Post a Comment On: Backreaction

Chad Orzel recently pointed me towards an article in Physics World according to which “Dark energy emerges when energy conservation is violated.” Quoted in the Physics World article are George Ellis, who enthusiastically notes that the idea is “no more fanciful than many other ideas being explored in theoretical physics at present,” and Lee Smolin, according to whom it’s “speculative, but in the best way.” Chad clearly found this somewhat too polite to be convincing and asked me for some open words:

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I had seen the headline flashing by earlier but ignored it because – forgive me – it’s obvious energy non-conservation can mimic a cosmological constant.

Reason is that usually, in General Relativity, the expansion of space-time is described by two equations, known as the Friedmann-equations. They relate the velocity and acceleration of the universe’s normalized distance measures – called the ‘scale factor’ – with the average energy density and pressure of matter and radiation in the universe. If you put in energy-density and pressure, you can calculate how the universe expands. That, basically, is what cosmologists do for a living.

The two Friedmann-equations, however, are not independent of each other because General Relativity presumes that the various forms of energy-densities are locally conserved. That means if you take only the first Friedmann-equation and use energy-conservation, you get the second Friedmann-equation, which contains the cosmological constant. If you turn this statement around it means that if you throw out energy conservation, you can produce an accelerated expansion.

It’s an idea I’ve toyed with years ago, but it’s not a particularly appealing solution to the cosmological constant problem. The issue is you can’t just selectively throw out some equations from a theory because you don’t like them. You have to make everything work in a mathematically consistent way. In particular, it doesn’t make sense to throw out local energy-conservation if you used this assumption to derive the theory to begin with.

Upon closer inspection, the Physics World piece summarizes the paper:
Dark Energy from Violation of Energy Conservation
Thibaut Josset, Alejandro Perez, Daniel Sudarsky
Phys. Rev. Lett. 118, 021102 (2017)
arXiv:1604.04183 [gr-qc] which got published in PRL a few weeks ago, but has been on the arxiv for almost a year. Indeed, when I looked at it, I recalled I had read the paper and found it very interesting. I didn’t write about it here because the point they make is quite technical. But since Chad asked, here we go.

Modifying General Relativity is chronically hard because the derivation of the theory is so straight-forward that much violence is needed to avoid Einstein’s Field Equations. It took Einstein a decade to get the equations right, but if you know your differential geometry it’s a three-liner really. This isn’t to belittle Einstein’s achievement – the mathematical apparatus wasn’t then fully developed and he was guessing its way around underived theorems – but merely to emphasize that General Relativity is easy to get but hard to amend.

One of the few known ways to consistently amend General Relativity is ‘unimodular gravity,’ which works as follows.

In General Relativity the central dynamical quantity is the metric tensor (or just “metric”) which you need to measure the ratio of distances relative to each other. From the metric tensor and its first and second derivative you can calculate the curvature of space-time.

[Image] General Relativity can be derived from an optimization principle by asking: “From all the possible metrics, which is the one that minimizes curvature given certain sources of energy?” This leads you to Einstein’s Field Equations. In unimodular gravity in contrast, you don’t look at all possible metrics but only those with a fixed metric determinant, which means you don’t allow a rescaling of volumes. (A very readable introduction to unimodular gravity by George Ellis can be found here.)

Unimodular gravity does not result in Einstein’s Field Equations, but only in a reduced version thereof because the variation of the metric is limited. The result is that in unimodular gravity, energy is not automatically locally conserved. Because of the limited variation of the metric that is allowed in unimodular gravity, the theory has fewer symmetries. And, as Emmy Noether taught us, symmetries give rise to conservation laws. Therefore, unimodular gravity has fewer conservation laws.

I must emphasize that this is not the ‘usual’ non-conservation of total energy one already has in General Relativity, but a new violation of local energy-densities does that not occur in General Relativity.

If, however, you then add energy-conservation to unimodular gravity, you get back Einstein’s field equations, though this re-derivation comes with a twist: The cosmological constant now appears as an integration constant. For some people this solves a problem, but personally I don’t see what difference it makes just where the constant comes from – its value is unexplained either way. Therefore, I’ve never found unimodular gravity particularly interesting, thinking, if you get back General Relativity you could as well have used General Relativity to begin with.

But in the new paper the authors correctly point out that you don’t necessarily have to add energy conservation to the equations you get in unimodular gravity. And if you don’t, you don’t get back general relativity, but a modification of general relativity in which energy conservation is violated – in a mathematically consistent way.

Now, the authors don’t look at all allowed violations of energy-conservation in their paper and I think smartly so, because most of them will probably result in a complete mess, by which I mean be crudely in conflict with observation. They instead look at a particularly simple type of energy conservation and show that this effectively mimics a cosmological constant.

They then argue that on the average such a type of energy-violation might arise from certain quantum gravitational effects, which is not entirely implausible. If space-time isn’t fundamental, but is an emergent description that arises from an underlying discrete structure, it isn’t a priori obvious what happens to conservation laws.

The framework proposed in the new paper, therefore, could be useful to quantify the observable effects that arise from this. To demonstrate this, the authors look at the example of 1) diffusion from causal sets and 2) spontaneous collapse models in quantum mechanics. In both cases, they show, one can use the general description derived in the paper to find constraints on the parameters in this model. I find this very useful because it is a simple, new way to test approaches to quantum gravity using cosmological data.

Of course this leaves many open questions. Most importantly, while the authors offer some general arguments for why such violations of energy conservation would be too small to be noticeable in any other way than from the accelerated expansion of the universe, they have no actual proof for this. In addition, they have only looked at this modification from the side of General Relativity, but I would like to also know what happens to Quantum Field Theory when waving good-bye to energy conservation. We want to make sure this doesn’t ruin the standard model’s fit of any high-precision data. Also, their predictions crucially depend on their assumption about when energy violation begins, which strikes me as quite arbitrary and lacking a physical motivation.

In summary, I think it’s a so-far very theoretical but also interesting idea. I don’t even find it all that speculative. It is also clear, however, that it will require much more work to convince anybody this doesn’t lead to conflicts with observation.