Scientists used observations of more than 3000 quasars, discovered by the SDSS (Sloan Digital Sky Survey) project, to measure with unprecedented precision the cosmic clustering of hydrogen gas into clusters.
Press release on behalf of SDSS
Scientists used observations of more than 3000 quasars, discovered by the SDSS (Sloan Digital Sky Survey) project, to measure with unprecedented precision the cosmic clustering of hydrogen gas into clusters. The quasars used for the study, whose number is 100 times greater than the number of quasars used in similar studies in the past, are eight to ten billion light-years away from us, making them the most distant objects known to us.
Gas accumulations between the Earth and the quasars absorb light from the spectra of the quasars. Absorbing the light allows researchers to map the distribution of the gas and measure the extent of its clustering on orders of magnitude of a million light years. The degree of clustering of the gas may answer basic questions, such as whether neutrinos have mass and what is the nature of dark energy, which is supposed to be the cause of the accelerated expansion of the universe.
"Scientists have been studying galaxy clusters for a long time to learn about cosmology," explained Eros Seljak of Princeton University and one of the SDSS researchers. "However, the physics behind the formation of galaxies and their clustering is extremely complicated. Since most of the mass in the universe consists of dark matter, our lack of understanding regarding the relationship between the distribution of galaxies (which we can see) and dark matter (which we cannot see, but cosmological models predict its existence and behavior) creates uncertainty." It is estimated that the gas, which is detected in the spectra of the quasars, is distributed in a very similar way to the dark matter, which contributes to reducing the uncertainty.
"We have known for several years that the spectra of quasars are a unique tool for studying the distribution of dark matter in the early universe, but the quantity and quality of the data from the SDSS made the vision a reality," said David Weinberg (Weinberg) of Ohio State University and a member of the SDSS team. "It's amazing how much we can learn about the structure of the universe 10 billion years ago."
Seliak and his SDSS colleagues combined the analysis of the data from the quasar spectra with measurements of galaxy clustering, gravitational dusting and ripples in the cosmic background radiation, measured by NASA's WMAP (Wilkinson Microwave Anisotropy Probe) satellite. Combining the information produced the most accurate picture to date of the clusters of matter in the universe at distances of a million light years to several billion light years. The comprehensive research allows careful comparisons between the information and theoretical models of the history and composition of the universe.
"This is the most severe test so far for the predictions of the inflationary cosmological model. The model passes it with great success," added Seliak.
The expansion model of the universe holds that immediately after the big bang the universe went through a period of extremely rapid acceleration, during which slight fluctuations in density turned into folds of astronomical sizes in space, folds whose imprint can be seen in clusters of astronomical objects. The theory of the expanding universe predicts a certain dependence between the degree of clustering and the scale being looked at. The new research supports the theory's predictions. Other scenarios regarding the universe, such as the cyclic universe theory, offer very similar predictions, and they are also consistent with the recent results.
Preliminary analyzes by the WMAP team and others hinted at deviations of the cosmic clusters from the predictions of the expanding universe model. If these deviations do exist, a serious shift in the current paradigm of origin and structure in the universe would be necessary.
"The new data and their analysis substantially improve the observational accuracy of testing the predictions of the swelling model," said Patrick McDonald (McDonald) of Princeton University and one of the authors of the study. "The new results correspond almost exactly to the predictions of the expanding model."
"The clustering of matter is an accurate and powerful test of cosmological models, and the current study is consistent with and extends previous studies," agreed Adrian Pope of Johns Hopkins University, who led a team that conducted a previous analysis of galaxy clustering at SDSS.
Also, the new analysis provides the best information about the neutrino mass. Experiments on Earth, which in 2002 gave rise to the Nobel Prize in Physics, showed beyond any doubt that neutrinos have mass, but these experiments could only measure the difference in masses between the three known types of neutrinos. The presence of neutrinos affects the cosmic clustering per year on a scale of millions of light years, exactly the scales on which the research on the spectra of quasars shed light.
According to the new study, the mass of the lightest neutrino is less than twice the last mass difference measured. The new measurement also rules out the possibility of the existence of another family of neutrino particles, which some experiments on Earth suggested.
"Cosmology, the science of the largest structures, can tell us about the properties of elementary particles such as neutrinos," said Lam Hui of the Fermi National Accelerator Laboratory, which belongs to the United States Department of Energy. Hoy conducted a separate analysis of the data together with Scott Burles (Burles) from MIT and other scientists.
In addition, the new research provides additional support for the existence of dark energy and the fact that it is constant in time. The research puts the best constraints yet on the evolution of dark energy in time.
"No evidence was found for the existence of dark energy changing in time, so the possibility that the universe will disintegrate in the future in the 'big rip' is significantly reduced following the new study," said Aleksey Makarov from Princeton University, who also took part in the study.
Translation: Dikla Oren
