Can String Theory Explain Dark Energy?

CAN STRING THEORY EXPLAIN DARK ENERGY?
Category: Romance and Relationships

A new paper by Cambridge physicist Stephen Hawking and Thomas Hertog of CERN (hertog@mail.cern.ch) suggests that it can. The leading explanation for the observed acceleration of the expansion of the universe is that a substance, dark energy, fills the vacuum and produces a uniform repulsive force between any two points in space---a sort of anti-gravity.

Quantum field theory allows for the existence of such a universal tendency. Unfortunately, its prediction for the value of the density of dark energy (a parameter referred to as the cosmological constant) is some 120 orders of magnitude larger than the observed value. In 2003, cosmologist Andrei Linde of Stanford University and his collaborators showed that string theory allows for the existence of dark energy, but without specifying the value of the cosmological constant. String theory, they found, produces a mathematical graph shaped like a mountainous landscape, where altitude represents the value of the cosmological constant.

After the big bang, the value would settle on a low point somewhere between the peaks and valleys of the landscape. But there could be on the order of 10^500 possible low points---with different corresponding values for the cosmological constant---and no obvious reason for the universe to pick the one we observe in nature. SOME EXPERTS HAILED THIS MULTIPLICITY OF VALUES AS A VIRTUE OF THE THEORY. FOR EXAMPLE, STANFORD UNIVERSITY'S LEONARD SUSSKIND IN HIS BOOK "THE COSMIC LANDSCAPE: STRING THEORY AND THE ILLUSION OF INTELLIGENT DESIGN, "ARGUES THAT DIFFERENT VALUES OF THE COSMOLOGICAL CONSTANT WOULD BE REALIZED IN DIFFERENT PARALLEL WORLDS---THE POCKET UNIVERSES OF LINDE'S "ETERNAL INFLATION" THEORY. WE WOULD JUST
HAPPEN TO LIVE IN ONE WHERE THE VALUE IS VERY SMALL. BUT CRITICS SEE THE LANDSCAPE AS EXEMPLIFYING THE THEORY'S INABILITY TO MAKE USEFUL PREDICTIONS.

THE HAWKING/HERTOG PAPER IS MEANT TO ADDRESS THIS CONCERN. It looks at the universe as a quantum system in the framework of string theory. Quantum theory calculates the odds a system will evolve a certain way from given initial conditions, say, photons going through a double slit and hitting a certain spot on the other side.

You repeat your experiment often enough and then you check that the odds you predicted were the correct ones. In Richard Feynman's formulation of quantum theory, the probability that a photon ends up at a particular spot is calculated by summing up over all possible trajectories for the photon. A PHOTON GOES THROUGH MULTIPLE PATHS AT ONCE AND CAN EVEN INTERFERE WITH ITS OTHER PERSONAS IN THE PROCESS. HAWKING AND HERTOG ARGUE THAT THE UNIVERSE ITSELF MUST ALSO FOLLOW DIFFERENT TRAJECTORIES AT ONCE, EVOLVING THROUGH MANY SIMULTANEOUS, PARALLEL HISTORIES, OR "BRANCHES." (THESE PARALLEL UNIVERSES ARE NOT TO BE CONFUSED WITH THOSE OF ETERNAL INFLATION, WHERE MULTIPLE UNIVERSES COEXIST IN A CLASSICAL RATHER THAN IN A QUANTUM SENSE.)

WHAT WE SEE IN THE PRESENT WOULD BE A PARTICULAR, MORE OR LESS PROBABLE, OUTCOME OF THE "SUM" OVER THESE HISTORIES. IN PARTICULAR, THE SUM SHOULD INCLUDE ALL POSSIBLE INITIAL CONDITIONS, WITH ALL POSSIBLE VALUES OF THE COSMOLOGICAL CONSTANT. BUT APPLYING QUANTUM THEORY TO THE ENTIRE UNIVERSE---WHERE THE EXPERIMENTERS ARE PART OF THE EXPERIMENT---IS TRICKY. HERE YOU HAVE NO CONTROL OVER THE INITIAL CONDITIONS, NOR CAN YOUR REPEAT THE EXPERIMENT AGAIN AND AGAIN FOR STATISTICAL SIGNIFICANCE. INSTEAD, THE HAWKING-HERTOG APPROACH STARTS WITH THE PRESENT AND USES WHAT WE KNOW ABOUT OUR BRANCH OF THE UNIVERSE TO TRACE ITS HISTORY
BACKWARDS.

Again, there will be multiple possible branches in our past, but most can be ignored in the Feynman summation because they are just too different from the universe we know, so the probability of going from one to the other is negligible. For example, Hertog says, knowledge that our universe is very close to being flat could allow one to concentrate on a very small portion of the string theory landscape whose values for the cosmological constant are compatible with that flatness. That could in turn lead to predictions that are experimentally testable. For example, one could calculate whether our universe is likely to produce the microwave background spectrum we actually observe. (Physical Review D, upcoming article; contact Thomas Hertog, hertog@mail.cern.ch)

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Invisibility at the flick of a switch



This is what you'd see if the system worked with the atoms in your handAdverts for x-ray specs have tantalised kids throughout the decades. Sadly the reality is always a pair of useless plastic glasses, but this could all change due to a breakthrough made at Imperial College London. By exploiting the way that atoms move in solids the researchers have made solid materials turn completely transparent. 'This real life x-ray specs effect relies on a property of matter that is usually ignored that the electrons it contains move in a wave-like way', says Chris Phillips. 'What we have learnt is how to control these waves directly'.

The secret to this breakthrough at Imperial College London is specially patterned crystals made up of nanoscale boxes that hold electrons. 'Basically we have made 'designer atoms'', says Chris. 'By choosing the size and shape of our little boxes, we can use the rules of quantum mechanics to choose the energy levels of the electrons that are trapped inside them'. When light is shone on these crystals it becomes entangled at a molecular level rather than being absorbed, causing the material to become transparent. 'You can think of the effect as similar to the way that the peaks and troughs of water waves cancel each other out to create calm water', explains Mark Frogley. 'In the materials created it is the wave patterns of the electrons that cancel each other allowing light to travel through the material and making it transparent'. At the moment the effect can only be produced in a lab under specific conditions but future applications could include seeing through rubble at earthquake sites, or looking at parts of the body obscured by bone.

. Imaging with a thermal infrared camera shows whether you're hotDespite the almost magical feat of making solids transparent the key finding of this research is the fundamental physical effect creating the transparency. This effect has potential in the development of new efficient lasers, data security and quantum computing.

A stumbling block for the development of lasers has always been the need to create something called population inversion in the material that amplifies the light, normally glass or crystal. 'Einstein showed that, to make lasers, you need to excite the molecules into this population inversion condition, where they no longer absorb light', explains Mark. The breakthrough at Imperial College London demonstrates that light can now be amplified without the need to create population inversion. This contradicts Einstein's long-standing rule, and opens up the way for the development of a whole new range of lasers.

Data security could be improved due to the discovery that as light passes through these crystals it slows right down and could potentially be stopped and stored. Chris explains, 'When we send information as light pulses down optical fibres, it can only be accessed by making a form of measurement, which disturbs the information. This technology means we could send light signals through a network without having to disturb them ourselves. So, if confidential information was being spied on, the disturbance would show up and we could nab the eavesdropper with 100% certainty'.

Dr Mark Frogley and Professor Chris Phillips
Imperial College London

Dr James Dynes
University College London

Dr Mattias Beck
ETH Zurich, Switzerland

Prof. Jerome Faist
University of Neuchatel, Switzerland