A new study looking at the large-scale structure of the Universe has found that it is, in fact, flat, with a constant dark energy throughout space. Dark energy is a mysterious material that makes up around 70% of the Universe and is the driving force behind its accelerated expansion. The new measurements, combined with well-established methods, have provided the best current measure of the dark energy content of the Universe and its “flatness”- described scientifically as “curvature”.
The study, led by scientists from the Institute of Cosmology and Gravitation at the University of Portsmouth, looked at the shapes of large, mostly empty expanses of space containing few galaxies, known as voids. Cosmic voids are, on average, spherical, with sizes between 5 and 20 megaparsecs, or 150-620 quintillion kilometres (with eighteen zeros!). Their spherical shape means that observing them to be stretched can give insight into the expansion and curvature of the Universe.
The “flatness” of the Universe is not a new problem- it has long been up for debate. A Universe with positive curvature is called a closed Universe, which can be thought of like a sphere. A Universe with negative curvature is called an open Universe- imagine the shape of a saddle. A flat Universe has no curvature, yet this isn’t as simple as it seems. This does not necessarily mean that the Universe is like a piece of paper stretching to infinity- it can be bent into shapes like a torus, while still being classed as cosmologically flat. In everyday life we know that the angles inside a triangle sum to 180 degrees, and when drawing three points far apart in a flat Universe, this is true. However, in curved space, the angles in this triangle won’t add up to 180 degrees.
The dark energy content of the Universe and curvature are linked by cosmological models. The model to best describe these strange properties of our Universe is obtained by combining the results of multiple techniques, as is shown in the new paper. The oldest method of probing the expansion and curvature of the Universe comes from the observation of massive exploding stars, known as supernovae. Baryon acoustic oscillations (BAO) are another useful tool, which are remnants of sound waves that travelled through the Universe not long after the Big Bang. Measurement of the cosmic microwave background (CMB)- radiation that remains from the early Universe- is the final common technique.
The simplest model of dark energy is known as the cosmological constant, named for its uniformity in space and time, which is favoured by the data in the new study. Other forms of dark energy have been proposed that are dynamic in space and time, but these are disfavoured by the majority of studies. Though still little is known about the elusive dark energy, what we do know is that dark energy is driving the observed accelerated expansion of our Universe- but will it keep expanding forever?
The fate of our Universe is related to its curvature. A closed Universe will expand to a maximum size before collapsing in a “Big Crunch”- the opposite of the Big Bang. An open Universe will expand forever, with the effect of dark energy ripping the Universe apart or cooling the Universe to a “Big Freeze”. A flat Universe, as ours is thought to be, will similarly expand forever. None of these possible endings sound idyllic, however all of which would occur long after our own Sun has stopped shining and so are not worth worrying about.
The field of cosmology is a cutting-edge one, filled with questions that are yet to be answered. The lead author of the paper, Dr Seshadri Nadathur, has said: “This result shows the power of galaxy surveys to pin down the amount of dark energy and how it evolved over the last billion years. We’re making really precise measurements now and the data is going to get even better with new surveys coming online very soon.”
Illustration by Puya Mirkarimi