aa r X i v : . [ phy s i c s . pop - ph ] A ug First Life in the Universe
Ren´e Liseau
Department of Space, Earth and Environment, Chalmers University of Technology,Onsala Space Observatory, SE-439 92 Onsala, Sweden, e-mail: rene . liseau@chalmers . se This presentation is aimed at first year students of physics, but also at the general, scienceinterested, public. It is therefore deliberately kept free from technical details. These canbe found in the references provided in footnotes.The paper is organised in several chapters, some of which might be skipped by theknowledgeable reader. INDEXCHAPTER PAGEShort Summary 31. What this is all about 32. Things we call life 42.1 What is life made of? 42.1.1 Is life restricted to the matter world? 42.1.2 Could it be possible that there exists life made of antimatter? 42.1.3 Could it be possible that there is life in the dark matter world? 42.2 How do we recognise living things? 52.3 What is life doing? 52.4 What is the structural architecture of life? 53. How it all started 53.1 What is the age of the universe? 53.2 The first elements 63.3 The first stars 71.3.1 Population III: low mass stars 73.3.2 Population III: high mass stars 83.4 The Dark Ages of the universe 93.4.1 The standard model 93.4.2 How dark were the Dark Ages and for how long? 104. Coming back to the issue of life 115. Possibilities to detect extraterrestrial life 125.1 The nearby universe 125.1.1 Solar system exploration 135.1.2 Stars and planets and the ISM in the Milky Way 135.2 The distant universe 146. Conclusions 147. Resum´e 15Bibliography 16Appendix 192 hort Summary.
Here, we ask a simple question, i.e. “at what cosmic time, at theearliest, did life first appear in the universe?” Given what we know about the universetoday, there may be some partial answers to this question, but much will still have to beleft to speculation. If life in general requires stars as its primary energy source and useselemental building blocks heavier than those initially produced in a Big Bang scenario,first life could have appeared, when the universe was considerably less than 0.1 billionyears old. At that time, heavy element producing hypernovae exploded at correspondingredshifts z > ∼
45, significantly higher than commonly assumed ( z ≈ z > One of the questions that I have been wondering about for a long time is “When, at thevery earliest, could life have emerged in the universe? And was that a unique event ordid it happen several times, over and over again?”One is generally thinking of life that originated and evolved on Earth. But, couldthere have been occasions in other places and at other times for this to happen? Inother words, at what stage, in the history of the universe, were the absolutely necessary ingredients abound in sufficient abundance and the overall conditions acceptable for lifeto get started? A less restrictive question would be “when were the necessary buildingblocks available” and life would have had the possibility to develop? The latter questionis often used by geocentric-oriented people, with the proviso “life as we know it”.However, this presents us immediately with a problem, leading us into a blind all´e:we don’t know the life as we know it , since we, in spite of numerous serious attempts todo so, have not found a way to unambiguously defining it nor to making it in the lab .One of the major difficulties to really know life in a more general sense stems from thefact that we are dealing with the statistics of one - we are aware of its existence only onEarth. The statistics of one is a contradiction in terms and of course nonsense. But itillustrates the important and well-known fact that we have nothing else to compare withand to learn from, opening the field to enormous speculation.Most often, discussions about life are hampered already at the very start, dependingon what axioms one is equipped with. For instance, a teleological standpoint, based onpersonal belief and conviction, excludes a meaningful communication with other views.Often, the axiom is a divine plan, according to which humans are the crown of creation,fulfilling the plan. With this axiom, explanations for “why?” and “what is the plan?”need not to be offered by its defenders and non-fitting ideas need not to be considered.Another phalanx of debaters represents people that are expressing essentially the op-posite view, i.e. life is a number of random processes without external intervention,involving more or less coincidental accidents. These can lead to the idea of a darwinistickind of evolution, the outcome of which is not pre-determined and, hence, not foreseeable.Here, we will take a rather pragmatic, but non-the-less scientific, approach requiringthat a logical line of thought called theory has to be able to withstand testing throughrepeated experiments or observations. We will try to argue free from prejudice. At the The observed wavelength of a spectral line can produce a shift z = ( λ obs − λ lab ) /λ lab . If the observedwavelength is larger than that measured in the lab, the shift is toward the red of the electromagneticspectrum. z is a directly measured quantity and as such independent of any model. See, e.g., the review by Bich, L. & Green, S. (2018, Synthese 195, 3919)
Is defining life pointless?Operational definitions at the frontiers of biology
First, we need, however, agree on a few assumptions. Even though one has not been ableto provide a satisfactory definition of life, we may still ask
Without supporting and scientifically verifiable evidence for life to exist in an immaterial,spiritual world, we can assume that life belongs to the material world. We may then ask
We are aware of life forms in only one manifestation of ordinary matter. That does notnecessarily mean that life is entirely bound to matter we are generally familiar with. Aslightly irritating fact might be that ordinary matter, like us and the stars, is not by farthe most abundant phase state of gravitating stuff.
