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The sea !


Gravity is an attractive force. Or at least this is what we knew up until 20 years ago when astronomers discovered that distant objects were not slowed down by gravity but accelerated. More precisely, they updated measurements of the relationship between velocity and distance for galaxies, first initiated by Slipher in the early 20th century, which led to the discovery of the expansion of the Universe by Lemaitre and Hubble in the 1920's. Of course the Universe does not expand into something, unlike a gas which takes up the maximal volume in a chamber. No, what expands is the physical distance between any two astrophysical objects. Galaxies recede away from us. They do so because initially way back in the past something forced them to do so. Commonly known as the Big Bang, this still mysterious phenomenon, beyond the present understanding of physics, serves as the initial seed for this expansion. So it expands, but gravity lurks and sooner or later it should fall towards a final Big Crunch or even a Big Bounce to expand again into an eternal cycle. Empedocles already thought so. So distant objects should eventually slow down.


This is not what has been observed. Although this might come as a surprise, this was already suggested as soon as the end of the 1910's by the Dutch physicist de Sitter. De Sitter spacetime expands exponentially fast powered by a constant and uniform energy density. Here and everywhere lies a source of acceleration. This sounds very much like the Aether of the 19th century when scientists thought that light can only propate through an invisible medium, the Aether. The concept died with the advent of relativity and hence reappeared in a new guise with the discovery of the cosmic acceleration. Funnily enough, particle physics experiments at CERN, which have discovered the Higgs particle, have also vindicated the existence of another Aether: the Higgs field is constant throughout space-time and gives masses to all particles. Unfortunately these explanations are the most problematic in current theoretical physics. For instance, the effective way of describing the cosmic acceleration is to postulate that all spacetime is filled with a constant energy density. A natural way of generating this energy would be to appeal to the quantum fluctuations of all the known particles in the vacuum. This would give rise to an energy density way bigger than the measured one. In other words, we have no clue if the Universe is accelerating because of a quantum Aether, but if we assume that this is the case, well-tested physics gives a completely nonsensical answer to its magnitude. So goes the world. The sea then? not yet...


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