Black holes and string theory
Artist's rendition of a black hole
A black hole possesses a surface, called the event horizon, which separates the interior of the black hole from the outside region. The area of this surface determines the thermodynamic Bekenstein-Hawking entropy of the black hole. This entropy should, according to Boltzmann, have a statistical interpretation in terms of microstates of the system, i.e. of the black hole. Thus, any candidate theory for a consistent theory of quantum gravity has to be able to identify these microstates, and their subsequent counting has to reproduce the thermodynamic entropy of the black hole.
String theory is a leading contender for a consistent theory of quantum gravity. With the pioneering work of Strominger and Vafa, it has become evident that it is possible to identify and to count black hole microstates in the context of string theory. At the current juncture, the black holes that are best understood are those that arise as solutions to theories of gravity with a certain amount of supersymmetry. Supersymmetry is a symmetry that relates bosons and fermions, and the amount of supersymmetry is calculated by the number of generators of this symmetry. These generators are called supercharges, and the black holes arising in string theory that are best understood are those with eight supercharges. Black holes of this type are supported by scalar fields, and this exhibit an interesting flow mechanism termed the attractor mechanism. This mechanism states that as one moves towards the event horizon of the black hole, the scalar fields flow to specific values at the event horizon, thereby losing all memory of their initial values far away from the horizon.
The attractor mechanism is at the heart of the recent progress in string theory in reproducing the thermodynamic black hole entropy by microstate counting. The latter exhibits fascinating connections with topological string theory and with the theory of automorphic forms. Topological string theory is a simplified topological version of full-fledged string theory that appears to capture the microstates of super symmetric black holes. Automorphic forms are an extension to several complex variables of the concept of analytic functions on the upper half-plane satisfying a certain functional equation.
In addition, the microstates should also be captured by the enigmatic AdS2/CFT1 correspondence. The AdS/CFT correspondence, discovered by Maldacena, describes a deep connection between gravitational theories in d-dimensional spacetimes and field theories in one dimension lower. It states that quantum gravity (closed string theory) in anti-de Sitter spacetimes has an equivalent (dual) description in terms of conformal field theories (open string theory) in one dimension lower, living at the boundary of anti-de Sitter spacetime. The AdS2/CFT1 correspondence is a poorly understood example of this duality.
Despite much work in this area, our understanding of black holes with eight supercharges is far from being complete, and it is likely that many more surprises will emerge. Ultimately the goal is to obtain an understanding of realistic black holes, i.e. of black holes with no super symmetry at all.
The writer is a student at the Dept. of Computer Science and Engineering, BRAC University
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Newly discovered seventh and eighth bases of DNA -- called 5-formylcytosine and 5 carboxylcytosine -- are actually versions of cytosine
For decades, scientists have known that DNA consists of four basic units -- adenine, guanine, thymine and cytosine. Those four bases have been taught in science textbooks and have formed the basis of the growing knowledge regarding how genes code for life. Yet in recent history, scientists have expanded that list from four to six.Now, with a finding published online in the July 21, 2011, issue of the journal Science, researchers from the UNC School of Medicine have discovered the seventh and eighth bases of DNA.
These last two bases -- called 5-formylcytosine and 5 carboxylcytosine -- are actually versions of cytosine that have been modified by Tet proteins, molecular entities thought to play a role in DNA demethylation and stem cell reprogramming.
Thus, the discovery could advance stem cell research by giving a glimpse into the DNA changes -- such as the removal of chemical groups through demethylation -- that could reprogram adult cells to make them act like stem cells.
"Before we can grasp the magnitude of this discovery, we have to figure out the function of these new bases," said senior study author Yi Zhang, Ph.D., Kenan Distinguished Professor of biochemistry and biophysics at UNC and an Investigator of the Howard Hughes Medical Institute. "Because these bases represent an intermediate state in the demethylation process, they could be important for cell fate reprogramming and cancer, both of which involve DNA demethylation."
Much is known about the "fifth base," 5-methylcytosine, which arises when a chemical tag or methyl group is tacked onto a cytosine. This methylation is associated with gene silencing, as it causes the DNA's double helix to fold even tighter upon itself.
Last year, Zhang's group reported that Tet proteins can convert 5 methylC (the fifth base) to 5 hydroxymethylC (the sixth base) in the first of a four step reaction leading back to bare-boned cytosine. But try as they might, the researchers could not continue the reaction on to the seventh and eighth bases, called 5 formylC and 5 carboxyC.
The problem, they eventually found, was not that Tet wasn't taking that second and third step, it was that their experimental assay wasn't sensitive enough to detect it. Once they realized the limitations of the assay, they redesigned it and were in fact able to detect the two newest bases of DNA. The researchers then examined embryonic stem cells as well as mouse organs and found that both bases can be detected in genomic DNA.
The finding could have important implications for stem cell research, as it could provide researchers with new tools to erase previous methylation patterns to reprogram adult cells.
It could also inform cancer research, as it could give scientists the opportunity to reactivate tumor suppressor genes that had been silenced by DNA methylation.
The research was funded by the Howard Hughes Medical Institute and the National Institutes of Health.
Study co-authors from UNC include Shinsuke Ito, Ph.D.; Li Shen, Ph.D.; Susan C. Wu, Ph.D.; Leonard B. Collins and James A. Swenberg, Ph.D
Source: Science Daily
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Nokia slips from 1 to 3 in smartphone sales
Fri, Jul 29th, 2011 10:24 pm | Dial 2000 from your GP mobile for latest news |
Nokia has dominated the smartphone market ever since its 1996 launch of the Communicator model, but competition from its two nearest rivals and a slump in its own sales sent it straight from first to third place in the three months to June as growth in the sector starts to slow.
Apple sold a record 20.3 million iPhones in the quarter despite the fact that its iPhone 4 model is now more than a year old. Usually success of smartphone models does not last so long.
Apple unveiled its sales last week, but on Friday analysts also estimated Samsung sold 19 million smartphones in the quarter, well ahead of Nokia's 16.7 million as it was able to benefit from booming demand with smartphones using Google's Android software.
"Samsung's Galaxy portfolio has proven popular, especially the high-tier S2 Android model," said Neil Mawston, analyst at Strategy Analytics."
Strategy Analytics estimated smartphone market volume grew 76 percent from a year ago in the second quarter. ABI Research was somewhat more cautious estimating market grew 62 percent.
SLOWDOWN WORRIES
Growth on the overall cellphone market slowed too in the April-June period, as sales of basic phone models dropped for the first time in seven quarters due to consumers reining in spending, research firm IDC said on Friday.
IDC said strong smartphone demand boosted the market to still grow 11.3 percent from a year ago to 365.4 million phones, but this was a clear slowdown from the 16.8 percent growth seen in the previous quarter.
Strategy Analytics estimated the total market at 361 million cellphones in the quarter.
In a Reuters poll, 29 analysts' average forecast for the total market stood at 374 million phones.
IDC said sales of simpler so-called feature phones fell 4 percent from a year ago due to conservative spending and continued shift to smartphones, most visible in developed markets, such as the United States, Japan and Western Europe.
"The shrinking feature phone market is having the greatest impact on some of the world's largest suppliers of mobile phones," analyst Kevin Restivo said in a statement.
"Stalwarts such as Nokia are losing share in the feature phone category to low-cost suppliers such as Micromax, TCL-Alcatel and Huawei."
Struggling Nokia, still the world's largest phone maker by volume, saw its phone sales shrinking 20 percent from a year ago. This helped Samsung to close the gap to the Finnish firm in the overall cellphone market to the lowest level ever.
Some analysts already forecast for Samsung to become the world's largest cellphone vendor next year.