Antimatter has the same physical properties as matter, but with opposite electric chargeand, if appropriate, opposite baryon number, or opposite spin. An example is antihydrogen ,i.e. an antielectron (called positron) bound to an antiproton. In addition, some particlesare their own matter and antimatter, such as the photon and some other bosons.In principle, one cannot see any plausible reason, why there should not exist any “an-tilife” somewhere in the universe. We should avoid to shake hands with any hypotheticalantihumans, though. When matter and antimatter meet and bounce into each other, theycompletely annihilate into high-energy radiation ( γ -ray photons).However, the chances to encounter or even detect antilife may be rather slim, asthe observable universe consists of an overwhelmingly larger amount of matter, withantimatter being highly elusive. Until this day, this matter-antimatter imbalance hasremained one of the biggest mysteries of the universe. The question, whether life could conceivably also has manifested itself in the dark matter (DM) world appears highly academic. However, the way by which DM has been definedwould make it highly unsuitable for chemical reactions, as it does not interact in anyother way than gravitationally . However, in contrast to antimatter, we do not have aclue what DM is, in spite of the fact that dark matter stands apparently for about 85%of all gravitating matter in the universe. So, we need to leave this question open. Oelert W., for the PS210-Collaboration (1997), Nuc.Phys. B, Proceedings Suppl., 56, 319:
Productionof anti-hydrogen at CERN Possible interactions between baryonic and dark matter have been speculated about by Bowman etal., 2018, Nat., 555, 67,
An absorption profile centred at 78 megahertz in the sky-averaged spectrum , andreferences therein. .2 How do we recognize living things? We all have a clear idea about what non-living things are, including those that once werealive but now are dead. We have an instinctive understanding of what life is, but havedifficulties to phrase this insight into strictly defining words. Maybe, one way to go aboutit, is to take an operational path, asking a different question, viz.
Living individuals eat, digest and excrete, i.e., they metabolize. However, this wouldalso be applicable to a car engine, which we would not call a living thing: it is made ofordinary matter, consumes fuel, transforms that nutrition to energy in the form of heatand expels gases through its exhaust. And, like life, it ages and in the end, it dies.It is clear, therefore that the points made above are necessary, but obviously notsufficient, conditions for life. However, living organisms do also something else, i.e., theybuild self-replicating entities and produce offsprings. A car engine does not do that, buton the other hand, crystals show growth patterns and make copies of themselves. Theytoo, we feel, are non-living things. In summary, the doings of life seem not to be unique.
It is reasonable to assume that life follows universal recipes and patterns that have beenapplied throughout the history of the universe, i.e. the building from smaller to largerthings in a hierarchical fashion, from quarks to nucleons to atoms to molecules and so on,up to superclusters of galaxies. On Earth too, life forms developed from the simplest tomore complex aggregates of matter. Made of biomolecules, these display an interesting(and unexplained) feature: as building blocks, life on Earth uses exclusively homochiralmolecules, i.e. molecules with the same “handedness” (chirality) .In addition, all life on Earth seems confined, in cell-like “bags”, rather than beingdistributed, like in a gas or fluid . It is not clear, to what extent this is a general propertyof life. It is also not clear, whether life always needs a planet or whether there exist(ed)life forms in the vast volumes of space in between the stars, in the so called interstellarmedium (ISM). When addressing the physicochemical possibility for the first appearance of life, we needto know when to start the clock. A first rough estimate would be given by the inverse ofthe Hubble constant, H , which measures the current expansion of the universe, and where1 /H has the units of time. The actual value of H has become a hotly debated issue. e.g., L (left-handed) for amino acids and D (right-handed) for sugars; see articles in: Lough, W. J.,& Wainer, I. W., eds., 2002, Chirality in natural and applied science , Oxford: Blackwell Science. Hypothetical alternatives can be found in, e.g., the books “The Black Cloud”, by Fred Hoyle (1957)or “Solaris” by Stanislaw Lem (1961). and the late universe, two values have been foundthat differ by more than a billion years, viz. 1 /H = 14 . . H are based on three studies that are exploiting completelydifferent methods and are entirely independent of each other. To determine the actual ageof the universe requires the use of a particular theoretical, or cosmological, model. In fact,this discrepancy in the values of H may have profound implications for our understandingof the universe, and hence require the development of complementary or even alternatecosmologies .However, for the currently widely accepted ΛCDM cosmology (Λ Cold Dark Matter),one derives an age of 13.8 Gyr in the case of the “early time”, and of 12.6 Gyr in thecase of the “late time” measurement . Obviously, at most one of these numbers can becorrect. The younger age seems to be in conflict with ages that have been quoted in theliterature for the oldest stars in the Galaxy .As our interests lie in the early phases of the universe, throughout this paper andunless otherwise stated, the value of H derived by the Planck-collaboration will be used. The success of the Big Bang model has largely been based on its feat to correctly reproduceobserved abundances of the elemental species . In particular, the Big Bang synthesizedmerely the lightest nuclei with the number of protons and neutrons A ≤
7, i.e. H, He, Liand their isotopes. After these first few minutes ( ≈ µ yr) of the explosion, the extremelyrapid expansion resulted in the dramatic decrease of the density and the temperature to From the analysis of data from ESAs Planck mission the value of H = 67 . ± . − Mpc − hasbeen derived (The Planck collaboration, 2019, arXiv:1807.06209v2, Planck 2018 results. VI. Cosmologicalparameters ; ESA stands for European Space Agency). This results in a Hubble time 1 /H = 14 . ± . The Atacama Cosmology Telescope: DR4 Maps and CosmologicalParameters ) yielding H = 67 . ± . − Mpc − . The CMB (Cosmic Microwave Background) is therelict radiation from the hot Big Bang at the beginning of the universe. The photons of the CMB wereceive today originated when the universe was about 470 thousand years old. Observations of standard candles (Cepheids and Supernovae type Ia, Riess et al. 2019, ApJ 876, 85)on one hand and of gravitational lenses on the other (Wong et al. 2020, MNRAS, arXiv:1907.04869v2)in the local universe ( z < ∼
2) have resulted in the combined value H = 73 . ± . − Mpc − , i.e.1 /H = 13 . ± . Bernal J.L., Verde L. & Riess A.G. (2016, JCAP 019, 1) discuss
The trouble with H at considerabledepth. Since the appearance of their article, the discrepancy has become even more severe: Riess A.G.2019, Nat. Rev. Phys., The expansion of the Universe is faster than expected.
