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GW170817: An Extragalactic SETI Opportunity?

While we pursue SETI by listening to and looking at nearby stars within our own galaxy, the possibility of going extragalactic remains. Consider the activity at Penn State, where Jason Wright and colleagues Matthew Povich and Steinn Sigurðsson have been conducting the Glimpsing Heat from Alien Technologies (G-HAT) project, looking at infrared data from both the Wide-field Infrared Survey Explorer mission (WISE) and the Spitzer Space Telescope. An unusual infrared signature could conceivably be a sign of waste heat from such a culture.

Turning up the signature of a Kardashev Type III civilization, one capable of tapping the energy output of an entire galaxy, would be a spectacular find, a search well worth continuing. But there are other ways of looking for evidence that might fit what Wright has written about in terms of ‘Schelling points.’ The idea is to draw on game theory to analyze a situation in which two players who cannot communicate are engaged in a cooperative activity. They are, perhaps, asking themselves questions like when to transmit to a given target and in which directions to listen.

Thus we look at SETI as a mutual activity, and the game theory analyzed by the American economist Thomas Schelling comes into play. Wright looks at this in a paper called “Exoplanets and SETI” (citation below), noting that one of Schelling’s examples is particularly germane to SETI: Imagine you find yourself in a large city and are looking for someone who is also looking for you. Let me quote Wright on this:

Guessing the times and places to meet in the city, and guessing the frequencies to tune to in radio SETI, are superior strategies to random ones. In the city, this might include the locations of famous landmarks and times that bells chime or other coordinated actions occur; in radio SETI this might mean astrophysically significant frequencies and their multiples. Makovetskii (1980) called this a “mutual strategy of search” for “synchrosignals”(Makovetskii 1977), and Filippova et al. (1991) described this concept as a “convergent strategy of mutual searches” for SETI (both apparently unaware of Schelling’s prior art).

Image: Nobel Prize-winning economist and arms control theorist Thomas Schelling (1921-2016). Credit: Harvard University.

It is clearly useful to identify Schelling points when thinking about where to observe if we are looking for a SETI signal. We started SETI off back in the Project Ozma days by looking for Sun-like stars that might host habitable planets like our own, and we have target lists for continuing searches today. Would ETI do the same? The ecliptic might be a good place to look for a signal because Earth would transit the Sun from the viewpoint of astronomers around distant stars there. Another Schelling point: Exoplanet transits, which could be considered a communications window for any civilization deliberately trying to draw our attention.

Notice that these Schelling points assume a civilization that is attempting to be found, which is what Yuki Nishino and Naoki Seto (Kyoto University) do in a paper that extends Schelling to other galaxies. Whereas finding waste heat from a Type III civilization would assume no intent to communicate, Nishino and Seto ask whether there are any windows that an ETI culture might choose if it wanted to announce its presence to another galaxy. Given the challenge of the distances involved, such a signal would have to be sent at an obvious Schelling point.

Let’s leave aside the unknowable issue of motives (who knows why an alien species might want to broadcast its existence to beings in a culture millions of light years away?), and ask whether there are any obvious Schelling points that civilization might choose. The authors go through the literature, finding proponents of synchronizing signals via supernovae or gamma rays bursts, but here they question the timing accuracy involved and doubt the utility of the concept.

But the researchers argue that there is one natural phenomenon that could be forecast well enough in advance to allow a civilization to synchronize a signal: A binary neutron star merger. They point to the detection by the LIGO-Virgo network of a gravitational wave from the binary neutron star (BNS) system GW170817. Soon after the detection, the host galaxy NCG4993 was identified by electromagnetic observations, at a distance of ~40 Mpc. Nishino and Seto believe that a sufficiently advanced civilization could predict such a merger with high accuracy:

These Galactic BNSs contain recycled pulsars as one of the binary components, allowing high-precision measurements of the binary parameters. If ETI can detect the recycled pulsar of an inspiraling BNS system in their galaxy, they can also make a high-precision measurement for the quantities required for the signal transmission to be synchronized with the BNS merger. This synchronization scheme seems reasonable to us as a signal receiver…

A natural event lights up in a way sure to draw the attention of astronomers in other galaxies. A signal timed to coincide with it is sent to take advantage of the attention it has created.

