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The Fermi paradox has given rise to various attempts to explain why no evidence of extraterrestrial civilisations was found so far on Earth and in our Solar System. Here, we present a dynamical model for the development of such civilisations, which accounts for self-destruction, colonisation and astrophysical destruction mechanisms of civilisations including gamma-ray bursts, type Ia and type II supernovae as well as radiation from the supermassive black hole. We adopt conservative estimates regarding the efficiency of such processes and find that astrophysical effects can influence the development of intelligent civilisations and change the number of systems with such civilisations by roughly a factor of 2; potentially more if the feedback is enhanced. Our results show that non-equilibrium evolution allows for solutions in-between extreme cases such as "rare Earth" or extreme colonisation, including scenarios with civilisation fractions between 10^{-2} and 10^{-7}. These would imply still potentially large distances to the next such civilisations, particularly when persistence phenomena are being considered. As previous studies, we confirm that the main uncertainties are due to the lifetime of civilisations as well as the assumed rate of colonisation. For SETI-like studies, we believe that unbiased searches are needed considering both the possibilities that the next civilisations are nearby or potentially very far away., 8 pages, 6 figures, 1 table. Accepted for publication in the International Journal of Astrobiology

The theory of lithopanspermia proposes the natural exchange of organisms between solar system bodies through meteorites. The focus of this theory comprises three distinct stages: planetary ejection, interplanetary transit and planetary entry. However, it is debatable whether organisms transported within the ejecta can survive all three stages. If the conjecture is granted, that life can indeed be safely transmitted from one world to another, then it is not only a topic pertaining to planetary science but also biological sciences. Hence, these stages are only the first three factors of the equation. The other factors for successful lithopanspermia are the quality, quantity and evolutionary strategy of the transmitted organisms. When expanding into new environments, invading organisms often do not survive in the first attempt and usually require several attempts through propagule pressure to obtain a foothold. There is a crucial difference between this terrestrial situation and the one brought about by lithopanspermia. While invasive species on Earth repeatedly enters a new habitat, a species pragmatically arrives on another solar system body only once; thus, an all-or-nothing response will be in effect. The species must survive in the first attempt, which limits the probability of survival. In addition, evolution sets a boundary through the existence of an inverse proportionality between the exchanges of life between two worlds, thus further restricting the probability of survival. However, terrestrial populations often encounter unpredictable and variable environmental conditions, which in turn necessitates an evolutionary response. Thus, one evolutionary mode in particular, bet hedging, is the evolutionary strategy that best smooth out this inverse proportionality. This is achieved by generating diversity even among a colony of genetically identical organisms. This variability in individual risk-taking increases the probability of survival and allows organisms to colonize more diverse environments. The present analysis to understand conditions relevant to a bacterial colony arriving in a new planetary environment provides a bridge between the theory of bet hedging, invasive range expansion and planetary science.

In this paper we revisit the Dysonian approach and assume that a superadvanced civilisation is capable of building a cosmic megastructure located closer than the habitable zone (HZ). Then such a Dyson Sphere (DS) might be visible in the optical spectrum. We have shown that for typical high melting point meta material - Graphene, the radius of the DS should be of the order of $10^{11}$cm, or even less. It has been estimated that energy required to maintain the cooling system inside the DS is much less than the luminosity of a star. By considering the stability problem, we have found that the radiation pressure might stabilise dynamics of the megastructure and as a result it will oscillate, leading to interesting observational features - anomalous variability. The similar variability will occur by means of the transverse waves propagating along the surface of the cosmic megastructure. In the summary we also discuss the possible generalisation of definition of HZs that might lead to very interesting observational features., Comment: 7 pages, 2 figures

