Saturday, September 11, 2021



As the universe ages on incomprehensibly long timescales, the stars disappear from sight before matter itself begins to decay.

 When looking at a dark night sky, astronomers and laypeople are often surprised by the large number of bright stars visible to the naked eye.  From a good vantage point on a clear night and with little light pollution, the average person can see about 2,500 individual stars.

 It's easy to imagine our ancestors looking up at the same night sky, with twinkling headlights evoking the same (or stronger) emotions they arouse today.  But while it may seem like the universe is immutable — with stars that will always shine to any creature curious enough to look up — actually, that's not the case.

 astronomical ages

 We live in what he calls the "Stelliferous Era", which extends from when the universe was 1 million years old to when the universe is 100 trillion years old.  This period is most notable for its many churning galaxies, composed of countless bright stars burning hydrogen.  But massive stars burn their fuel quickly, so larger stars die faster than smaller ones (millions of years compared to hundreds of billions).  Eventually, the latest generation of stars will deplete any remaining hydrogen fuel available for consumption.

 During the Star Age, some supermassive stars will dramatically perish as supernovae.  But most simply fail after going through several phases of stellar evolution.  When that happens, the universe will be filled only with a variety of so-called degenerate stellar debris: black holes, white dwarfs, brown dwarfs, and neutron stars.  Adams and Laughlin called this period the Degenerate Era, hypothesized that it will occur between 1015 (1 quintillion) and 1039 (1 duodecillion or 1 followed by 39 zeros) years after the Big Bang.

 While black holes, white dwarfs, brown dwarfs and neutron stars exist today, during the Degenerate Era they dominate the universe.  Degenerate stellar remnants are generally much cooler and darker than most stars in our current era.  The night sky we see today will no longer exist, being replaced by one with fewer stars – and distinctly more obscure.  Brown dwarfs, which were too small to undergo regular fusion reactions, will inherit much of the hydrogen left in the universe.  Black holes will grow larger during the Degenerate Era, fueled by whatever matter they can accumulate, including other degenerate stellar remnants.

 degenerate life

 Would life still exist during the Degenerate Era?  It seems unlikely when the stars run out of hydrogen fuel, life as we know it will be long gone.

 The universe will be burnt and cold and will only get colder with time.

 Aging stars and stellar remnants will result in most worlds being too frigid to sustain life.  Planets of this time, which were once capable of supporting large organisms, will either be pushed out of their orbits or spiral into the remnant of degenerate star remnants.  Or, as Pogge said, "the remaining stars will eat your children or throw them away."  This means that star systems as we think of them today will no longer exist.

 During the Degenerate Era, stellar debris and even galaxies will experience a series of collisions and near collisions.  In our current era, these collisions often produce merged galaxies and encourage star formation.  But during the Degenerate Era, that won't happen.

 Some chaotic and fused galaxies from discarded stellar corpses may still form, but the lack of available free hydrogen will mean that new stars will generally not exist.  In many cases, the degenerate stellar remnants will simply be launched into intergalactic space through close collisions, scattering them widely.

 The final phase of the Degenerate Era will happen unexpectedly.

 Matter trapped in dead stars will literally separate, with neutrons decaying to protons, electrons, and antineutrinos.  Protons themselves are supposed to have an extremely long half-life of approximately 1034 years, but as far as we know, that time is destined to arrive during the Degenerate Era, causing white dwarfs, brown dwarfs, and neutron stars to simply degrade and fade away.

 “Eventually, the protons simply dissolve”, .  At that point, the universe will be composed of abundant free radiation, subatomic particles and countless black holes.  These black holes will reign supreme during the next era, aptly dubbed the Black Hole Era, which is a topic for another day.

 Though impossibly distant – and on some levels deeply disturbing to think about – the Degenerate Era will one day happen.  In Woody Allen's iconic 1977 romantic comedy Annie Hall, a school-aged version of Allen stops doing his homework after reading in a newspaper that the universe is expanding, prompting the character to ask the pediatrician: "What's the point?"

 You can empathize with Allen's character.  But the Degenerate Era is undoubtedly a fascinating topic to contemplate, especially considering how unrecognizable the future cosmos will be compared to the already bizarre universe we now know.


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