Dark energy doubts

In 1998, physicists discovered from observations of distant exploding stars that the universe is not only expanding but also accelerating in its expansion instead of slowing down due to the gravitational interactions between neighbouring galaxies, which was the generally held belief. This was independently confirmed by measurements of the cosmic microwave background and the distribution of galaxies in the sky, which implied that some mysterious force was acting against the pull of gravity, causing galaxies to fly away from each other at ever increasing speeds.

This acceleration was attributed to 'dark energy', a dominant force with a strong repellent action that scientists can to this day neither prove nor understand, but is widely thought to account for up to two-thirds of the entire universe. Einstein introduced dark energy to his famous equations as a way of obtaining a 'cosmological constant', but scientists and theorists alike have struggled to fully explain dark energy.

Dr Weller, Olga Mena and Jose Santiago from Fermilab have studied a new model of gravity that discounts the existence of dark energy. The analysis, published in 'Physical Review Letters' shows that one can account for the acceleration of the universe without any need for dark energy, by relying instead on modifications to the laws of gravity.

According to the team, their modified model of gravity would not make any noticeable difference to the current model in near-space, but further towards the outer parameters of the observable universe - where the effects of acceleration are apparent - the effects of the new model would become gradually more exaggerated.

Dr Weller explained: "At these distances, the curvature of space is so small that the universe appears flat. These modifications 'turn on' at very large distances or small curvatures, giving a geometrical origin to the accelerated expansion of the universe."

According to the model, Newton's law of gravity is not modified within the solar system, meaning that the team's theory fits with all existing solar system experiments and theories. Even Einstein's theory of relativity stands up to modification, as these measurements were calculated at small distances. Despite this, the team's model still allows for around 25 per cent of the universe to be made up of 'dark' or 'invisible' matter.

The UCL Astrophysics Group is also involved in the Dark Energy Survey, which aims to map 300million galaxies in order to measure dark energy or alternatives to dark energy.

• Links:
• UCL Physics and Astronomy
• Fermilab
• Full paper
• Dark Energy Survey