Dark Energy vs. Dark Matter
What is Dark Energy?
How Can We Find It?
Studying Supernovae
Sound Waves Method
Gravitational Lensing


Light twists its way through a lumpy universe

As a light ray travels from a distant galaxy to Earth, it gets pushed around a little. The gravity of intervening objects, such as galaxies, galaxy clusters, or clouds of dark matter, "warps" the space around it. This warp deflects the path of any passing light ray. Across billions of light-years of space, a light ray can have many such gravitational encounters, so its path across the universe is twisted.

This effect is called gravitational lensing, and it could provide important clues to the nature of dark energy.

There are two types of gravitational lensing.

Strong lensing clearly distorts the view of distant galaxies, sometimes producing several images of a single galaxy. Strong lensing occurs when a distant galaxy lines up directly behind a massive galaxy or galaxy cluster, which exerts a strong gravitational pull.

strong gravitational lensing

A Hubble Space Telescope image shows a strong gravitational lensing effect. The gravity of a cluster of galaxies distorts the view of more distant galaxies into several bright arcs. Weak lensing, which will be used to study dark energy, produces much more subtle effects. [NASA/STScI]

Weak lensing occurs when the light from a distant galaxy passes a good distance from a massive galaxy, galaxy cluster, or dark-matter concentration, or closer to less-massive objects . It produces a slight distortion in the shape of a distant galaxy. The effect is so subtle that you can’t notice a difference just by looking at the galaxy. Instead, astronomers must analyze the shapes of millions of galaxies to search for patterns. These patterns will allow them to produce three-dimensional maps of the distribution of matter throughout the universe.

These maps will clearly show the distribution of dark matter. But they also will help scientists understand the nature of dark energy.

As with other search methods, weak lensing will allow astronomers to probe much of the history of universe, from shortly after the Big Bang to today. That will show how the distribution of matter has changed over time — a function that is controlled in part by dark energy.

Dark energy causes space itself to expand, so the universe gets bigger. As it does so, matter becomes more thinly distributed and the light that provides the weak lensing has to travel a greater distance across the universe. Since astronomers know how much matter the universe contains, measuring both how widely it is spread out and the distance traveled at different times will show how the universe has expanded. Different models for dark energy predict different expansion histories, so determining how the universe has expanded will help select the correct explanation.

At the same time, dark matter would have required that galaxies clump together to form large-scale structures like clusters and superclusters early in the universe, when matter was more densely packed and gravity overwhelmed dark energy’s repulsive force. As the universe grew, dark energy became more dominant, preventing galaxies from forming large clusters. The weak-lensing method will show when galaxies stopped clumping together, indicating how and when dark energy exerted its dominance over gravity.

Sound Waves Method

Weak Gravitational Lensing

Weak gravitational lensing causes many subtle shifts in the path of a light ray as it crosses the universe. This diagram shows the path of light from several galaxies (blue ovals at the back of the box) as it journeys toward us (the blue ovals at the front, showing the position of the galaxies in a telescopic view).