[image: fran.png]
*Endymion:  Eternal slumber*
Consider it the mythological realm's greatest irony.    Zeus, the
incorrigible philanderer, couldn't stand the thought of his wife Hera ever
having an affair.  (Colossal egos tend to be the most fragile.)  So,
whenever any man showed an interest in Hera, Zeus was quick to intervene.
  One such suitor was Endymion, a stunningly handsome shepherd-prince.
 According to one version of the story, Endymion was the son of Zeus and
one of his many mistresses.  Endymion was raised by a shepherd and, when a
young man, became one, himself.   While tending his flock, Zeus
materialized in front of him and professed his paternity.  As proof, he
brought Endymion up to Olympus and had intended to let him remain
indefinitely as a guest.   However, Endymion soon met and was enraptured by
Hera, much to Zeus' chagrin.   Zeus shooed him away back to Earth and
commanded him to resume his duties as a simple shepherd.      Soon
thereafter,Selene, the moon goddess, saw Endymion tending his sheep.  To
her, he was the most beautiful young man she had ever seen and yearned to
see him every night forever.   She pleaded with Zeus to grant him eternal
youth.   Zeus consented, but with something a twist.  He appeared before
Endymion again and said, "I know that you shall soon die.  However, I shall
give you an option.    You may either die at the time the fates foretold or
you may have eternal life and youth, but only if you lapse into a slumber
from which you shall never awaken."   Endymion immediately chose the second
option and at once collapsed onto the ground and fell asleep.   Each night
thereafter Selene passed over him and spent hours simply admiring his
beauty as he slept for all eternity.    Endymion's is a rather short -and
somewhat sad- story.  Though he never perished and will forever retain this
youthful beauty, he and Selene never became lovers.     She merely admired
him from afar while he slept his immortal life away.

THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 24591168.16
2020-2021:  XLVI

THE DAILY ASTRONOMER
Tuesday, November 17, 2020
Remote Planetarium 125: Dark Energy II

Consider it the greatest of all cosmic enigmas.   Dark energy.   Though it
may pervade the Universe and account for more than half of it, little to
nothing is known about it.    In fact, the "dark" in dark energy refers to
our utter ignorance of its nature.  We should quickly review the salient
points introduced yesterday.


   - According to the Big Bang Theory, the Universe formed approximately
   13.8 billion years ago.  Ever since its inception, the Universe has
   expanded.   The greatest expansion occurs during the inflationary epoch
   10^-36 to 10^-32 seconds after the Big Bang.
   - Most cosmologists initially assumed that the expansion would
   eventually decelerate as material within the Universe exerted a 'braking
   effect' on it.
   - In 1998, two teams of astronomers announced the discovery that some
   Type Ia supernovae were fainter -and therefore more distant-than expected.
    They concluded that the Universal expansion is accelerating with time due
   to a mysterious force now known as* dark energy.  *This acceleration
   likely started about nine billion years after the Big Bang..
   - Astronomers believe that dark energy accounts for about 68% of the
   Universe!

So, with all that having been said (written), the question remains: how can
scientists possibly find dark energy?   By their own admission, they don't
know why it exists or what form it might assume.   Is it related to
Einstein's "cosmological constant," or it is not?         Cosmologists have
a short list of potential dark energy candidates:


*-The intrinsic energy of empty space*
One of the 20th century's greatest discoveries is that empty space is,
well, hardly empty.     Particles are literally coming into and then going
out of existence constantly.   Moreover, space can produce more space and,
according to General Relativity, contain energy.     It is perhaps possible
that the intrinsic energy of outer space and the continual creation of new
space could accelerate the expansion.    The problem is that nobody quite
knows the nature of this energy or why it should assume a specific value.

*-An incomplete view of gravity*
 The notion that our perception of gravity could possibly be incomplete is
one that makes most researchers uncomfortable.    From Newtonian mechanics
to General Relativity, gravity has always been modeled as an "attractive
force" mediated between all massive objects.      Is it possible that
gravity might exhibit repulsive properties over large distances?   Such
repulsion could explain the acceleration of the expansion.  Despite any
misgivings researchers might have pertaining to this issue,  dark energy is
such an important force that it might necessitate the modification of well
established gravitational laws.

*-Quintessence*
The fanciful name for a new type of energy altogether. It is called
"Quintessence" because it would effectively be a fifth force.     Unlike
empty space energy, Quintessence would be able to exhibit different
properties at different times.   The issue with this energy is that it
seems to be the scientific equivalent of a Deus Ex Machina, a device that
an author contrives to solve what would have otherwise been an intractable
problem.   This other energy might exist, of course, but many researchers
are skeptical.

[image: auniverseof1.jpg]
*-Extra Dimensions?*
We are accustomed to four dimensions:  breadth, length, width and duration,
three spatial and one temporal.   However, it is possible that the Universe
might contain many more dimensions beyond those we can perceive.     A
branch of theoretical physics called "String theory," which posits that all
matter is a manifestation of strings, just as musical notes are produced by
different string frequencies.   String theory suggests that the Universe
consists of ten dimensions, including the four dimensions already
mentioned.  One can envision these extra dimensions as being akin to rough
textures along the membrane of a fruit.  These other dimensions might
somehow accelerate the expansion of the three spatial dimensions.   The
profound problem with String Theory is that it doesn't lend itself to
experimentation.   Without observational evidence, String theory is
regarded by some as just so much mind play.*

Considering all these possibilities, the task of detecting dark energy is a
daunting one.    In 2022, the European Space Agency (ESA) will launch
EUCLID, a probe designed in part to study the redshifts of distant
galaxies.    The aim is to observe how the Universe has evolved over the
last 10 billion years by both observing the expansion at different eras
through the analysis of redshifts and also weak gravitational lensing.
Lensing is the process by which foreground objects cause the light of more
distant objects to become magnified or multiplied.     The analysis of
these observations will hopefully give scientists some insight into how
galaxies  at different distances appear to recede from us.    These
observations might show them how dark energy changes through time and how
it affects different regions of the Universe.


[image: Euclid_Transparent.png]
*ESA's Euclid probe.*
Scheduled to launch in 2022.

Tomorrow, we devote a class to questions pertaining to dark energy.  We
invite you to send your queries, as well.


*Someone was alleged to have asked Stephen Hawking for his opinion about
astrology.  Hawking reportedly replied, "Well, it's better than String
Theory."




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