Just when cosmologists have a workable theory for when and how galaxy collisions happened in the early Universe, something challenges it. In this case, the challenger is a collision of two massive galaxy clusters that combined to form a gigantic galaxy cluster.
They happened when the Universe was half its age and according to theory, that’s too early. The result of the collision is the El Gordo galaxy cluster. And, its existence challenges the best-accepted theory of cosmology, called the Lambda-cold dark matter (usually abbreviated as LCDM) standard model.
LCDM basically sets constraints (or parameters) on the origin and evolution of structure in the Universe. It has three parts. One is the cosmological constant Lambda (L). It’s associated with dark energy. The second denotes dark matter (labeled CDM). The third is basically ordinary matter (often referred to as “baryonic”). This is a simple way of expressing how the properties of the cosmos that we see came into existence. Essentially, it explains the large-scale structure of the Universe as seen in the Cosmic Microwave Background. It also describes the distribution of galaxies, the abundances of hydrogen, helium, and lithium, and the accelerating expansion of the universe.
So, how does El Gordo and its time-clashing birth process play havoc with LCDM? In that model, galaxies form first in the Universe. Then, later on, they begin to combine to create larger galaxies and clusters of galaxies. That takes time, so having them show up so early—as El Gordo did—in cosmic history is a problem.
Understanding Gigantic Galaxy Cluster Formation
A research team led by Elena Asencio from the University of Bonn, figured out when this collision happened by looking at simulations of the interaction. After detailed analysis, they estimated the mass of the resulting “El Gordo” cluster by using gravitational lensed light from background galaxies. This weak lensing and the mass of the cluster come from Hubble Space Telescope observations and compared with more recent studies by JWST.
The Bullet Cluster is another of several gigantic galaxy clusters challenging the Lambda-cold dark matter theory of structure formation in the early Universe. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.
The authors then searched through less detailed cosmological LCDM simulations covering a very large volume. They were hunting for simulated cluster pairs. The aim was to count how many of these are broadly analogous to what El Gordo was like shortly before the collision. The team used an innovative “lightcone tomography” method. It considers that more distant objects are viewed further back in time when there was less structure.
The results revealed that the tension with LCDM is very severe for any plausible collision velocity. Moreover, the remaining uncertainty in El Gordo’s mass isn’t as significant. But, there are still other issues to work out. One of them is timing in the early Universe. As mentioned, the collision that produced El Gordo happened too soon. You could make it happen if the collision takes place more quickly. And a simulation shows that. But, while it is possible to get a simulation that looks like El Gordo after the quick collision, such an event is too rare in LCDM. It would be very unusual to find two such massive clusters within striking distance so early in cosmic history. Then, they have to be headed towards each other at a really high speed. That really stretches credibility.
El Gordo and Other Clusters Raise the Tension
The team’s new study and the more precise mass measurement may lead to more efforts to simulate El Gordo to better understand this enigmatic object. That’s because Ascensio and her colleagues were able to get a reliable mass measurement for El Gordo. It actually seems to work better with the LCDM model. However, some issues remain. “This does reduce the tension with LCDM,” said Ascensio, “but it is still highly significant for any plausible collision velocity. Hundreds of detailed simulations
Finally! Astronomers are Starting to See the First Galaxies Coming Together With JWST
One of the James Webb Space Telescope’s principal science goals is to observe the epoch where we think that the first galaxies were created, to understand the details of their formation, evolution, and composition. With each deep look back in time, the telescope seems to break its own record for the most distant galaxy ever seen. Science papers are now are starting to trickle in, as astronomers are finally starting to collect enough data from JWST to build a deeper understanding of the early Universe.
In a new study published in Nature Astronomy, a team of researchers in Denmark believe they have observed some of the very first, earliest galaxies with JWST. These galaxies are so old, they are likely still in the process of being formed.
One known standard is that the ratio between galaxies and their heavy elements has held constant in the local Universe through the last 12 billion years of history, or about 5/6 of the age of the Universe. But with JWST, astronomers are now seeing that the youngest galaxies look different. They don’t have that same ratio of stars to heavier elements because they haven’t gone through the cycles of star formation and star death yet, enriching gas clouds with metals, i.e., elements heavier than hydrogen and helium.