Current ideas of how toresolve the H issue is to introduce further ad hoc assumptions into the existing models. For the redshift-age relation, τ (Ω m , Ω Λ ; z ), we adopt the integrated Friedman equation by Thomas &Kantowski (2000, Phys. Rev. D 62, 103507, Eq. 18), i.e. valid for a Friedman-Lemaˆıtre-Robertson-Walkermetric with Ω = Ω Λ + Ω m = 1, Ω Λ = 0 and Ω κ = 0. The last term describes the geometrical flatnessof the universe, the value of which, on the basis of Planck data, has been challenged in an article by DiValentino E., Melchiorri A. & Silk J. (2020, Nature Astronomy, 4, 196). The CMB stems from the time when matter and photons decoupled, at redshift z ≈ z ≈
2; e.g. Smith etal. 2018, ApJ 854, 37. See, e.g., Howes et al. 2015, Nat. 527, 484 and https: // en . wikipedia . org / wiki / List of oldest stars and to predict (R. Alpher, 1948, arXiv:1411.0172v1) the existence of the isotropic background radi-ation (CMB), that was serendipitously discovered in 1964 by Penzias and Wilson (1965, ApJ 142, 419).The discovery was theoretically explained by Dicke et al. (1965, ApJ 142, 414) in the same journal. . This prevented the nucleosynthesis of heavierspecies, the accomplishment of which would need energetic nuclear burning inside stars. After the initial Big Bang nucleosynthesis was completed, most of the matter in theuniverse was in the form of hydrogen nuclei, with slightly less than a quarter of the massbeing in the form of helium nuclei . In addition, the remaining species accounted foranother small fraction.Stars with masses around that of the sun (M ⊙ ) convert hydrogen to helium by predom-inantly the so called proton-proton cycle. In the pp-cycle, four hydrogen nuclei combineinto one helium nucleus (4 p → α ). Above about 1.3 M ⊙ , the so called CNO cycle dominates the helium production, but also this process converts four H-nuclei into oneHe-nucleus. In all fusion processes, a number of other particles are also produced thatcarry away some part of the generated energy. The total energy released through fusionis the binding energy difference of the involved species.The great majority of the observed stars are stars that produce most of their energythrough fusion of hydrogen. Because of that, their basic physics is the same and applyto all of them and, consequently, they constitute what is called the main sequence (MS)of stars . They are stable and change only relatively slowly, producing energy overconsiderable amounts of time. However, their exact lifetimes are critically depending ontheir mass, with stars of lower mass living longer.After the MS phase, the fusion of helium nuclei yields primarily carbon via the triple-alpha process (3 α → C). By fusing with He-nuclei, the α -process proceeds after carbonto produce O, Ne and so on, i.e. predominantly nuclei consisting of multiples of fourwith even numbers of protons.
In roundish numbers, the MS-life of the sun is 10 billion years. To order of magnitude,the ages of stars of various masses can be obtained from t MS = 10 /M m yr, where m=3for solar masses M = 0 . M = 0 .
