Put two massive galaxy clusters into collision and you have an astronomical laboratory for the study of dark matter, that much discussed and controversial form of matter that does not interact with light or a magnetic field. We learn about it through its gravitational effects on normal matter. In new work out of Caltech, two such clusters, each of them containing thousands of galaxies, are analyzed as they move through each other. Using data from observations going back decades, the analysis reveals dark and normal matter velocities decoupling as a result of the collision.

Collisions on galactic terms have profound effects on the vast stores of gas that lie between individual galaxies, causing the gas to become roiled by the ongoing passage. Counter-intuitively, though, the galaxies themselves are scarcely affected simply because of the distances between them, and for that matter between the individual stars that make up each.

We need to keep an eye on work like this because according to the paper in the Astrophysical Journal, so little of the matter in these largest structures in the universe is in the form we understand. That’s a telling comment on how much work we have ahead if we are to make sense of the structure of a cosmos we would like to explore. In fact, the authors make the case that only 15 percent of the mass in the clusters under study is normal matter, most of it in the form of hot gas but also locked up in stars and planets. That would make 85 percent of the cluster mass dark matter.

The clusters in question are tagged with the collective name MACS J0018.5+1626. All matter, including dark matter, interacts through gravity, while normal matter is also responsive to electromagnetism. That means that normal matter slows down in these clusters as the gas between the individual galaxies becomes turbulent and superheated, while the dark matter within the clusters moves ahead in the absence of electromagnetic effects. Lead author Emily Silich (a Caltech grad student working with principal investigator Jack Sayers) likens the effect to that of a collision between dump trucks carrying sand. “The dark matter is like the sand and flies ahead.”

Image: This artist’s concept shows what happened when two massive clusters of galaxies, collectively known as MACS J0018.5+1626, collided: The dark matter in the galaxy clusters (blue) sailed ahead of the associated clouds of hot gas, or normal matter (orange). Both dark matter and normal matter feel the pull of gravity, but only the normal matter experiences additional effects like shocks and turbulence that slow it down during collisions. Credit: W.M. Keck Observatory/Adam Makarenko.

Some years back we looked at the two colliding galaxy clusters known collectively as the Bullet Cluster (see A Gravitational Explanation for Dark Matter). There, the behavior of the component materials of the clusters has been analyzed in the study of dark matter, but the clusters are seen from Earth with a spatial separation. In the case of MACS J0018.5, the clusters are oriented such that one is moving toward us, the other away. These challenging observations made it possible to analyze the velocity differential between dark and normal matter for the first time in a cluster collision.

Caltech’s Sayers explains:

“With the Bullet Cluster, it’s like we are sitting in a grandstand watching a car race and are able to capture beautiful snapshots of the cars moving from left to right on the straightway. In our case, it’s more like we are on the straightway with a radar gun, standing in front of a car as it comes at us and are able to obtain its speed.”

I’m reminded of my previous post on Chris Lintott’s book, where the astrophysicist takes note of the role of surprise in astronomy. In this case, the scientists used the kinetic Sunyaev-Zel’dovich effect (SZE), a distortion of the cosmic microwave background spectrum caused by scattering of photons off high-energy electrons, to measure the speed of the normal matter in the clusters. With the two clusters moving in opposite directions as viewed from Earth, untangling the effects took Silich to data from NASA’s Chandra X-ray Observatory (another reminder of why Chandra’s abilities, in this case to measure extreme temperatures of interstellar gas, are invaluable).

Adds Sayers:

“We had this complete oddball with velocities in opposite directions, and at first we thought it could be a problem with our data. Even our colleagues who simulate galaxy clusters didn’t know what was going on. And then Emily got involved and untangled everything.”

Nice work! The analysis tapped many Earth- and space-based facilities. Data from the Caltech Submillimeter Observatory (CSO), now being relocated from Maunakea to Chile, go back fully twenty years. The European Space Agency’s Herschel and Planck observatories, along with the Atacama Submillimeter Telescope Experiment in Chile, were critical to the analysis, and data from the Hubble Space Telescope were used to map the dark matter through gravitational lensing. With the clusters moving through each other at 3000 kilometers per second – one percent of the speed of light – collisions like these are in Silich’s words “the most energetic phenomena since the Big Bang.”

