Matteo Cantiello edited But_let_s_forget_intelligence__.tex  almost 8 years ago

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But let's forget intelligence for a second and just focus on the likelihood of Life\footnote{Life as we know it, that is life involving carbon-based chemistry in liquid water} in the Universe.  In a paper \href{http://arxiv.org/abs/1606.08448}{paper}  just posted on the arxiv, Avi Loeb (Harvard), Rafael Batista and David Sloan (Oxford), try to calculate what is discussed  the relative probability of Life emerging in the Universe as function of time. This is an interesting question, as is,  inthe standard cosmological model (called ``$\Lambda$CDM'') our Universe is only about 13 billion years old and expected to exist and expand for  a much longer (infinite?) amount of time. So the number of stars that are yet sense, similar  to be born ask the question ``What  isvery large. Not only, it turns out that  the most common type likelihood  of star in the Universe is not being born on Earth on  a star like the Sun, but one about 1/10th its mass. Also, these very-low mass stars are much dimmer: They burn their candle very slowly and live about 1000 times longer than the Sun. Thermonuclear reactions allows our Sun to shine for about 10 billion years. A star 0.1 times the mass certain year?'' assuming some knowledge  of the Sun lives for 10 trillion years. number of humans alive as function of time.  This is an interesting question, as in the standard cosmological model (called ``$\Lambda$CDM'') our Universe is only about 13 billion years old and expected to exist and expand for a much longer (infinite?) amount of time. So the number of stars that are yet to be born is still large.  Obviously, even in an eternal Universe the amount of gas available to form stars is limited. In an astrophysical sense, we recently passed the baby boom phase: The peak in star formation happened a few billion years ago, and the cold gas in galaxies in expected to be effectively exhausted in about 20-30 billion years or so from now. Long story short, it is possible to count how many stars have been born so far and what is their mass, and also how many more will be born in the future.  Now, it turns out that the most common type of star in the Universe is not a star like the Sun, but one about 1/10th its mass.  The particularity of these very-low mass stars is that they are much dimmer: They burn their candle very slowly and live about 1000 times longer than the Sun. Thermonuclear reactions allows our Sun to shine for about 10 billion years. A star 0.1 times the mass of the Sun lives for 10 trillion years. This offers plenty of opportunity for life to emerge in the future around stars smaller than the Sun. These stars will still be shining when no more star formation will be occurring in the Universe.  Putting all this information together It then seems obvious that, assuming these low-mass stars provide similar environment for life as our Sun, the relative probability of life-emergence has to peak in the far future. This is the main result of Loeb and collaborators.  The results is particularly nice since it is relative: It doesn't state how many planets hosting life are present at a certain time. It just calculates the relative probability as function of time.  can As the authors point out, ``The question is then, why do we find ourselves orbiting a star like the Sun now rather than a lower mass star in the future?''  Putting together this information with the rate   Avi Loeb is not new at writing interesting papers on the likelihood of ``life as we know it'' in the Universe. Together with Rafael Batista and David Sloan from Oxford, Loeb just posted on the arXiv a very interesting article titled ``The relative likelihood of Life as a Function of Cosmic Time''.   In this work the authors try to calculate what is the relative probability of Life\footnote{Life as we know it, that is life involving carbon-based chemistry in liquid water} emerging in the Universe as function of time. This is an interesting question, as in the standard cosmological model (called ``$\Lambda$CDM'') our Universe is only about 13 billion years old and expected to exist and expand for a much longer (infinite?) amount of time.   Loeb and collaborators, on the other hand, looked at the complete history of the cosmos.   The   , trying to assess what is the relative likelihood  This work in a sense follows a recent paper pointed out that many, many stars have already been born (and died) since the beginning of the Universe, making the existence of life somewhere in the Universe ``Before'' the Earth very likely \citep{Frank_2016}