A Million Points Of Light

In Cosmology by Brian Koberlein4 Comments

The Baryon Oscillation Spectroscopic Survey (BOSS) is part of the Sloan Digital Sky Survey-III. It looks at the positions and redshifts of more than a million galaxies. Redshift can be used as a measure of distance, though due to cosmic expansion you have to be a little careful about what that distance is. Using such a large sample, we can look at how the redshifts of galaxies (and therefore their distances) cluster on average, and this allows us to test whether the standard model of cosmology is correct

The key is to measure what is known as baryon acoustic oscillation (BAO). In the Universe there are two main forces driving the way in which galaxies cluster on large scales. One is dark matter, which causes galaxies to clump together. The other is dark energy, which causes these clumps to spread apart. The scale at which clumping occurs allows us to compare the ratio of dark matter to dark energy. It also allows us to study whether that ratio has changed over time. If, for example, dark energy were stronger in the early Universe, the gaps between clumps would be larger at greater distances.

The standard model of cosmology is known as the LCDM model, and assumes that dark energy is a constant, rather than varying over time and space. This new result found that the level of dark energy was constant over time to the limits of its observation, so once again the standard model holds up. The survey also calculate a value of dark energy through the Hubble parameter, and got a value of  67.3 km/s per megaparsec, which is much lower than the value of 73 recently found by Hubble observations, and even lower than the “official” value of 69.3 km/s per megaparsec. This variation of results within different observations, known as tension in the model, seems to be common these days. While the results aren’t completely contradictory when you take into account their uncertainties, they could hint at aspects of the model we don’t fully understand.

So overall the standard cosmological model holds up once again, but there are hints that a new chapter of the story may be unfolding as well.

Paper: Shadab Alam et al. The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological analysis of the DR12 galaxy sample. Monthly Notices of the Royal Astronomical Society (2016) arXiv:1607.03155 [astro-ph.CO]


  1. Brian, Could you please tell me ( if space between the object’s within the universe is empty and a vacuum) what is the source that vehicle’s in space use to push against for moving around?

    1. Author

      They push against themselves. Newton’s third law of motion says that a force on an object produces a resultant force back. So when the hot fuel races out the back end of a rocket, the rocket moves forward as a result.

  2. This is a very cool result, one that relies upon some pretty high-powered statistical analyses. There are an awful lot of astronomers (and other scientists?) listed as authors, part of a pretty big consortium.

    The tension re the estimates value(s) of H0 is interesting; based simply on what’s reported in the various papers, there does indeed seem to be an inconsistency (or perhaps more than one). Despite the firepower deployed for this paper, it would be very nice to have an independent confirmation, based on completely different observations, analyses, and teams. Likely once the LSST has been running for a while? Or perhaps from the Pan-STARRS-based data?

  3. Hi Brian, If i was a galaxy i would probably know the answers to Dark matter and energy. If the nature of clumping and dispersion could be considered as nourishment and activations it gives me a visual on the position. Otherwise I got nothing haha.

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