Dark matter explains many phenomena including galaxy rotation curves, which imply more mass than we can see, and gravitational lensing has been used to show that visible mass is insufficient to explain the lensing effect. But we still don’t know what this stuff is, assuming it is real and not a demonstration of our need to refine General Relativity through theories like Modified Newtonian Dynamics (MOND). What we need is direct detection of dark matter particles, an ongoing effort whose resolution will shape our understanding of galactic structure and conceivably point to new physics.

The paper is Silich et al. 2024. “ICM-SHOX. I. Methodology Overview and Discovery of a Gas–Dark Matter Velocity Decoupling in the MACS J0018.5+1626 Merger,” Astrophysical Journal 968 (2): 74. Full text.

One night a few years back I had a late night call from a friend who was involved in Breakthrough Starshot, the attempt to design a probe that could reach nearby stars and return data with transit times of decades rather than centuries. His news was surprising. The Parkes radio dish in Australia, then being used by the Breakthrough Listen SETI project, had detected a signal that seemed to come from Proxima Centauri. “What’s interesting,” said he, “is that when you move the dish off Proxima, the signal disappears.” You probably remember this episode, which had a brief moment in the news and may well live on among the conspiracy-minded in the wackier regions of cyberspace.

We know now that the signal was some form of radio frequency interference, commonly abbreviated RFI. In any case, our conversation was relatively tame because the idea of a terrestrial explanation seemed inevitable, no matter how tantalizing the first look at this signal. After all, with all the years of SETI effort since the original Project Ozma, was it likely that we would pick up a signal from the nearest of all stars? What were the odds that there would be a radio-using civilization so close to home?

Here I’m being deliberately provocative, because in fact we couldn’t know the odds. We know absolutely nothing about alien civilizations including whether or not they exist. To go science fictional, suppose Earth had triggered a nearby ‘lurker’ probe that had been in our Solar System monitoring our activities and had learned about our interest in the Alpha Centauri system. Would they possibly use a signal from Proxima as an introduction to first contact? Maybe from a lurker probe in that system? The scenarios can get as wild as anyone might wish. Best, then, to keep an eye on that signal.

It’s instructive to see what happened following the Proxima ‘detection,’ which occurred on April 29, 2019, and I’m reminded of it by Chris Lintott’s fine new book Accidental Astronomy (Basic Books, 2024). An astrophysicist at Oxford and well known radio presenter for the BBC program ‘Sky at Night,’ Lintott writes with his usual grace about the often serendipitous way astronomical discoveries happen, from the appearance of ‘Oumuamua to the surprises Cassini found at Enceladus. The overall point is that you have to look to find something, and in astronomy keeping the lenses pointed without preconceptions often churns up something rare and strange.

So what was interesting about the April 29, 2019 event? One eye-catching thing was that the radio signature was extremely narrow, an interesting fact given that naturally emitted radio waves tend to cover a wide range of wavelengths. Narrow-band signals are the kind of thing we think of as the stuff of radio broadcasts. In other words, technologies. I mentioned that the signal disappeared when the Parkes dish was moved off Proxima, and it also reappeared when the dish was returned to the target. This process, known as ‘nodding,’ is a handy way to rule out background radio sources.

So the signal definitely had the attention of the scientists at Breakthrough Listen, who dubbed it BLC1 (Breakthrough Listen Candidate 1). It also seemed to show a Doppler effect, changing in frequency slightly as time passed in ways that would be expected for a transmitter on a planet orbiting a star. If you think back to the famous ‘Wow!” signal of August 1977, detected at the Big Ear radio telescope run by Ohio State University, you can place it and BLC1 in context. The key is to look for signals that are narrowband, and if they repeat, so much the better. The Wow! Signal didn’t repeat, but BLC1 showed up more than once in 2019.

After that, the repetitions ceased, with no appearances in the following years. Now what? Transmissions from Earth satellites were ruled out given their much greater frequency drift, and deep space probes like the Voyagers and New Horizons were not aligned to match the Proxima signal. At this point scientists were turning over rarer and rarer explanations, including a transmitter on an asteroid, or an Earth-based transmitter that was deliberately being used to mimic a legitimate SETI signal. None of this fit BLC1, which in any case tracked Proxima’s motion across the sky.

The problem with anomalies like this one is that they go up in smoke if similar events occur with mundane explanations. When analyzed more closely, the Parkes data from the relevant period between April and May of 2019 contained four new detections of BLC1. But they also contained the same signal some fifteen times during periods in which the telescope was not pointing at Proxima. And at least one of these detections persisted as the telescope moved on and then off the target, which ruled out a signal from an alien civilization and pointed to an explanation much closer to home.

