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Low-temperature physicists at Lancaster University may have found a laboratory test of the untestable string theory.
The test which uses two distinct phases of liquid helium - is reported online this week in Nature Physics (published 23 December). Their paper will also be published as the cover article in the paper edition of Nature Physics in January.

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Title: Inflationary Constraints on Type IIA String Theory
Authors: Mark P. Hertzberg (MIT), Shamit Kachru (Stanford), Washington Taylor (MIT), Max Tegmark (MIT)
(Version v2)

We prove that inflation is forbidden in the most well understood class of semi-realistic type IIA string compactifications: Calabi-Yau compactifications with only standard NS-NS 3-form flux, R-R fluxes, D6-branes and O6-planes at large volume and small string coupling. With these ingredients, the first slow-roll parameter satisfies epsilon >= 27/13 whenever V > 0, ruling out both inflation (including brane/anti-brane inflation) and de Sitter vacua in this limit. Our proof is based on the dependence of the 4-dimensional potential on the volume and dilaton moduli in the presence of fluxes and branes. We also describe broader classes of IIA models which may include cosmologies with inflation and/or de Sitter vacua. The inclusion of extra ingredients, such as NS 5-branes and geometric or non-geometric NS-NS fluxes, evades the assumptions used in deriving the no-go theorem. We focus on NS 5-branes and outline how such ingredients may prove fruitful for cosmology, but we do not provide an explicit model. We contrast the results of our IIA analysis with the rather different situation in IIB.

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Title: APOLLO: the Apache Point Observatory Lunar Laser-ranging Operation: Instrument Description and First Detections
Authors: T.W. Murphy, Jr., E.G. Adelberger, J.B.R. Battat, L.N. Carey, C.D. Hoyle, P. LeBlanc, E.L. Michelsen, K. Nordtvedt, A.E. Orin, J.D. Strasburg, C.W. Stubbs, H.E. Swanson, E. Williams

A next-generation lunar laser ranging apparatus using the 3.5 m telescope at the Apache Point Observatory in southern New Mexico has begun science operation. APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) has achieved one-millimetre range precision to the moon which should lead to approximately one-order-of-magnitude improvements in the precision of several tests of fundamental properties of gravity. We briefly motivate the scientific goals, and then give a detailed discussion of the APOLLO instrumentation.

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String theory is having trouble producing inflation the rapid expansion of space thought to have occurred in the early universe at least in some of the theory's simplest incarnations, according to a new study.
The work suggests squaring string theory with the well-accepted notion of inflation will be challenging at best and some even say that one or both theories may have to be abandoned.
String theory is a leading contender for the "theory of everything", which would unify all the forces of physics in one framework. Though there are many different versions of string theory, all posit that elementary particles are actually tiny vibrating strings, and that the universe contains extra spatial dimensions beyond the three that we can see.
Now, a new study suggests it may be difficult to reconcile string theory with the widely accepted theory of inflation, which explains several key cosmological observations such as why the universe appears to have the same properties in whichever direction astronomers look.
The study was carried out by a team of researchers led by Mark Hertzberg of MIT in Cambridge, US. The team tried to produce inflation in three versions of string theory in which the extra dimensions are shaped like a doughnut the simplest possibility. But they found that the conditions needed for inflation appear to be impossible to achieve in these simple versions.

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Being 4 minutes late doesn't usually cause physicists around the world to fizz with excitement - but it's a different matter if the latecomer is a photon, and its tardiness could indicate a breakdown of relativity on cosmic scales. What's more, this delay could provide us with our first hints of quantum gravity at work, and thus be a unique way of testing string theory.
Last month, the MAGIC gamma-ray telescope collaboration based on La Palma in the Canary Islands announced that they had measured a 4-minute time difference between the arrival of high and low-energy gamma rays released at the same time in a flare from the Markarian 501 galaxy, some half a billion light years away. According to Einstein's theory of special relativity, both sets of photons should have arrived simultaneously, and the team is controversially claiming that the discrepancy is due to the first detected effects of quantum gravity.
Theories of quantum gravity - which attempts to shoehorn gravity into quantum mechanics - predict that space-time fluctuates rapidly on so-called Planck scales of about 10^-35 metres. These fluctuations could have slowed down high-energy gamma-rays, causing MAGIC's observed time delay a model for quantum gravity that is based on an unconventional version of string theory, which they say predicts the 4-minute delay exactly. Alternative quantum gravity models based on standard versions of string theory can't explain the effect.

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Title: Probing Quantum Gravity using Photons from a Mkn 501 Flare Observed by MAGIC
Authors: J. Albert, et al., for the MAGIC Collaboration, John Ellis, N.E. Mavromatos, D.V. Nanopoulos, A.S. Sakharov, E.K.G. Sarkisyan

We use the timing of photons observed by the MAGIC gamma-ray telescope during a flare of the active galaxy Markarian 501 to probe a vacuum refractive index ~ 1-(E/M_QGn)^n, n = 1,2, that might be induced by quantum gravity. The peaking of the flare is found to maximise for quantum-gravity mass scales M_QG1 ~ 0.4x10^18 GeV or M_QG2 ~ 0.6x10^11 GeV, and we establish lower limits M_QG1 > 0.26x10^18 GeV or M_QG2 > 0.39x10^11 GeV at the 95% C.L. Monte Carlo studies confirm the MAGIC sensitivity to propagation effects at these levels. Thermal plasma effects in the source are negligible, but we cannot exclude the importance of some other source effect.

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The BL Lacertae object Markarian 501 was identified as a source of gamma-ray emission at the Whipple Observatory in March 1995.
Markarian 501 is more than 300 million light years distant from the earth. It has an active nucleus and is the energy source for the jet of fast-moving particles.

Variable VHE -ray emission from Markarian 501
The blazar Markarian 501 (Mrk 501) was observed at energies above 0.10 TeV. with the MAGIC telescope from May through July 2005.

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R.A. = 16:53:52.2, Dec. = 39:45:36 (J2000)
V = 14mag

Markarian 501.kmz
Google Sky file

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Title: Brane Worlds in Collision
Authors: G. W. Gibbons, H. Lu, C.N. Pope
(Version v3)

We obtain an exact solution of the supergravity equations of motion in which the four-dimensional observed universe is one of a number of colliding D3-branes in a Calabi-Yau background. The collision results in the ten-dimensional spacetime splitting into disconnected regions, bounded by curvature singularities. However, near the D3-branes the metric remains static during and after the collision. We also obtain a general class of solutions representing p-brane collisions in arbitrary dimensions, including one in which the universe ends with the mutual annihilation of a positive-tension and negative-tension 3-brane.

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