La Théorie des Cordes paraît aujourd’hui en mesure de devenir une théorie de Tout, c’est-à-dire apporter une explication à tout ce qui était jusqu’ici incompréhensible pour la science, de la gravité au Big Bang et aux Trous noirs. Le seul problème est qu’elle ne peut expliquer un univers comme le nôtre, en expansion à une vitesse accélérée.
En fait nul n’a compris encore la raison de cette expansion. On évoque une mystérieuse énergie noire. Mais, selon les théories actuelles, rien ne devrait se produire.
Cependant aujourd’hui certains physiciens proposent une solution à cette interrogation. Pour eux, notre univers ne serait qu’un point dans un ensemble beaucoup plus vaste dont une expansion accélérée serait la règle naturelle, entre un hyper-espace de grande dimension et un vide absolu, selon la description qu’en donne Antonio Padilla, de l’Université de Nottingham (UK).
La théorie des cordes ,aujourd’hui célébré, avait commencé modestement. Ce n’était alors qu’une simple équation destinée à donner un sens aux collisions entre protons et neutrons mus par ce que l’on nommait la « strong force ». Celle-ci l’emporte sur la gravité sur les courtes distances quand il s’agit de tenir réunis les protons dans les noyaux des atomes. Mais dans un sens étendu, les physiciens suggérèrent que chaque particule fondamentale, électron, quark, boson de Higgs, pouvait être l’extrémité d’une corde minuscule vibrant sur un rythme distinct.
Le premier succès de la théorie des cordes fut de pouvoir décrire la gravité . Plus tard les autres forces furent aussi analysées. La théorie des cordes devint ainsi une « théorie du Tout ».
Ses mathématiques sont complexes. Nous n’en connaissons que trois, trois d’espace et une de temps. Cependant certains théoriciens des cordes ont suggéré qu’elles étaient dix séparées par une membrane poreuse. mais elles seraient si minuscules qu’elles seraient inobservables par tout instrument que ce soit. Elles ont été nommés des branes. Malheureusement la théorie des cordes est si flexible qu’elle peut décrire de nombreux objets imaginaires au nombre d’au moins 10 puissance 500 Quelle autorité peut-elle avoir en ce cas précis ?
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It might seem odd that string theory struggles with something so apparently mundane as an accelerating universe. The reason is that the rate of expansion comes from the very geometry of space-time, as defined by Albert Einstein’s general theory of relativity and later descibed in detail by cosmologist Willem de Sitter. In one solution to Einstein’s equations, space-time is spherical and expanding at an accelerating rate – what’s now known as a de Sitter (dS) space. In the alternative solution, space-time is saddle-shaped and cannot expand at all, called an anti-de Sitter (AdS) space. String theory strongly implies that only AdS space-times are stable and able to support themselves, even though a wealth of astronomical data, including from those distant supernovae, confirms that our present-day universe is dS.
What we see and feel would be just a projection of a greater reality, beyond our senses
The seed of an idea to get around this impasse was planted even before physicists had fully got to grips with dark energy and the accelerating universe. In 1999, in an attempt to solve an entirely different problem in string theory, theorists Lisa Randall and Raman Sundrum toyed with the concept of high-dimensional branes, albeit in a less extravagant, easier-to-describe, five-dimensional (5D) setting.
In geometry, the surface of an object always requires one dimension fewer than the object itself – for instance, each face of a 3D cube is a 2D square. Likewise, Randall and Sundrum discovered that it was possible to have a pair of 5D AdS space-times separated by a brane that was merely 4D, just like our universe. Moreover, Randall’s later work with theorist Andreas Karch showed that this brane would have an accelerating, dS geometry – again, just like our universe. Did we live on a brane? The possibility was tantalising.
Alas, despite its superficial promise, the Randall-Karch brane never much helped to ease string theory’s woes. The reason was that, sandwiched between two mammoth AdS space-times, it wasn’t much more stable than the few pure dS universes that could be wrenched out of string theory. About five years ago, however, Danielsson and his colleagues at Uppsala University had an epiphany. “We thought, what if instability wasn’t a problem?” he says. “What if we could turn it to our advantage?”
Every space-time model has a certain level of energy woven into its fabric, governing the types and behaviours of the particles, strings, branes and other entities that may be contained within it. If a space-time doesn’t reside at the lowest possible energy, quantum mechanics says it is inherently unstable and has the risk of “decaying”, suddenly transforming into a new universe in which the energy is lower. Danielsson’s group considered a 5D AdS space-time that begins high on this energy ladder, and found that if even a tiny part of it decays, this fragment quickly forms a “dark bubble” of lower-energy 5D AdS space-time. As in the Randall-Karch scenario, this bubble is enclosed by a 4D dS space-time like our own – yet crucially, it arises out of instability, rather than being at the mercy of it.
In this new scenario, the bubble membrane on which our cosmos is poised still wouldn’t be perfectly stable. But that just means another dark bubble would occasionally pop up, or nucleate, within its inner 5D space-time, which we can’t see. In fact, new dark bubbles would continually nucleate within each other, each enclosed by a dS brane – in effect, a new universe. In this grand multiverse, what we refer to as “the” big bang would be just the moment our parent bubble gave birth to ours.
Danielsson argues that this idea is actually more intuitive than accepted big-bang cosmology. “A favourite picture of the big bang is that it’s like a balloon expanding into space,” he says. “Usually someone tells you that’s wrong: the balloon isn’t expanding into space, because that extra space – the beyond – simply doesn’t exist. But with the dark bubble, actually it does.”
Leaking gravity
However, there is a fear that, on closer inspection, the bubble may burst. The issue has to do with that most pesky of nature’s forces, gravity, and, in particular, why it is so weak. A common illustration of gravity’s weakness is the comparative strength of a fridge magnet, which is able to exert enough electromagnetic force to stick to a metal door and, in so doing, counteract the gravitational pull of the entire planet. String theory offers a generic, hand-waving answer to gravity’s feebleness: with 10 dimensions to act in, gravity is somehow diluted more than the others. But if gravity is leaking away, theorists still have to explain why it isn’t so weak as to let planets escape their orbits and spinning galaxies fly apart.
The trick is to somehow confine gravity so that only a little – just the right amount – seeps away into the extra dimensions. Padilla argues that dark bubbles don’t successfully do this. After all, the dark bubble multiverse is infinite, so gravity can potentially leak anywhere. As a result, Danielsson’s group has had to introduce additional strings in their fifth dimension to tether gravity to the bubble membranes.
“To us, it looked like you have to jump through hoops to get gravity to look 4D,” says Padilla. For this reason, he and his colleagues Ben Muntz and Paul Saffin at the University of Nottingham began to toy with an alternative solution: get rid of the multiverse and have just one bubble. With no infinite space-time, gravity’s leakage is stemmed, allowing it to be weak, but not too weak. And while our world is still the 4D brane surrounding a 5D bubble, it is now also the barrier between a 5D cosmos and pure nothingness. In other words, it is literally the edge of the universe, an “end of the world” brane. “Of course, this also presents a problem,” says Padilla. “What’s nothing, and how can something come out of it?”
How to get something from nothing is a vexed question, to say the least. From the dawn of philosophical thought, people have wondered how space, time, substance – even the rules governing those things – can arise if there is nothing there to begin with. As physics deals with relationships between entities, there seems little hope that it can ever fully answer this mystery.
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