1) is thus ten thousand billion years, a thousand times the age ofthe universe ! In contrast, a star only three times as massive as the sun lives for only370 million years . For still higher masses, the time dependence becomes less steep, sothat, e.g., m(10) = 2 . ≥
30) = 2, resulting in MS-lifetimes, respectively, of about30 million and less than 10 million years. The latter is comparable to the evolutionaryhistory of mankind.It becomes evident, that stars lighter than the sun and which were born at the earliesttimes of the universe, should still be around, lingering on the main sequence. In favor-able cases, some of them may be directly observable by us and provide us with valuable In the early, radiation dominated, universe, the temperature drops approximately as the inverse ofthe scale factor, T ∝ /a , hence as t − . . The scale factor is related to the Hubble parameter H ( t ) = ˙ a/a ,with the definition a = 1 for H ( t ) = H . The standard model primordial value of the helium abundance is Y p = 0 . ± . CNO stands for Carbon C, nitrogen N and oxygen O. These act merely as catalysts and do notcontribute to the nuclear energy generation. When plotting the stellar surface temperature against the radiated power, the data lie in a relativelynarrow band across this so called Hertzsprung-Russell diagram (HR diagram). . Concepts becomehere a little fuzzy and some astronomers term these stars pop III, others reserve thisepithet for the truly very first stars. So far, searches for zero-metallicity stars in theGalaxy have been unsuccessful and fruitless. On the main sequence, hydrogen fusion results in the build-up of a helium core at thecenter of the stars. After the MS life, stars evolve rather rapidly and expel, by meansof stellar winds or supernova explosions (stars heavier than about 8 M ⊙ will end theirlives as supernovae), their nuclear ashes into space. That material then can give rise tonew generations of stars, which are made of progressively more enriched elements. Thesun, for instance, being born nearly ten billion years after the Big Bang, shows elementalabundances that are much higher than the primordial ones.According to the theory of stellar evolution, stars with masses larger than the sunare generating their thermonuclear energy running the CNO-cycle. In order to ignite theinitial steps of this process, appreciable amounts of carbon as catalytic and nuclear fuelare needed. In the very early universe, however and since carbon is produced only afterthe main sequence phase, stars of masses lower than a certain mass will not have hadtime to evolve off the MS.Following the Big Bang nucleosynthesis, stars that are made of only hydrogen andhelium are difficult to form. In order to accomplish that, stars have to be extremelyheavy, maybe up to 1000 M ⊙ , which is much larger than the highest masses of currentlyobserved stars ( <
100 M ⊙ ).The evolution of Pop III (“zero metallicity”) stars of several hundred solar masses hasbeen computed using detailed theoretical models . For masses between 120 and 500 M ⊙ ,these models yield MS lifetimes between 2.35 and 1.74 million years. However, accordingto models of the post-MS evolution of pop III stars, stars of masses above 260 M ⊙ willdirectly collapse into black holes . This might have generated a pool of numerous massivestellar black holes, yet to be discovered.For MS masses in the interval 70 - 260 M ⊙ , pair-instabilities lead to violent supernovaeruptions (hypernovae) with energies in excess of 10 erg. Only recently has such anenergetic hypernova been reported in a metal-poor dwarf galaxy at redshift z = 0 . z ≈ [Fe/H] = − . ± .
2, for an 1d LTE model; Nordlander et al., 2019, MNRAS 488, L109. Aside fromiron, the spectrum of this particular star displays also trace amounts of other heavy elements (C, Ca, Mgand Ti), indicating that these were formed in a supernova of stellar progenitor mass of ∼
10 M ⊙ . Baraffe, Heger & Woosley, 2001, ApJ 550, 890 Heger & Woosley, 2002, ApJ 567, 532. see the review by Woosley, Heger & Weaver 2002, RevModPhys 74, 1015 SN2016aps: Nicholl et. al. 2020, arXiv:2004.05840, Nature Astronomy, 13 April 2020; pair-instabilitysupernovae do not produce r-, s- or p-process elements (see Meyer B.S. 1994, ARAA 32, 153), nor speciesheavier than Zn. . So, within less than10 million years ( z ≈ . These clouds would need to have fragmented intoa few hundred smaller and denser pieces, corresponding to the seeds of the heavy Pop IIIstars.Less than 10 million years would thus constitute the earliest time at which the universecould have been enriched in heavy elements by means of supernova mass ejections, whilethe less massive SN-progenitors would still be spending time on the main sequence andexplode later. Nature’s experimenting to generate systems that might eventually becomeliving could have started already then. There is, however, a little aber with the scenario outlined above. Most people are familiarwith the fact that gas that is compressed heats up, increasing its pressure build-up andresisting further compression. Something, i.e. some process, would have to get rid of theexcess heat, so that the gas could be further compressed .In the standard model, one is following the matter, i.e. the baryonic matter (BM),which, during the period of recombination, only slightly earlier had gone from a completelyionized to an essentially neutral state. At that stage ( z ≈ ≈
470 thousandyears), the temperature of the adiabatically expanding universe is about 3000 K, with allthe hydrogen and helium in their respective electronic ground states.The CMB consisted of photons with wavelengths around 1 µ m and these hardly inter-acted with the gas in the clouds. Photons with wavelengths shorter than at least ten timesthe ones of the CMB would have been required to do anything about this situation. Onlythese much more energetic photons would have been able to transport the heat away. Insummary, it is believed that atomic neutral gas, consisting of hydrogen and helium (H Iand He I), cannot do the trick. That is, there would not have been luminous things likestars in galaxies illuminating the universe, hence the universe would have appeared darkto the human eye.A number of scientists have developed a way out of this dilemma. If one instead ofatomic gas uses molecular gas, the heating problem could become largely alleviated. Forinstance, the hydrogen molecule H has energy levels that correspond to wavelengths ofsome micrometers ( µ m), i.e. in the energy regime of the CMB at that time. Within anorder of magnitude, the formation time of H is t (H ) > ∼ yr / ( n H cm − ). This very slowbuild-up of sufficient amounts of coolants resulted in essentially no radiation of opticalphotons for extended periods of time, hence the term “Dark Ages” . Adopting H = 67 . − Mpc − in a standard flat ΛCDM cosmology, 2 million years after the BigBang correspond to a redshift z = 420, temperature T CMB = 1150 K and density n (H) = 100 cm − . In cosmology, these clouds are called regions of overdensity (see, e.g., Herrera, Waga & Jor´as 2017,PhysRevD, 95-6, 4029). In models, an overdensity parameter is introduced, commonly ad hoc with thevalue δ c = 200, but other values have also been used. The density of the overdense region is ρ ( t ) = δ c × ρ c ,where ρ c = 3 / (8 πG ) × H ( t ) is the critical density. The distribution of the overdense regions is assumedto be Gaussian. Isothermal self-gravitating clouds with density contrasts between centre and outer edge ρ /ρ out ≥ . Gould R.J. & Salpeter E.E. 1963, ApJ 138, 393; the formation efficiency is believed to be within therange 0 . − For a review, see Miralda-Escud´e J. 2003, Sci. 300, 1904.
