South Korean astronomers are challenging the notion that the universe’s expansion is accelerating, an observation in the 1990s that lead to the theory of dark energy. This is currently very controversial, and may simply fizzle away or change our understanding of the fate of the universe.

In the 1990s astronomers used data from Type Ia supernovae to determine the rate of the expansion of the universe. Type Ias are known as standard candles because they put out the exact same amount of light. The reason for this is the way they form. They are caused by white dwarfs in a double star system – the white dwarfs might pull gas from their partner, and when that gas reaches a critical amount its gravity is sufficient to cause the white dwarf to explode. Because the explosions occur at the same mass, the size of the explosion, and therefore its absolute brightness, is the same. If we know the absolute brightness of an object, and we can measure its apparent brightness, then we can calculate its exact distance.

The astronomers used data from many Type Ia supernova to essentially map the expansion of the universe over time. Remember – when we look out into space we are also looking back in time. They found that the farther away galaxies were the slower they were moving away from each other, as if the universal expansion itself were accelerating over time. This discovery won them the Nobel Prize. The problem was, we did not know what force would cause such an expansion, so astronomers hypothesized the existence of dark energy, as a placeholder for the force that is pushing galaxies away from each other. This dark energy force would have to be significant, stronger than the gravitational force pulling galaxies together.

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The universe is a big place, and it is full of mysteries. Really bright objects, that can be seen from millions or even billions of light years away, can therefore be found, even if they are extremely rare. This is true of fast radio bursts (FRBs), which are extremely bright and very brief flashes of light in the radio frequency. They typically last about one thousandth of a second (one millisecond). Even though this is very brief, they still represent a massive energy output, and their origins have yet to be confirmed.

Recently astronomers have detected the brightest FRB so far seen, and it was relatively close, only 130 million light years away. That may seem far, but most FRBs are billions of light years away (again, indicating that they are relatively rare, because we need a huge volume of space to see them). Because this FRB was bright and close, it gives us an opportunity to examine it in more detail than most. But – this is also made possible by recent upgrades to the equipment we use to detect FRBs.

The primary instrument we use is CHIME (Canadian Hydrogen Intensity Mapping Experiment). As the name implies, this was developed to map hydrogen in the universe, but it is also well-suited to detect FRBs. So far, since 2018, it has detected about 4,000 FRBs. But because they are so brief, it is difficult to localize them precisely. We can see what direction they are coming from, and if that intersects with a galaxy we can say it probably came from that galaxy. But astronomers want to know where within that galaxy the FRB is coming from, because that may provide clues to confirm their origin. So they built “outriggers” – small versions of CHIME spread around North America to effectively increase the size of the CHIME detection area and significantly increase its precision. It was this new setup that detected the recent FRB. What did they find?

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Avi Loeb is at it again. He is the Harvard astrophysicist who first gained notoriety when he hypothesized that Oumuamua, the first detected interstellar object, might be an alien artifact. His arguments were pretty thin, not taken very seriously by the scientific community, and mostly did not pan out. However, Oumuamua has left the solar system and so any unanswered questions will remain forever unanswered. But Loeb has been riding his fame and his alien artifact narrative ever since, founding the Galileo Project dedicated to looking for alien technological artifacts. Recently, NASA discovered the third every interstellar object, and the first interstellar comet, 3I/ATLAS. (“3I” is simply the nomenclature for the third interstellar object, and “ATLAS” for the Asteroid Terrestrial-impact Last Alert System that made the discovery.) Loeb recently published a paper and is blogging that this too shows “anomalies” deserving of exploring the ET technology hypothesis. And again – I am not impressed.

Let me jump ahead a bit and say up front – I am not against exploring the alien hypothesis, pretty much in any context. Even though the probability may be low, the payoff would be huge, and it’s worth a consideration. I am not against looking for alien technological signatures. This may, in fact, be the best method for detecting an alien technological civilization. I also think that serious academics and scientists should be taking such efforts seriously and there should be no academic shame in engaging in them. So I am with Loeb to that extent.

