Even In Another Galaxy, Einstein’s Theory Of Relativity Is Still Correct

This is the Hubble Space Telescope's imaging of the gravitational lens ESO325-G004. NASA, ESA, Hubble Heritage Team (STScI / AURA).

When it comes to understanding gravity and the universe, the equations we use are the ones set out by Einstein over a century ago. General relativity is the best theory we currently have, but that doesn’t mean it shouldn’t be confirmed. A new study has just tested its validity in another galaxy.

An international group of researchers have tested predictions of general relativity in ESO325-G004, or E325 for short, a galaxy 500 million light-years from Earth. The galaxy is what is known as a gravitational lens and objects such as this are perfect test beds for Einstein’s theory. The work is reported in Science.

Gravitational lenses are produced when massive objects deform space-time so that light from background objects can get to us distorted and magnified. General relativity has some very specific predictions of how much the light bends and these depend on the mass of the lensing galaxy.

“We used data from the Very Large Telescope in Chile to measure how fast the stars were moving in E325 – this lets us infer how much mass there must be in E325 to hold these stars in orbit,” lead author Dr Thomas Collett, from the University of Portsmouth, said in a statement. “We then compared this mass to the strong lensing image separations that we observed with the Hubble Space Telescope and the result was just what GR predicts with 9 percent precision. This is the most precise extrasolar test of GR to date, from just one galaxy.”

Gravitational lenses are divided into three broad categories. This galaxy belongs to the strong gravitational lenses, which are known to create stunning displays called Einstein arcs. It is estimated that a few hundred of these strong gravitational lenses are known. Hubble’s Frontier Fields project has given us some of the most famous examples. E325 was chosen because it is relatively nearby, giving the researchers better constraints on its mass.  

The gravitational lens from LRG 3-757 galaxy taken with the Hubble Space Telescope's Wide Field Camera 3. ESA/Hubble & NASA.

“The universe is an amazing place providing such lenses which we can then use as our laboratories," added team member Professor Bob Nichol, also from the University of Portsmouth. "It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was.”

The standard model of cosmology that we use to describe the universe relies on general relativity being correct. The model includes everything we see as well as two mysterious components, dark matter and dark energy, which together account for 94 percent of the universe. We are yet to find out what dark matter and dark energy actually are, so tests like this serve to confirm that these “substances” are real and not just artifacts resulting from a dodgy theory.

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