9. Palla and collaborators have made detailed models of H formation in the earlyuniverse, including a reasonably large number of chemical reactions . According to thesemodels, an appreciable fraction of H has formed at z ≈
300 [3.3 Myr, 820 K, 35 cm − ] andreached equlibrium at z ≈
100 [17 Myr, 275 K, 1 cm − ]. These models contained alreadyin their initial set-up molecular “impurities”, as the simplest reaction, i.e. ground stateH + H → H + hν , exceeded the age of the universe and would not have been viable .In the presence of dust, H formation can be speeded up considerably . The afore-mentioned models had not any dust included.So, about 20 million years after the Big Bang, the evolution toward unstable self-gravitating entities could have begun. Free-fall times would be some 40 million years,i.e. the first stars could have lightened up within less than 100 million years. However,theoretical models of structure formation need considerably more time to make anythinglike stars, putting that into the redshift interval z ≈
20 to 10, which corresponds to 400million to 1 billion years. In these models, the main actor for the gravitational pulling-together is the dark matter (DM), which does not care much about any cooling radiation,as dark matter does not interact with photons. The ordinary matter (BM) of the becomingstars is thought to have been dragged along, taking a piggy-back ride on the dark matter.
Once the first stars had formed, their energetic radiation was re-ionizing the universe.The subsequent recombinations of the protons and electrons gave rise to optical photons,lightening up the universe again. According to the Planck Collaboration , the mid-pointredshift of this recombination period was z rec = 7 . ± .
7, corresponding to the age of700 ±
100 million years .It has been realized that supernovae can produce copious amounts of dust . If therehad been supernova explosions early on in the the universe, the evolution of the matteruniverse would have been different from what is now the established view. But it is alsoclear that our understanding of the evolution of the universe, after the decoupling ofmatter and radiation, depends critically on what the dark matter (DM) was doing, bothbefore and after. Obviously, being ignorant about the true nature of DM, any assumptionsconcerning its physics must remain rather speculative .To summarise, we can identify at least two, more or less well-founded, ideas regardingthe history of the first stars in the early universe. According to the widely establishedstandard model, the first stars did not form before the universe was almost a billion yearsold. On the other hand, if the Dark Ages had been very much shorter, or did not really Palla F., 1999, in: Proceedings of Star Formation 1999, held in Nagoya, Japan, June 21 - 25, 1999,ed. T. Nakamoto, Nobeyama Radio Observatory, p. 6-11, and references therein. My friend Francescopassed prematurely away in 2016. see, e.g., Latter W.B. & Black J.H., 1991, ApJ 372, 161. These authors address this reaction byassuming that one of the hydrogen atoms is in an excited state ( ≈
10 eV). see the review by Wakelam V. et al., 2017, Molecular Astrophysics 9, 1. The Planck collaboration, Planck 2018 results. VI. Cosmological parameters, arXiv:1807.06209v2.For z = 7 .
7, I find T CMB = 24 K, n (H) = 8 . − cm − and t ff = 1 . Harikane Y. et al. (2020, arxivv: 1910.10927v3) discover a large population of galaxies at z >
6. Theredshifts were determined from spectral lines of carbon and oxygen. Matsuura et al. 2011, Sci 333, 1258:
Herschel Detects a Massive Dust Reservoir in Supernova 1987A The pro s and con s of current dark matter theories, including the plethora of proposed carriers,some sort of astro-particle, are discussed by Ostriker J.P. & Steinhardt P. 2003, Sci. 300, 1909,
NewLight on Dark Matter . The authors also discuss the concept of Dark Energy, the identity of which isequally unknown, although assumed to be some sort of homogeneous “fluid”. Unlike Dark Matter, whichis gravitationally self-attractive, Dark Energy is gravitationally self-repulsive, being the driver of thecurrently accelerated expansion of the universe. . This interval cor-responds to the time during which CMB-temperatures were between 100 ◦ and 0 ◦ C, i.e.the temperatures of water on a rocky planet in the liquid phase. Clearly, the idea of lifebeing of the terrestrial kind has been implicitly assumed.The ever improving sensitivity of the observations of remote galaxies, due to largertelescopes and more sophisticated detectors, has widened our horizons since the time ofEdwin Hubble to increasingly larger redshifts, which at the moment of writing correspondsto z >