What bothers me about Loeb is that his arguments are so terrible. He is just another classic example of an academic and scientist who has no apparent experience with scientific skepticism and therefore is falling for common pitfalls. He also appears to not have learned anything in the last seven years, which is greatly disappointing. In fact, I would argue that he is hurting his stated greater cause (with which I largely agree), to make searching for alien technology academically respectable. Loeb is essentially engaging in anomaly hunting, and shows no signs of understanding what that means. Let’s take a look at his latest list of apparent “anomalies” to see what I mean.

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Let’s talk about climate change and life on Earth. Not anthropogenic climate change – but long term natural changes in the Earth’s environment due to stellar evolution. Eventually, as our sun burns through its fuel, it will go through changes. It will begin to grow, becoming a red giant that will engulf and incinerate the Earth. But long before Earth is a cinder, it will become uninhabitable, a dry hot wasteland. When and how will this happen, and is there anything we or future occupants of Earth can do about it?

Our sun is a main sequence yellow star. The “main sequence” refers to the Hertzsprung-Russell diagram (HR diagram), which maps all stars based on mass, luminosity, temperature, and color. Most stars fall within a band called the main sequence, which is where stars will fall when they are burning hydrogen into helium as their source of energy. More massive stars are brighter and have a color more towards the blue end of the spectrum. They also have a shorter lifespan, because they burn through their fuel faster than lighter stars. Blue stars can burn through their fuel in mere millions of years. Yellow stars, like our own, can last 10 billion years, while red dwarfs can last for hundreds of billions of year or longer.

Which stars are the best for life? We categorize main sequence stars as blue, white, yellow, orange, and red (this is a continuum, but that is how we humans categorize the colors we see). Interestingly, there are no green stars, which has more to do with human color perception than anything else. Stars at an otherwise “green” temperature have enough blue and red mixed in to appear white to our color perception. The hotter the star the farther away a planet would have to be to be in its habitable zone, and that zone can be quite wide. But hotter stars are short-lived. Colder stars last for a long time but have a small and close-in habitable zone, so close they may be tidally locked to their star. Red dwarfs are also relatively unstable and put out a lot of solar wind which is unfriendly to atmospheres.

So the ideal color for a star, if you want to evolve some life, is probably in the middle – yellow, right where we are. However, some astronomers argue that the optimal temperature may be orange, which can last for 15-45 or more billion years, but with a comfortably distant habitable zone. If we are looking for life in our galaxy than orange stars are probably the way to go.

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Exoplanets are pretty exciting – in the last few decades we have gone from knowing absolutely nothing about planets beyond our solar system to having a catalogue of over 5,000 confirmed exoplanets. That’s still a small sample considering there are likely between 100 billion and 1 trillion planets in the Milky Way. It is also not a random sample, but is biased by our detection methods, which favor larger planets closer to their parent stars. Still, some patterns are starting to emerge. One frustrating pattern is the lack of any worlds that are close duplicates of Earth – an Earth mass exoplanet in the habitable zone of a yellow star (I’d even take an orange star).

Life, however, does not require an Earth-like planet. Anything in the habitable zone, defined as potentially having a temperature allowing for liquid water on its surface, will do. The habitable zone also depends on variables such as the atmosphere of the planet. Mars could be warm if it had a thicker atmosphere, and Venus could be habitable if it had less of one. Cataloguing exoplanets gives us the ability to address a burning scientific question – how common is life in the universe? We have yet to add any data points of clear examples of life beyond Earth. So far we have one example of life in the universe, which means we can’t calculate how common it is (except maybe setting some statistical upper limits).

Finding that a planet is habitable and therefore could potentially support life is not enough. We need evidence that there is actually life there. For this the hunt for exoplanets includes looking for potential biosignatures – signs of life. We may have just found the first biosignatures on an exoplanet. This is not 100%. We need more data. But it is pretty intriguing.

The planet is K2-18b, a sub-Neptune orbiting a red dwarf 120 light years from Earth. In terms of exoplanet size, we have terrestrial planets like Earth and the rocky inner planets of our solar system. Then there are super-Earths, larger than Earth up to about 2 earth masses, still likely rocky worlds. Sub Neptunes are larger still, but still smaller than Neptune. They likely have rocky surfaces and thick atmospheres. K2-18b has a radius 2.6 times that of Earth, with a mass 8.6 times that of Earth. The surface gravity is estimated at 12.43 m/s^2 (compared to 9.8 on Earth). We could theoretically land a rocket and take off again from its surface.