Image: NASA’s Spitzer Space Telescope has provisionally detected the faint afterglow of the explosive merger of two neutron stars in the galaxy NGC 4993. The event, labeled GW170817, was initially detected nearly simultaneously in gravitational waves and gamma rays, but subsequent observations by many dozens of telescopes have monitored its afterglow across the entire spectrum of light. Spitzer’s observation on September 29th comes late in the game, just over 6 weeks after the event was first seen, but if this weak detection is verified, it will play an important role in helping astronomers understand how many of the heaviest elements in the periodic table are created in explosive neutron star mergers. This image is a color composite of the 3.6 and 4.5 micron channels of the Spitzer IRAC instrument, rendered in cyan and red. The faint light from the explosion is to faint to be easily seen mixed in the light of the other stars in the galaxy. Credit: JPL/Caltech.

Thus we have a mechanism for creating a targeted signal that could announce the existence of a civilization to another galaxy, for whatever purposes it chose to do this. The detection of the cataclysmic binary neutron star merger would be accompanied shortly thereafter by a communication signal meant to be detected by astronomers studying the merger event itself. The authors estimate that a 1 terawatt transmitter in a galaxy 130 million light years away could send 10 megabytes of data to a receiver like the Square Kilometer Array.

The odds of a civilization choosing to deploy such methods seem long indeed, but it fascinates me to see that gravitational wave astronomy is now beginning to work its way into the SETI discussion. Thus the authors discuss the synchronization of the signal:

… the ETI would be able to estimate the location and the epoch of the highly energetic event in advance. Most optimistically, we might actually find an artificial signal by reanalyzing the electromagnetic data already taken from GW170817. Additionally, the LIGO-Virgo network will start the next observational run in early 2019, and a new BNS merger might be identified. The early and deep radio observation for its host galaxy might also be worth considering from the perspective of SETI.

The authors also note that to demonstrate the synchronization of the signal (i.e., to show that it was not a coincidence), the extraterrestrial civilization might include the intrinsic information of the particular binary neutron star merger involved. It’s an ingenious notion that extends directed beacon signals into the extragalactic realm and one that also calls into question the issue of galactic gravitational lensing. In any case, the transmitting culture is adjusting the timing and direction of its transmissions to coincide with the arrival of the gravitational wave signal.

Given the year-long observations of the GW170817 afterglow, Nishino and Seto argue that the difference between the arrival time of the GW signal and the artificial signal should be in the range of one year. Thus a transient, flaring phenomenon drawing our attention could become the announcement of a SETI opportunity, with a binary neutron star merger the brightest transient that can be predicted sufficiently in advance for the signal to be prepared.

Thomas Schelling, it goes without saying, never dreamed his work on game theory might be turned to extragalactic SETI, but the exercise of defining observational windows will continue.

The paper is Nishino and Seto, “The Search for Extra-Galactic Intelligence Signals Synchronized with Binary Neutron Star Mergers,” The Astrophysical Journal Letters Vol. 862, No. 2 (1 August 2018). Full text. The paper by Jason Wright is “Exoplanets and SETI,” available as a preprint.


Comments on this entry are closed.

  • hiro August 24, 2018, 14:07

    There is no type III civilization because we either use an outdated or wrong definition.

    • Andrei August 25, 2018, 8:09

      Also I think that such a civilization is unlikely to exist.
      But if they do, it would make sense to try to contact others by broadcasting to a completely different galaxy.
      For us who listen for others without broadcasting a very strong signal.
      This article point out some important strategies, and this might be important as this alien might have completely different preferences.
      (And their choice might be logical even to us when discovered. Such as a frequency choosen from a matematical formula, or a physical constant.)

      • Alex Tolley August 25, 2018, 14:01

        If civs, even KIIIs, want to appear silent, then they would select anti-Schelling points for communications.

    • hiro August 25, 2018, 14:57

      An ant might define the supreme species on this planet in a way such that each being is able to move +100 tons objects and run faster than 10000 mph; well none of us can do these but we have nukes!

      Anyway, forget about communicate via prime numbers; some advanced civilizations might have sent out maps of complex surfaces which contain the Riemann’s Zeta Zero on the critical line, but we are completely clueless because we only look for prime numbers, talking about misunderstanding.

