Far out cosmology: dark energy, string theory incompatable (Introduction)

by David Turell , Friday, August 17, 2018, 20:46 (2078 days ago) @ David Turell
edited by David Turell, Friday, August 17, 2018, 20:53

A new set of papers on dark matter, string theory and inflation as related to a population of universes as predicted by string theory:

https://www.quantamagazine.org/dark-energy-may-be-incompatible-with-string-theory-20180...

"The conjectured formula — posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri and Lev Spodyneiko and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal and Paul Steinhardt — says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth.

"Since 1998, telescope observations have indicated that the cosmos is expanding ever-so-slightly faster all the time, implying that the vacuum of empty space must be infused with a dose of gravitationally repulsive “dark energy.”

"In addition, it looks like the amount of dark energy infused in empty space stays constant over time (as best anyone can tell).

"But the new conjecture asserts that the vacuum energy of the universe must be decreasing.

"Vafa and colleagues contend that universes with stable, constant, positive amounts of vacuum energy, known as “de Sitter universes,” aren’t possible. String theorists have struggled mightily since dark energy’s 1998 discovery to construct convincing stringy models of stable de Sitter universes. But if Vafa is right, such efforts are bound to sink in logical inconsistency; de Sitter universes lie not in the landscape, but in the “swampland.” “The things that look consistent but ultimately are not consistent, I call them swampland,” he explained recently. “They almost look like landscape; you can be fooled by them. You think you should be able to construct them, but you cannot.”

"The conjecture, if true, would mean the density of dark energy in our universe cannot be constant, but must instead take a form called “quintessence” — an energy source that will gradually diminish over tens of billions of years.

***

"No less dramatically, the new swampland conjecture also casts doubt on the widely believed story of the universe’s birth: the Big Bang theory known as cosmic inflation. According to this theory, a minuscule, energy-infused speck of space-time rapidly inflated to form the macroscopic universe we inhabit. The theory was devised to explain, in part, how the universe got so huge, smooth and flat.

***

"To abide by the formula, the inflaton field’s energy would probably have needed to diminish too quickly to form a smooth- and flat-enough universe, he and other researchers explained. Thus, the conjecture disfavors many popular models of cosmic inflation. In the coming years, telescopes such as the Simons Observatory will look for definitive signatures of cosmic inflation, testing it against rival ideas.

***

"Whether the de Sitter swampland conjecture and future experiments really have the power to falsify string theory remains to be seen. The discovery in the early 2000s that string theory has something like 10500 solutions killed the dream that it might uniquely and inevitably predict the properties of our one universe. The theory seemed like it could support almost any observations and became very difficult to experimentally test or disprove.

***

"Vafa thinks a concerted search for definitely stable de Sitter universe models is long overdue. His conjecture is, above all, intended to press the issue. In his view, string theorists have not felt sufficiently motivated to figure out whether string theory really is capable of describing our world, instead taking the attitude that because the string landscape is huge, there must be a place in it for us, even if no one knows where. “The bulk of the community in string theory still sides on the side of de Sitter constructions [existing],” he said, “because the belief is, ‘Look, we live in a de Sitter universe with positive energy; therefore we better have examples of that type.'’”

Comment: It is time for work like this, critical of all cosmological theories