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Huge cosmic voids could probe dark energy

Nothing to see here? Despite being vast expanses of nothingness, gigantic voids in the universe could lead us to a new theory of gravity – one that may give a clue to the nature of dark energy.

Thought to be a mysterious force pushing the universe apart, dark energy was dreamed up to explain the discovery that the expansion of the universe is accelerating. The leading model holds that space-time has an inherent amount of dark energy for any given volume and that this does not change over time – this is called a cosmological constant.

But because there is no reason why dark energy should take any specific form, some physicists find this model frustrating and so try alternative theories. "When we say dark energy, most people use it as a general label for whatever it is that we need to stick in to make things work," says Martin Sahlén of the University of Oxford.

Behaviour change

Sahlén and his colleagues invoke a new field that is added to a cosmological constant to change dark energy's density, depending on the local density of ordinary matter. That means dark energy would have behaved differently at different times. In the early universe, matter's density was high, and dark energy would have had very little effect. But as the universe expanded and became less dense, dark energy's effects became more prominent, causing the expansion to accelerate as observed.

Such dark energy has an observable consequence: it causes space-time in large, under-dense regions known as voids to behave differently than if dark energy were a cosmological constant alone. Where regions of high density, such as galaxy clusters, warp space-time to bend light towards us like a magnifying glass, voids bend light away like a concave lens. Measuring the amount of such "demagnification" could help test the team's model.

In addition to probing the nature of dark energy, the model could put gravity on the scales. In Einstein's theory of general relativity, our current best understanding of gravity, the particle that carries the force – the graviton – must be massless. But the new field in Sahlén's theory would give the graviton a small mass. Testing this idea could lead to a more fundamental theory of gravity.

Big, big news

In September, members of the Sloan Digital Sky Survey reported the first-ever measurement of voids causing demagnification, but the data is not yet sharp enough to distinguish between general relativity and "massive gravity" theories, in which the graviton has mass. However, forthcoming experiments, such as the European Space Agency's Euclid space telescope, could put this question to the test.

"The results are interesting, and could in principle be tested in the next decade or so," says cosmologist and massive gravity researcher Mark Wyman at New York University. But he is concerned that Sahlén's model continues to include the cosmological constant.

"One of the original reasons I and many others were interested in massive gravity was to discover if we could do away with the need for a cosmological constant when the graviton is massive," says Wyman. "Discovering that the graviton has a mass would, however, be big, big news."

Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.111.241103



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