THE structure of the universe seems to have undergone a dramatic rearrangement a few billion years after its creation. Startling results announced by the most comprehensive galaxy-mapping project ever undertaken suggest that the distribution of matter in the universe shifted suddenly from very dense clusters, to being much more evenly dispersed. The finding leaves astronomers at a loss to explain what could have caused the change.
Researchers at the Sloan Digital Sky Survey are using a pair of dedicated telescopes at Apache Point, New Mexico, to create a detailed three-dimensional map of the position and brightness of 100 million objects, mostly galaxies, spanning a quarter of the sky. Even in its incomplete state 鈥� with 15 million objects mapped so far 鈥� the project dwarfs previous surveys of galaxy distribution, which have never encompassed more than a few hundred thousand galaxies.
Tamas Budavari of Johns Hopkins University in Baltimore, Maryland, and his colleagues used a series of filters to classify the galaxies picked up by the survey according to their colours, which correlate with shape and age. Redder galaxies are older, elliptical and larger. Having formed very soon after the big bang, they have already gone through their episodes of star formation and are mostly populated with older, stable stars. Bluer galaxies are smaller spirals that formed a few billion years or more later, and are still in the throes of active star formation.
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The researchers already knew that the early, red galaxies that formed when the Universe was smaller and denser are clustered more strongly than the later, blue ones, which would have been farther apart when they formed and therefore won鈥檛 have had enough time to form such dense clusters. They expected to see a gradual change in the pattern of distribution as they moved down the scale from older galaxies to younger ones.
But when the researchers looked at galaxies on different parts of the scale, they found that their distribution was far from continuous. Instead they saw a sudden switch between two very distinct populations, with the older, redder galaxies far more intensively clustered. 鈥淭his is surprising,鈥� says Andrew Connolly of the University of Pittsburgh, a co-investigator on the project. 鈥淚 would have expected there to be a whole range of clustering signatures, not just two.鈥�
This means that at some point in the past, galaxies suddenly started clumping differently. And not just galaxies. Physicists believe that most matter in the universe is actually invisible, or 鈥渄ark鈥�, to our telescopes. They assume that gravity causes dark matter to clump in the same way as normal matter, so its distribution will mirror that of galaxies. If the team鈥檚 finding is correct, it has uncovered a dramatic shift in the structure of all matter in the universe.
The result is so unexpected, no researchers contacted by 快猫短视频 were willing to speculate on what could have caused this shift. Perhaps the rate of expansion of the universe changed suddenly, or the strength of gravity shifted. But it鈥檚 more likely to reflect something like a phase shift, where the density of matter crossed a threshold beyond which it suddenly started clumping differently. Whatever the cause, the team is convinced the answer will be fundamental to explaining why the universe now looks the way it does. 鈥淭he clustering is directly related to the distribution of matter in the universe today,鈥� says Budavari.
