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To do this, they use the results of the Dark Energy Survey DES , a collaboration of research institutions in the US, South America and Europe that studies observations made by the Victor M Blanco telescope in Chile, which is fitted with specialised instruments for dark energy detection.

Presenting the first tranche of results from the survey, Abbott and colleagues reveal progress towards constraining the nature of dark energy. They also report that the results suggest the universe is spatially flat, and derive a tighter constraint on the density of baryon matter.

Further planned DES surveys, they conclude, will likely sharpen up knowledge of the impact of dark energy in the universe by orders of magnitude. The research is soon to be published in the journal Science, and is currently available on the preprint server arXiv. Digital Issues Buy a back issue. Renew my subscription Give a Gift Manage my subscription.

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What are 'dark matter' and 'dark energy'? / Space Science / Our Activities / ESA

News Space 06 May Multiple measurements close in on dark energy. A huge collaboration of scientists reports important new findings about the shape of the universe and the mysterious force that shapes it.

The search for dark energy

Andrew Masterson reports. Explore dark energy Big Bang universe. Andrew Masterson is editor of Cosmos. Looking for more science? Click here to see our subscription options. Click here to see our gift options. Recommended for you.


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Before we delve into them, it is important to recognize the difference between the accelerating expansion and dark energy. The former has been shown by observations, but the latter is just the interpretation of those observations. Any interpretation has to take into account all the observations: the supernovae results, the BAOs, the CMB and its acoustic peaks, and the growth of galaxy clusters. Riess has already been involved in a public skirmish along these lines.

Riess disagreed with their assessment. Sarkar counters this by arguing that some controversial analyses, such as that by Isaac Tutusaus in a paper in Astronomy and Astrophysics However, the consensus remains among cosmologists that BAOs are strong evidence for accelerated expansion. Indeed, challenges to the existence of dark energy often focus on our most precious cosmological models. The Cosmological Principle states that the distribution of matter in the universe is both homogenous and isotropic. However, on smaller scales matter is lumpy, arranged into galaxies and clusters of galaxies, which form great chains and walls of clusters that stretch hundreds of millions of light-years.

Crucially, though, these largest structures, such as the Sloan Great Wall, are not gravitationally bound. In-between these islands of matter are vast voids where the density of matter is far lower. Gravity will affect the expansion of space differently depending on whether you are in a cluster or a void. This is an exact solution to the Friedmann equation, which solves the general theory of relativity for an expanding universe consistent with the Cosmological Principle and where the curvature of space, which is zero, is the same everywhere.

Because the universe is dominated by voids where the lower gravity allows the universe to expand faster, it is only by averaging all the different rates of expansion that it would seem like the expansion is accelerating. In particular, he is critical of the fact that in FLRW cosmology, the scales that matter are the largest scales that ignore the coarse graininess of individual galaxies and clusters. On scales less than million light-years, the universe is lumpy, filled with those voids and clusters that affect space and its expansion differently. Add in all the smaller voids, and they account for more than half the universe, so they have a big say in how the universe appears to expand.

In timescape cosmology, clocks run faster in voids than in more densely populated regions of space. Billions more years would have passed in voids than in galaxy clusters, and in those extra billions of years there will have been more expansion of space. Averaging the expansion rate across all of space — that is, the voids and the clusters — makes it seem like the expansion is getting faster because the voids dominate.

Wiltshire says his group has just begun work tackling the challenge of reducing the BAO data without assuming the FLRW metric, and says that the initial results show promise. Fitting the heights of the acoustic peaks in the CMB data is even more of a challenge, as it requires rewriting the mathematics that describes the growth of the tiny anisotropies that are the seeds of cosmic structures.

Multiple measurements close in on dark energy

In standard cosmology, the FLRW metric is assumed to exactly describe the average growth of the universe on arbitrarily large scales. However, in a generally inhomogeneous universe as described in timescape cosmology, this is no longer the case. Even if the deviations from homogeneity are small, as the anisotropies of the CMB show, their average growth may not exactly follow the Friedmann equation on large scales.

Wiltshire, however, has the FLRW metric in his sights. It looks for a relation between the Hubble constant — a measure of the expansion of space — and the luminosity distance a relation between the absolute and apparent magnitude of an object. This relation holds only for a universe where the curvature of space is the same everywhere, as per the Friedmann equation. If the test supports the predictions of the FLRW metric, then timescape cosmology is probably wrong. The Hubble constant, which is so fundamental to the expansion of space, is also a source of consternation.

In Riess led a team making the most precise measurement of the Hubble constant in the local universe. As with his discovery of the accelerating expansion 20 years ago, Riess made this measurement using type Ia supernovae, initially those that had exploded in galaxies that also host visible Cepheid variable stars — another cosmic yardstick with which to measure stellar distances.

Riess draws an analogy with the growth of a human body. A doctor might measure the height of a child and plot that on a growth chart to predict how tall that child will be when they are an adult.

So why the difference in the Hubble constant at the opposite ends of history? One possibility is that our assumptions about the early universe are wrong. Perhaps dark-matter particles are less stable or interact more than we thought, which would affect the properties of the CMB.

Perhaps there was an earlier spurt of dark energy sometime in the first billion years. Make no mistake, dark energy — be it the cosmological constant or quintessence — is the leading theory with plenty of observational evidence to support it. The alternatives remain highly controversial. With the Dark Energy Survey — an international collaboration using a million-pixel camera called DECam on the Blanco 4 m telescope at the Cerro Tololo Inter-American Observatory in Chile — releasing its first data from a survey of million galaxies, these are exciting times.

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