SLAC: Dark Energy Sheds New Light on Expansion

[bystander]

Dark Energy Measurement Sheds New Light on Universe's Expansion
SLAC National Accelerator Laboratory | 15 July 2010

In this optical and X-ray image of galaxy cluster Abell 2219, X-ray brightness from the Chandra X-ray observatory (pink) is superimposed on an optical image showing the distribution of starlight and galaxies in the cluster. The very hot gas that fills the cluster shines brightly at X-ray wavelengths. [i](Courtesy: Anja von der Linden et al)[/i]

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Through observations of massive galaxy clusters, scientists have made the most precise measurements to date of the effects of dark energy and gravity on cosmological scales. This work is an important step toward understanding why the expansion of the universe is accelerating. Something is pushing our universe apart, faster and faster, with each passing moment, and future work using similar methods should determine whether that something is dark energy or a change in the way gravity works on cosmological scales.

The analysis, contained in four papers published this month in the Monthly Notices of the Royal Astronomical Society, was led by a team based at the Kavli Institute for Particle Astrophysics and Cosmology, located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University.

Clusters of galaxies, the largest gravitationally bound objects in the universe, began forming about 10 billion years ago. Because it takes a long time for light from the farthest reaches of the universe to arrive at Earth, the most distant clusters appear as they did when they were much younger, while the closest ones look more their actual ages. By looking at clusters both near and far, researchers were able to study the evolution of clusters and deduce how changes in the universe over billions of years helped shape their growth. The results offer insights into the forces that made the universe we see today.
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By combining their observations of galaxy clusters with other cosmological data, the scientists made the most precise dark energy measurements to date. The new measurements are consistent with the simplest model, in which dark energy is a "cosmological constant"—an energy field that is uniform throughout space and time. The idea of a cosmological constant was introduced by Albert Einstein in 1917, but soon fell out of favor. In recent years, the idea has become popular again as a way of explaining the accelerating expansion of the universe.

The observations also weigh against so-called “modified gravity" models, in which gravity is either stronger or weaker than predicted by Einstein’s Theory of General Relativity. The new results show that the growth of cosmic structure is consistent with the predictions of General Relativity, supporting the view that dark energy drives cosmic acceleration.

The observed growth of massive galaxy clusters – I. Statistical methods and cosmological constraints (p )

• Monthly Notices of the Royal Astronomical Society 406 2 (July 2010) DOI: 10.1111/j.1365-2966.2010.16992.x

The observed growth of massive galaxy clusters – II. X-ray scaling relations (p )

• Monthly Notices of the Royal Astronomical Society 406 2 (July 2010) DOI: 10.1111/j.1365-2966.2010.16993.x

The observed growth of massive galaxy clusters – III. Testing general relativity on cosmological scales (p )

• Monthly Notices of the Royal Astronomical Society 406 2 (July 2010) DOI: 10.1111/j.1365-2966.2010.16799.x

The observed growth of massive galaxy clusters – IV. Robust constraints on neutrino properties (p )

• Monthly Notices of the Royal Astronomical Society 406 2 (July 2010) DOI: 10.1111/j.1365-2966.2010.16794.x