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Nobel-prize winning theory put to the test
MEASURING DISTANCES IN SPACE USING BLACK HOLES
A person who watches the starlit sky at leisure will be able to observe how the flickering celestial objects
slowly move across the firmament. Viewed from Earth, space looks serene. No indication of galaxies breaking
apart at breath-taking speed, of planets careering around stars and of black holes devouring matter. No
indication? A person who looks closely will find evidence of the celestial bodies’ dynamics and evolution.
Prof Dr Rolf Chini’s team from the Institute of Astronomy does look closely. The researchers watch certain areas
of the sky over a period of weeks and months, searching for, for example, periodic brightness oscillation or shifting
spectral lines which they use to uncover binary stars. Because of their irregular brightness bursts, active black holes
(info), so-called quasars, are likewise under scrutiny from Bochum’s telescopes located in the Atacama Desert in
Chile.
The astronomers are working, for example, on a new method for measuring the distance between quasars and
Earth. They could thus reassess the theory regarding the expansion of the universe, which was awarded the Nobel
Prize in Physics in 2011, and thus gain fundamental knowledge about the universe. Using supernova explosions,
three cosmologists had calculated that the expansion of the universe is speeding up. That method is based upon the
assumption that all supernovas have the same level of brightness. “Our distance measurement would do without
such an assumption,” says Chini. “I don’t wish to upset the worldview, but we would like to have an independent
method for determining distances in space, in addition to the supernova measurements.” In four to five years,
estimates the scientist from Bochum, he might have enough data to create a statistics.
Until then, other interesting conclusions may be drawn from the measurements, for example those regarding the
structure of black holes. Or more precisely: the structure of their surroundings. “A black hole is something infinitely
small, a thing that has a lot of mass but no expansion,” describes Chini. “We wanted to know what the vicinity of a
black hole looks like.”
Matter does not simply fall directly into a black hole; rather, it moves in a spiral towards its centre. In the vicinity
of the black hole, matter gathers, forming the so-called accretion disc. It is surrounded by a dust ring. In 2014, Chini’s
team was the first one to describe the form of the dust mantle, thus solving the decade-old astronomical puzzle
regarding the structure of quasars.
The researchers proceeded as follows: when matter falls onto the accretion disc, it lights up brightly. Light in the
visible spectrum reaches the observers on Earth directly from the accretion disc. A certain percentage of the light,
however, crashes into the dust mantle surrounding the accretion disc and heats the dust. The dust emits that heat in
the form of infrared radiation which can likewise be measured from Earth. However, it arrives there several weeks
later than the accretion disc’s visible light. Based on that precisely measured time delay, Chini's team determined for
the first time the internal concave form of the dust mantle and, consequently, the correct distance from the black
hole which now corresponds with the theoretical prognoses.
Julia Weiler
Info
QUASARS: ACTIVE BLACK HOLES
The centres of most galaxies contain a black hole which might be up to several hundred million times heavier
than our sun. It pulls in all matter that approaches it too closely. Before the matter disappears forever, it falls onto
the gas disc surrounding the black hole and lights up brightly one last time. The bright galaxy cores are also referred
to as quasars. About ten per cent of all galaxy cores are active, that means that the black hole in their centre sucks in
a lot of matter. Whether or not a galaxy is active changes in the course of time. Once a black hole has sucked in all
stars and all gas in its immediate surroundings, it becomes inactive for a while – until new celestial objects move into
its vicinity that it can devour. The black hole in the centre of the Milky Way is quiet at present; however, it may
become active again.