The interplay between dark energy and the shape of the universe leaves the fate of the cosmos hanging in the balance, warns physicist Pedro Ferreira.
We live in a special time. For the past two decades, most of my colleagues and I have been working under the assumption that we can know everything about the universe. We know the amount of matter and energy it contains. We know its shape is flat. We can trace its history from the earliest moments after the big bang and we can even predict its fate. Or at least we thought we could.
Why were we so confident? Exquisite measurements of the radiation left over from the big bang led us to believe that we could work out the curvature of the universe to within a few per cent. In doing so, we have determined how much energy the universe contains and that most of it is in an exotic form called dark energy, which is driving the expansion of space.
However, recent discoveries have left me wondering if these claims were premature. As we learn more about dark energy and its effect on the expansion of space and time, we find that dark energy and the shape, or geometry, of the universe are worryingly intertwined.
By changing our assumptions about dark energy we can radically modify our constraints on the shape of the universe. Equally, without a much more precise measurement of the geometry, it is impossible to determine the nature and evolution of dark energy. Our picture of the universe has, to some extent, been blown wide open again.
This state of affairs has serious implications for how we proceed in our exploration of the universe. A host of missions are being planned to pin down dark energy, but unless we measure the geometry much more accurately, the whole endeavour could be futile. In short, we will remain resolutely in the dark about dark energy.
A host of missions are being planned to pin down dark energy, yet they could all be in vain
It was Einstein who showed that what we mean by gravity is really the geometry of space and time. His general theory of relativity describes how space-time is deformed and stretched by whatever it holds and, in return, the different constituents of the universe will coast along the bends and warps of space-time. If we apply Einstein's idea to the whole of space-time, we find that the universe expands according to the underlying geometry of space itself.
The geometry of the universe can take three possible forms, each of which is intimately related to the total amount of matter and energy in each unit-volume of space. If there is too much stuff, the universe will have a positive curvature. This means it curves like the surface of a ball and may collapse in a big crunch. Too little and the curvature will be negative: the universe will curve like a saddle, flying apart unfettered by the gravitational pull (see diagram below).
Only if the universe has exactly the right density, corresponding to a few protons per cubic metre, will it be flat and have zero curvature. It will continue expanding forever because the energy of all the constituents that are flying apart carefully balances their gravitational pull.
Because it is so central to the evolution of the universe, understanding the curvature of space is one of cosmology's great goals. Before the late 1990s, we suspected that the universe had to be almost flat. Otherwise it would have either flown apart or collapsed very early on in its existence.
But all we had was a rough idea and little else. In fact, at any given conference on cosmology at the time, a few different models would be discussed: one with flat geometry and full of dark matter, another, also flat and containing dark energy, and yet another one which was emptier than all the others and so had negative curvature. Every now and then, the possibility that we lived in a positively curved universe would be mooted but without firm, precise observations, there was much rhetoric and no conclusions.
This changed as measurements of the cosmic microwave background (CMB), the radiation left over from the big bang, became more accurate and a simple m