Axions, one of the leading candidates for dark matter, may be able to be converted to photons (and vice versa) under the right conditions. If we can cause and control their conversion, we might discover our first particle beyond the Standard Model, and possibly solve the dark matter and strong CP problems as well. (Credit: Sandbox Studio, Chicago, Symmetry Magazine/Fermilab and SLAC)

Astrophysically, even with all the different forms that normal matter can take, it simply can’t explain the Universe we observe on its own. Beyond all the stars, planets, gas, dust, plasma, black holes, neutrinos, photons and more, there’s an overwhelming suite of evidence that the Universe contains two ingredients whose…

This artist’s impression shows the cigar-shaped interpretation of ‘Oumuamua. Although it was initially suspected to be rocky in nature, the lack of observed outgassing combined with its anomalous acceleration instead suggests that it may be made of nitrogen ice. That remains the leading hypothesis, even after a recent analysis (faultily) suggested otherwise. (Credit: ESO/M. Kornmesser; nagualdesign)

In 2017, scientists discovered an object in our Solar System that was unlike anything else we’d ever seen before. Whereas all the other bodies in our own cosmic backyard — planets, moons, comets, asteroids, Kuiper belt objects and more — had properties that indicated they originated from within our own…

The lunar horizon glow, shown here as imaged by the Clementine Spacecraft in the 1990s, had actually been seen numerous times during the Apollo mission, but its existence was treated as dubious until an explanation for the lunar atmosphere was fully developed. This didn’t occur until 1998, when the sodium Moon spot, and a sodium tail extending from the Moon, were discovered. (Credit: NASA)

For a number of very good reasons, you wouldn’t expect the Moon to have an atmosphere at all. Compared to planets that do have substantial atmospheres — like Earth, Venus, and even Mars — the Moon is tremendously low in mass. At just 1.2% the mass of the Earth, it…

The idea that two quanta could be instantaneously entangled with one another, even across large distances, is often talked about as the spookiest part of quantum physics. If reality were fundamentally deterministic and were governed by hidden variables, this spookiness could be removed. Unfortunately, attempts to do away with this type of quantum weirdness have all failed. (Credit: Alan Stonebraker/American Physical Society)

For all of history, there’s been an underlying but unspoken assumption about the laws that govern the Universe: that if you know enough information about whatever system you’re dealing with, you can predict precisely how that system will behave in the future. That’s what deterministic means: if you know enough…

This illustration of the Moon shows it as it will appear during maximum eclipse, 99.1% obscured, on November 19, 2021. (After midnight in most parts of the world.) This is the longest partial eclipse in nearly 600 years. (Credit: TimeandDate.com)

On November 19, 2021, Earth will see the longest partial lunar eclipse in centuries.

Black holes, when you fall into them, inevitably lead you towards the central singularity. Because they don’t emit light, they are worth considering as a potential candidate for our Universe’s dark matter. (Credit: ESO, ESA/Hubble, M. Kornmesser)

In many ways, the most extreme objects in all the Universe are black holes. They pack more mass into a smaller volume of space than anything else, curving the fabric of space so severely that once anything enters a certain region, it can never escape, not even if it travels…

This simulation shows the radiation emitted from a binary black hole system. In principle, we should have neutron star binaries, black hole binaries, and neutron star-black hole systems, covering the entire allowable mass range. In practice, we saw a longstanding ‘gap’ in such binaries between about 2.5 and 5 solar masses. With the newest LIGO data, that gap seems to disappear. (Credit: NASA’s Goddard Space Flight Center)

How massive can the most massive neutron star be, and how light can the lightest black hole be? For the entire history of astronomy up until 2015, our understanding of both of these phenomena was limited. While both neutron stars and black holes were thought to have formed by the…

The Solar System formed from a cloud of gas, which gave rise to a proto-star, a proto-planetary disk, and eventually the seeds of what would become planets. The crowning achievement of our own Solar System’s history is the creation and formation of Earth exactly as we have it today, which turns out to be the Solar System’s densest planet. Whereas you’d expect, being closest to the Sun, it would’ve been Mercury. (Credit: NASA/Dana Berry)

When it comes to the Solar System, the elements that make up each planet are determined by how they all formed. Closest to the Sun, the high temperatures, large amounts of solar radiation, and intense solar winds can easily kick the lightest elements off of any protoplanets that are forming…

This artist’s impression shows how J043947.08+163415.7, a very distant quasar powered by a supermassive black hole, may look close up. This object has the largest apparent brightness of any quasar from the early Universe, and is undoubtedly powered by a supermassive black hole. (Credit: ESA/Hubble & NASA, M. Kornmesser)

At the center of practically every large, massive galaxy in the Universe is the same type of structure: a supermassive black hole. Ranging from a few million solar masses up to tens of billions of solar masses, these astrophysical engines are the most extreme single objects known to humanity. …

The Universe itself, through a variety of nuclear processes involving stars and stellar remnants, as well as other means, can naturally copiously produce nearly 100 elements of the periodic table. There are only 8 total processes, both natural and human-made, that cause them all. (Credit: ESO/L. Calçada/M. Kornmesser)

The Universe’s normal matter consists, humbly, of atoms.

Ethan Siegel

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

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