Hydrodynamical simulations of coupled and uncoupled quintessence models I: Halo properties and the cosmic web

Slightly under the weather as a very nice man removed four of my wisdom teeth yesterday, so a quick post for today. But we've got a new and exciting paper accepted.

Cosmology has come a long way, and we now have exquisite data which point us to the make up of the Universe, a curious mix of dark matter, dark energy, and a few baryons like ourselves.

But there's a problem. It's a boring Universe. What, you say! Boring. Yes, we have dark matter and dark energy, things that we still really need to discover in terms of their "quantum properties" such as are they this particle or that, this field or that, but we know what their effects on the expansion of the Universe are; dark matter is is gravitationally attractive, just like everyday matter, and dark energy is repulsive, driving the expansion acceleration of the Universe.

But as far as we can tell from the data, dark energy appears to be the same as Einstein's Cosmological Constant. And the completely boring thing about this is that it is boring. Its properties don't change, its energy density remains constant, and eventually it will completely dominate the Universe with its boring properties.

And this gives astronomers a headache. If we've nailed all the properties of the Universe, what's the point of building new telescopes and looking at the sky? Do we just want to keep improving on the accuracy of the numbers we measure?

Well, no. The question is "is there anything exciting happening, but at a level below detection in the current data?" And that's where there is a new growth market in cosmology.

The fundamental properties of dark matter and dark energy are governed by the rules of quantum mechanics, where "if it's not forbidden, it's bound to happen". And as we don't know the true fundamental properties of the dark sector, we can envisage various interactions, such as dark energy changing its spots, changing its influence  on the expansion. Or maybe dark matter decaying into dark energy, also changing their overall history of the expansion. How much could such interactions be hidden in the data?

This is where this new paper comes in. Understanding how such interactions influence cosmic expansion is quite straight-forward, but the effects on what we see, such as the brightness of distant supernovae, is very subtle.

But if we are messing about with changes in the dark sector, these might have implications on what happens to the baryons, influencing where and how stars and galaxies form. I've written about this before, but the formation of galaxies is a very nonlinear process and so it's very difficult to keep track of with a paper and pen.

So we turn to supercomputers (I love supercomputers) to follow the evolution of matter in such novel cosmologies. These usually combines the expansion of the universe, the influence of gravity and the complexities of gas physics and star-formation, but what PhD students, Edoardo Carlesi and Scott Wales, developed was the inclusion of this new physics into cosmological codes for supercomputers.

Now, the mathematics involved are a little messy, but for those in the know, what we do is modify the Lagrangian; while it is not generally appreciated, the Lagrangian is the mathematical framework for much of modern physics, from general relativity to quantum field theory. We modified the Lagrangian of dark energy to include interaction terms;
For the terms in the brackets, the first one is the "standard term" for dark energy, whereas the second term is a "self-interaction" term, that allows dark energy to change its spots. We use
The last term is an interaction term, that allows dark energy to interact with dark matter (so one can decay into the other). Again, we choose a particular form, namely
So, with this we can make some synthetic universe, messing about with the parameters in there to change the strengths of the various interactions and seeing what the effects are. Remember, don't want to produce universes that are significantly different to the one we observe, we're looking for universes that can he hidden within the limitations of current observations.

This first paper concentrated on the properties of the cosmic web, namely how galaxies are distributed through space. This means we have to identify empty regions (voids), dense regions (clusters) and the joiny-inbetweeny-bits - the filaments. And once we have those, we can start to make pictures.
The paper has lots of comparison of the statistics here, what's going on with the distribution of clusters, filaments and voids, how are the galaxies distributed in mass, and and how are their spins correlated. Our conclusions;
but the signals are still subtle and will take a lot of extra observing with future facilities to tease out if they are there. Yay, there is a use of the telescopes being built.

It's time to take my pain-killers, so I'll wrap up here. This is a first paper looking at the influence of novel cosmologies, and I'll post our second one shortly. But until then, well done Edoardo!

Hydrodynamical simulations of coupled and uncoupled quintessence models I: Halo properties and the cosmic web

We present the results of a series of adiabatic hydrodynamical simulations of several quintessence models (both with a free and an interacting scalar field) in comparison to a standard \LCDM\ cosmology. For each we use 2×10243 particles in a 250\hMpc\ periodic box assuming WMAP7 cosmology. In this work we focus on the properties of haloes in the cosmic web at z=0. The web is classified into \emph{voids}, \emph{sheets}, \emph{filaments} and \emph{knots} depending on the eigenvalues of the velocity shear tensor, which are an excellent proxy for the underlying overdensity distribution. We find that the properties of objects classified according to their surrounding environment shows a substantial dependence on the underlying cosmology; for example, while Vmax shows average deviations of 5 per cent across the different models when considering the full halo sample, comparing objects classified according to their environment, the size of the deviation can be as large as 20 per cent.
We also find that halo spin parameters are positively correlated to the coupling, whereas halo concentrations show the opposite behaviour. Furthermore, when studying the concentration-mass relation in different environments, we find that in all cosmologies underdense regions have a larger normalization and a shallower slope. While this behaviour is found to characterize all the models, differences in the best-fit relations are enhanced in (coupled) dark energy, thus providing a clearer prediction for this class of models.


  1. "dark energy is repulsive, driving the expansion of the Universe."

    No. Consider the fact that the universe can be expanding with a zero cosmological constant, or with a negative (attractive) cosmological constant. Also consider that a universe with a positive cosmological constant can collapse in the future. So, it is not correct to say that the cosmological constant drives the expansion of the universe. At most it drives the acceleration, and there can be no acceleration without a cosmological constant, though there can be deceleration even if the cosmological constant is positive.

    1. Mea culpa - I meant acceleration. I blame the painkillers....


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