The stuff that makes up our Universe is tricky to measure, to put it mildly. We know that most of the Universe's matter-energy density consists of dark energy, the mysterious unknown force that's driving the Universe's expansion. And we know that the rest is matter, both normal and dark.
Accurately figuring out the proportions of these three is a challenge, but researchers now say they've performed one of the most precise measurements yet to determine the proportion of matter.
According to their calculations, normal matter and dark matter combined make up 31.5 percent of the matter-energy density of the Universe. The remaining 68.5 percent is dark energy.
"To put that amount of matter in context, if all the matter in the Universe were spread out evenly across space, it would correspond to an average mass density equal to only about six hydrogen atoms per cubic meter," said astronomer Mohamed Abdullah of the University of California, Riverside and the National Research Institute of Astronomy and Geophysics in Egypt.
"However, since we know 80 percent of matter is actually dark matter, in reality, most of this matter consists not of hydrogen atoms but rather of a type of matter which cosmologists don't yet understand."
Understanding dark energy is actually crucial to our understanding of the Universe. We don't know what it is, exactly - the 'dark' in the name refers to that mystery - but it appears to be the force that drives the expansion of the Universe, the velocity of which has proven incredibly difficult to narrow down past a certain point.
Once we have a better understanding of the expansion rate, that will improve our grasp of the evolution of the Universe as a whole. Hence, constraining the properties of dark energy is a pretty important undertaking for cosmology in general, and there are a number of ways to do so.
Abdullah and his team employed a method based on the way things move around in galaxy clusters - groups of up to thousands of galaxies gravitationally bound together.
Generally, galaxy clusters are a good tool for measuring matter in the Universe. That's because they're made up of matter that has come together over the lifetime of the Universe, about 13.8 billion years, under gravity.
The number of clusters we can observe in a volume of space is highly sensitive to the amount of matter, so counting them can give a reasonable measurement. But, again, that's not a simple task.
"A higher percentage of matter would result in more clusters," Abdullah said.
"The 'Goldilocks' challenge for our team was to measure the number of clusters and then determine which answer was 'just right'. But it is difficult to measure the mass of any galaxy cluster accurately because most of the matter is dark so we can't see it with telescopes."
The team found a way around this problem with a technique called GalWeight. It uses the orbits of galaxies in and around a cluster to determine which galaxies actually belong to any given cluster, and which do not, with over 98 percent accuracy. This, they said, provides a more accurate census of that cluster, in turn leading to a more accurate mass calculation.
"A huge advantage of using our GalWeight galaxy orbit technique was that our team was able to determine a mass for each cluster individually rather than rely on more indirect, statistical methods," explained astronomer Anatoly Klypin of New Mexico State University.
The team applied their technique to observations collected by the Sloan Digital Sky Survey, and created a catalogue of galaxy clusters. These clusters were then compared to numerical simulations of galaxies to calculate the total amount of matter in the Universe.
The team's result - 31.5 percent matter and 68.5 percent dark energy - is in close agreement with other measurements of the Universe's matter-energy density.
"We have succeeded in making one of the most precise measurements ever made using the galaxy cluster technique," said astronomer Gillian Wilson of UC Riverside.
"Moreover, this is the first use of the galaxy orbit technique which has obtained a value in agreement with those obtained by teams who used noncluster techniques such as cosmic microwave background anisotropies, baryon acoustic oscillations, Type Ia supernovae, or gravitational lensing."
This result, the team says, demonstrates that GalWeight could prove to be a very useful tool for continuing to probe and constrain the cosmological properties of the Universe.
The research has been published in The Astrophysical Journal.