Sunday, July 26, 2015

Space and Time

Those of an anthropic bent have often made much of the fact that we are only 13.7 billion years into what is apparently an open-ended universe that will expand at an accelerating rate forever.  The era of the stars will last a trillion years; why do we find ourselves at this early date if we assume we are a ‘typical’ example of an intelligent observer?  In particular, this has lent support to lines of argument that perhaps the answer to the ‘great silence’ and lack of astronomical evidence for intelligence or its products in the universe is that we are simply the first.  This notion requires, however, that we are actually early in the universe when it comes to the origin of biospheres and by extension intelligent systems.  It has become clear recently that this is not the case. 

The clearest research I can find illustrating this is the work of Sobral et al, illustrated here via a paper on arxiv  and here via a summary article.  To simplify what was done, these scientists performed a survey of a large fraction of the sky looking for the emission lines put out by emission nebulae, clouds of gas which glow like neon lights excited by the ultraviolet light of huge, short-lived stars.  The amount of line emission from a galaxy is thus a rough proxy for the rate of star formation – the greater the rate of star formation, the larger the number of large stars exciting interstellar gas into emission nebulae.  The authors use redshift of the known hydrogen emission lines to determine the distance to each instance of emission, and performed corrections to deal with the known expansion rate of the universe.  The results were striking.  Per unit mass of the universe, the current rate of star formation is less than 1/30 of the peak rate they measured 11 gigayears ago.  It has been constantly declining over the history of the universe at a precipitous rate.  Indeed, their preferred model to which they fit the trend converges towards a finite quantity of stars formed as you integrate total star formation into the future to infinity, with the total number of stars that will ever be born only being 5% larger than the number of stars that have been born at this time. 

In summary, 95% of all stars that will ever exist, already exist.  The smallest longest-lived stars will shine for a trillion years, but for most of their history almost no new stars will have formed.

At first this seems to reverse the initial conclusion that we came early, suggesting we are instead latecomers.  This is not true, however, when you consider where and when stars of different types can form and the fact that different galaxies have very different histories.  Most galaxies formed via gravitational collapse from cool gas clouds and smaller precursor galaxies quite a long time ago, with a wide variety of properties.  Dwarf galaxies have low masses, and their early bursts of star formation lead to energetic stars with strong stellar winds and lots of ultraviolet light which eventually go supernova.  Their energetic lives and even more energetic deaths appear to usually blast star-forming gases out of their galaxies’ weak gravity or render it too hot to re-collapse into new star-forming regions, quashing their star formation early.  Giant elliptical galaxies, containing many trillions of stars apiece and dominating the cores of galactic clusters, have ample gravity but form with nearly no angular momentum.  As such, most of their cool gas falls straight into their centers, producing an enormous burst of low-heavy-element star formation that uses most of the gas.  The remaining gas is again either blasted into intergalactic space or rendered too hot to recollapse and accrete by a combination of the action of energetic young stars and the infall of gas onto the central black hole producing incredibly energetic outbursts.   (It should be noted that a full 90% of the non-dark-matter mass of the universe appears to be in the form of very thin X-ray-hot plasma clouds surrounding large galaxy clusters, unlikely to condense to the point of star formation via understood processes.)  Thus, most dwarf galaxies and giant elliptical galaxies contributed to the early star formation of the universe but are producing few or no stars today, have very low levels of heavy element rich stars, and are unlikely to make many more going into the future.

Spiral galaxies are different.  Their distinguishing feature is the way they accreted – namely with a large amount of angular momentum.  This allows large amounts of their cool gas to remain spread out away from their centers.  This moderates the rate of star formation, preventing the huge pulses of star formation and black hole activation that exhausts star-forming gas and prevents gas inflow in giant ellipticals.  At the same time, their greater mass than dwarf galaxies ensures that the modest rate of star formation they do undergo does not blast nearly as much matter out of their gravitational pull.  Some does leave over time, and their rate of inflow of fresh cool gas does apparently decrease over time – there are spiral galaxies that do seem to have shut down star formation.  But on the whole a spiral is a place that maintains a modest rate of star formation for gigayears, while heavy elements get more and more enriched over time.  These galaxies thus dominate the star production in the later eras of the universe, and dominate the population of stars produced with large amounts of heavy elements needed to produce planets like ours.  They do settle down slowly over time, and eventually all spirals will either run out of gas or merge with each other to form giant ellipticals, but for a long time they remain a class apart.

Considering this, we’re just about where we would expect a planet like ours (and thus a biosphere-as-we-know-it) to exist in space and on a coarse scale in time.  Let’s look closer at our galaxy now.  Our galaxy is generally agreed to be about 12 billion years old based on the ages of globular clusters, with a few interloper stars here and there that are older and would’ve come from an era before the galaxy was one coherent object.  It will continue forming stars for about another 5 gigayears, at which point it will undergo a merger with the Andromeda galaxy, the nearest large spiral galaxy.  This merger will most likely put an end to star formation in the combined resultant galaxy, which will probably wind up as a large elliptical after one final exuberant starburst.  Our solar system formed about 4.5 gigayears ago, putting its formation pretty much halfway along the productive lifetime of the galaxy (and probably something like 2/3 of the way along its complement of stars produced, since spirals DO settle down with age, though more of its later stars will be metal-rich).

