Astrophysicists have long noticed that other galaxies are moving away from us, clear evidence that the universe is expanding. That’s not really surprising given that the whole thing started with a Big Bang some 14 billion years ago.

But in the late 90s, astronomers spotted something much more unexpected and shocking. Not only is the universe expanding but the rate of expansion is increasing. In other words, every galaxy in the cosmos is accelerating away from us.

Today, this accelerating expansion of the universe is one of the great puzzles of modern cosmology—nobody knows how it can come about. So cosmologists are intensely interested in finding more evidence of this extraordinary acceleration, particularly in experiments on Earth itself, if that’s at all possible.

Now one physicist has calculated that scales at which these effects are observable. It turns out that quantum mechanics prevents any observation of the accelerating cosmos at scales smaller than about 60 metres, says Craig Hogan at Fermilab and the University of Chicago. He says this fundamental limit defines a natural boundary between the quantum and cosmic aspects of our universe.

Hogan’s conclusion is based in a simple thought experiment. He imagines two quantum particles in an accelerating universe like the one we live in. As the universe expands, the separation between these particles increases, an effect that is measurable by their redshift, for example (which is how astronomers measure the movement of distant galaxies ).

But Hogan points out that there is another factor that needs to be taken into account: quantum mechanics. This naturally introduces some uncertainty into the position of both particles. The question is at what length scale does this uncertainty swamp any distance changes caused by cosmic acceleration.

That answer is straightforward to calculate: a distance of about 60 metres. “There is no measurement, even in principle, that could unambiguously reveal local classical effects of cosmic expansion on much smaller scales,” says Hogan. “There is no sense in which a region of space can be said to expand on smaller scales [than about 60 metres].”

That’s an entirely new limit that separates the quantum universe from the macroscopic one. It suggests that quantum systems only “notice” that they are in expanding universe at scales greater than 60 metres.

That’s something that many physicists will find fascinating. And while Hogan says the limit prevents any measurement of cosmic expansion on this scale, the converse might be true as well: that cosmic expansion prevents certain quantum effects occurring on scales larger than this.

Quantum theorists have long puzzled over is why we never observe quantum effects, such as superpositions, in macroscopic objects. One idea put forward by physicists such as Roger Penrose is that the influence of gravity at macroscopic scales acts like a measurement causing a quantum superposition to collapse into a single observable state. One problem with this idea is that it does not predict the scale at which this quantum classical boundary must occur.

But Hogan’s approach raises an interesting alternative. In this scenario, it’s not gravity that causes the collapse of quantum systems but the accelerating expansion of spacetime itself. And this new formulation has the advantage of predicting exactly the scale at which this quantum boundary exists—around 60 metres or so.

So an important next step is to determine whether Hogan’s formulation makes any experimentally observable predictions. It’s certainly true that physicists have created entangled pairs of photons that maintain their quantum character at distances of over 100 kilometres for short periods of time.

The accelerating cosmos may place a limit on these kinds of experiments. at what scale does it become impossible to make the quantum measurements necessary to detect entanglement?

That’s a back of an envelope calculation for somebody with an hour or two to spare. Answers, please, as comments here!

Ref: arxiv.org/abs/1312.7797 : Quantum Indeterminacy In Local Measurement Of Cosmic Expansion