Science  09 May 2014:
Vol. 344, Issue 6184, pp. 565

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  1. Stellar Explosion Illuminates Cosmic Dark Ages

    Lit up.

    Hawaii's Subaru Telescope saw GRB 130606A.


    A rare ultra–high-energy stellar explosion called a gamma ray burst (GRB) offers evidence that the "cosmic dark ages"—when no galaxies or stars illuminated the heavens—lasted at least a billion years after the big bang.

    Post–big bang, the universe was filled primarily with neutral hydrogen gas. Light from the first stars and galaxies broke apart, or reionized, the hydrogen atoms. To find out when that happened, researchers look for abundances of neutral hydrogen in the universe's past.

    One way to do this is to study GRBs, which occur when a large, rapidly rotating star collapses. The explosion shoots a beam of radiation into space; if pointed toward Earth, it appears as a flash of gamma rays, followed by an afterglow. Astronomers study this radiation to search for the absorption of telltale wavelengths of light by nearby neutral hydrogen.

    Tomonori Totani, an astronomer at the University of Tokyo, and colleagues measured the spectrum of the afterglow from GRB 130606A, a very distant object that exploded when the universe was just 1 billion years old. Neutral hydrogen made up 10% of the intergalactic gas around the burst—suggesting that the reionization process that ended the dark ages was not yet complete, they report in a paper to be published in June in the Publications of the Astronomical Society of Japan.

  2. Polar Bear Evolution—Fast and Furious


    Polar bears split from the brown bear, their closest relative, less than 500,000 years ago—far more recently than the several million years previously estimated, according to a new genomic study published this week in Cell.

    Researchers from Denmark, China, and the United States sequenced the genomes of 80 polar bears from Greenland and 10 brown bears from a variety of locations. The team also examined the effects of Arctic living on the bears by comparing the species' genomes. Although little more than a blink in time from an evolutionary perspective, 500,000 years was long enough for key genetic differences to evolve between the two species, they found. In polar bears, the most strongly selected genes included several related to lipid processing and to the development of the heart and circulatory system, adaptations that may help polar bears cope with an extremely high-fat diet. One that stood out was a gene called APOB, which helps transfer fat from blood into cells.

    "The work helps us understand what makes a polar bear a polar bear and how fast this evolution occurred," comments Charlotte Lindqvist, an evolutionary biologist at the University at Buffalo in New York.