Multiple measurements close in on dark energy

An extensive analysis of four different phenomena within the universe points the way to understanding the nature of dark energy, a collaboration between more than 100 scientists reveals.

Dark energy – the force that propels the acceleration of the expanding universe – is a mysterious thing. Its nature, writes telescope scientist Timothy Abbott from the Cerro Tololo Inter-American Observatory, in Chile, and colleagues, “is unknown, and understanding its properties and origin is one of the principal challenges in modern physics”.

Indeed, there is a lot at stake. Current measurements indicate that dark energy can be smoothly incorporated into the theory of general relativity as a cosmological constant; but, the researchers note, those measurements are far from precise and incorporate a wide range of potential variations.

“Any deviation from this interpretation in space or time would constitute a landmark discovery in fundamental physics,” they note.

Thee heart of the problem, of course, is that dark nature is observable only indirectly, by its effects.

These fall into two categories. First, it deforms galactic architectures through accelerating the expansion of the universe. Second, it suppresses growth in some parts of the cosmic structure.

However, it is not the only force that can produce such results, and the danger thus always exists that what is assumed to be evidence of dark matter activity may in fact be something else altogether.

Current approaches to measuring dark matter are problematic. All of them begin with the cosmic microwave background (CMB), the relic radiation that fills space, generated just 400,000 years after the Big Bang.

At that point in the history of the universe the influence of dark matter was minimal. It increased significantly as spacetime expanded ever more and ever faster. 

The second pillar for measuring it, thus, comprises observations of “low-redshift” phenomena – wavelengths stretched over vast distances, allowing calculations of conditions within the universe the past several billion years.

Combining the two measurements and then extrapolating forwards to the present day, Abbott and colleagues note, “can be a powerful test of our models, but it requires precise, independent constraints from low-redshift experiments”.

It follows, then, that any increase in the precision of low-redshift measurements will also increase the precision of dark energy calculations, reducing (or perhaps increasing) the chances that a previously undiscovered physics is in play in the universe.

The researchers approach this challenge by invoking a combination of multiple observational probes for low-redshift phenomena – namely, those measuring Type Ia supernova light curves, fluctuations in the density of visible (or “baryonic”) matter, weak gravitational lensing, and galaxy clustering.

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.

The DES findings, they report, absolutely – and independent of CMB-based research – rule out a universe in which dark energy doesn’t exist. They also report that the results suggest the universe is spatially flat, and derive a tighter constraint on the density of baryon matter.

These results, they suggest, constrain the state of “of dark energy and its energy density in the Universe” … “to a precision that is almost a factor of three better than the 7 previous best single-experiment result from the CMB”.

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.

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