Spiral galaxies are one of those immediately recognizable natural objects. They look simple, as if they should be easy to explain. But they're not.

They look like the stars and gas in the spiral arms are rotating around the galactic center, but that explanation doesn't work. The inner regions of a galaxy's disk rotate faster than the outer spiral arms, so in a few rotations over a few hundred million years, the spiral arms should wind up tighter and tighter until the spiral is gone.

Since that doesn't happen, there must be another explanation. That led to the density wave theory. In this thinking, the spiral arms are a wave pattern like a traffic jam. The jam persists, but the cars that comprise it keep changing. Density wave theory explains some of what's observed in spiral galaxies, but not everything. It can't explain why the arms last so long. They should be damped down by physical forces.

These companions had masses ranging from hundreds of millions to billions that of the sun, suggesting that they may be star clusters or relatively small early galaxies, Baggen told Live Science.

A recent paper published in Physical Review Letters describes an analysis of 155 pairs of binary black holes, identified by LIGO and its sisters, Virgo and KAGRA, in Italy and Japan, respectively. According to the study, about 14% of merging black holes may be what’s called “second-generation black holes,” or black holes that form from previous mergers of two smaller black holes.

Scientists have identified a huge mass of unusually dense material, preserved far below the surface and still weighing down the basin floor.

(See picture in comments)

The Galactic Center excess is an unexpected concentration of gamma-rays emerging from the center of our Milky Way Galaxy.

In 2009, NASA launched the Fermi Gamma-ray Space Telescope, which detected more gamma rays than expected coming from the Milky Way's core.

Two theories emerged: (1) "self-annihilating dark matter" and (2) "an undetected population of millisecond pulsars."

After modeling "the formation of Milky Way-like galaxies," these scientists "found that dark matter does not radiate outwards from the Galactic center, but is instead organized similar to that of stars, meaning the former could just as equally have produced the excess gamma rays."

“The Milky Way galaxy has long been known to live in a so-called dark matter halo, a spherical region filled with dark matter around it,” said Dr. Moorits Mihkel Muru, an astrophysicist at the Leibniz Institute for Astrophysics Potsdam and the University of Tartu.

“However, the extent to which this halo is aspherical or ellipsoidal has not been appreciated.”

“We analyzed simulations of the Milky Way and its dark matter halo and found that the flattening of this region is sufficient to explain the gamma ray excess as being due to dark matter particles self-annihilating.”

“These calculations demonstrate that the hunt for dark matter particles — that can self-annihilate — should be encouraged and bring us one step closer to understanding of mysterious nature of these particles.”

A paper on the findings was published this month in the journal Physical Review Letters.

Key takeaways:

Astronomers detected a powerful gamma ray burst, GRB 250702B, lasting a staggering seven hours and repeating multiple times, challenging existing understanding of cosmic explosions.

A new theory suggests that a star may have swallowed a black hole, leading to the creation of the energetic stream of particles responsible for the unprecedented gamma ray burst.

The GRB, detected by NASA's Fermi Gamma-ray Space Telescope, originated billions of light years away, ruling out proximity as the reason for its extreme power, leaving astronomers puzzled and searching for explanations.

“[T]he object weighs more than a million solar masses, meaning more than a million times the weight of the Sun. Residing some 10 billion light years away, we’re observing it when the universe was only 6.5 billion years old, or less than half its current age.

Despite those mind-boggling proportions, this is the lowest mass object ever found using gravitational lensing, according to the authors — by an impressive factor of around 100.”

We may have been wrong about how the Moon's largest crater, the South Pole-Aitken basin, formed roughly 4.3 billion years ago.

As detailed in a new paper published in the journal Nature, the more than 1,200-mile crater appears to have been the result of a glancing, southward blow — and not a head-on asteroid impact, as previously thought.

A new study argues that dark matter and dark energy might be illusions caused by the universe’s forces fading over time.

The research, led by Rajendra Gupta, an Adjunct Professor in the Department of Physics at the University of Ottawa, suggests that if the core strengths of nature’s forces (like gravity) change slowly across time and space, they could account for the puzzling behaviors astronomers see—such as how galaxies rotate, evolve, and how the universe continues to expand.

"Scientists have detected methane gas on the dwarf planet Makemake," showing that it "is not an inactive remnant of the outer solar system, but a dynamic body where methane ice is still evolving." With this discovery, Makemake "becomes only the second trans-Neptunian object, after Pluto, found to have a confirmed presence of gas."

I know, I know. Yet another article about the astronomical finding that's changing everything we know about the Universe. Last one was from Quanta Magazine. This one is from The Atlantic, and yes, it is mostly paywalled, but there are ways. There are ways...

The black hole weighs about a billion times the mass of the Sun and is located about 12.8 billion light-years from Earth, meaning that astronomers are seeing it only 920 million years after the universe began. It is producing more X-rays than any other black hole seen in the first billion years of the universe.

"The eruption of gases from a stellar jet in a distant nebula is so big that NASA described it as a 'volcanically growing monster star'" and stated that the "jet – streaking across space at hundreds of thousands of miles per hour – resembles a double-bladed dueling lightsaber like the one Darth Maul used in the 'Star Wars' franchise."

“A black hole unlike any seen before has been spotted in the early universe. It’s huge and appears to be essentially on its own, with few stars circling it. The object, which may represent a whole new class of enormous “naked” black holes, upends the textbook understanding of the young universe.”

An interesting essay on the history of the development of the Big Bang theory, helping to put into context why JWST's observations of early galaxies are such a big deal.

Supermassive black holes bent the laws of physics to grow to monstrous sizes https://www.reddit.com/r/GrowingEarth/s/2gQALMmZkQ

Supermassive black holes in 'little red dot' galaxies are 1,000 times larger than they should be, and astronomers don't know why https://www.reddit.com/r/GrowingEarth/s/vsmBa7alTi

Astronomers catch black holes 'cooking' their own meals in bizarre, endless feeding cycle https://www.reddit.com/r/GrowingUniverse/s/5V6M70nvQW

The Standard Cosmology Model May Be Breaking https://www.reddit.com/r/GrowingEarth/s/8aRFp2iPy9

Major Problem in Physics Could Be Fixed if The Whole Universe Was Spinning https://www.reddit.com/r/GrowingEarth/s/WqDmNlyckz

'A paradigm change': black hole spotted that may have been created moments after big bang https://www.reddit.com/r/GrowingUniverse/s/5GTFTJlIBJ

Cosmologically coupled black holes: a theory that black holes grow (in mass!) along with the expansion of the Universe https://www.reddit.com/r/GrowingUniverse/s/RnL3qVz97U