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In 2006 (yes, it was that long ago – yikes) the International Astronomical Union (IAU) officially adopted the definition of dwarf planet – they are large enough for their gravity to pull themselves into a sphere, they orbit the sun and not another larger body, but they don’t gravitationally dominate their orbit. That last criterion is what separates planets (which do dominate their orbit) from dwarf planets. Famously, this causes Pluto to be “downgraded” from a planet to a dwarf planet. Four other objects also met criteria for dwarf planet – Ceres in the asteroid belt, and three Kuiper belt objects, Makemake, Haumea, and Eris.

The new designation of dwarf planet came soon after the discovery of Sedna, a trans-Neptunian object that could meet the old definition of planet. It was, in fact, often reported at the time as the discovery of a 10th planet. But astronomers feared that there were dozens or even hundreds of similar trans-Neptunian objects, and they thought it was messy to have so many planets in our solar system. That is why they came up with the whole idea of dwarf planets. Pluto was just caught in the crossfire – in order to keep Sedna and its ilk from being planets, Pluto had to be demoted as well. As a sort-of consolation, dwarf planets that were also trans-Neptunian objects were named “plutoids”. All dwarf planets are plutoids, except Ceres, which is in the asteroid belt between Mars and Jupiter.

So here we are, two decades later, and I can’t help wondering – where are all the dwarf planets? Where are all the trans-Neptunian objects that astronomers feared would have to be classified as planets that the dwarf planet category was specifically created for? I really thought that by now we would have a dozen or more official dwarf planets. What’s happening? As far as I can tell there are two reasons we are still stuck with only the original five dwarf planets.

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There really is a significant mystery in the world of cosmology. This, in my opinion, is a good thing. Such mysteries point in the direction of new physics, or at least a new understanding of the universe. Resolving this mystery – called the Hubble Tension – is a major goal of cosmology. This is a scientific cliffhanger, one which will unfortunately take years or even decades to sort out. Recent studies have now made the Hubble Tension even more dramatic.

The Hubble Tension refers to discrepancies in measuring the rate of expansion of the universe using different models or techniques. We have known since 1929 that the universe is not static, but it is expanding. This was the famous discovery of Edwin Hubble who notice

d that galaxies further from Earth have a greater red-shift, meaning they are moving away from us faster. This can only be explained as an expanding universe – everything (not gravitationally bound) is moving away from everything else. This became known as Hubble’s Law, and the rate of expansion as the Hubble Constant.

Then in 1998 two teams, the Supernova Cosmology Project and the High-Z Supernova Search Team, analyzing data from Type 1a supernovae, found that the expansion rate of the universe is actually accelerating – it is faster now than in the distant past. This discovery won the Nobel Prize in physics in 2011 for  Adam Riess, Saul Perlmutter, and Brian Schmidt. The problem remains, however, that we have no idea what is causing this acceleration, or even any theory about what might have the necessary properties to cause it. This mysterious force was called “dark energy”, and instantly became the dominant form of mass-energy in the universe, making up 68-70% of the universe.

I have seen the Hubble Tension framed in two ways – it is a disconnect between our models of cosmology (what they predict) and measurements of the rate of expansion, or it is a disagreement between different methods of measuring that expansion rate. The two main methods of measuring the expansion rate are using Type 1a supernovae and by measuring the cosmic background radiation. Type 1a supernovae are considered standard candle because they have roughly the same absolute magnitude (brightness). The are white dwarf stars in a binary system that are siphoning off mass from their partner. When they reach a critical point of mass, they go supernova. So every Type 1a goes supernova with the same mass, and therefore the same brightness. If we know an object’s absolute magnitude of brightness, then we can calculate its distance. It was this data that lead to the discovery that the universe is accelerating.

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Astrophysicists come up with a lot of whacky ideas, some of which actually turn out to be possibly true (like the Big Bang, black holes, accelerating cosmic expansion, dark matter). Of course, all of these conclusions are provisional, but some are now backed by compelling evidence. Evidence is the real key – often the challenge is figuring out a way to find evidence that can potentially support or refute some hypothesis about the cosmos. Sometimes it’s challenging to figure out even theoretically (let alone practically) how we might prove or disprove a hypothesis. Decades may go buy before we have the ability to run relevant experiments or make the kinds of observations necessary.

