Do we have gravitational lenses

What gravitational lenses reveal about space

The largest known accumulations of matter in the universe are galaxy clusters. Due to their enormous attraction, they even deflect the light - they act like a gigantic lens.

Galaxy clusters combine a multitude of huge galaxies and dominate large parts of their surroundings with their enormous attraction. They even bend the light from objects behind them. Because gravity, gravitation, is responsible for the deflection here, one speaks of gravitational lenses.

On Earth, astronomers see the object lying far behind the cluster distorted, enlarged and, in some cases, significantly amplified. Often researchers also see several images of a single object - what sounds like a cosmic sleight of hand is one of the most spectacular and aesthetic phenomena in the entire cosmos.

The galaxy cluster Abell 2218

A particularly beautiful example is the Abell 2218 galaxy cluster observed by the Hubble Space Telescope. The cluster is located two billion light-years away in the constellation Dragon - its galaxies appear in the image in yellowish tones, typical for quite old stars. The massive elliptical galaxies cluster in the center of the cluster - smaller cluster members can be seen in the entire field of vision.

At the top right there seems to be another small cluster center. A clear sign that two galaxy clusters are merging here. Dozens of small arcs extend around the cluster in many colors and shapes - the bent light of young galaxies more than six billion light years behind Abell 2218. Without the amplifying and magnifying effect of the gravitational lensing effect, these young galaxies would not be visible at all. Abell 2218 is a natural magnifying glass - a huge natural telescope!

The galaxy cluster Cl2244-02

In the case of the cluster of galaxies Cl2244-02, astronomers have used the Very Large Telescope of the European Southern Observatory ESO to examine a huge glowing arc. This shows an enormous amount of detail. Thanks to the precise VLT technology, the researchers were able to use the spectra to determine the distance between the cluster and the object being lined up: the cluster is almost five billion light-years away, the arc originates from an object that is almost twelve billion light-years away, believe it or not.

Weigh galaxy clusters with gravitational lenses ...

Gravitational lenses have long been an indispensable tool for astronomers. Because in “lined up” images there is also information about the heap that acts as a lens. The researchers simply count the galaxies in the observed cluster and use them to estimate its mass. It is immediately noticeable that the visible matter is nowhere near enough to produce the observed effects. The galaxy cluster must be full of matter that we cannot see. The bright, yellow galaxies that cluster there are only a very small fraction of the actual matter.

But there is another, literally all-encompassing aspect: Gravitational lenses reveal a lot about the structure of the cosmos as a whole. The large-scale structure of matter in space directly influences the image splitting, i.e. the distance between the multiple images. In order to make statements about the structure of the universe, astronomers have to examine many gravitational lenses. Since such multiple images of background galaxies are unfortunately quite rare, the researchers switch to quasars, the most luminous and most distant objects in space - more gravitational lenses are known from them.

Researchers measure the splitting of the images and then compare their results with values ​​that models predict about the structure of the world. Joachim Wambsganß, astronomer at the University of Potsdam, has been working with gravitational lenses for years - from the number and splitting of the multiple quasars he draws conclusions about the type, amount and distribution of matter in the universe: “If there were very many galaxy clusters in the universe, then one would be very much expect many multiple quasars with large splits. However, if there are fewer galaxy clusters and many more small, compact structures like galaxies, then one would expect smaller splits. "

Bent light and the expansion of the cosmos

The number of quasars depicted several times and the splitting of their images thus unravels the distribution of matter in the cosmos. Because quasars are extremely bright and can therefore still be seen at a great distance. Even if the galaxy cluster acting as a lens could no longer be seen because it is too far away, the lensed quasar should still be noticed. “It looks like the so-called Standard Cold Dark Matter Universe - one of the best studied cosmological models - would produce very many large split images. But you don't find it in the real universe, ”reports Wambsganß. The number of known gravitational lenses is still too small to make really reliable statistical statements about the models of the structure of the world.

Make four out of one

In the case of multiple images of quasars, it is not only their splitting that is of interest. The light of the individual images travels different ways through space - in some cases the distances are even of different lengths. This can be measured when the quasar shown several times flickers a little - then the images do not flicker simultaneously, but one after the other, because the light in the individual images travels for different lengths of time. With some quasars - many of which have strong fluctuations in brightness - the "light transit time" differs by more than 400 days!

In the mid-1960s, the young Norwegian astronomer Sjur Refsdal - now a professor in Hamburg - developed a method for determining the Hubble constant directly from the time delay in the images (provided you know something about the galaxy acting as a lens). The Hubble constant is one of the fundamental quantities for our understanding of the structure and development of the universe - and the subject of decades of heated debate. The flickering of a quasar twelve billion light years away provides researchers with one of the most sought-after parameters in astronomy.

Incidentally, when this method was developed, gravitational lenses were still purely hypothetical objects - the first lens was only discovered in 1979. Within just a few years, gravitational lenses have become one of the big issues in astronomy. There are "flickering" double quasars - but observations are tedious. For years you have to measure the brightness of the individual quasar images every night if possible, but at least once a week. The large telescopes are only awarded for a few nights - bad chances for such an elaborate project. To date, only a few really good "observation series" have come about. A few more experiments are in progress - there may be a promising direct value of the Hubble constant soon.

Crooked images due to strong masses

In the case of multiple images and spectacular arcs, astronomers speak of the strong gravitational lensing effect. In the case of the “weak gravitational lensing effect”, the images of the galaxies that are close to a mass accumulation when viewed from the earth are only slightly deformed. The light of distant galaxies does not pass a massive pile of dark matter without being disturbed. Astronomers therefore pay attention to the shape of the galaxies in the sky. What has made its way as a sharp image of an elliptical-looking galaxy reaches the earth slightly distorted, as it were in the shape of a banana.

The VLT in search of dark matter

The dark matter between us and the galaxy has in a way distorted the picture - just like the floor tiles of a swimming pool appear deformed when viewed through the water; only this effect is much, much smaller in the sky, explains Peter Schneider, astronomer at the University of Bonn: “The typical changes in the image ellipticities are in the range of one to two percent. These are effects that are very difficult to measure, and we are slowly getting into the field of precision cosmology. "

Measuring a single galaxy is useless - it could also have an unusual shape purely by chance. Peter Schneider's team is looking for areas in the sky in which all galaxies are characteristically deformed: “Our best result at the moment is based on around half a million galaxies whose shape has been measured. But these are not so dramatic numbers because we live in a universe in which, if you look deep enough, the sky is full of faint galaxies. "

Astronomers are now drawing almost cosmic maps of how dark matter pervades space. “On the matter maps we also see clearly pronounced density maxima, which is an indication that there is a concentration of dark matter at the point, for example a cluster of galaxies. Some of them have now also been discovered in visible light and identified directly with galaxy clusters, ”explains Schneider. So the scientists have also found large clusters of glowing galaxies where there must be a lot of dark matter.

“The results that come out of this method form one of the pillars of modern cosmology. We measure the distribution of dark matter at a time when the universe was around ten billion years old, ”says Schneider. Dark matter has spun a vast web of walls and piles over time. The shining galaxies are, as it were, only the whitecaps in the sea of ​​dark matter.