Looking for similar signals at different frequencies then popped up many more examples of what Lintott calls “annoying chirps, caused by human-made sources emitting at many frequencies at once, interfering with the quest for aliens.”

Too bad. It was exciting for a while, but frankly, everyone I know who is mixed up with SETI studies more or less assumed that while an explanation had to be found, it would be one that involved RFI, and so it was. But notice what happened around this event. The news of the data-delving re Proxima Centauri got out to the Guardian, whose reporting on it eschewed sensationalism but nonetheless made a point: Almost anything that happens within a project exploring subjects as sensitive as SETI will come to someone’s attention outside the community sooner than you think.

That makes any idea that a future ‘first contact’ will be covered up by scientists or governments rather ludicrous. Lintott comments:

The idea of a clandestine network squirreling away evidence of signals in the sky is hard to reconcile with the fact that the most interesting signal found by SETI in decades ended up in the press almost immediately. There are something like fifteen thousand professional astronomers in the world, including PhD students, making up essentially a small village, and news travels fast, especially when telescopes are pressed into service globally to follow some new occurrence in the sky.

As Lintott notes, the same thing happened in 2017, when a gravitational wave event was matched with a visual signal detected by spaceborne instruments. This was the first time that newly detected gravitational waves could be correlated with a visual event, which swung telescopes worldwide in its direction. You can’t put a worldwide effort to study a single object into effect without thousands of people becoming aware of it, if only because these observations need to be coordinated.

Image: This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope. Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani.

There is much in Accidental Astronomy that will bring anyone with a casual interest in the field up to date quickly. Current controversies and surprises include interstellar visitors like ‘Oumuamua and 2I/Borisov, the former of which produced a slight acceleration that inevitably raised questions (Lintott is circumspect in his treatment and clearly supports a natural explanation, though an open-minded one). Dyson spheres come up in the discussion of Boyajian’s Star, with its odd changes in brightness that are now thought to be unusual bands of dust that themselves are unexplained. The panoply of observing techniques and deep sky searches come into play in lucid and friendly prose. Befitting his BBC work, Lintott is a fine communicator.

The overall theme is a healthy one. Keep your eyes open, your lens covers off, your mind open. The inevitable corollary is: Don’t get locked into your own thinking to the point that you spend your career defending a hypothesis just because it’s yours. I always think of Voyager approaching Io and sending back images of volcanoes that only one team – Stan Peale, Patrick Cassen, and R. T. Reynolds – had thought would be there. Their paper in Nature appeared with Voyager 1 just three days out from the Jovian system. Talk about timing! Here’s Lintott on the matter:

We astronomers like being surprised, to wallow for the moment in the sense that there is more to understand. It’s a different feeling, utterly, from the way science and scientific progress are often portrayed on screen or in print, where you’re likely to hear stories about singularly clever people who have been blessed, with some clap of thunder, with a dose of cosmic truth before spending their careers trying to prove themselves right. The astronomy I know and love is more likely to involve a bunch of people staring at a screen and looking confused than to feature someone running down a corridor shouting “Eureka.”

So here’s to looking confused. Chris Lintott should be able to keep expanding on this theme in future editions because as the James Webb Space Telescope reminds us, every time we significantly upgrade our hardware, we see things we hadn’t expected to see. Ahead of us is the Vera Rubin Observatory, not to mention a generation of Extremely Large Telescopes (ELTs) that should be able to delve into exoplanet atmospheres around the closest stars. In a few scant decades we’ve gone from the idea that exoplanets are probably uncommon to the realization that they are ubiquitous. Who knows what the next transient in optical or radio wavelengths may bring?

“We were dreamers, dreaming greatly, in the man-stifled town;
We yearned beyond the sky-line where the strange roads go down.
Came the Whisper, came the Vision, came the Power with the Need…”

— Kipling, from “The Song of the Dead”

We’re lucky that science fiction fans are such packrats. They not only keep beloved books and magazine issues from their past but also catalog them relentlessly. Because of both these traits, I can turn to my own bookshelf and pull out the November, 1957 issue of Astounding Science Fiction to see P. Schuyler Miller’s review of John Campbell’s Islands of Space, in which he described the novel as “very characteristic of the best ‘hard’ science fiction of its day.” Miller had a lot to do in subsequent book reviews for the magazine with establishing ‘hard SF’ as a category.