9. In the context of the present paper the most important point is that the redshiftof this galaxy, MACSJ 1149-JD1, has been determined from the observation of a spectralline of oxygen . This implies that at such high redshifts generations of supernovae hadalready produced significant amounts of this non-primordial element. One of the prime arguments for the possible ubiquity of life has been that life on Earthuses the universally most abundant elements as its basic building blocks, i.e. H, O, Cand N: carbon based chemistry in water solution together with nitrogen (for, e.g., theamino acids). It may appear odd therefore that the terrestrial type of life is composed ofthe least available form of matter, viz. of the “ordinary” (baryonic) matter contributingmerely 4% to the total energy budget, which is dominated by Dark Energy (73%) andDark Matter (23%).On the other hand, in highly complex structural forms, such as terrestrial DNA, thelow-abundance element of phosphorus is incorporated. Like oxygen, that element toorequired supernova explosions for its production . In the biomolecular literature, therehas been a vivid discussion regarding as to what extent important molecules, used by lifeon Earth, could be replaced by others that are very similar in their functionality. A primeexample would be the possible substitution of 15th column phosphorus by dito arsenic .This has actually been observed in some bacteria on Earth, but, in the end, did not gainthe winning hand: phosphates are vastly more exploited by living organisms on Earththan arsenates . In addition, arsenic is more than a thousand times less abundant thanphosphorus; so, in this case, nature actually picked the more plentiful one.In summary, for arriving at universal beginnings and evolutions of life, large relativequantity seems not to be of primary importance. Loeb A. 2014, Int. Journal of Astrobiology 13, 337:
The Habitable Epoch of the Early Universe . At z + 1 ≈ − > ∼ σ ) in the probability distribution of the initial overdensity regions. Hashimoto T. et al. (https://almascience.eso.org/alma-science/science-highlights-highz-oxygen) re-port the discovery of a galaxy at z = 9 . z ≈
15, i.e. about 250 million years after the Big Bang. Except helium, which is a noble, i.e. chemically non-reactive, species. Nitrogen is less abundant thanneon and iron, being the seventh most abundant element in the Milky Way. Cescutti C., Matteuchi F. Caffau E. & Franois P. (2012, A&A 540, A33)
Chemical evolution of theMilky Way: the origin of phosphorus Knodle R., Agarwal P. & Brown M. (2012, Biomolecules 2, 282)
From Phosphorous to Arsenic:Changing the Classic Paradigm for the Structure of Biomolecules . Elias, M., et al., 2012, Nat, 491, 134,
The molecular basis of phosphate discrimination in arsenate-richenvironments Possibilities to detect extraterrestrial life
This chapter will be deliberately kept brief, since there exists a very large body of modernliterature to which the reader is referred (see below). In general, that literature coversaspects of astrobiology, biochemistry, geology and the physics of the habitats of exoplan-etary and terrestrial life. Here, we will also address some topics that are generally notcovered by the conventional literature.
Not having a clear definition of the concept of life makes searching for it difficult, notknowing what to look for. However, one giant thought leap was taken when it wasproposed already early on to look for, not simply bacterial but what is called “intelligent”,life with the SETI project . SETI represents the anthropocentric vision that life, givenenough time, develops beings similar to ourselves . SETI has mostly been listening to,but occasionally also been sending, radio messages at well defined wavelengths. Theunderlying assumption is that every technically advanced civilisation will know about the21 cm (1.4 GHz) line of hydrogen . And use it for interstellar communication. So far, noconvincing result has been obtained.A couple of comments regarding the universality of the 21 cm line may be in orderhere. Firstly, our status of technically advanced civilisation is still in its infancy, meaningthat these hypothetical others may be more developed and might have been using otherchannels of communication . This is like comparing smoke signals with a blanket tohigh speed broad band communication through optical fibres and/or with global satellitesystems. Most people are not looking for smoke signals and hence would not be aware thatsomebody might want to talk to them. Conversely, the smoke signaller would be entirelyignorant of the ever ongoing intercontinental telephone conversations and broadcastings.In the process of smoke signalling, some puffs might have become distorted, say by thewind, and a message saying “dinn atf seabnd brin trump” may be difficult to interpret.Which leads to the basic problem of communicating, i.e. to be able to convey an under-standable message. Even when it is undistorted, humans have not been able to do so withvarious species even on their own home planet. In addition and like on Earth, differentspecies may live in separated and completely different environs. Terrestrial examples are The Astrobiological Copernican Weak and Strong ) who attempt toestimate the number of such civilisations (sic!), where their case is built on a stereotypical parallel to thehuman history. But recall the example of the sharks, which for nearly 400 million years haven’t changed that much. The wavelength of 21 cm corresponds to the energy difference of the hyperfine states ( F = 1 and F = 0) of the H I 1 s ground state. This spin flip transition has a lifetime of nearly 11 million years( A ≈ × − s − ), which, because of its implied low absorption probability, would make it verysuitable for interstellar communication. The line was discovered by Ewen H.I. & Purcell E.M. 1951, Nat.168, 356: Observation of a Line in the Galactic Radio Spectrum: Radiation from Galactic Hydrogen at1,420 Mc./sec Instead of photons, using e.g. neutrinos, which are travelling close to the speed of light, and havingrest masses P m , i c < .
