Cosmologists produce new maps of dark matter dynamics

July 25, 2017, University of Portsmouth
Slice through the celestial equator showing the radial component of the velocity field (in kilometres per second). Blue regions are falling towards us and red regions are flying away from us. Galaxies of the Sloan Digital Sky Survey main galaxy sample are overplotted. In the centre of the slice, the infalling dynamics of the Sloan Great Wall, one of the largest structure of the known universe, can be observed. Credit: University of Portsmouth

New maps of dark matter dynamics in the Universe have been produced by a team of international cosmologists.

Using advanced computer modelling techniques, the research team has translated the distribution of galaxies into detailed maps of streams and velocities for the first time.

The research was carried out by leading cosmologists from the UK, France and Germany.

Dr Florent Leclercq from the University of Portsmouth's Institute of Cosmology and Gravitation said: "Dark matter is a substance of yet unknown nature that scientists believe makes up more than 80 per cent of the total mass of the Universe. As it does not emit or react to light, its distribution and evolution are not directly observable and have to be inferred."

The researchers used legacy data obtained during 2000 – 2008 from the Sloan Digital Sky Survey (SDSS), a major three-dimensional survey of the Universe. The survey has deep multi-colour images of one fifth of the sky and spectra for more than 900,000 .

The new maps cover the Northern Sky up to a distance of 600 megaparsecs, which is the equivalent of looking back about two billion years.

The researchers used a set of phase-space analysis tools and built on research from 2015, which reconstructed the initial conditions of the nearby Universe.

Detection probabilities for different structure types (from left to right: void, sheet, filament, and cluster), in the cosmic web as observed by the Sloan Digital Sky Survey. Structure types are classified using DIVA, an algorithm reflecting the dynamical trend, instead of the current density configuration. The red dot on the map shows our location. Credit: University of Portsmouth

Dr Leclercq said: "Adopting a phase-space approach discloses a wealth of information, which was previously only analysed in simulations and thought to be inaccessible using observations.

"Accessing this information in galaxy surveys opens up new ways of assessing the validity of theoretical models in light of observations."

The research is published in the Journal of Cosmology and Astroparticle Physics.

Explore further: Mapping dark matter

More information: Florent Leclercq et al. The phase-space structure of nearby dark matter as constrained by the SDSS, Journal of Cosmology and Astroparticle Physics (2017). DOI: 10.1088/1475-7516/2017/06/049

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Jul 25, 2017
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1 / 5 (7) Jul 25, 2017
If the "Blue regions are falling towards us" how can there be much touted cosmic expansion and acceleration or even a "Big Bang" creation? Even if it happens only within 2 billion light years around us.

The blue and red regions moving towards each other should merge and form one large block! Frailty, thy name is "Big Bang" cosmology!
2.3 / 5 (3) Jul 25, 2017
The Filaments pic makes it look a lot like the greater cosmic structures form Megacosmic proteins and crystalline structures. I understand that is a visual take, however, I do not think it should be discounted that such structures would form as a fractal iteration of what we see at submicroscopic quantum levels expanded to what we see as megacosmic scales. It may be that we can learn a lot about the sub-quantum world by looking at the macrocosmic in a new way. As Above, So Below.
5 / 5 (7) Jul 25, 2017
how can there be much touted cosmic expansion

Imagine placing some strips of tape on a partially inflated balloon. As you continue to inflate the balloon, the areas taped over will stay a constant size. So even though there's global expansion of the balloon, little local areas need not expand themselves.

GR produces a variety of results depending on what inputs you feed it. We don't have the capacity to really solve the whole universe in one approach, so we have a few model-based assumptions we can start with. At the very simplest level, one is to have a point or spherical mass and then free space beyond it; another is to have a uniform distribution of mass and energy throughout a boundary-free volume. The first produces an effect approximately the same as Newtonian gravity, and the second, depending on relative densities, allows for expansion. Our universe is neither model, but both models allow us to understand some parts of it overall.
1 / 5 (1) Jul 26, 2017
I will admit, I've never quite understood that explanation of expansion. It simply doesn't make any sense to me how space inside of galaxies doesn't expand while space outside does. The only thing I can see that would be different is the gravitational fields inside galaxies. What the correlation is between gravity fields and expansion would be though is beyond me.

3 / 5 (4) Jul 26, 2017
If the "Blue regions are falling towards us" how can there be much touted cosmic expansion

The image includes only peculiar velocity, i.e. it removes the effect of the Hubble Flow.

The blue and red regions moving towards each other should merge and form one large block

They do, if you actually read the caption you can see gap between the largest red and blue regions is the Sloan Great Wall. This is gravitational collapse into massive structures.

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