Since its discovery in 2008, astronomers have been puzzled by a cosmic mystery so vexing that it has even led some to question whether the general theory of relativity – Einstein’s masterpiece theory of gravity – is wrong on cosmic scales. The trouble is that light travelling through the universe does not seem to be affected by the gravity of large structures such as galaxy clusters in the way that Einstein had predicted.
Now we have created the largest ever map of the universe’s voids – empty regions or “holes” in space – and superclusters, which are regions with more galaxies and matter than average. This has proven Einstein right, but has reintroduced another mystery.
To understand the origin of the puzzle, we need to grasp the subtle effect that gravity has on the leftover radiation from the primordial hot plasma at the birth of the universe, known as the “cosmic microwave background”. This radiation has been travelling through the universe for 13.8 billion years, before finally being picked up by instruments on the Planck satellite – allowing us to create temperature maps that have been crucial in understanding the universe. On their journey, the light particles, or photons, of this radiation have encountered both voids and superclusters in the universe.
Einstein’s theory tells us that light experiences the effects of gravity just like matter. When a photon enters an empty void, it at first loses energy due to the greater attraction of the mass behind it. After crossing the halfway point, it then gains energy again due to the pull of matter on the other side. The net effect is similar to the change in speed a runner would experience if she ran up and down an intervening hill on her route.
Sloan Digital Sky Survey
But what if the size of the hill changed while she was running? In fact, the universe is expanding, which stretches the “hill”, reducing its height while the photon is crossing the void. The analogy with our runner would be if she had to run further uphill than downhill on the other side: she’d slow down more going up than she sped up coming back down. Similarly, photons from the cosmic microwave background passing through voids lose a tiny bit of their energy in the process, thus appearing very slightly cooler. Conversely, photons passing through superclusters appear very slightly hotter than normal. This is called the “integrated Sachs-Wolfe effect”.
However, in 2008 a team of astronomers from the University of Hawaii attempted to measure this effect. To do so, they first identified 50 individual voids and superclusters each in the distribution of galaxies in the sky, and then measured the average temperature of each type of structure using Planck’s cosmic microwave background map. Sure enough, the radiation appeared colder when seen through voids than through superclusters. However, the apparent size of the effect was more than five times larger than predicted by calculations.
This apparent drastic failure of the theory has proved very hard to understand. Alternative theories proposed to explain it have included a completely new model of the Big Bang, a different interpretation of the properties of “dark energy” – the theoretical form of energy causing the expansion of the universe to accelerate – or even an overhaul of the theory of gravitation itself.