THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian date:  2457708.16   

              "The more things change...."

THE DAILY ASTRONOMER
Tuesday, 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.