This plot shows the observed galaxies in an “element-stellar mass diagram”: The farther to the right a galaxy is, the more massive it is, and the farther up, the more heavy elements it contains. The gray icons represent galaxies in the present-day Universe, while the red show the new observations of early galaxies. These ones clearly have much less heavy elements than later galaxies, but agree roughly with theoretical predictions, indicated by the blue band. Credit: Kasper Elm Heintz, Peter Laursen.
For this study, the astronomers looked at 16 galaxies, some of the earliest galaxies ever observed. Their observations revealed that the chemical abundances in these galaxies are one-fourth of that seen in galaxies that were formed later. In their paper, the astronomers wrote that “these findings suggest that galaxies at this time are still intimately connected with the intergalactic medium and subject to continuous infall of pristine gas, which effectively dilutes their metal abundances.”
As gravity gathered together the first clumps of gas,the first stars and galaxies were formed.
“When we analyzed the light from 16 of these first galaxies, we saw that they had significantly less heavy elements, compared to what you’d expect from their stellar masses and the amount of new stars they produced,” said Kasper Elm Heintz, leader of the study and assistant professor at the Cosmic Dawn Center at the Niels Bohr Institute and DTU Space in Copenhagen, Denmark, in a press release.
These results, the astronomers say, are in stark contrast to the current model where galaxies evolve in a form of equilibrium throughout most of the history of the Universe, where there is a relationship between how many stars have formed and how many heavy elements have formed.
Illustration of galaxy formation: Diffuse gas from intergalactic space plummets toward the center, sparking star formation and becoming part of the galaxy’s rotating disk. When stars die, they return their gas to the galaxy (and the intergalactic space), now enriched with heavy elements. Credit: Tumlinson et al. (2017)
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Why Build Megastructures? Just Move Planets Around to Make Habitable Worlds
In 1960, Freeman Dyson proposed how advanced civilizations could create megastructures that enclosed their system, allowing them to harness all of their star’s energy and multiplying the habitable space they could occupy. In 2015, the astronomical community was intrigued when the star KIC 8462852 (aka. Tabby’s Star) began to dim inexplicably. While an analysis of the star’s light curve in 2018 revealed that the dimming pattern was more characteristic of dust than a solid structure, Tabby’s Star focused attention on the concept of megastructures and their associated technosignatures.
Dyson’s ideas were proposed at a time when astronomers were unaware of the abundance of exoplanets in our galaxy. The first confirmed exoplanet was not discovered until 1992, and that number has now reached 5,514! With this in mind, a team of researchers from Bangalore, India, recently released a paper that presents an alternative to the whole megastructure concept. For advanced civilizations looking for more room to expand, taking planets within their system – or capturing free-floating planets (FFP) beyond – and transferring them into the star’s circumsolar habitable zone (HZ) is a much simpler and less destructive solution.
The research was led by Raghav Narasimha, a physics graduate student at Christ University in Bangalore, India. He was joined by Margarita Safonova and Chandra Sivaram, a Department of Science and Technology (DST) Woman Scientist and a professor of astrophysics (respectively) at the Indian Institute of Astrophysics (IIAP) in Bangalore, India. The preprint of their paper, “Making Habitable Worlds: Planets Versus Megastructures,” recently appeared online and is being reviewed for publication in Astrophysics and Space Science.
The Problem with Megastructures
The possibility of advanced civilizations building giant structures to harness the energy of their stars is time-honored, with examples going back to the early 20th century. The earliest examples include John Desmond Bernal’s “Bernal Sphere,” which he detailed in his 1929 work “The World, the Flesh & the Devil.” According to Bernal, the source of the material for building such structures “would only be in small part drawn from the Earth; for the great bulk of the structure would be made out of the substance of one or more smaller asteroids, rings of Saturn or other planetary detritus.”
