How gravity gives astronomers a powerful lens on the universe

Newswise — In 1919, astronomers Arthur Eddington and Andrew Crommelin captured photographic images of a total solar eclipse. The Sun was then in the constellation of Taurus and a handful of its stars were visible in the photographs. But the stars weren’t quite in their expected place. The Sun’s enormous gravity had bent the light from these stars, making them appear slightly out of place. It was the first demonstration that gravity could alter the path of light, as Albert Einstein predicted in 1915.

The bend of light by the mass of a star or galaxy is one of the central predictions of general relativity. Although Einstein first predicted the deflection of light from a single star, others like Oliver Lodge argued that large mass could act as a gravitational lens, distorting the path of light in the same way. that a glass lens focuses the light. In 1935, Einstein demonstrated how light from a distant galaxy could be distorted by a galaxy in front of it to create a ring of light. Such an Einstein ring, as it was called, would cause the distant galaxy to appear as a ring or arc of light around the nearest galaxy. But Einstein thought that this effect would never be observed. These arcs of light would be too faint to be captured by optical telescopes. Einstein was right until 1998, when the Hubble Space Telescope captured a ring around the galaxy B1938+666. It was the first observed optical ring, but it was not Einstein’s first ring. The first ring was seen in radio light and was captured by the Very Large Array (VLA).

In 1987, a team of students from the MIT Research Lab in Electronics under Professor Bernard Burke, and led by doctoral student Jackie Hewitt, used the VLA to create detailed images of known radio-emitting objects. One of them, known as MG1131+0456, had a distinct oval shape with two bright lobes. Hewitt and his team considered several models to explain the unusual shape, but only an Einstein ring matched the data. Einstein’s galactic prediction has finally been observed.

Radio astronomy is particularly good at capturing lensed galaxies. They have become a powerful tool for radio astronomers. Just as a glass lens focuses light to make an object appear brighter and larger, so does a gravitational lens. By observing lensed galaxies, radio astronomers can study galaxies that would be too distant and faint to be seen on their own. Einstein’s rings can be used to measure the mass of the nearest galaxy or galactic cluster since the amount of gravitational lensing depends on the mass of the foreground galaxy.

One of the most interesting aspects of the gravitational lens is that it can be used to measure the rate at which the universe is expanding. Light from a distant galaxy can take many different paths as it passes in front of the foreground galaxy. Each of these paths can have different distances, which means that the light can reach us at different times. We might see a burst of light from the galaxy multiple times, each from a different path. Astronomers can use it to calculate galactic distance, and therefore the scale of the cosmos.

Since the VLA first detected an Einstein ring, radio astronomers have found more of them and captured them in greater detail. In 2015, for example, the Atacama Large Millimeter/submillimeter Array (ALMA) captured a detailed image of the lens arcs of a distant galaxy named SDP.81. The image was sharp enough for astronomers to trace arcs back to their source to study star formation in the galaxy.

Einstein’s rings are now commonly seen in astronomical images, especially in deep-field images, such as those from the James Webb Space Telescope and others. As radio astronomy has shown, they are more than beautiful. They give us a new lens on the cosmos.

Brian Koberleina science writer for NRAO. He has a doctorate. in Physics from the University of Connecticut, and has published research in physics and astrophysics. With David Meisel, he is the author of Astrophysics Through Computation, published by Cambridge University Press.

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