Scientists are claiming that they have calculated the total amount of matter in the Universe. 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 the matter; both normal and dark.
Accurately figuring out the proportions of these three is a challenge; but researchers now say they have performed one of the most precise measurements; yet to determine the proportion of matter.
According to the 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.
Astronomer Mohammad Abdullah of the University of California, Riverside and the National Research Institute of Astronomy and Geophysics in Egypt said that “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.”
He added, “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 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 are 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. Abdullah says “A higher percentage of matter would result in more clusters. 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 the telescope.”
The team’s result – 31.5 percent matter and 68.5 percent dark energy; is in close agreement with other measurement;s of the Universe’s matter-energy density.
Astronomer Gillian Wilson of UC Riverside said that “We have succeeded in making one of the most precise measurements; ever made using the galaxy cluster technique. Moreover, this is the first use of the galaxy orbit technique; which has obtained value in agreement with those obtained by teams; who used non-cluster techniques such as cosmic microwave background anisotropies, baryon acoustic oscillations; Type la supernovae or gravitational lensing.”
