Galaxy clusters puzzle cosmologists

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In order to reconcile the standard model of cosmology with the amount of galaxy clusters observed in X-ray, a team of French researchers (1) shows that the masses of the clusters should be 70% higher than the current estimates. These results confirm the Planck results obtained in 2013 by observing the clusters in the microwave domain in the more distant universe. Nevertheless, the topic goes on to puzzle the cosmologists and might be an indication of a “new physics.”

A cluster of galaxies seen by two satellites: X (XMM) and SZ (Planck). © ESA

Clusters of galaxies, the largest structures that have collapsed on themselves under the action of gravity, can be observed with a variety of techniques. Among others, those of imagery and of X-ray spectroscopy that reveal the presence and the properties of a hot gas at a temperature of several tens of millions of degrees. They can also be observed in the millimeter and submillimeter domains, by measuring the imprint that this hot gas leaves on the cosmic microwave background radiation: this is the Sunyaev-Zeldovich effect (“SZ effect”). These clusters are a valuable source of information about our Universe and its history, in particular through measures of their abundance – the number of clusters per unit volume. Indeed, this abundance results from the number of structures that have had time to be formed by the collapse of the matter fluctuations whose origin lies in the early universe.

A trace of these primordial fluctuations is in the cosmic microwave background (CMB), the “first picture” of the Universe. Its observation and analysis by the Planck satellite has significantly consolidated the foundations of the standard cosmological model: a cosmological constant (or dark energy) representing approximately 70% of the Universe density and a cold dark matter component representing 25%. The remaining 5% are made of atoms, the only content directly accessible to astronomers observations.

Knowing the amplitude of the primordial fluctuations (through the CMB), the abundance of galaxy clusters can then be predicted and compared to observations, and so compel the models of Universe. But Planck has also observed the abundance of clusters through their imprint by the SZ effect on the CMB. The comparison conducted by the IAS researchers and introduced in 2013 (2) showed that, given the masses estimates obtained in the past, based on the application of the equation of the hydrostatic equilibrium gas within clusters, the amount of observed clusters is 3-4 times lower than the expected values! In order to reconcile these abundances with the standard model, the masses of the clusters should be much higher, about 70%, than the previous estimates based upon the X-ray observations. Many cosmologists are not ready to take this step, which would imply that the standard cosmological model needs to be revised, for example by invoking the existence of massive neutrinos.

A French team studied this issue with a new approach (3). At first, they took over the data that lead to determine the abundance of clusters (local / relatives) detected by their X-ray emission. Then, they evaluated the masses that these clusters should have to be in agreement with the model “ΛCDM” as it appears to us through the fluctuations of the cosmic microwave background measured by Planck. Their results then show a very good agreement with the findings mentioned above: the masses of the clusters must be greater than previous estimates of about 70%. So there is great consistency between the conclusions drawn from X-ray observations and those obtained from the “SZ” Planck sample. The question of the actual mass of the clusters remains a subject debated among specialists: are the masses of clusters obtained by different methods of direct observation really underestimated or do these measures suggest a “new physics”? The availability of accurate and reliable measurements of clusters masses thus appears more than ever as a crucial issue for cosmology. This topic is expected to make decisive progress in the forthcoming years thanks to the development of instruments dedicated to the realization of large surveys of galaxies. Among them, there is the Euclid satellite, which will allow a more accurate and reliable measurement of the masses of clusters by gravitational lensing effect.

The masses of selected galaxy clusters are reported. In abscissa: the masses estimated by hydrostatic equilibrium from X-ray observations. Y-axis: the masses needed for the galaxy cluster abundance to be compatible with the Standard Cosmological Model (SCM). The red line marks the mass fit and shows that the masses of the standard model are 70% greater than the masses deduced from direct X-ray observations.


  • The French researchers at the origin of this discovery work at the Institut de Recherche en Astrophysique et Planétologie, IRAP (CNRS, Université de Toulouse Paul Sabatier) and at the Institut d’Astrophysique Spatiale, IAS (CNRS, Université Paris Sud, CNES). This work was also supported by the OCEVU LabEx, founded by the ANR, within the “Investissements d’Avenir” program.
  • These results were published in 2014, A&A 571, A20
  • These results were the topic of a publication in Astronomy and Astrophysics,A&A 582, A79

Further Resources

IRAP Contact

  • Alain Blanchard,, 05 61 33 28 42



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