With the advent of modern astronomy, humans might now have acquired the technological and intellectual requirements to communicate with other intelligent beings beyond the solar system, if they exist. Radio signals have been identified as a means for interstellar communication about 60 years ago. And the Square Kilometer Array will be capable of detecting extrasolar radio sources analogous to terrestrial high-power radars out to several tens of light years. The ultimate question is: will we be able to understand the message, or, vice versa, if we submit a message to extraterrestrial intelligence first, how can we make sure that they understand us? Here I report on the largest blind experiment of a pretend radio message received on Earth from beyond the solar system. I posted a sequence of about two million binary digits ("0" and "1") to the social media that encoded a configuration frame, two slides with mathematical content, and four images along with spatial and temporal information about their contents. Six questions were asked that would need to be answered to document the successful decryption of the message. Within a month after the posting, over 300 replies were received in total, including comments and requests for hints, 66 of which contained the correct solutions. About half of the solutions were derived fully independently, the other half profited from public online discussions and spoilers. This experiment demonstrates the power of the world wide web to help interpreting possible future messages from extraterrestrial intelligence and to test decryptability of our own deliberate interstellar messages., Comment: accepted by the International Journal of Astrobiology (Cambridge University Press), 11 pages, 3 figures (2 col, 1 b/w), 1 table, Fig. 2 (top) updated, original SETI Decrypt Challenge call at https://twitter.com/DrReneHeller/status/724935476327624704, python code available at http://bit.ly/2rs9jw9

Pulsars have at least two impressive applications. First, they can be used as highly accurate clocks, comparable in stability to atomic clocks; second, a small subset of pulsars, millisecond X-ray pulsars, provide all the necessary ingredients for a passive galactic positioning system. This is known in astronautics as X-ray pulsar-based navigation (XNAV). XNAV is comparable to GPS, except it operates on a galactic scale. I propose a SETI-XNAV research program, to test the hypothesis that this pulsar positioning system might be an instance of galactic-scale engineering by extraterrestrial beings (section 5). The paper starts with a critique of the rejection of the extraterrestrial hypothesis when pulsars were first discovered (section 2), continues with some highlights on the rich pulsar phenomenology (section 3), and their usefulness for various purposes (section 4). The core section 5 proposes tests and predictions of SETI-XNAV, related to: the pulsar distribution and power in the galaxy, their population, their evolution, extragalactic pulsars, possible pulse synchronizations, pulsar usability when navigating near the speed of light, decoding galactic coordinates, directed panspermia, and information content in pulses. Even if pulsars are natural, they are likely to be used as standards by ETIs in the galaxy (section 6). Such a common galactic timing and positioning standard have deep consequences for SETI and METI. I discuss potential policy issues, as well as benefits for humanity, whether the research program succeeds or not., Working paper, comments are welcome.

We discuss how visions for the futures of humanity in space and SETI are intertwined, and are shaped by prior work in the fields and by science fiction. This appears in the language used in the fields, and in the sometimes implicit assumptions made in discussions of them. We give examples from articulations of the so-called Fermi Paradox, discussions of the settlement of the Solar System (in the near future) and the Galaxy (in the far future), and METI. We argue that science fiction, especially the campy variety, is a significant contributor to the "giggle factor" that hinders serious discussion and funding for SETI and Solar System settlement projects. We argue that humanity's long-term future in space will be shaped by our short-term visions for who goes there and how. Because of the way they entered the fields, we recommend avoiding the term "colony" and its cognates when discussing the settlement of space, as well as other terms with similar pedigrees. We offer examples of science fiction and other writing that broaden and challenge our visions of human futures in space and SETI. In an appendix, we use an analogy with the well-funded and relatively uncontroversial searches for the dark matter particle to argue that SETI's lack of funding in the national science portfolio is primarily a problem of perception, not inherent merit., 26 pp, 66 footnotes, 1 appendix, International Journal of Astrobiology (2017) First View https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/visions-of-human-futures-in-space-and-seti/FAE7433E55B26DC162DF888F9628558D

