The Earth formed at a redshift of z∼0.53. That means that when our world was young, the universe was quite a bit smaller than it is today.

Question

The nearest galaxies to our own are around 10 kpc (kilo-parsecs) in size, lie at a distance of around 1 Mpc from us (Mega-parsecs), and have luminosities of around 1037 W. Back when the Earth was young, however, they would have appeared closer, and hence larger and brighter.

If we assume that the nearest galaxies are just being carried along by the expansion of the universe (actually not true for the nearest galaxies), then how far from us would they have been when the Earth was young (in Mpc)?


Back, then, what would the apparent angular size of a galaxy at that distance be (in arcsec)? Note that the size of the galaxy will not be affected by the expansion of the universe (since galaxies are gravitationally bound), but the distance to it will be affected.


And what flux would we have measured back then from a galaxy at that distance? (in Wm−2 )? You may assume that the inverse square law works (in reality, the stretching of photons would modify it somewhat). Assume that a parsec is 3.09×1016m.

Answer 1

At the time when the Earth was young, the nearest galaxies would have been around 0.53 Mpc away from us. The apparent angular size of a galaxy at that distance would be around 0.0017 arcsec. The flux we would have measured back then from a galaxy at that distance would be around 1.7×1041 Wm−2.