What Are Some Challenges to Dark Energy?
Like all frontiers of science, it’s the unknown that draws the greatest intrigue. Dark energy may be the greatest mystery we currently have. We don’t know what it is, but we do have theories that try to identify it. But like all ideas, variants exist and dark energy has some interesting and potentially crippling challenges that face it. Here are a few of the hurdles it faces in becoming a sound theory.
Scientists postulate the existence of dark energy based on readings they have of distant objects moving away from us faster than anticipated, accelerating faster and faster. Dark energy has been the solution to this by enforcing space time with an energy density that speeds up movement as the universe expands…right? Not so according to work by Istavan Szapudi (University of Hawaii in Honolulu) and colleagues. They took a computer simulation of the universe and found no need to evoke the mysterious dark energy as a reason for the acceleration. Instead, it’s all about the arrangement of the stuff inside the Universe. As gravity condenses, empty spaces grow and via backreaction these places speed up the expansion. Its presence defines the space and therefore endows it with the property. This is the situation one would find in a non-homogenous scenario, of which the young Universe seems to have been in. It would be a great find if true, especially since the dark energy density scientist expect to see is not even close to observations to explain away. It would also imply that different places in the Universe age differently, meaning that many of our standard rulers may not be as reliable and give the erroneous readings we find to be dark energy based their nature (Skibba, Wiltshire).
Energy Conservation Laws
Physicists have concepts that are so convincing in their truth that they are laws. One of these pertains to energy, meaning that in a closed system energy is neither created nor destroyed. One alternate version of general relativity, dubbed unimodular gravity, may show how dark energy violates this principle according to work from researchers at Aix-Marseille University and the National Autonomous University. If dark energy is true, then the model shows how it is energy leaving the Universe. This would be contrary to the foundations of physics. Therefore, if true, dark energy would have to be something else entirely (Patel).
Another rather convincing (but not fully affirmed) concept in physics is the theory of relativity, developed by Einstein in the early 20th century. It informs us as to the nature of space-time and the effects of reference frames on the world of physics. Amongst the work was what he considered a mistake – a cosmological constant - that he inserted into his equations to ensure a static, non-changing Universe as he felt God would have created. Nowadays, we give that constant the role that dark energy plays…or does it? Blake Temple and Zeke Vogler (both at University of California) and Joel Smoller (University of Michigan at Ann Arbor) tried to work the constant out of Einstein’s equations. It’s really only there because of dark energy, so it doesn’t interfere with the other results of relativity if left out. That means the consequence for leaving it out would be a Universe without stability, i.e. a non-Friedman solution to relativity. But if this is so, then any changed to the matter-energy density of the Universe would result in the effects we attribute to dark energy and therefore we would have no need to invoke the mysterious constant (Fell).
One of the leading candidates for a theory of everything is string theory, which can tie all the aspects of physics together but has low (low) predictive power. Mathematically, over 10500 possible solutions exist and to help reconcile this scientists wonder if it implies a multiverse, where every possible solution does exist. But a development from Cumrun Vafa (Harvard University) brought this into question when an idea known as the de Sitter swampland conjecture establishing what possible universes can exist was found. It shows that as a “universe expands, the density of energy in the vacuum of empty space must decrease faster” than a particular rate. And ours was on the cut list of possible universes (known as de Sitter for the geometries that reign there), because both dark energy and the Big Bang offer conditions that violate that conjecture. Dark energy embedded in empty space is thought to be constant over the span of the Universe but it should be deceasing with this situation via a mechanism known as quintessence and eventually lead to the universe collapsing back onto itself instead of expanding forever. This means that either string theory is a bust of that some underlying feature that we associate with our universe is wrong (Wolchover).
So who knows where this road goes. Maybe dark energy will become an outdated concept, a stepping stone to the real truth of the situation. Maybe it’s completely right. It should be exciting to find out what is really happening, because like a good show there could be a great twist just around the bend…
Fell, Andy. “Doing without Dark Energy.” Sciencedaily.com. Science Daily, 14 Dec. 2017. Web. 18 Dec. 2018.
Patel, Neel V. “New Theory About Dark Energy Breaks Law of Conservation of Energy.” Inverse.com. Inverse, 11 Jan. 2017. Web. 18 Dec. 2018.
Skibba, Ramin. “Astrophysicists Envision a Universe Without Dark Energy.” Insidescience.com. AIP, 06 Apr. 2017. Web. 14 Dec. 2018.
Wiltshire, David and Alan Coley. “Can we ditch dark energy by better understanding general relativity? Cosmosmagazine.com. Cosmos. Web. 17 Dec. 2018.
Wolchover, Natalie. “Dark Energy May Be Incompatible With String Theory.” Quantamagazine.org. Quanta. Web. 13 Dec. 2018.
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