A potential deviation from ΛCDM described via "eating"
At least sixteen news outlets ran stories about "dark energy that is devouring dark matter" in recent two days. I think that the journalists started with a University of Portsmouth press release that described the recent publication of a British-Italian paper in Physical Review Letters.
The article by Salvatelli and 4 co-authors has been available since June:
What is the paper about? They try to reconcile the cosmic microwave background (CMB) data from WMAP, Planck, and others, and design a model that correctly incorporates cold dark matter (CDM) as well as the growth rate of the large-scale structure (LSS) – that rate currently looks lower than previously thought.
And when they construct their model, which combines these things slightly differently than existing papers in literature, they find some deviation slightly greater than 2 standard deviations somewhere. So the next question is how this deviation can be cured if it is real. And they effectively propose two possibilities to change the successful ΛCDM model (cosmological constant plus cold dark matter). Either to allow the neutrinos to be massive – that's represented by adding \(m_\nu\) in front of ΛCDM – or by adding "vacuum interactions" i.e. switching to iVCDM (interacting vacuum cold dark matter).
I would say that the neutrino masses should be favored in that case but all the celebrations are directed at the second possibility which is described by the phenomenological equations\[
\dot \rho_c + 3 H \rho_c &= -Q,\\
\dot V &= +Q.
\] Here, \(V\) is the dark energy (density) – it's equal to \(\Lambda/8\pi G\) in the normal cosmological constant case – and \(\rho_c\) is the cold dark matter (density). The terms \(\pm Q\) have to be the same due to some kind of energy conservation and their nonzero value is what adds "iV" to the iVCDM model. This phenomenological adjustment was first proposed in the mid 1970s so it's very old. A nonzero \(Q\) is supposed to arise from some interactions in the dark matter sector but I think that no convincing particle-physics-based model of this kind exists as of now.
You know, the whole concept that \(V\), the dark energy, is non-constant is highly problematic. A cosmological constant has to be constant in time (that's why it's called a constant) and it's a far more convincing explanation to keep it constant and add something else (like the neutrino masses). If you want to make \(V\) variable, you have to add at least "some degrees of freedom", and if you add too many, so that it will resemble some other particles, the pressure will be much closer to zero than the \(p=-\rho\) value that you approximately need. However, it's a possibility that \(V\) is (and especially was) changing and the authors make various Bayesian and other exercises to quantify how much they think that it's favored over the ΛCDM.
OK, the content of the paper is rather clear. It makes some sense but it's not conclusive and there's no revolution yet (or around the corner). But let's have a look what the media have made out of this technical paper on cosmology.
The Daily Mail's title is
Is dark energy turning the universe into a 'big, empty, boring' place? Mysterious force may be swallowing up galaxiesAs you can see, the title has pretty much nothing to do with the paper. First of all, the positive cosmological constant (dark energy) has been known to turn the Universe into a big, empty, boring place for almost a century, and this scenario has been expected to occur in our actual Universe since the experimental discovery of the cosmological constant in the late 1990s. So the new paper they are trying to describe – but they utterly fail – has surely not discovered that this is what the future of the Universe is going to look like.
Are galaxies being swallowed by a mysterious force? First, the sign of \(Q\) may be positive or negative which converts some dark matter into dark energy or vice versa. The sign is really just a technicality and there shouldn't be too much ado about nothing. If you care about the sign, they prefer \(Q=-q_V HV\) with \(q_V\approx -0.15\) so the minus signs cancel and \(Q\) is positive. The displayed equations above indeed say that the dark matter goes up and the dark matter goes down so the press is right and Richard Mitnick is wrong.
And is it right to suggest that the galaxies are being swallowed now? Swallowed is surely a very strong word here – if something is reducing the amount of dark matter, it is very slow today and it occurs rather uniformly everywhere. Equally importantly, most of the paper is about past cosmological epochs. They divide the life of the Universe to four bins, \(q_1,q_2,q_3,q_4\), and it's the fourth bin that contains the present.
They see some small but potentially tantalizing deviations in all the bins. But because this is based on cosmological observations that basically study the distant past only, it is very problematic to suggest that the finding, even if it were real, primarily tells us something about what is happening today.
The other popular articles mainly differ in the choice of verbs. Sometimes the dark matter is being "swallowed up", sometimes it is being "gobbled up". But whatever the wording is, I think it is fair to say that most cosmologists wouldn't be able to reconstruct the point of the paper – even approximately – from these popular stories in the media.
And make no doubts about it. It is very important whether there exists a functioning, particle-physics-based model (ideally embedded in string theory) that describes the microscopic origin of the required extra term – otherwise the extra term looks like a nearly indefensible fudge factor. For those reasons, I think that the neutrino masses would be preferred as an explanation if the deviation were real (which I find very uncertain). There may be other explanations, too. We just need some correction at a generic place in the equations of energy densities. Perhaps, some cosmic strings or cosmic domain walls could help, too.
Whatever the right calculation of these processes and the explanation of any possible deviations is, don't imagine that a big goblin is walking throughout the Universe and gobbles up the galaxies (or the dark matter in them). Even if this term existed, it's just another boring term that the laymen (and perhaps even most experts) would be annoyed by and most likely, the microscopic explanation would be in terms of some basic particle reactions similar to hundreds of particle reactions that are already known.
The journalists' desire to present even the most mundane and inconclusive suggestions as mysterious discoveries that change absolutely everything is unfortunate. It may be needed to get the readers' attention – but this is largely a fault of the previous overhyped stories. The dynamics is similar to a p@rn consumer who demands increasingly more hardc@re p@rn. Where does this trend lead?
By the way, one more news on theoretical work on the identity of dark matter. Three days ago, Science promoted SIMP (strongly interacting massive particle) models as a replacement for WIMP. SIMPs could account for the observed multi-keV line and offer you their own of a SIMP miracle, matching the WIMP miracle. See e.g. this paper by Feng and others or this paper in PRL for extra ideas.
On Tuesday, SciAm and others promoted a paper arguing that Hooper-like gamma rays from the center of the Milky Way may be created by dark matter explosions instead.
Guth, Linde, Dijkgraaf, and some other well-known characters were hired as instructors at the World Science U(niversity). Register now.
A new team has created an app to detect cosmic rays with people's (and your) smartphones – with the apparent goal to make the Auger experiment obsolete. ;-)