12 eV ( m < × − g) for all three flavours i (e-, µ -, τ -neutrino; PlanckConsortium 2018). In quantum physics, elementary particles exhibit wavelike behaviour and their wave-length is called the de Broglie wavelength, λ dB = ( h/m υ ) p − ( υ/c ) . Here λ is the wavelength, h isPlanck’s constant, υ is the speed of the particle and c that of light, and m is the particle’s rest mass.The shorter the wavelength (the higher the frequency) the more information can be transmitted. Sinceneutrinos travel close to the speed of light, the neutrino wavelength would be significantly shorter than21 cm. In addition, neutrinos have a very small absorption cross section (Bethe & Peierls 1934, Nat. 133,532): it needs a column of water that is 500 pc long to make sure to catch one. The Search for Life in the ComingDecades, An Astrobiology Strategy for the Search for Life in the Universe .Top priority is the search for signs of past or present liquid water as a proxy for life(in different physical conditions and for diverse chemistries, methane or ammonia, forinstance, could perhaps also do the job).
Current plans for future observations build on the conditions for terrestrial life and arebased on “what life is doing”, i.e. to tracing signs of biotic influence on the planets thatlife is inhabiting. On Earth, life forms have apparently transformed the archaic oxygenpoor atmosphere to the oxygen rich one of today .In particular, one is aiming at searches for biomarkers toward exoplanetary atmospheres .Currently, much effort is devoted to the finding of a twin of Earth, i.e. more or less anexact copy of this particular - our - planet. The underlying assumption seems to bethat life could develop only in terrestrial conditions throughout the history of the Earth.Specifically taken into consideration are, of course, the abundant existence of liquid water,the obliquity of the planet’s rotation axis, the presence of the large moon, the dynamicsof surface plate tectonics including volcanism, radioactivity and the interior generation ofthe global magnetic field and so on.In addition, during the history of the Earth, viz. on time scales of billions of years,these conditions had changed profoundly. What regards life, the planet has undergoneseveral documented mass extinctions, when nearly all multicellular organisms had beenwiped out . In spite of this, new life forms did emerge and continued to strive anddeveloped further. Apparently, in the long run, life is stronger than death.A considerable number of large molecules have been discovered in the dense interstellarmedium , with emphasis on regions that are protected against the harmful ultravioletradiation from the stars. Of special interest is the detection of prebiotic molecules , in Grenfell J.L. et al. 2010, AsBio, 10, 77:
Co-Evolution of Atmospheres, Life, and Climate . Inparticular regarding oxygen, see: Meadows V.S. et al. 2018, AsBio, 18, 630,
Exoplanet Biosignatures:Understanding Oxygen as a Biosignature in the Context of Its Environment Kaltenegger L. 2011, Proc. IAU Symp 280, 302, The molecular universe, eds. J. Cernicharo & R.Bachiller:
Biomarkers of Habitable Worlds: Super-Earths and Earths . more than 75% to 90%, see, e.g., https://en.wikipedia.org/wiki/Extinction event McGuire, B. A., 2018, ApJS, 239, 17, see, e.g, McGuire, B. A., Carroll, P. B., & Garrod, R. T., 2018, ASPC, 517, 245, Prebiotic Molecules Joergensen, J. K., Favre, C., Bisschop, S. E., Bourke, T. L., van Dishoeck, E. F., & Schmalzl, M., . However, without in situmeasurements, the risk of false positive results will always be present. High z -experiments regarding the detection of life in the early universe have not beenconsidered in earnest: the technical difficulties aside, we have to remember that any beingsof animate material would have lived there billions of years ago. In addition, observingredshift distributions of life would strongly indicate that life originates whenever theconditions are right.Indirect, and non-unique, evidence may be obtained from observations of star forma-tion beyond some thresholding redshift, say z ts > ∼
20, when the universe would have beenless than 180 million years old .Another, perhaps very remote, possibility could come from a very speculative idea.Assume that an ancient civilisation had reached a high level of understanding of the natureof dark matter. Assume further that DM is, as one generally suspects, made of highlynon-interacting heavy particles and that this civilisation had learned how to produce andto control these. Speculating that the DM beams would be kept better collimated overlong distances than laser beams, they might have been used for cosmic communication.We would not be aware of it .Because of their exotic status and our ignorance regarding dark matter and darkenergy these speculations may provide yet another set of solutions to the Fermi paradox . In an attempt to understand the origin(s) of life in the history of the universe, I have beenquite naively concentrating on what might have been possible and what not. I tried toavoid as much as possible to lean too much on our experience of life on Earth. Similarly,I tried not to favour (or to disfavour) any particular cosmological model. The one I haveused throughout, I picked essentially for calibration purposes: anyone can recalculate the Handbook of Exoplanets , eds. Deeg H.J. & Belmonte J.A., Springer, https://doi.org/10.1007/978-3-319-55333-7 Bowman et al., 2018, (Nat., 555, 67,
An absorption profile centred at 78 megahertz in the sky-averaged spectrum ), attributed an observed broad absorption trough, centred at 78 MHz and 19 MHzwide (FWHM), to the 21 cm line at 20 > z >
15. The frequency is given by ν lab = 1420 / ( z + 1) MHz. According to Vega, Salucci & Sanchez (2012, New Astronomy 17-7, 653), cold DM particles havea rest mass in the range 1 to 2 keV. Being by definition non-relativistic (“cold” means “slow”), theytravel at speeds below 10% of the speed of light, and their de Broglie wavelength would be λ dB > ∼
80 ˚A.The extremely different mass scale of m a ≈ − eV was considered by Marsh & Silk (2014, MNRAS,437, 2652, A model for halo formation with axion mixed dark matter ), which illustrates the difficulty toidentify the nature of dark matter. Dark energy (DE) comprises 68% of all the energy density in the universe, with normal matter (BM)contributing just 5%, with the rest being DM. Dark energy is introduced by Λ in Einstein’s field equationsthat counteracts gravity. For some inexplicable reason, Λ began to dominate at z ≈ .
