Hubble Proves Einstein Right on Galactic scales

Credit: ESO, ESA/Hubble, NASA

Using the NASA/ESA Hubble Space Telescope and European Southern Observatory’s Very Large Telescope (VLT), a team led by Thomas Collett (University of Portsmouth, UK), was able to perform the most precise test of general relativity outside the Milky Way to date.

The theory of general relativity predicts that objects deform spacetime, causing any light that passes by to be deflected and resulting in a phenomenon known as gravitational lensing. This effect is only noticeable for very massive objects. A few hundred strong gravitational lenses are known, but most are too distant to precisely measure their mass. However, the elliptical galaxy ESO 325-G004 is amongst the closest lenses at just 450 million light-years from Earth.

Einstein’s general theory of relativity predicts that objects deform spacetime, causing any light that passes by to be deflected. This effect is only noticeable for very massive objects. One result of this deformation of spacetime is that light from distant sources is deflected around a massive intervening object, such as a galaxy. ESO 325-G004 is the large haze of light in the centre of the screen which is deforming light from the background galaxies.

Credit: ESO/L. Calçada

Using the MUSE instrument on the VLT the team calculated the mass of ESO 325-G004 by measuring the movement of stars within it. Using Hubble the scientists were able to observe an Einstein ring resulting from light from a distant galaxy being distorted by the intervening ESO 325-G004. Studying the ring allowed the astronomers to measure how light, and therefore spacetime, is being distorted by the huge mass of ESO 325-G004.

This infographic compares the two methods used to measure the mass of the galaxy ESO 325-G004. The first method used ESO’s Very Large Telescope to measure the velocities of stars in ESO 325-G004. The second method used the NASA/ESA Hubble Space Telescope to observe an Einstein ring caused by light from a background galaxy being bent and distorted by ESO 325-G004. By comparing these two methods of measuring the strength of the gravity of ESO 325-G004, it was determined that Einstein’s general theory of relativity works on extragalactic scales — something that had not been previously tested.

Credit: ESA/Hubble, ESO, NASA
Collett comments: “We know the mass of the foreground galaxy from MUSE and we measured the amount of gravitational lensing we see from Hubble. We then compared these two ways to measure the strength of gravity — and the result was just what general relativity predicts, with an uncertainty of only nine percent. This is the most precise test of general relativity outside the Milky Way to date. And this using just one galaxy!”

General relativity has been tested with exquisite accuracy on Solar System scales, and the motions of stars around the black hole at the centre of the Milky Way are under detailed study, but previously there had been no precise tests on larger astronomical scales. Testing the long range properties of gravity is vital to validate our current cosmological model.

This video pans across NASA/ESA Hubble Space Telescope observations of the elliptical galaxy ESO 325-G004 that lies about 450 million light-years away. The galaxy is part of a diverse collection of galaxies in the cluster Abell S0740.

Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

These findings may have important implications for models of gravity alternative to general relativity. These alternative theories predict that the effects of gravity on the curvature of spacetime are “scale dependent”. This means that gravity should behave differently across astronomical length-scales from the way it behaves on the smaller scales of the Solar System. Collett and his team found that this is unlikely to be true unless these differences only occur on length scales larger than 6000 light-years.

“The Universe is an amazing place providing such lenses which we can use as our laboratories,” adds team member Bob Nichol (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.”

This image from the NASA/ESA Hubble Space Telescope shows the diverse collection of galaxies in the cluster Abell S0740 that is over 450 million light-years away in the direction of the constellation Centaurus. The giant elliptical ESO 325-G004 looms large at the cluster’s centre. Hubble resolves thousands of globular star clusters orbiting ESO 325-G004. Globular clusters are compact groups of hundreds of thousands of stars that are gravitationally bound together. At the galaxy’s distance they appear as pinpoints of light contained within the diffuse halo. This image was created by combining Hubble science observations taken in January 2005 with Hubble Heritage observations taken a year later to form a 3-colour composite. The filters that isolate blue, red and infrared light were used with the Advanced Camera for Surveys aboard Hubble.

Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

This research was presented in a paper entitled “A precise extragalactic test of General Relativity” by Collett et al., to appear in the journal Science.

Thomas Collett discusses the methods behind his research into whether Einstein’s general theory of relativity is correct on extragalactic length scales. This research was published in the journal, Science, in June 2018

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Credit: University of Portsmouth
The international team is comprised of: Thomas E. Collett (Institute of Cosmology and Gravitation, University of Portsmouth, UK), Lindsay J. Oldham (Institute of Astronomy, University of Cambridge, UK, and Harvard College, Harvard-Smithsonian Center for Astrophysics, USA), Russell Smith (Centre for Extragalactic Astronomy, University of Durham, UK), Matthew W. Auger (Institute of Astronomy, University of Cambridge, UK), Kyle B. Westfall (ICG, Portsmouth, UK, and University of California, Santa Cruz, USA), David Bacon (ICG, Portsmouth, UK), Robert C. Nichol (ICG, Portsmouth, UK), Karen L. Masters (ICG, Portsmouth, UK), Kazuya Koyama (ICG, Portsmouth, UK) and Remco van den Bosch (Max Planck Institute for Astronomy, Garching, Germany).
Links

Contacts and sources:
Thomas Collett
University of Portsmouth

Bob Nichol
University of Portsmouth

Mathias Jäger
ESA/Hubble, Public Information Officer

Science paper