As a very quick postscript to my previous post about the amazing performance of Hubble’s spanking new camera, let me just draw attention to a fresh paper on the ArXiv by Rychard Bouwens and collaborators, which discusses the detection of galaxies with redshifts around 8 in the Hubble Ultra Deep Field (shown below in an earlier image) using WFC3/IR observations that reveal galaxies fainter than the previous detection limits.
Amazing. I remember the days when a redshift z=0.5 was a big deal!
To put this in context and to give some idea of its importance, remember that the redshift z is defined in such a way that 1+z is the factor by which the wavelength of light is stretched out by the expansion of the Universe. Thus, a photon from a galaxy at redshift 8 started out on its journey towards us (or, rather, the Hubble Space Telescope) when the Universe was compressed in all directions relative to its present size by a factor of 9. The average density of stuff then was a factor 93=729 larger, so the Universe was a much more crowded place then compared to what it’s like now.
Translating the redshift into a time is trickier because it requires us to know how the expansion rate of the Universe varies with cosmic epoch. The requires solving the equations of a cosmological model or, more realistically for a Friday afternoon, plugging the numbers into Ned Wright’s famous cosmology calculator.
Using the best-estimate parameters for the current concordance cosmology reveals that at redshift 8, the Universe was only about 0.65 billion years old (i.e. light from the distant galaxies seen by HST set out only 650 million years after the Big Bang). Since the current age of the Universe is about 13.7 billion years (according to the same model), this means that the light Hubble detected set out on its journey towards us an astonishing 13 billion years ago.
More importantly for theories of galaxy formation and evolution, this means that at least some galaxies must have formed very early on, relatively speaking, in the first 5% of the time the Universe has been around for until now.
These observations are by no means certain as the redshifts have been determined only approximately using photometric techniques rather than the more accurate spectroscopic methods, but if they’re correct they could be extremely important.
At the very least they provide even stronger motivation for getting on with the next-generation space telescope, JWST.