      • Andrei August 28, 2018, 3:23

        Hello Hiro.
        Well our SETI scientists look for a strong signal.
        So if the message isn’t prime numbers, they still will be investigating it.
        One strategy used is search many frequency at once without any assumption which one the aliens will use.
        The problem with that is that a certain frequency only will be looked at for a fraction of a second, and soon they move on to next star. We look for those who send continuously, but that is unlikely as it cost so much energy.
        A message like the ‘Wow’ signal could easily be missed, and if it contained a burst information package it was lost as it only recorded as a set of letters on a paper.
        Aliens assume every civilization have decent computers and a modem connected to their listening devices, but we didn’t and lost the information they sent us.
        It was example, but listening to millions of frequences could be a good strategy, only we need to listen longer and money, radiotelescope time and resources do not allow for that.
        We might miss something, and this article is the reason for that worry – to improve the strategy.

  • Alex Tolley August 24, 2018, 15:05

    The idea of Schelling points is interesting, although I have to wonder the motives of extra-galactic ETI in sending signals to targets millions and 10’s of millions of lightyears away. Even if they do, which frequencies to look at harks back to traditional SETI and the “water hole” frequency (or multiples of).

    My sense is that Wright’s approach to look for artifacts is a better approach. Even using Schelling points, in this case possible locations in space) it is more akin to looking for spoor near a waterhole or on a trail, rather than hoping to see the animal at dusk or dawn. Such signs will exist even if teh animal has long gone. As so it is with civilizations. Their artifacts will exist for longer than they have been active, and it makes no assumptions about motivations for signaling or communication.

    • Harold Shaw August 26, 2018, 10:45

      If ETI lifespans are long enough, distances of millions of light years are less of an obstacle. Lifespans in the millions of years should be possible, especially for synthetic or digital lifeforms.
      As you say, the hard part is knowing what to look for.

  • Jason Wright August 24, 2018, 15:19

    It’s probably true that Schelling didn’t expect his work to end up in extragalactic SETI, but one of the first examples in his book is actually the 21 cm line of hydrogen in radio SETI!

    Also, the “find a stranger in New York” example has actually been performed:


    • Paul Gilster August 24, 2018, 15:22

      Really! Had not realized he used that example. Thanks, Jason.

  • Jason Wright August 24, 2018, 15:20
  • jurov August 24, 2018, 16:40

    Reminds me of another interesting paper recently: “Fast Radio Bursts from Extragalactic Light Sails” https://arxiv.org/abs/1701.01109

  • John walker August 24, 2018, 17:03

    First a small numerical correction, Nishino and Seto claim data reception volume of 10e4 bits (w/ phase 2 SKA from 1TW at 40Mpc). That’s 1.25kB not 10MB.

    While the basic idea of an eti data blitz correlated to rare astrophysical events is interesting, the authors make an incredibly large number of assumptions only some of which are referred to in their discussions section. Many of those assumptions on developmental level, time line, signal type and strength etc. seem largely arbitrary. There is also no mention of expected event frequency. This is relevant simply from an instrument allocation perspective. Whereas LIGO and co detect 1 or 2 events per year, the ET or Cosmic Explorer would be detecting over 1 million pa. How many BNS mergers amongst those there would be, I don’t know.

    Hopefully, the GW instruments referred to in the paper will be built. Einstein Telescope, DECIGO and much later the BBO(Big Bang Observer). Then we will see!

  • Robin Datta August 24, 2018, 17:28

    The New York experiment involved beings of the same species and the same general cultural background: they knew what cities with their buildings, roads, etc. were, some of the major landmarks of the particular city they were in; what humasn looked like, how they communicated, what language they likely used, etc.

    Take away all those (and more!) commonalities and one may get some idea of the magnitude of the difficulties with SETI.

    • J. Jason Wentworth August 29, 2018, 1:27

      I agree–an interesting game which might help us is “What Aliens Know.” A scientific extraterrestrial civilization would, by definition, have to know at least some things that we know, and vice-versa. (It is an assumption to accept the idea that there is at least one other such civilization. In his book “The Eerie Silence,” Paul Davies made a saddening but plausible case that we’re the only one, by showing how the discovery of science depended on a fortunate chain of contingent events and mindset-preparing occurrences.) Also:

      If another scientific civilization exists (one that has developed high technology based on the predictive powers of science, rather than having “tinkered-up” inventions and devices, which are limited in scope), we can pretty safely assume that they know something about geometry (Euclidean at least), mathematics, shapes that are universal in nature (spheres, “teardrop” liquid droplets, etc.), conic section trajectories, acoustics, and general physics, including the principles upon which telescopes, microscopes, radios, engineering, and spacecraft are based, even if their sensory organs differ from ours. They might well know things that we don’t, especially if they’re an older society, but it would be astounding if we and they had no such knowledge in common, and:

      For this reason, it should be feasible to “play interstellar seek and find” (rather than “hide and seek”) with another such race–and win–provided that the other ‘player’ also desires contact, or at least isn’t actively hiding in order to avoid contact. If this is the case (they ^aren’t^ hiding, that is), deliberately making any electromagnetic, gravity wave, or probe-borne messages that we send easy to decipher (by employing anti-cryptography, as Carl Sagan and other SETI researchers have advocated) should make them both identifiable as artificial and understandable to intelligent aliens.

