In addition, it looks like the amount of dark energy infused in empty space stays constant over time as best anyone can tell. You think you should be able to construct them, but you cannot.
Several telescope experiments are underway now to more precisely probe whether the universe is expanding with a constant rate of acceleration, which would mean that as new space is created, a proportionate amount of new dark energy arises with it, or whether the cosmic acceleration is gradually changing, as in quintessence models.
Instead of tearing apart in a Big Rip, a quintessent universe would gradually decelerate, and in most models, would eventually stop expanding and contract in either a Big Crunch or Big Bounce. 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. 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. In the meantime, string theorists, who normally form a united front, will disagree about the conjecture. Eva Silverstein , a physics professor at Stanford University and a leader in the effort to construct string-theoretic models of inflation , thinks it is very likely to be false.
But in her view, accelerating-expansion models are no more disfavored now, in light of the new papers, than before. Matthew Kleban , a string theorist and cosmologist at New York University, also works on stringy models of inflation. And yet he acknowledges that, based on existing evidence, the conjecture could well be true.
That obviously would be very interesting. 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 s that string theory has something like 10 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. In , Vafa and a network of collaborators began to think about how to pare the possibilities down by mapping out fundamental features of nature that absolutely have to be true.
The intuition about where landscape ends and swampland begins derives from decades of effort to construct stringy models of universes. This is not a successful approach towards creating a free quark, but this realization did give rise to the string model of the strong interactions.
The story begins in the late s, when particle accelerators were just entering their heyday. After the discovery of the antiproton in the s, larger and more energetic particle accelerators began to be constructed, leading to an enormous suite of new particles that arose from colliding charged particles into other charged particle.
The newly discovered particles came in three types:. But one interesting thing to note was that mesons, before decaying, were like bar magnets. If you break a bar magnet with a north and south pole , you don't get an independent north and south pole, but rather two magnets each with their own north and south poles.
Similarly, if you try to pull a meson apart, eventually it "snaps," creating two separate mesons in the process.
Magnetic field lines, as illustrated by a bar magnet: a magnetic dipole, with a north and south pole These permanent magnets remain magnetized even after any external magnetic fields are taken away. If you 'snap' a bar magnet in two, it won't create an isolated north and south pole, but rather two new magnets, each with their own north and south poles.
Mesons 'snap' in a similar manner. This was initially where string theory began: as the string model of the strong nuclear interactions. If you envision a meson as a string, then pulling it apart increases the tension in the string until you reach a critical moment, resulting in two new mesons.
The string model was interesting for this reason, but predicted a number of strange things that didn't appear to match reality, such as a spin-2 boson which wasn't observed , the fact that the spin-1 state doesn't become massive during symmetry breaking i.
Then the idea of asymptotic freedom was discovered and the theory of quantum chromodynamics QCD came to be, and the string model fell out of favor. QCD described the strong nuclear force and interactions extraordinarily well without these pathologies, and the idea was abandoned. The Standard Model, now complete, didn't need this new, esoteric, and simultaneously ineffective framework.
At high energies corresponding to small distances , the strong force's interaction strength drops At large distances, it increases rapidly. This idea is known as 'asymptotic freedom,' which has been experimentally confirmed to great precision. But a decade or so later, this idea was reborn into what's now known as modern string theory. Instead of working at the energy scales where nuclear interactions are important, the idea was put forth to take the energy scale all the way up to the Planck energy, where the spin-2 particle that made no sense could now play the role of the graviton: the theoretical force-carrying particle responsible for a quantum theory of gravity.
That spin-1 particle could be the photon, and other excited states could be associated with the known Standard Model particles. All of a sudden, a long sought dreamed seemed within reach in this new framework. For one, string theory suddenly made it plausible that the Standard Model of particles and interactions could be reconciled with General Relativity. By viewing each of the elementary particles as either an open or closed string that vibrated at specific, unique frequencies, and the fundamental constants of nature as various states of the vacuum in string theory, physicists could finally hope to unify all the fundamental forces together.
Feynman diagrams top are based off of point particles and their interactions. Converting them into In string theory, all particles are simply different vibrating modes of an underlying, more fundamental structure: strings. But what you get out of string theory isn't exactly as simple as this.
You don't simply get the Standard Model and General Relativity, but rather something much, much larger and more grandiose that contains both the Standard Model and General Relativity, but also much more. String theory, even in the low-energy limit, demands a much greater degree of symmetry than even this, which means that a low-energy prediction of superpartners should arise.
The fact that we have discovered exactly 0 supersymmetric particles, even at LHC energies, is an enormous disappointment for string theory. Lacking traditional tests, they are seeking validation of string theory by a different route.
And if string theory is the only possible approach, then its proponents say it must be true — with or without physical evidence. If they are successful, the researchers acknowledge that such a proof will be seen as controversial evidence that string theory is correct. Meanwhile, outside experts caution against jumping to conclusions based on the findings to date. They are calculations, but there are weasel words in certain places.
Over the past century, physicists have traced three of the four forces of nature — strong, weak and electromagnetic — to their origins in the form of elementary particles. Only gravity remains at large. This picture perfectly captures gravity on macroscopic scales.
Physicists use a mathematical framework called quantum field theory to describe the probabilistic interactions between particles. But unifying the laws of nature in this way has proven immensely difficult.
But the argument had a flaw: While some hadrons do consist of pairs of quarks and anti-quarks and plausibly resemble strings, protons and neutrons contain three quarks apiece, invoking the ugly and uncertain picture of a string with three ends. Soon, a different theory of quarks emerged. But ideas die hard, and some researchers, including Green , then at the University of London, and Schwarz , at the California Institute of Technology, continued to develop string theory.
Problems quickly stacked up. And because each of the innumerable ways of knotting up the extra dimensions corresponds to a different macroscopic pattern, almost any discovery made about our universe can seem compatible with string theory, crippling its predictive power.
On the plus side, researchers realized that a certain vibration mode of the string fit the profile of a graviton, the coveted quantum purveyor of gravity. And on that stormy night in Aspen in , Green and Schwarz discovered that the graviton contributed a term to the equations that, for a particular version of string theory, exactly canceled out the problematic anomaly.
But only a year passed before another version of string theory was also certified anomaly-free. In all, five consistent string theories were discovered by the end of the decade.
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