Olaf Stapledon took things a step further in his science fiction novel “Star Maker,” where he described how a future advanced human civilization was “able to construct, out in space, artificial planets for permanent habitation. These great hollow globes of artificial super-metals and artificial transparent adamant, ranged in size from the earliest and smallest structures, which were no bigger than a very small asteroid, to spheres considerably larger than the Earth.” These sources may have been the source of inspiration for Dyson’s 1960 proposal paper.
In this seminal paper, “Search for Artificial Stellar Sources of Infrared Radiation,” Dyson reasoned that a civilization’s motivations for building an “artificial biosphere” (later dubbed a “Dyson Sphere” by Nikolai Kardashev) would include harnessing energy but also multiplying the amount of space they could inhabit. Beginning with the likely possibility that civilization observed at cosmic distances would be much older and more advanced than humanity, he argued that:
“[It is] a reasonable working hypothesis that their habitat will have been expanded to the limits set by Malthusian principles. We have no direct knowledge of the material conditions which these beings would encounter in their search for lebensraum… One should expect that, within a few thousand years of its entering the stage of industrial development, any intelligent species should be found occupying an artificial biosphere which completely surrounds its parent star.”
However, at the time of writing, Dyson was working with the limits of habitable space within our Solar System, which was confined to Earth. Nevertheless, the various bodies of the Solar System (particularly Jupiter and the gas giants) have a tremendous amount of material that could (in theory) be repurposed to create an artificial biosphere. Using Jupiter as an example, Dyson argued that the planet’s mass was sufficient to create a spherical shell around the Sun about 2 to 3 meters (6.5 to 10 ft) in thickness with a density of “200 grams per square centimeter.”
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The Darkest Parts of the Moon are Revealed with NASA’s New Camera
While the surface of the Moon has been mapped in incredible detail over the last several decades, one region has eluded orbital cameras due to the lack of sunlight, which are aptly called the permanently shadowed regions (PSRs) of the Moon. However, two cameras operating on two different lunar orbiters have recently worked in tandem to produce a stunning mosaic image of the lunar south pole’s Shackleton Crater, a portion of which resides directly on the lunar south pole and whose depths have been shrouded in complete darkness for possibly the last few billion years. As a result, scientists hypothesize that water ice could have accumulated within its dark depths that future astronauts could use for fuel and life support.
These two cameras are NASA’s Lunar Reconnaissance Orbiter Camera (LROC) mounted on the Lunar Reconnaissance Orbiter (LRO), and NASA’s ShadowCam that is mounted onboard the Korea Aerospace Research Institute’s (KARI) Danuri spacecraft, with each spacecraft having been in lunar orbit since 2009 and 2022, respectively, and each camera providing their unique capabilities to help construct this unique and first-time mosaic image.
LROC features two narrow-angle cameras (NACs) capable of capturing images at 0.5 meters per pixel and a wide-angle camera (WAC) that can capture 100 meters per pixel, which is how LROC has delivered thousands of high-resolution images of the Moon during its 13 years of service. Despite this, LROC is unable to take images that are not in direct sunlight. Enter ShadowCam, whose light sensitivity is 200 times greater than LROC and can capture details of the lunar surface that have remained unseen until now. It accomplishes this by capturing the reflected sunlight from the myriad of geologic features on the Moon or light from the Earth, also known as Earthshine. The downside is ShadowCam can’t capture images in direct sunlight, which is why images from both cameras were used to create this stunning mosaic, as ShadowCam was used to capture the unlit portions of Shackleton Crater, such as the interior walls and floor, and LROC was used to capture the sunlit portions, such as the flanks and rim of the crater.
Artist’s impression of NASA’s ShadowCam mapping permanently shadowed regions (PSRs) on the Moon. (Credit: Arizona State University/Malin Space Science Systems)
LROC image of the rim crest and upper portion of the rim of Shackleton Crater with the lunar south pole located on Shackleton’s rim near the upper right corner of the image. The upper slopes of Shackleton’s interior are very steep, often greater than 30 degrees. Due to the Moon’s low axial tilt (~1.5 degrees), Shackleton’s shadowed interior never receives direct sunlight. (Credit: NASA/Goddard Space Flight Center/Arizona State University)
This new mosaic of Shackleton Crater on the Moon was obtained with a combination of images from NASA’s Lunar
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