  • Harold Shaw August 25, 2018, 10:58

    A few months ago, there was an article here that described using celestial phenomena, such as gravitational lensing and stellar pumping, as a galaxy wide network to propagate or embed messages. My search of the site’s database failed but I think the idea could be applicable here. An embedded message that flairs and subsides with the star merger could be mistaken as natural. However, it would hopefully serve to attract enough interest to look deeper and be much less expensive than sending transmitters to the merger event.

  • Infinite123Lifer August 25, 2018, 12:55

    Fascinating concept! Working with what could be feasible is the realm of SETI it seems to me.

    I personally don’t think there are any Type 3 civs either … but that’s just my opinion based off what my primitive meatsack can phathom within its imagination and subject to change, certainly not a closed book, end of discussion finality sort of thing. A perhaps infinitely small probability … in a perhaps infinitely large and lasting universe. Even if the universe is limited to it’s size it may not be limited in time … a rinse and repeat sort of deal. Was there ever a type 3 in any universe? Awful waste of space if there hasn’t been.

  • AlexT August 25, 2018, 13:05

    “Nishino and Seto… GW signal and the artificial signal should be in the range of one year…”
    It seams that authors suppose that ETI simultaneously know nothing about us , and ,in same time , they know well what is one Earth’s year i.e. how many “ETI’s time measurement units” it takes to Earth to make the full turn aroung the Sun :-).
    It sounds like story from Star Track serial … OMG…

  • Geoffrey Hillend August 25, 2018, 15:42

    An intergalactic signal would have to be very powerful to travel that distance and be detectable. It might have to use gamma rays. This idea makes sense.

  • J. Jason Wentworth August 26, 2018, 3:59

    I also doubt that there are any Type III civilizations. But *if* there are any, they might be more likely to be machine civilizations (because they would, by definition–considering their spatial extent–have to be very old and thus very long-lived [unless FTL travel is possible, and they discovered how to do it]). To a machine Type III civilization, the multi-million-year transit times–even involving replies–of gravity wave or electromagnetic signals would be of little consequence, since they would be effectively immortal (even if their ^individual^ members wore out and had to be replaced at intervals).

  • Harold Shaw August 26, 2018, 12:47

    As long as a the same artificial signal is used and the the signal is broadcast over many merger events, then the question of where to place the signal may not be relevant. The broadcaster would be depending on an observer who is cataloging the general qualities of celestial objects and not looking at any place in particular. The signal would just need to stand out as being only associated with merger events.

  • Michael Fidler August 27, 2018, 2:13

    Could we be looking at something that is actually doing quantum entanglement of all the particles at the same instance. Not a collapse of the whole object but an entangle particle spaghetti that happen all at the same time. Like a collapse of the wave function at the quantum level but the whole object turns into a giant wave wormhole like a rip tide. Dr. Who uses that to jump from universe to universe.♾

  • Andre August 27, 2018, 21:42

    “one capable of tapping the energy output of an entire galaxy”

    If the KIII civilization is so powerful in the first place, why wouldn’t such a civilization simply create it’s own gigantic artificial energy beacon that signals in a repeatable pattern so that intelligences in other galaxies could see and attempt decode? That would seem simpler. Also, it would be very coincidental that by chance they happened to choose to communicate via good old reliable radio technology discovered here on Earth only in the 19th century.

  • Michael Fidler August 30, 2018, 1:57

    So instead of using the whole energy of a galaxie to watch reruns of Lassie how about a KIV civilization that CREATES whole universes.

  • Michael Fidler September 2, 2018, 2:02

    New and very interesting idea in looking for Dysonian SETI.

    Multi-Stellar SETI Candidate Selection (Kaggle Kernels)
    By Jose Solorzano | September 1st 2018

    “That multi-stellar extraterrestrial civilizations exist and that there’s spatial clustering in their colonization patterns. The hypothesis is not far-fetched: If a civilization is able to build detectable astro engineering, interstellar travel (and colonization) shouldn’t be far beyond their capabilities. Additionally, spatial clustering should be expected for the same reasons that human populations cluster on Earth.”