Campbell’s book, extensively revised from its original appearance in the spring, 1931 issue of Amazing Stories Quarterly, is an interesting curiosity in being the first appearance of a ‘warp drive’ in science fiction. Here the concept emerges as a way of folding spacetime in ways that allow superluminal travel. The concept flows freely in science fiction of all sorts, led directly to Star Trek’s famous ‘warp factor,’ and was inspiration for Miguel Alcubierre’s investigation of whether or not ‘bending’ spacetime could actually be achieved, and how much energy it would take to do that.

It also moves between the ‘hard SF’ Miller describes and the more picaresque ‘space opera,’ heavy on adventure and short on technical detail. Whatever the subgenre, Islands of Space is an early fanciful leap in which protagonists Arcot and Morey discuss how their ‘space strain’ drive works and why other forms of propulsion are also useful:

“See here; with this new space strain drive, why do we have to have the molecular drive at all?”

“To move around near a heavy mass—in the presence of a strong gravitational field,” Arcot said. “A gravitational field tends to warp space in such a way that the velocity of light is lower in its presence. Our drive tries to warp or strain space in the opposite manner. The two would simply cancel each other out and we’d waste a lot of power going nowhere. As a matter of fact, the gravitational field of the sun is so intense that we’ll have to go out beyond the orbit of Pluto before we can use the space strain drive effectively.”

And look, here’s the first appearance of another science fiction motif, a higher dimension through which spacecraft can move without violating Einsteinian relativity. It solved a lot of problems in the days when hard SF inevitably edged into space opera as it approached c:

They were well beyond the orbit of Pluto when they decided they would be safe in using the space strain drive and throwing the ship into hyperspace.

I can only speculate how many writers’ careers were saved in those days by being able to deploy hyperspace to wave away all those bothersome problems with physics.

I’m not going to linger on Campbell’s novel, which the extraordinary E. F. Bleiler, who seems to have read every science fiction tale published as the genre was emerging, described as “greatly overloaded with unnecessary (although at times ingenious) exposition, hence almost unreadable; weak novelistically; and clichéd in its action plot.” All too true, alas, as I remember from reading it in my grad student days. But how stuffed with ideas Campbell’s writings could be, even when they went far off the rails in some of his later editorials in Astounding and Analog.

Giancarlo Genta, whose work in automotive engineering at Politecnico di Torino in Italy is highly regarded, is also a SETI theorist who has authored numerous papers in astronautics as well as Lonely Minds in the Universe (Copernicus, 2007). On top of this, Genta has written science fiction tales of his own, like The Hunter (Springer, 2013), which explores first contact with a highly dangerous alien civilization. His most recent paper is a look at interstellar exploration as it moves from fictional musings into actual hardware, with numerous SF references on the way.

Genta has recourse to the ‘hard science fiction’ terminology, referring to it as “science fiction strictly based on scientific knowledge.” That’s a handy, vest-pocket definition and I like it. We can add the idea that hard SF attempts to present innovative technologies with consistency and intellectual rigor, so that it demands a level of detail that can be glossed over in SF oriented more toward the social sciences. Poul Anderson could work in both camps but is probably best known for hard SF like Tau Zero. Arthur C. Clarke’s credentials at hard SF are foundational to the field. A trip through Greg Benford’s ‘Galactic Center’ novels is a master class in how hard SF is done.

A touchstone volume for those interested in the continuing vitality of the form is the Hartwell and Cramer collection The Hard SF Renaissance (Tor, 2003), which assembles work from the major creators in the subgenre. These are likely familiar names to most Centauri Dreams readers, and some have appeared in these pages: Stephen Baxter, David Brin, Hal Clement, Alastair Reynolds, Kim Stanley Robinson, Karl Schroeder, Allen Steele. That hardly exhausts the list, and I’ll direct you to this volume for others. You’ll find 960 pages of hard SF to work with inside.

In the Genta paper, I enjoyed being reminded of A. W. Bickerton’s quote from 1926. The British scientist was as outspoken about lunar travel as some scientists were about interstellar travel a few decades ago, saying:

This foolish idea of shooting at the moon is an example of the absurd length to which vicious specialisation will carry scientists. To escape Earth’s gravitation a projectile needs a velocity of 7 miles per second. The thermal energy at this speed is 15,180 calories [per gram]. Hence the proposition appears to be basically impossible.