Black holes fell into that category. They were predicted by physics long before we could find evidence of their existence. There is a category of black hole, however, that we still have not confirmed through any observation – primordial black holes (PBH). As the name implies, these black holes may have been formed in the early universe, even before the first stars. In the early dense universe, fluctuations in the density of space could have lead to the formation of black holes. These black holes could theoretically be of any size, since they are not dependent on a massive star collapsing to form them. This process could lead to black holes smaller than the smaller stellar remnant black hole.

In fact, it is possible that there are enough small primordial black holes out there to account for the missing dark matter  – matter we can detect through its gravitational effects but that we cannot otherwise see (hence dark). PBHs are considered a dark matter candidate, but the evidence for this so far is not encouraging. For example, we might be able to detect black holes through microlensing. If a black hole happens to pass in front of a more distant star (from the perspective of an observer on Earth), then gravitational lensing will cause that star to appear to brighten, until the black hole passes. However, microlensing surveys have not found the number of microlensing events that would be necessary for PBHs to explain dark matter. Dark matter makes up 85% of the matter in the universe, so there would have to be lots of PBHs to be the sole cause of dark matter. It’s still possible that longer observation times would detect larger black holes (brightening events can take years if the black holes are large). But so far there is a negative result.

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I wrote earlier this week about the latest successful test of Starship and the capture of the Super Heavy booster by grabbing arms of the landing tower. This was quite a feat, but it should not eclipse what was perhaps even bigger space news this week – the launch of NASAs Clipper probe to Europa. If all goes well the probe will enter orbit around Jupiter in 2030.

Europa is one of the four large moons of Jupiter. It’s an icy world but one with a subsurface ocean – an ocean that likely contains twice as much water as the oceans of Earth combined. Europa is also one of the most likely locations in our solar system for life outside Earth. It is possible that conditions in that ocean are habitable to some form of life. Europa, for example, has a rocky core, which may still be molten, heating Europa from the inside and seeding its ocean with minerals. Chemosynthetic organisms survive around volcanic vents on Earth, so we know that life can exist without photosynthesis and Europa might have the right conditions for this.

But there is still a lot we don’t know about Europa. Previous probes to Jupiter have gathered some information, but Clipper will be the first dedicated Europa probe. It will make 49 close flybys over a 4 year primary mission, during which it will survey its magnetic field, gravity, and chemical composition. Perhaps most exciting is that Clipper is equipped with instruments that can sample any material around Europa. The hope is that Clipper will be able to fly through a plume of material shooting up geyser-like from the surface. It would then be able to detect the chemical composition of Europa material, especially looking for organic compounds.

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It is now generally accepted that 66 million years ago a large asteroid smacked into the Earth, causing the large Chicxulub crater off the coast of Mexico. This was a catastrophic event, affecting the entire globe. Fire rained down causing forest fires across much of the globe, while ash and debris blocked out the sun. A tsunami washed over North America – one site in North Dakota contains fossils from the day the asteroid hit, including fish with embedded asteroid debris. About 75% of species went extinct as a result, including all non-avian dinosaurs.

For a time there has been an alternate theory that intense vulcanism at the Deccan Traps near modern-day India is what did-in the dinosaurs, or at least set them up for the final coup de grace of the asteroid. I think the evidence strongly favors the asteroid hypothesis, and this is the way scientific opinion has been moving. Although the debate is by no means over, a majority of scientists now accept the asteroid hypothesis.

But there is also a wrinkle to the impact theory – perhaps there was more than one asteroid impact. I wrote in 2010 about this question, mentioning several other candidate craters that seem to date to around the same time. Now we have a new candidate for a second KT impact – the Nadir crater off the coast of West Africa.

Geologists first published about the Nadir crater in 2022, discussing it as a candidate crater. They wrote at the time:

“Our stratigraphic framework suggests that the crater formed at or near the Cretaceous-Paleogene boundary (~66 million years ago), approximately the same age as the Chicxulub impact crater. We hypothesize that this formed as part of a closely timed impact cluster or by breakup of a common parent asteroid.”

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