.

This article 5 big astronomy “facts” that changed in the 21st century is featured on Big Think.

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