## The Great Dark Energy Poll

Yesterday was a very busy day: up early to check out of my hotel and head to the third day of the Euclid Consortium meeting for the morning session, then across to the Institute of Physics for a Diversity and Inclusion Panel meeting, then back to the Euclid Consortium meeting for the last session of the day, then introducing the two speakers at the evening event, then to Paddington for the 7.15 train back to Cardiff. I was not inconsiderably tired when I got home.

I had to bale out of the evening session to get the train I was booked on, but it seemed to be going well. Before I left, Ofer Lahav asked for an informal show of hands about a few possibilities relating to the nature of Dark Energy. Since today is polling day for the 2017 General Election, I thought it might be a good idea to distract people from politics for a bit by running a similar poll on here.

There are lots of possibilities for what dark energy may turn out to be, but I’ve decided to allow only six broad classes into which most candidate explanations can be grouped:

- The cosmological constant, originally introduced as a modification of the left hand side of Einstein’s general theory of relativity – the side that describes gravity – but more often regarded nowadays as a modification of the right-hand-side representing a vacuum energy. Whichever interpretation you make of this, its defining characteristic is that it is
*constant*. - Modified gravity, in other words some modification of the left-hand-side of Einstein’s equations that manifests itself cosmologically which is more complicated than the cosmological constant.
- Dynamical dark energy, i.e. some other modification of the energy-momentum tensor on the right-hand side of Einstein’s equation that looks like some form of “stuff” that varies dynamically rather than being cosmologically constant.
- Violation of the cosmological principle by the presence of large-scale inhomogeneities which result in significant departures from the usual Friedman-Robertson-Walker description within which the presence of dark energy is
- Observational error, by which I mean that there is no dark energy at all: its presence is inferred erroneously on the basis of flawed measurements, e.g. failure to account for systematics.
- Some other explanation – this would include the possibility that the entire standard cosmological framework is wrong and we’re looking at the whole thing from the wrong point of view. If you choose this option you might want to comment through the box below what you have in mind.

Well, there are the six candidates. Make your choice:

Follow @telescoper
June 8, 2017 at 5:41 pm

I find the question a bit muddled, philosophically speaking. To me, the cosmological constant is simply a mathematical entity allowed by relativity, which we use to represent the physical phenomenon known as dark energy. Thus option 1 is a tautology, in the sense that it is true for all the other options (with the exception of option 2).The interesting question is surely: (i) assuming GR is valid at cosmic scales, what is the physical basis of dark energy, i.e., what physical phenomenon does the cosmological constant represent? (ii) assuming GR is not a complete theory, what are the alternatives?

June 8, 2017 at 5:52 pm

Dark energy has a much wider range of possible behaviour than the cosmological constant – none of the other options requires a constant vacuum energy (or equivalently a constant linear modification of gravity).

June 8, 2017 at 9:53 pm

But that’s the muddle for me. Surely the so-called cosmological constant term in the field equations is simply a mathematical entity that could represent all sorts of things? It doesn’t have to represent a vacuum energy, constant or not, does it?

June 9, 2017 at 5:01 pm

“Surely the so-called cosmological constant term in the field equations is simply a mathematical entity that could represent all sorts of things?”Two problems here. First, as it is usually denoted by Lambda in the Einstein equations, it is constant. Second, while it is possible that the cosmological constant could be caused by some sort of vacuum energy (this boils down to the debate on which side one puts it), which Einstein debated with Schrödinger, it doesn’t have to be. It could “just be there”, the same way that we can’t explain the value of

G, or even why it is non-zero, on deeper principles.June 9, 2017 at 8:34 am

Though I must agree with Peter’s argument, I think that the cosmological constant wouldn’t be a satisfying answer anyway.

If I throw a ball, space is expanding between me and the ball. A good theory should explain this in relation to the big bang. The cosmological constant represents at best an average or if you prefer a mean. What produce the variations outside the mean? You can’t invoke the anthropic principle here.

I think Dark Energy is simply a part of the energy of expansion which is recycled by super massive black holes at the center of galaxies. Matter falling into a black hole has equal and opposite momentum to the black hole. But energy is never lost. The energy of expansion is launched in opposite direction at the center of the black hole. The black hole is in fact an extreme Kerr black ring where spacetime is matter, including Dark Matter. There are possibly Dark Photons interacting only with Dark Matter.

The right part of the equation is the same as the left part. It is EQUAL. Spacetime is matter and energy…

June 9, 2017 at 2:08 pm

I don’t understand, if dark energy is all around, don’t the pulls cancel each other out?

June 9, 2017 at 4:58 pm

Right. Same for matter as well. There is no gradient, but there can still be acceleration.

June 9, 2017 at 5:02 pm

“I’ve decided to allow only six broad classes into which most candidate explanations can be grouped”To get back to politics, surely a coalition is also possible. It is not the simplest case, but there is no

a priorireason to rule it out.June 10, 2017 at 9:34 am

My choice is 6 because I read G = T as Gravity = Time.

June 11, 2017 at 10:31 pm

I mentioned this issue at a maths conference this weekend. Most of the mathematicians I talked to seemed to take the approach I expected – that the cc term is a respectable term in the field equations, allowed by mathematics, no great mystery. It may be constant or it may not, the term’s name is purely historic. That the measured acceleration of the universe is so much smaller than estimates of the quantum energy of the vacuum is an interesting problem for physicists, but that’s a different matter

June 12, 2017 at 7:55 am

A good answer to this and other questions: https://arxiv.org/abs/1002.3966