The Stars Uncounted

In Cosmology by Brian Koberlein

As we’ve recently seen, the cosmos is much larger than we’ve thought, with more than 2 trillion galaxies in the observable universe. Actually observing many of the most distant and faint galaxies is a real challenge, but more of them are being detected thanks to a trick that relies on relativity. 

The more distant a galaxy, the more dim it can appear. This is due in part to the fact that the apparent brightness of an object decreases with the square of its distance (known as the inverse square law). For galaxies, this effect is even more dramatic due to cosmic expansion, which further dims objects billions of light years away. Because of this dimming, small dwarf galaxies can be difficult to observe. This is a problem because in the nearby universe dwarf galaxies are the most numerous, so we could be missing a lot of galaxies when we look across great distances.

But it turns out that relativity can help, thanks to an effect known as gravitational lensing. The path of starlight can be deflected by the gravity of a nearby mass, as Arthur Eddington first demonstrated in 1919. This means that light from a distant galaxy can be deflected and focused if a closer galaxy is between us and it. Through gravitational lensing, the distant galaxy can appear brighter than it would otherwise, just as a glass lens can magnify and brighten a distant star.

Recently, a team used this method to observe faint dwarf galaxies at a redshift between z = 1 and z = 3.  We see these galaxies as they were when the Universe was 2 to 6 billion years old, which is a period of peak star formation. They found that dwarf galaxies were most abundant at the greatest redshifts, and thus the earliest period. Since most of the stars in these early dwarf galaxies were hot and bright, they flooded the Universe with ultraviolet light, driving the reionization period of the early Universe.

When the James Webb telescope launches in 2018, we should have an even better view of these dim and distant galaxies. Until then, gravitational lensing will help us explore this critical period of galaxy formation.

Paper: Anahita Alavi, et al. The Evolution Of The Faint End Of The UV Luminosity Function During The Peak Epoch Of Star Formation (1<z<3). The Astrophysical Journal, Volume 832, Number 1 (2016)  arXiv:1606.00469