M-dwarf stars are more abundant than G-dwarf stars, so our position as observers on a planet orbiting a G-dwarf raises questions about the suitability of other stellar types for supporting life. If we consider ourselves as typical, in the anthropic sense that our environment is probably a typical one for conscious observers, then we are led to the conclusion that planets orbiting in the habitable zone of G-dwarf stars should be the best place for conscious life to develop. But such a conclusion neglects the possibility that K-dwarfs or M-dwarfs could provide more numerous sites for life to develop, both now and in the future. In this paper we analyze this problem through Bayesian inference to demonstrate that our occurrence around a G-dwarf might be a slight statistical anomaly, but only the sort of chance event that we expect to occur regularly. Even if M-dwarfs provide more numerous habitable planets today and in the future, we still expect mid G- to early K-dwarfs stars to be the most likely place for observers like ourselves. This suggests that observers with similar cognitive capabilities as us are most likely to be found at the present time and place, rather than in the future or around much smaller stars., Published in International Journal of Astrobiology

The Zoo solution to Fermi's Paradox proposes that extraterrestrial intelligences (ETIs) have agreed to not contact the Earth. The strength of this solution depends on the ability for ETIs to come to agreement, and establish/police treaties as part of a so-called "Galactic Club". These activities are principally limited by the causal connectivity of a civilisation to its neighbours at its inception, i.e. whether it comes to prominence being aware of other ETIs and any treaties or agreements in place. If even one civilisation is not causally connected to the other members of a treaty, then they are free to operate beyond it and contact the Earth if wished, which makes the Zoo solution "soft". We should therefore consider how likely this scenario is, as this will give us a sense of the Zoo solution's softness, or general validity. We implement a simple toy model of ETIs arising in a Galactic Habitable Zone, and calculate the properties of the groups of culturally connected civilisations established therein. We show that for most choices of civilisation parameters, the number of culturally connected groups is greater than 1, meaning that the Galaxy is composed of multiple Galactic Cliques rather than a single Galactic Club. We find in our models for a single Galactic Club to establish interstellar hegemony, the number of civilisations must be relatively large, the mean civilisation lifetime must be several millions of years, and the inter-arrival time between civilisations must be a few million years or less., 12 pages, 5 figures, accepted to the International Journal of Astrobiology

If advanced civilizations appear in the universe with an ability and desire to expand, the entire universe can become saturated with life on a short timescale, even if such expanders appear rarely. Our presence in an apparently untouched Milky Way thus constrains the appearance rate of galaxy-spanning Kardashev type III (K3) civilizations, if it is assumed that some fraction of K3 civilizations will continue their expansion at intergalactic distances. We use this constraint to estimate the appearance rate of K3 civilizations for 81 cosmological scenarios by specifying the extent to which humanity is a statistical outlier. We find that in nearly all plausible scenarios, the distance to the nearest visible K3 is cosmological. In searches for K3 galaxies where the observable range is limited, we also find that the most likely detections tend to be expanding civilizations who have entered the observable range from farther away. An observation of K3 clusters is thus more likely than isolated K3 galaxies., 11 pages, 3 figures. In press, International Journal of Astrobiology. v2: Discussion added. Figures re-formated. Typos corrected. References added and updated

The history of life on Earth and in other potential life-bearing planetary platforms is deeply linked to the history of the universe. Since life as we know it relies on chemical elements forged in dying heavy stars, the universe needs to be old enough for stars to form and evolve. Current cosmological theory indicates that the universe is 13.7$\pm 0.13$ billion years old and that the first stars formed hundreds of millions of years after the big bang. At least some stars formed with stable planetary systems wherein a set of biochemical reactions leading to life could have taken place. In this lecture, I argue that we can divide cosmological history into four ages, from the big bang to intelligent life. The Physical Age describes the origin of the universe, of matter, of cosmic nucleosynthesis, as well as the formation of the first stars and galaxies. The Chemical Age begun when heavy stars provided the raw ingredients for life through stellar nucleosynthesis and describes how heavier chemical elements collected in nascent planets and moons to give rise to prebiotic biomolecules. The Biological Age describes the origin of early life, its evolution through Darwinian natural selection, and the emergence of complex multicellular life forms. Finally, the Cognitive Age describes how complex life evolved into intelligent life capable of self-awareness and of developing technology through the directed manipulation of energy and materials. We conclude discussing whether we are the rule or the exception., 7 pages, Opening plenary talk delivered at the S\~ao Paulo Advanced School of Astrobiology, S\~ao Paulo, December 2011. In press, Int. J. Astrobio. Reference updated