From COSMOS Magazine: “Multiple measurements close in on dark energy”

Cosmos Magazine bloc

From COSMOS Magazine

06 May 2019
Andrew Masterson

Cerro Tololo Inter-American Observatory, located on Cerro Tololo in the Coquimbo Region of northern Chile, Altitude 2,207 m (7,241 ft)

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 Survey

Dark Energy Camera [DECam], built at FNAL

NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

Timeline of the Inflationary Universe WMAP

The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

DES is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

Over six years (2013-2019), the DES collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.

Dark energy – the force that propels the acceleration of the expanding universe – is a mysterious thing. It’s nature, write 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.

The 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.

CMB per ESA/Planck

ESA/Planck 2009 to 2013

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.

Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex Mittelmann Cold creation

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.

Red shift and wave length shift-The Earliest Stars And Galaxies In The Universe Science at ESA

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 published in the journal Physical Review Letters.

See the full article here .

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