On a stellar and planetary scale, we once again find ourselves where and when we would expect your average complex biosphere to be.  Large stars die fast – star brightness goes up with the 3.5th power of star mass, and thus star lifetime goes down with the 2.5th power of mass.  A 2 solar mass star would be 11 times as bright as the sun and only live about 2 billion years – a time along the evolution of life on Earth before photosynthesis had managed to oxygenate the air and in which the majority of life on earth (but not all – see an upcoming post) could be described as “algae”.  Furthermore, although smaller stars are much more common than larger stars (the Sun is actually larger than over 80% of stars in the universe) stars smaller than about 0.5 solar masses (and thus 0.08 solar luminosities) are usually ‘flare stars’ – possessing very strong convoluted magnetic fields and periodically putting out flares and X-ray bursts that would frequently strip away the ozone and possibly even the atmosphere of an earthlike planet. 

All stars also slowly brighten as they age – the sun is currently about 30% brighter than it was when it formed, and it will wind up about twice as bright as its initial value just before it becomes a red giant.  Depending on whose models of climate sensitivity you use, the Earth’s biosphere probably has somewhere between 250 million years and 2 billion years before the oceans boil and we become a second Venus.  Thus, we find ourselves in the latter third-to-twentieth of the history of Earth’s biosphere (consistent with complex life taking time to evolve).

Together, all this puts our solar system – and by extension our biosphere – pretty much right where we would expect to find it in space, and right in the middle of where one would expect to find it in time.  Once again, as observers we are not special.  We do not find ourselves in the unexpectedly early universe, ruling out one explanation for the Fermi paradox sometimes put forward – that we do not see evidence for intelligence in the universe because we simply find ourselves as the first intelligent system to evolve.  This would be tenable if there was reason to think that we were right at the beginning of the time in which star systems in stable galaxies with lots of heavy elements could have birthed complex biospheres.  Instead we are utterly average, implying that the lack of obvious intelligence in the universe must be resolved either via the genesis of intelligent systems being exceedingly rare or intelligent systems simply not spreading through the universe or becoming astronomically visible for one reason or another. 

In my next post, I will look at the history of life on Earth, the distinction between simple and complex biospheres, and the evidence for or against other biospheres elsewhere in our own solar system.

Friday, July 24, 2015

What's all this about?

This blog is to be a repository for the thoughts and analysis I've accrued over the years on the topic of astrobiology, and the place of life and intelligence in the universe.  All my life I've been pulled to the very large and the very small.  Life has always struck me as the single most interesting thing on Earth, with its incredibly fine structure and vast, amazing history and fantastic abilities.  At the same time, the vast majority of what exists is NOT on Earth.  Going up in size from human-scale by the same number of orders of magnitude as you go down through to get to a hydrogen atom, you get just about to Venus at its closest approach to Earth - or one billionth the distance to the nearest star.  The large is much larger than the small is small.  On top of this, we now know that the universe as we know it is much older than life on Earth.  And we know so little of the vast majority of the universe.

There's a strong tendency towards specialization in the sciences.  These days, there pretty much has to be for anybody to get anywhere.  Much of the great foundational work of physics was done on tabletops, and the law of gravitation was derived from data on the motions of the planets taken without the benefit of so much as a telescope.  All the low-hanging fruit has been picked.  To continue to further knowledge of the universe, huge instruments and vast energies are put to bear in astronomy and physics.  Biology is arguably a bit different, but the very complexity that makes living systems so successful and so fascinating to study means that there is so much to study that any one person is often only looking at a very small problem.

This has distinct drawbacks.  The universe does not care for our abstract labels of fields and disciplines - it simply is, at all scales simultaneously at all times and in all places.  When people focus narrowly on their subject of interest, it can prevent them from realizing the implications of their findings on problems usually considered a different field.

It is one of my hopes to try to bridge some gaps between biology and astronomy here.  I very nearly double-majored in biology and astronomy in college; the only thing that prevented this (leading to an astronomy minor) was a bad attitude towards calculus.  As is, I am a graduate student studying basic cell biology at a major research university, who nonetheless keeps in touch with a number of astronomer friends and keeps up with the field as much as possible.  I quite often find that what I hear and read about has strong implications for questions of life elsewhere in the universe, but see so few of these implications actually get publicly discussed. All kinds of information shedding light on our position in space and time, the origins of life, the habitability of large chunks of the universe, the course that biospheres take, and the possible trajectories of intelligences seem to me to be out there unremarked.

It is another of my hopes to try, as much as is humanly possible, to take a step back from the usual narratives about extraterrestrial life and instead focus from something closer to first principles.  What we actually have observed and have not, what we can observe and what we cannot, and what this leaves open, likely, or unlikely.  In my study of the history of the ideas of extraterrestrial life and extraterrestrial intelligence, all too often these take a back seat to popular narratives of the day.  In the 16th century the notion that the Earth moved in a similar way to the planets gained currency and lead to the suppositions that they might be made of similar stuff and that the planets might even be inhabited.  The hot question was, of course, if their inhabitants would be Christians and their relationship with God given the anthropocentric biblical creation stories.  In the late 19th and early 20th century, Lowell's illusory canals on Mars were advanced as evidence for a Martian socialist utopia.  In the 1970s, Carl Sagan waxed philosophical on the notion that contacting old civilizations might teach us how to save ourselves from nuclear warfare.  Today, many people focus on the Fermi paradox - the apparent contradiction that since much of the universe is quite old, extraterrestrials experiencing continuing technological progress and growth should have colonized and remade it in their image long ago and yet we see no evidence of this.  I move that all of these notions have a similar root - inflating the hot concerns and topics of the day to cosmic significance and letting them obscure the actual, scientific questions that can be asked and answered.

Life and intelligence in the universe is a topic worth careful consideration, from as many angles as possible.  Let's get started.