Imagine if you were to throw your keys up in the air, and instead of slowing and falling back down, they sped up towards the ceiling. As counterintuitive as it might seem, this is one appropriate analogy for the way our universe behaves. According to fundamental laws of physics, since the Big Bang, physicists thought the expansion of the universe had to slow. The universe is full of matter and the attractive force of gravity pulls all matter together. However, in 1998, the Hubble telescope provided evidence that our amazing universe is actually expanding at an accelerating rate.
No one expected this, and no one knew how to explain it, but something was causing it. (Ironically, this Nobel-winning discovery came from a mission whose initial goal was to measure how much this expansion is expected to slow down over time).
Scientists still don’t have an explanation for this mysterious anti-gravity force, but they have given the phenomenon a name: dark energy. We know how energy exists in the universe from studying how it affects the expansion of the universe’s elements. Observing space with the naked eye would suggest that the universe is mostly empty, but dark energy is at work in 70 per cent of those ‘empty’ spaces, relentlessly pushing elements in the universe apart.
There is no direct way to interact with dark energy and measure its properties, and this is a profound problem inunravelling its mystery.
Unlike normal energy, dark energy does not seem to act through any of the fundamental forces of nature other than gravity. The evidence for dark energy is indirect. One of the ways researchers currently infer its existence is by watching massive galaxy clusters, some of the biggest elements in the universe, and mapping their movements to see how dark energy interacts with them.
In an effort to measure this, the South Pole Telescope (SPT), which measures 10 metres in diameter, was built at the southernmost point on Earth in Feb., 2007. The U.S. National Science Foundation-funded SPT initiative is an international collaboration between over a dozen mostly North American institutions, including McGill.
Tijme de Haan, a graduate student in physics at McGill and a lead author of a recent paper submitted to The Astrophysical Journal that analyzed galaxy clusters using SPT data, spoke to the Tribune about the project.
“The SPT is designed to detect the millimetre wavelength of light called the cosmic microwave background (CMB),” he said. “It is the earliest light in the universe emitted when it was 3,000 years old. By inferring its properties, we can capture a snapshot of the universe as it was long ago.”
Scientists believe that studying the CMB enables us to gather clues about the birth, evolution, and eventual fate of the universe. The CMB, which is just leftover radiation from the Big Bang, journeyed throughout the universe for 14 billion years, carrying information about cosmological evolution. It plays a role in mapping the geography of massive galaxy clusters from which we can derive the influence dark energy played in their evolution.
“Our paper is basically about counting galaxy clusters and looking at how many there are as a function of how far away they are and [then] inferring about dark energy,” de Haan said. “CMB leaves ‘shadows’ of very large galaxy structures across [the] history of the universe … Using the data from the SPT, we can go as far back in time as we want to see whether these galaxy clusters were formed very early on or just very recently. It gives us an idea of how fast galaxies were formed over cosmic time.”
Einstein’s cosmological constant was introduced in the theory of general relativity to accommodate a static universe, which was the theory during his era. To keep the universe from collapsing under gravity like a house of cards, Einstein hypothesized there was a repulsive force at work, called the cosmological constant, that counteracted gravity’s tug.
“When Einstein was applying his theory to the universe, he found that there was a factor that allowed for the universe to expand and accelerate, but he set that term to zero because he thought that couldn’t happen for a static universe … Now that we know universe is accelerating, people have started to investigate that term,” Alex van Engelen, another McGill graduate student involved in the project, said.
Since its commission in Feb., 2007, the 28-tonne SPT has looked at 2,500 square degrees of the sky (approximately one-fifth of the southernmost sky), but according to de Haan, only a small fraction of the data gathered has been analyzed. The complete analysis of the full data might bring exciting cosmological breakthroughs.
“As data analysis is going on, and from additional observations from other telescopes, we were able to trace galaxy clusters, and measure the mass of neutrinos—very light, almost massless particles, with some radioactive decays,” de Haan said. “If neutrinos have mass, it slightly changes the class of structures or how structures collapse.”
Such extensive and precise measurements would not have been possible without the SPT.
“First of all, Antarctica is very high up, and there’s a very large ice sheet. So there’s a high elevation and it’s very cold, hence the air there becomes very dry. There’s very little water vapour so the SPTcan give us a clear picture of the sky without being contaminated (water can absorb millimetre wave signals),” explained de Haan.
Hopefully this clear picture will help researchers accurately map the distribution of matter in the universe, and, one day, uncover the secret identity of dark matter.