General Relativity Yet Stays Unscarred, But Now on an Intergalactic Scale4 min read

In 1915 Abert Einstein proposed the General Theory of Relativity. Which, fundamentally, explains the workings of gravity around the cosmos. It postulates that massive bodies, like planets and stars, make steeper dips in the fabric of spacetime — space and time taken as a single entity, and those dips are the cause of gravity. Which, in turn, is the reason for the placement of planets and stars, the compactness of a galaxy, and why you are not floating in your room.

Apart from the theoretical part, GR is the reason for our smooth life due the the Global Positioning System, or GPS. Other than that General Relativity also explains Mercury’s eccentric orbit, which, by the way, Newton wasn’t able to.  Moreover, the recently discovered gravitation waves, which made quite a news-fuss last year, were predicted by GR a century back.

Although GR has many practical life applications, it’s has some limitations too, i.e. conditions inside a black hole or during the first instants of the Big Bang. But more importantly it has a troubled relationship with dark matter– a type a matter no one knows anything about except it’s theoretical existence.

Nature has been giving hints about dark matter for quite a time. In 1929 it was discovered that the Universe is expanding. But things got weird when, in 1983, two teams of astronomers revealed, which they later got a Nobel Prize on, that the Universe is expanding faster now than it was before. For this acceleration of the Universe’s expanding, dark energy was accused. However, this notion relies on GR, which is one reason why GR gets so much scientific attention.

Another reason for its popularity is the fact that GR has gone through intense validation in the past few decades, but still remains unscarred. Think of it as, if you could prove GR wrong, it would be a breakthrough with enough impact to tremble the very foundation of scientific community.

For this reason, a team of astronomers, led by Dr Thomas Collett of the Institute of Cosmology and Gravitation at the University of Portsmouth, used the phenomenon of gravitational lensing, to create a test of GR on an intergalactic scale. Let’s break down the process.

General Relativity, as we know, proposes that massive objects curve the fabric of spacetime to a great extent. If we consider the galaxy to be one single object, then a beam of light passing by it would experience a change in its route, due to the gravity of this object. Now if two galaxies are aligned along our line of sight, the galaxy in the foreground would act as a lens. Thus, this phenomenon is known as gravitation lensing.

The light emitted by the galaxy in the background, is deflected by the galaxy in the foreground which is acting as a lens. Thus, we can say that multiple “images” of the background galaxy is obtained. In this case all this is done with the help of Hubble Space telescope. At this point the astronomers calculated the separation between different images.

Although a few hundred strong gravitational lenses are known, but most are too distant to precisely measure their mass, so they can’t be used to accurately test GR. However, the recently registered galaxy ESO325-G004 is amongst the closest lenses.

After that, the astronomers, with the help of the Very Large Telescope (VLT) in Chile, measured how fast the stars were moving in the foreground galaxy. By this data, they calculated how much mass this galaxy must have to hold these stars in orbit. Finally this was compared to the data of image-separations obtained with the help of Hubble Space telescope. This result was, just as GR predicts, with only 9 percent error margin.

This error, as the researchers note, associated with the errors in the measurements of cosmological constants and the velocities of the stars in the ESO 325-G004, and they can be eliminated by watching other gravitational lenses.

It can also be said that these results, however indirectly, support the validation of GR with respect to the dark matter and dark energy issue.

“The Universe is an amazing place providing such lenses which we can then use as our laboratories,” adds team member Professor Bob Nichol, Director of the Institute of Cosmology and Gravitation. “It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was.”

The research is published in the journal Science.


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