7, about 6.5 billionyears ago. Why? See, e.g., https://en.wikipedia.org/wiki/Fermi paradox “where is everybody?” .Broadly speaking, one could imagine three scenarios for the first emergence of life inthe universe: (A) a very early epoch, (B) an early epoch, (C) an archaic terrestrial epoch.Requiring baryonic building blocks, these can briefly be summarised as (A) a very early epoch, age of the universe less than 0.1 billion years. Life originated atthe very first instance it had a chance to do so. Structure formation occurred in conditionsof high non-Gaussianity and the formation of life progressed after recombination. Theduration of “darkness” was very brief. Tests of this hypothesis would include evidencefor the very early existence of massive Pop III stars, at redshifts z ≫
10 (e.g. detectionin distant galaxies of Ly α at wavelengths considerably longer than 1.3 µ m with a GiantInfrared Space Telescope Array); (B) an early epoch, age of the universe about 1 billion years. The basis for this case isthe widely adopted standard cosmology. In that model, life might have originated afterre-ionisation, which is defining the end of the Dark Ages. Again, with regard to life,tests would be indirect and should be able to provide evidence that there are no stars ingalaxies at redshifts much larger than six ( z ≪ (C) an archaic terrestrial epoch, age of the universe approaching 10 billion years. Lifeoriginated first on Earth, once the planet had cooled to sustainable temperatures (aboutone billion years after formation). This is the opinion shared by many, but could be dis-proved by finding evidence for life elsewhere, either as relics or as signs of present activity.The possibility of panspermia exchange within the solar system could prohibit a con-clusive answer, unless molecular chiralities are different in different locations. Another,more stringent, hypothesis disproval would therefore have to come from extrasolar envi-ronments. An idea is to use biomarkers and to look for chemistry out of equilibrium .Maybe, it could be shown that life is the most probable driver and maintainer of thenon-equilibrium. This constitutes, in fact, the basis of recent proposals for large tele-scopes/arrays in space of the relatively near future. In the end, I was not able to provide a unique and definite answer to the question “whendid living organisms first emerge?” A couple of suggestions have been offered, none ofwhich might be true. These differ by a factor of a hundred in time and are based on theconventional assumption that life needs energy sources that generally take the form ofstars. And that life if thriving in the surrounding of these stars. The stars are made ofbaryonic matter.Only recently were we caught by complete surprise, when we had to realise that thereis something else than “the matter as we know it”. And this something, dark matter anddark energy, is ruling the universe. What will be the next big surprise? On a cosmological clock, Big Bang occurs at t = 0 and times refer to “standard” ΛCDM cosmologywith H = 67 . − Mpc − ( ρ c , = 8 . × − g cm − ), Ω = Ω Λ , + Ω m , = 1, Ω m , = 0 .
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The evolution andexplosion of massive stars ppendix A rough time line for events that might have had relevance for the development of life in the universe.The numbers refer to the age of the universe according to the cosmology referred to in the text.
370 thousand years ( z = 1300 ) : the ionized plasma recombines into neutral matter. Beginning of theDark Ages (standard theory).
470 thousand years ( z = 1100 ) : is as far back as we are able to observe the universe by means of thecosmic microwave radiation (CMB). Ordinary (baryonic) matter (BM) has already been more importantthan radiation since the age of 40 thousand years. z = 180 ) : is the age at which the very first stars might have formed (H I + DM). Thesewould have been massive (several hundred times the mass of the sun) and exploded within less than2 million years after their birth as hypernovae, sheding highly enriched material into the surroundingmedium.
10 million years ( z = 136 ) : dark matter (DM) begins to become more important than BM. The CMBtemperature is 100 ◦ C.
17 million years ( z = 99 ) : molecular hydrogen has formed, allowing the cooling of the gas throughvibrationally excited H . The CMB temperature is 0 ◦ C.
60 million years ( z = 45 ) : possibly, formation of the first stars (with H ). Their UV radiation wouldhave re-ionised the universe in an early, non-standard, ending of the Dark Ages.
700 million years ( z = 7 . ) : midpoint, within 100 million years, of the Dark Ages, according to thestandard model (ΛCDM cosmology).
900 million years ( z = 6 ) : the end of the Dark Ages, with the first stars re-ionising the universe(standard theory). z = 0 . ) : dark matter (DM) begins to become more important than “ordinary”matter (BM). z = 0 . ) : formation of the solar system and the birth of Earth. z = 0 . ) : humans write the history of the universe, as they understand it (or not).: humans write the history of the universe, as they understand it (or not).