Largest 3D galaxy map reveals expansion of the cosmos

Surveying 1.2 million galaxies speaks against a higher Hubble constant

This section shows about ten percent of the new galaxy map and provides a three-dimensional view of the galaxy distribution and its movements. The rectangle on the left shows a 1000 square-degree cut in the sky, which contains nearly 120, 000 galaxies, about ten percent of the entire BOSS catalog. © Jeremy Tinker / SDSS-III
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Five billion light-years deep and a quarter of the sky big: These are the dimensions of the largest three-dimensional galaxy map ever created by astronomers. Based on around 1.2 million galaxies, researchers have used it to precisely measure the cosmic expansion. The exciting thing is that their values ​​for the Hubble constant agree well with some previous ones - but not with all.

It is still the great unknown in the universe: the dark energy. It is the driving force behind the expansion of the cosmos, but what it consists of and how much it worked at different times is largely unexplained. Measurements of the expansion rate provide again and again for guesswork, because they differ significantly depending on the method.

650 billion cubic light years

A team of hundreds of astronomers has now completed the most comprehensive three-dimensional galaxy map to date. Two years ago, the Baryon Oscillation Spectroscopic Survey (BOSS) researchers used the first pre-release versions of this map to measure the distances of 1.2 million galaxies with one percent accuracy - more accurate than ever.

"For ten years, we've collected measurements of 1.2 million galaxies over a quarter of the sky to map the structure of the universe in a volume of 650 billion cubic light-years, " says Jeremy Tinker of New York University, one of the leaders of the project. The now completed map shows galaxies from two to seven billion years ago, capturing a crucial time: According to past data, cosmic expansion has been accelerating for about five to six billion years.

The baryonic acoustic oscillations left subtle fluctuations in the galaxy distribution. Zosia Rostomian / LBNL

"Frozen" density waves as a yardstick

To measure the rate of expansion and thus the effect of dark energy, the researchers in the new map analyze subtle variations in the distribution of galaxies. They were created by density waves in the early cosmos, so-called baryonic oscillations. Around 400, 000 years after the Big Bang, however, the coupling of light and matter collapsed, so that the pattern of these density waves was virtually "frozen". display

This is precisely what makes it possible to measure cosmic expansion. Because these primordial density waves are reflected in both the cosmic background radiation and the later galaxy distribution, researchers can conclude by comparing them on the expansion that has taken place since then,

Discrepancies in expansion

The result: The BOSS researchers come to a value of the Hubble constant of 67.6 kilometers per second per megaparsec (km / s / Mpc). However, this is very close to the value of 67.15 km / s / Mpc that the Planck satellite had determined in 2013 and significantly lower than the results of a team published just a few weeks ago had determined the expansion on the basis of supernovae and changeable stars.

Each point in this map shows the position of a galaxy six billion years ago. The color stands for their distance: yellow = near, purple = far. The image covers about 1/20 of the sky a section of the universe that is six billion light-years across, 4.5 billion light years high and 500 million light-years thick. Daniel Eisenstein / SDSS-III

"Our map tells us that the influence of dark energy in the period we are looking at has changed very slowly, if at all, " explains Florian Beutler of the University of Portsmouth.

The evaluations of the galaxy map thus seem to confirm the classical model of a flat universe with cold, dark matter (ΛCDM). After this, the influence of the mass and the dark energy balance so that the expansion will stop only after an infinite time. The behavior of the galaxies observed by the researchers also corresponds exactly to the predictions of the general relativity theory.

Open questions remain

However, the discrepancies with the latest supernova measurement results of the Hubble constant remain the same. "Whether this can be explained by a combination of statistical and systematic errors, or whether the flat ΛCDM model breaks down, is an exciting open question, " say the researchers.

In any case, the new galaxy map has already proven to be a valuable tool in cosmology: "We believe that the Baryon Oscillation Spectroscopic Survey marks an important cosmological milestone by providing accurate measurements of matter distribution in an enormous volume of the cosmos with detailed modeling and modeling Observations of the cosmic microwave background connects ", so the conclusion of the astronomers. (Monthly Notices of the Royal Astronomical Society, in press; arXiv: 1607.03155)

(Lawrence Berkeley National Laboratory / BOSS, 15.07.2016 - NPO)