  • ljk September 20, 2018, 9:20

    What do SETI terms mean? A committee weighs in.

    Inspired by a paper by Iván Almár’s paper on SETI terms:



    We didn’t agree on everything, but we came up with a lot of good recommendations and clarifications that I hope NASA will find useful. We did not go with “Communication SETI” because the committee thought it was too reminiscent of METI, and we did not recommend against “civilization” or “colonize” (mostly because both are in wide use and we did not want to endorse major changes without very good reason) but we did note the problems with those terms.

    Our final report is here:


  • ljk September 20, 2018, 9:31


    SETI Detection Strategies for Single Dish Radio Telescopes

    Gregory Hellbourg

    (Submitted on 10 Sep 2018)

    Radio Searches for Extra Terrestrial Intelligence aim at detecting artificial transmissions from extra terrestrial communicative civilizations. The lack of prior knowledge concerning these potential transmissions increase the search parameter space. Ground-based single dish radio telescopes offer high sensitivity, but standard data products are limited to power spectral density estimates.

    To overcome important classical energy detector limitations, two detection strategies based on asynchronous ON and OFF astronomical target observations are proposed. Statistical models are described to enable threshold selection and detection performance assessment.

    Subjects: Signal Processing (eess.SP); Instrumentation and Methods for Astrophysics (astro-ph.IM)

    DOI: 10.1109/SSP.2018.8450739

    Cite as: arXiv:1809.04533 [eess.SP]
    (or arXiv:1809.04533v1 [eess.SP] for this version)

    Submission history

    From: Gregory Hellbourg [view email]

    [v1] Mon, 10 Sep 2018 22:32:31 GMT (585kb)


  • ljk September 20, 2018, 9:34


    How Much SETI Has Been Done? Finding Needles in the n-Dimensional Cosmic Haystack

    Jason T. Wright, Shubham Kanodia, Emily G. Lubar

    (Submitted on 19 Sep 2018)

    Many articulations of the Fermi Paradox have as a premise, implicitly or explicitly, that humanity has searched for signs of extraterrestrial radio transmissions and concluded that there are few or no obvious ones to be found.

    Tarter et al. (2010) and others have argued strongly to the contrary: bright and obvious radio beacons might be quite common in the sky, but we would not know it yet because our search completeness to date is so low, akin to having searched a drinking glass’s worth of seawater for evidence of fish in all of Earth’s oceans.

    Here, we develop the metaphor of the multidimensional “Cosmic Haystack” through which SETI hunts for alien “needles” into a quantitative, eight-dimensional model and perform an analytic integral to compute the fraction of this haystack that several large radio SETI programs have collectively examined. Although this model haystack has many qualitative differences from the Tarter et al. (2010) haystack, we conclude that the fraction of it searched to date is also very small: similar to the ratio of the volume of a large hot tub or small swimming pool to that of the Earth’s oceans.

    With this article we provide a Python script to calculate haystack volumes for future searches and for similar haystacks with different boundaries. We hope this formalism will aid in the development of a common parameter space for the computation of upper limits and completeness fractions of search programs for radio and other technosignatures.

    Comments: 20 pages, 4 figures. Submitted to The Astronomical Journal on 17th August 2018, Accepted on 11th September 2018
    Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:1809.07252 [astro-ph.IM]
    (or arXiv:1809.07252v1 [astro-ph.IM] for this version)

    Submission history

    From: Shubham Kanodia [view email]

    [v1] Wed, 19 Sep 2018 15:39:31 GMT (536kb)


  • ljk September 20, 2018, 16:09

    The Cosmic Haystack

    I taught the first Penn State graduate course in SETI last spring, and I thought it went really well. We read a lot of papers, had great discussions about Schelling Points and all the different ways to search, and got to visit Green Bank to conduct original SETI observations. You can read all about the course at the class website here, along with student reactions to the papers (tweeted out by Sofia Sheikh here).

    One of the cuter terms in the syllabus was that any student whose final project got published in a refereed paper would get a retroactive ‘A’. Well, the first automatic ‘A’ has arrived! Shubham Kanodia and Emily Lubar, following a suggestion by Jill Tarter, calculated rigorous volumes of the Cosmic Haystack, and the paper is now on the arXiv.

