Nicknamed the Pandora Cluster, Abell 2744 is a supercluster of galaxies -

NASA et al./Cover Images / SIPA

  • In space, regions without galaxies are dominated by dark energy, according to a study published by our partner The Conversation.

  • A closer observation of these regions should make it possible to measure the evolution of the expansion of the universe.

  • The analysis of this hypothesis was carried out by Guilhem Lavaux, researcher at the Institut d'Astrophysique de Paris.

The universe is made up of many cosmic structures visible through the spatial arrangement of galaxies.

These structures reveal many voids, that is to say regions almost unoccupied by galaxies.

Many scientists are particularly interested in these cosmic voids.

How and why do we study them?

Voids between the great cosmological structures

Cosmic voids are regions of the universe seemingly devoid of galaxies.

They are also supposed to be poor in matter, hence their name “cosmic void”.

These regions have characteristic sizes of the order of a few million light years for the "smallest" among them.

The Milky Way, our galaxy, with a diameter of the order of a hundred thousand light years, is at least ten times smaller.

A cosmic void, of elongated and irregular shape, identified by the software VACUUM.

The red dots correspond to galaxies from the Sloan DR7 galaxy survey.

© Guilhem Lavaux

The first cosmic voids were actually observed in the late 1970s with the first systematic surveys of galaxies.

At that time, there were not yet scientifically established models of cosmology, that is, based on a converging set of astronomical observations.

In particular, observations of the distribution of galaxies were still relatively insufficient.

For example, it was impossible to see if the galaxies had a structured spatial organization or if they were distributed in a homogeneous way.

This debate, which had been going on since 1938, was initiated by Harlow Shapley.

It was not until the 1970s to have the first physical arguments on the spatial structuring of galaxies, which were quickly supported by real observation campaigns.

In the detection phase

The observation of the large structures formed by galaxies has led to numerous theoretical works in order to understand their formation.

These large structures have been called "cosmic web" with reference to the spider web.

The voids correspond to the holes in this canvas.

Density contrast inferred from galaxy readings.

Clusters of galaxies are shown in orange letters.

The names of a few notable voids have been spotted in purple letters.

The names of these structures, clusters and voids, are linked to the major constellations visible at the same position in the sky (Com = Coma / Hair of Berenice, Boo = Böötes, UMa = Ursa Major / Big Dipper).

© Florent Leclercq & Guilhem Lavaux

Nevertheless, the study of these voids has remained limited, because they suffer from two defects.

First of all, they are large, which requires surveys of very large and deep galaxies in order to detect and characterize them.

Then, the mathematical model of the evolution of the vacuum is more complex than that used to account for the small deviations in the homogeneity of the distribution of galaxies on the scales of the universe.

Cosmic voids are void of matter… and are dominated by dark energy

Cosmic voids are of scientific interest for the study of the universe: these regions, by definition, contain less matter than the rest of the universe.

But when you remove all the material, there is still something left: dark energy.

This has never been observed directly - but its existence, either as a cosmological constant or as a full-fledged physical object, is not prohibited by general relativity and is now required to interpret observations.

In particular, it is required to explain the acceleration of cosmological expansion.

The dark energy hypothesis admits of several variations.

The "simplest" dark energy would not form aggregates.

Cosmic voids hardly contain any matter, so they would be dominated by dark energy.

Privileged laboratories to test new hypotheses on dark energy through its effect on the dynamics of the small galaxies populating them, in short.

The study of these dynamics requires galaxy readings at extremely low luminosities, in order to see these small galaxies.

These surveys are under development, for example the “Legacy Survey of Space and Time” or Euclid.

Expansion of the universe and cosmic voids

Cosmic voids make it possible not only to test hypotheses about dark energy, but also to measure the evolution of the expansion of the universe.

Indeed, the "cosmological principle", very important in the discipline, says that the basic physical properties of the universe depend neither on the position of the observer nor on his direction of observation.

According to this principle, the cosmic voids must look on average like balls, whatever the moment.

This property is important because it makes it possible to compare the

apparent

extent

of a vacuum in depth, provided by the speed of distance of the galaxies which delimit it, with its extent over the sky, given by the distance over the sky between these same galaxies.

According to the cosmological principle, these two quantities are equal

on average

.

Imposing this equality informs us in return on the speed of expansion at the moment when the light was emitted by the galaxies delimiting this cosmic void, and thus to the contents of the universe at this same time.

New techniques for analyzing galaxy readings based on dynamic modeling make it possible to obtain a three-dimensional map of matter in the universe: blue for sparsely dense regions and red for very dense regions.

Cosmic voids are white.

© Guilhem Lavaux

Other types of structures can be used to perform this same expansion test - for example the acoustic oscillations of the primordial plasma, which are structures of much larger sizes than those of voids.

A turning point in the use of voids

As noted above, although the shape of cosmic voids is on average a ball, their individual shape can be quite varied - the head image of the article shows a very complex shape - and this variability makes their systematic identification complicated. .

The simultaneous, but separate, publication of two algorithms, based on the same principles, enabled a significant breakthrough in 2007. The open publication of their source codes DISPERSE and ZOBOV made it possible to use them for many applications.

For example, it was possible to follow the properties of galaxies according to the type of the web (a void, a cluster or a filament).

DISPERSE has also been used to find filaments of galaxies in images obtained with a detector in the X-ray domain, and on a much smaller scale, to analyze the structuring of molecular clouds.

"Databases" to understand the expansion of the universe

These algorithms are based on mathematical concepts of differential topology, which makes it possible, for example, to study water divides in geography.

They are more robust than the previous methods, and they make it possible to identify voids even if there are small distortions in the spatial distribution of galaxies, that is to say if we globally move the galaxies in any place. which direction (if the deformation is not too large).

With collaborators, we developed the “VACUUM” analysis chain, which in particular made it possible to build open databases of cosmic voids from the “

Sloan

 ”

sky survey 

.

Other catalogs of cosmic voids were subsequently constructed from other surveys such as DES (Dark Energy Survey).

These analyzes of cosmic voids resulted in new constraints on the expansion of the universe.

The latest results

The ways of studying and characterizing cosmic voids continue to evolve and make more and more use of open software.

Two significant developments are underway.

First, the evolution of computers and algorithms allows for ever finer analyzes of data: it is now possible to detect more voids and to reconstruct the cosmic web with ever more fidelity from galaxy surveys.

Furthermore, galaxy readings are getting bigger and bigger.

A new article has just been submitted for peer review, and, as is usual in astrophysics, made available to the community on an open pre-publication server.

This study demonstrates that cosmic voids allow better constraints on the material content of the universe than the techniques conventionally used, using the

same

data.

No significant deviation was noted from general relativity or cosmological parameters as constrained by the Planck mission.

The next major challenge will come from the exploitation of data from the Euclid mission which should make it possible to obtain unmatched constraints on dark energy.


This analysis was written by Guilhem Lavaux, researcher at the Institut d'Astrophysique de Paris.

The original article was published on The Conversation website.

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