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Tue, 30 Aug 2016

An extra Gigayear for the Universe?

Popular science descriptions of our present understanding of observational cosmology tend to say that we know the age of the Universe to be 13.80 gigayears, with an uncertainty of just 0.02 gigayears (20 megayears). But some of the oldest microlensed stars in the Galactic Bulge, within the central kiloparsec or so of our Galaxy, have best estimated ages of about 14.7 gigayears!. In the figure at left, our analysis of the probability distribution of the most likely age of the oldest of these stars is shown. The thin curves show probability densities for the ages of individual stars—several of these peak between about 14.5 and 15 gigayears. The thick curve shows the age of the oldest of these stars, supposing that we choose the individual star ages randomly according to their probability distributions. (This includes possible ages much lower than in the figure; we take the full asymmetric distributions into account.) So could the Universe be a gigayear older than is generally thought? The uncertainties are still big, but this is certainly an exciting prospect for shifting towards a more physically motivated cosmological model.

The more careful descriptions of the age of the Universe give a caveat—a warning of how or why the standard estimate might be wrong—the age estimate depends on fitting observations by using the standard ΛCDM model. Which is the standard model of cosmology. Meaning that it makes a non-standard assumption about gravity. Instead of allowing space to curve differently in regions where matter collapses into galaxies versus places where the Universe becomes more empty, which is what Einstein's general relativity says, the standard model is rigid (apart from uniform expansion). It doesn't allow general relativity to apply properly.

Several of us have been working on theoretical tools and observational analysis to see if we can apply general relativity better than in the standard model. At least so far, we generally find that doing our homework tells us that the would-be mysterious "dark energy" is really, until or unless proven otherwise, just a misinterpretation of space recently becoming negatively curved (on average) as voids and galaxies have formed during the most recent several gigayears.

This is where the age of the Universe comes in. In our new paper, arXiv:1608.06004, my colleagues and I summarise some key numbers that we argue are needed by any of the "backreaction" models similar to ours, which allow space to curve as galaxies and voids form, as required by the Einstein equation of general relativity. These simple constraints show that by fitting a no-dark-energy flat model (the Einstein–de Sitter model) at early times, the age of the Universe should be somewhat less than 17.3 gigayears, and quite likely somewhat more than the ΛCDM estimate of 13.8 gigayears. So we looked at published observations of stellar ages, which individually still have big uncertainties, but together favour the oldest stars having ages of around 14.7 gigayears. As expected, this is somewhere in between the two limits of 13.8 and 17.3 gigayears.

So will there be a race between detailed "backreaction" models versus stellar observers to get tight cosmological predictions of the age of the Universe versus accurate spectrosopic measurements of the oldest Galactic stars's ages (which have to be younger than the Universe, of course!)?

Barely had our paper become public on ArXiv, that we were reminded by colleagues studying cosmic microwave background (CMB) observations using the Einstein–de Sitter, no-dark-energy, flat cosmological model at early times that they also found an age of the Universe of something like 14.5 gigayears! Figure 4 bottom-right of arXiv:1012.3460 (PRD) shows our colleagues' estimates of the age of the Universe using the CMB and type Ia supernovae observations. Their most likely age is about 14.5 gigayears, give or take about half a gigayear. This is not so very different from the Galactic Bulge star best estimate! So we have very different, independent methods tending to give similar results. The uncertainties are still big. This story is not closed. But an extra Gigayear for the age of the Universe may be a clue that helps shift from the precise ΛCDM cosmology to the upcoming generation of accurate cosmology...

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