Hubble examines dark energy

Why is the expansion of the Universe speeding up? By combining observations from Hubble and super scopes on Earth, astronomers are getting closer to an answer.

 

The Abell 1689 galaxy cluster is 2.2 billion light-years from Earth. Due to its mass (the contribution of dark matter is symbolised in blue), it acts like a gravitational lens and warps the light coming from galaxies located behind it, which then appear as thin arcs of light (details on right). Some of these galaxies are over 10 billion light-years behind the cluster. Dark matter is matter which is invisible, but which gravitational effects are measured. Credit: NASA/ESA, E. Julio (JPL/LAM), P. Natarajan, J-P. Kneib (LAM) - Layout Enjoy Space

Since the Big Bang, the Universe has continued to grow. In addition, the further away a galaxy is from ours, the greater its “escape” velocity proves to be. However, at the end of the 1990s, astronomers discovered something totally unexpected, the expansion of the Universe is speeding up over time!

Mysterious dark energy
What is the cause of this acceleration and how does it work? Scientists do not know and have dubbed the mysterious forces at work “dark energy”. But, it may well be that spectacular progress could soon be made in the matter…  Since, after helping to record the existence of dark energy, the Hubble space telescope is now involved in determining the influence of this strange force.
It all began with an image from the orbital observatory belonging to NASA and the ESA (European Space Agency), an image of the Abell 1689 galaxy cluster. Due to its mass, the cluster is warping space-time and thus acts like a gigantic lens with regards light rays from objects behind it (see diagram below).

The principle of the gravitational lens applied to a cluster of galaxies. Due to its mass, the cluster warps space-time and bends the paths followed by light rays from objects located behind it. Credit: ESA

An effect that Einstein had predicted with his theory of relativity and which has since been observed on many occasions.  Astronomers then used super scopes on Earth (ESO’s VLT in Chile and Keck in Hawaii) to accurately establish the global mass of the cluster and its distance from Earth. Consequently, it is now possible to determine exactly how the Abell 1689 cluster is warping the image of the galaxies located behind it. As a result, any discrepancy recorded between observations and theoretical models reveals the consequence of the acceleration in the expansion of the Universe caused by dark energy. Researchers now hope to quantify the impact of this mysterious force on the geometry of the cosmos in a way that has never before been possible and perhaps also to understand its nature. Is the Universe destined to expand until it is "diluted" to the extreme because of dark energy, becoming increasingly empty and cold, causing a state of heat death? This new strategy for determining the effects of dark energy based on the study of gravitational lenses such as Abell 1689 could give us the answer and lift the veil what is largely considered the greatest mystery of modern astronomy.

Abell 1689 by the Hubble space telescope (left) and the representation of the dark matter in this cluster (right). In the Universe, visible energy/matter accounts for only 4%, the remaining being dark matter (22 %) and dark energy (74 %)! Credit: NASA/ESA, E. Julio (JPL/LAM), P. Natarajan, J-P. Kneib (LAM)

Published 19 August 2010