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 http://arxiv.org/abs/1202.3436
via a paper on arxiv and here http://www.sciencedaily.com/releases/2012/11/121106114141.htm
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.