    Full article with lots of links here:


    To quote:

    We also construct another one-parameter way to calculate search completeness: what density of uniformly spaced megawatt transmitters in the Milky Way can we rule out? We get about one per 0.27pc for the latest Breakthrough Listen paper (for L-band transmitters). The Galaxy could be filled with more transmitters than stars, and we wouldn’t have found them yet!

    So we haven’t done much searching, and people should not use the “failure” of radio SETI to date as evidence that we should stop looking.

    But it’s also important not to let the pendulum swing to far the other way! One might see numbers like -18 in the exponent and declare the entire project hopeless. On the contrary:

    1.New radio technology allows us to search through haystack volumes much faster than before; the Breakthrough Listen program is quickly catching up to the historical NASA programs by our metric, and MWA needed only 2 hours to dominate our calculation.

    2.If we expect to find transmitters near stars, our completeness is many orders of magnitudes higher, because we have concentrated our efforts there

    3.We don’t need to search the entire haystack unless there is nothing to find. Another way to look at it: you don’t have to drain the oceans to find marine life, unless they are sterile and you are trying to prove that. A bathtub’s worth of water is probably not enough to find a fish if you randomly sample the ocean, but if you look in the right places it’s about the right order of magnitude of search required.

  • ljk September 29, 2018, 12:52

    Student Project Scans Sky for Alien Laser Beams

    By Mike Wall, Space.com Senior Writer

    September 29, 2018 at 07:58 am ET

    An ambitious, student-run hunt for intelligent aliens is underway.

    The Trillion Planet Survey has begun scanning the huge Andromeda galaxy, as well as our own Milky Way, for beams of light that could have been produced by advanced alien civilizations. (The project’s name stems from the fact that Andromeda harbors about 1 trillion stars, and stars in general are thought to host at least one planet on average.)

    “First and foremost, we are assuming there is a civilization out there of similar or higher class than ours trying to broadcast their presence using an optical beam, perhaps of the ‘directed energy’ arrayed type currently being developed here on Earth,” lead researcher Andrew Stewart, a student at Emory University in Atlanta, said in a statement.

    Full article here:


    To quote:

    “Second, we assume the transmission wavelength of this beam to be one that we can detect,” Stewart added. “Lastly, we assume that this beacon has been left on long enough for the light to be detected by us. If these requirements are met and the extraterrestrial intelligence’s beam power and diameter are consistent with an Earth-type civilization class, our system will detect this signal.”

    That system employs an image-analysis pipeline and the global network of small (1-meter-class, or 3.3 feet), robotically controlled telescopes operated by the Las Cumbres Observatorye.

    Each telescope by itself captures a roughly 3 percent slice of Andromeda, which lies about 2.5 million light-years from Earth. The team will combine a set of these photos to create a single image, which will then be compared to a pristine “control” photo of the galaxy — one unsullied, for example, by satellites passing overhead.

    Any differences between the two images could theoretically result from light signals produced by E.T. (though there could of course be natural explanations as well). Such “transients” will be processed by the data pipeline, which will help weed out false positives, team members said.

    “One of the things the software checks for is, say, a satellite that did go through our image,” Kyle Friedman, a senior from Granada Hills Charter High School in Los Angeles, said in the same statement. “It wouldn’t be small. It would be pretty big, and if that were to happen the software would immediately recognize it and throw out that image before we actually even process it.”

    [Good, then we won’t have another Wow! signal that everyone will keep focusing on even though there have been much better ETI candidates in the 41 years since then.]

  • Harry R Ray November 15, 2018, 12:01

    The Gw170817 remenant has now been PROVEN to be a supermassive neutron star, and not a black hole YET, but in just a few centuries, it may spin down to the point where it may THEN collapse into a black hole.a

  • ljk January 18, 2019, 11:08

    16 January 2019

    Can you shroud a galaxy?


    So if aliens could cloak all stars fainter than 1,000 Suns, the galaxy would appear bluer than most galaxies. In red galaxies, most of the light comes from red giants, and the brightest of these are also the reddest. Cloaking the fainter red giants makes these galaxies redder, redder than natural galaxies.

    Over the next few years, astronomers will compile catalogs of the colors of billions of galaxies. Sunscreen points to a new way of looking for Type III societies among these billions of galaxies: we can look for galaxies with unusual colors.

    You can read the technical details in my paper, just published in the Publications of the Astronomical Society of the Pacific:


    The code used in the analysis is available at: