THE SOUTHWORTH PLANETARIUM
207-780-4249      www.usm.maine.edu/planet
70 Falmouth Street     Portland, Maine 04103
43.6667° N                   70.2667° W
Founded January 1970
Julian date:  2457708.16
              "The more things change...."



*THE DAILY ASTRONOMERTuesday, November 15, 2016*
*Dark Energy No More?*

More than thirteen billion years ago, according to cosmologists, the
Universe took form in a single colossal event colloquially known as the
"Big Bang."   All that now exists -space, time, matter and energy- arose
from this genesis event.   It emerged literally from nothing: a timeless
oblivion devoid of depth, breadth, width and duration.      Within a
fraction of a second after its inception, the Universe experienced a rapid
inflation followed by a slower, but still steady, expansion.      This is a
brief and altogether inadequate synopsis of the Big Bang Theory as
formulated by Belgian Priest/Astronomer  George Henri Joseph Eduoard
Lemaitre.  (1894-1966).*

During the years immediately following the development of the Big Bang
theory, scientists debated the fate of our Universe.  Was it a closed
Universe, meaning that the matter within it would exert enough of a
retarding effect on the expansion to cause the cosmos to stop and perhaps
even reverse back to a Big implosion? Or, was it open, meaning that amount
of material with the Universe would be insufficient to halt the expansion.
In this model, the Universe would literally expand forever.      The matter
was fiercely debated, but continued unresolved until the waning years of
the 20th century,when the High-Z Supernova Search Team shocked the world
with a announcement.  "The Universe is accelerating in its expansion!"

This pronouncement proved so shocking because astronomers knew of no
mechanism capable of inducing this acceleration.     Consequently,
theorists developed the notion of "Dark energy," an enigmatic "force"
responsible for the accelerating Universe.      The "dark" signified our
complete ignorance of this energy.    All that theorists knew was that the
mysterious dark energy comprised about 73% of the material Universe.
 Such a measurement proved humbling to cosmologists who were confronted
with the disquieting notion that the visible aspect of our cosmos
represented precious little of it.

Despite dark energy's elusive nature, many astronomical research groups had
embarked on ambitious efforts to demystify it.       The Mid-Scale Dark
Energy Spectroscopic Instrument (DESI), currently under construction, will
eventually be placed on the Mayall Telescope at the Kitt Peak Observatory
in Arizona.   The DESI will hopefully enable researchers to study dark
energy through its effects on the cosmic expansion.

However, such efforts might be unnecessary, for now the very existence of
dark energy is disputed.   Some astronomers assert that dark energy doesn't
exist because the Universe isn't actually accelerating in its expansion,
after all!     The issue pertains to Type Ia Supernovae: explosions which
occur in binary star systems consisting of a white dwarf stellar remnant
bound to a giant star.   The tidal forces exerted on the giant by the dwarf
constantly transfer matter from the latter onto the former.    When white
dwarf accumulates a certain amount of material, it will explode as a Type
Ia Supernova.       These supernovae serve as accurate distance
determination indicators because they should all be equally bright.   (The
explosions occur when the white dwarf becomes about 1.44 times as massive
as the Sun and so each one should experience approximately the same energy
release as any other.)


*​Type Ia supernova.  *  * This type of supernova occurs when a white dwarf
explodes after accumulating a certain amount of material from a companion
giant star.  Such supernovae serve as "cosmic yardsticks" because they
should be equally bright.*

The High-Z Supernova Search team based its findings on observations of Type
Ia Supernovae in distant galaxies.   They found the most distant of these
objects to be fainter than they predicted them to be.   From these
observations they assumed that the Universe must be conveying them away
faster than initially estimated.     This conclusion lead to the
development of the accelerating Universe model and with it, the concept of
dark energy.

Now, however, a research team at Oxford has published a paper indicating
that the Universe is not accelerating in its expansion.  They found the
expansion to be uniform, as it was once believed to have been.   These
researchers also based their observations on Type Ia Supernovae, but have
taken advantage of a larger database than was available to the High-Z
Supernova Search team in the late 1990's.     According to this new
research paper, dark energy might not exist simply because it was developed
to explain an expansion that might not actually be occurring.

From this paper we cannot pronounce the Dark Energy Theory dead.    Other
observations, even those not involving Type Ia Supernovae, support the
existence of dark energy.         This latest development might indicate
that dark energy might not only not pervade the Universe, but might not
even be real.

As for now,the issue remains uncertain.




*Edwin Hubble is often mistakenly credited with the formulation of this
theory.  Instead, he helped provide the observational evidence that
substantiated it.