In any case, science fiction has dealt with all of these themes, though as Genta points out, the further we move into relativistic realms, the more likely the author is engaging in space opera’s robust adventurism than the detailed physics of hard SF, which work best when dealing with concepts for which we have current solutions, no matter how imaginative. We don’t yet know how to produce a Von Neumann machine but the concepts are clear, and hard SF has emerged from the likes of Clarke and Fred Saberhagen, for example, to explore their Darwinian evolution and dangers.

Robert Forward’s Rocheworld has often appeared in these pages as an example of a manned interstellar mission whose physics is explained at a high level of detail. This journey to Barnard’s Star grew from Forward’s work on beamed propulsion systems and included an ingenious concept for crew return using the same beamer. Slower travel in space arks is a staple of science fiction with roots in novels like Heinlein’s Orphans of the Sky, drawn from two novellas and first published as a book in 1963. But the literature is rich and includes such classics as Brian Aldiss’ Non-Stop, Harry Harrison’s Captive Universe and many others.

Gene Roddenberry’s warp factor scale, developed for Star Trek, is nonlinear, with warp factor 6 requiring 7.32 days to reach Alpha Centauri, whereas getting to Vulcan (supposedly orbiting 40 Eridani) would require one month. But we have to have velocities like this to achieve science fictional goals whether of the hard SF, social sciences or space opera variety. Interstellar diplomacy? Poul Anderson couldn’t explore it without having a way to move not just faster than light but far beyond that limit. As Genta points out in the paper in Acta Astronautica:

“…to make it possible what science fiction describes, not only spacecraft must travel in FTL conditions, but their speed must be more than 2 orders of magnitudes greater than light speed. Just traveling slightly faster than speed has little advantages with respect to what in science fiction is called ‘subluminal’ travel.. True FTL travel is even more difficult to achieve. And even in this case, the situation will be like world wide travel in the nineteenth century: very costly (beyond the possibility of almost all people, except the very rich and the government officers) and slow (requiring weeks or months). In this situation very few people could travel, but empires spanning more than one continent, and international diplomacy were possible.

If we master a warp technology of some kind, we still have the problem of entering and exiting a warp condition, or for that matter entering a wormhole, if such exist and become a feasible way to travel. It may prove necessary to locate the departure point outside the stellar system the craft is in – Arcot and Morey noticed this problem in the Campbell novel. In this case, we need to factor in time to exit and enter the departure and arrival systems at speeds less than light. On the other hand, if wormholes exist and we can find a way to pass through them, we must first travel to them. Genta discussed these issues in a paper with Roman Kezerashvili in 2020 as well as in a 2023 paper on system-wide infrastructure as a precondition for interstellar expansion (citations below).

Science fiction tales often assume but rarely discuss the existence of inertial dampeners that allow high accelerations without seeming effect on the crew. Communications by FTL methods range from the ‘subspace messages’ of Star Trek (moving faster than light and verging on the instantaneous as the story lines progress), to recording information on a material substrate which could be put aboard a small probe and sent to destination (the probe is thus assumed to be faster than the craft that launches it). Early images of Earth from spy satellites were deorbited and returned on film, beginning our study of the best methods for re-entry from orbit.

I read a lot of novels, but these days most of my reading in interstellar topics is in scientific papers and books they have spawned. Nonetheless, we have to develop the imagination needed to know where necessary advances in technology will be made. Science fiction has been serving that purpose for a century now, and much longer depending on where you locate its origins – Brian Aldiss goes all the way back to Mary Shelley in Billion Year Spree, for example. The field remains robust and doubtless is encouraging as many new scientists as it did when I was a kid all goggle-eyed from Anderson’s The Enemy Stars. Long may it thrive.

The paper is Genta, “Interstellar Exploration: From Science Fiction to Actual Technology,” Acta Astronautica Vol. 222 (September 2024), pp. 655-660 (abstract). See also Genta and Kezerashvili, “Achieving the required mobility in the solar system through Direct Fusion Drive,” Acta Astronautica Vol. 173 (2020), 303-309 (abstract). The paper on system infrastructure is “Is a solar system-scale civilization a precursor to going interstellar,” in L. Johnson, K. Roy, Interstellar Travel, Elsevier, Amsterdam, 2023.