## Wednesday, October 14, 2015

### cosmologies without beginning

what would be observable consequences in a cosmological model where $a=0$ is not reached in a finite past? could you tweak the cosmological parameters in such a way that there's no CMB or no BBN?

#### 1 comment:

1. Of course, the de Sitter model doesn't have a scale factor of zero in the finite past and has neither CMB nor BBN. But that's probably not what you have in mind. On the other hand, if you mean tweaking the parameters compared to a universe similar to ours, then it can't be too similar, since cosmological observations in our universe indicate that the scale factor was zero in the finite past (at least within the framework of classical cosmology), which is known as the big bang.

Thus, the question is too vague. :-)

In the bounce models, where the universe contracts from infinity to a finite size then expands (one can speak of size here since all such models are spatially closed and have a finite mass and energy content), one can tweak the parameters such that the temperature at the minimum size is too low for BBN or for a CMB to form. One can also make the time the universe spends near the minimum scale factor arbitrarily long. If this is during the formation of helium, one could get all hydrogen to form helium and hence no heavier elements.

Again, though, I think the question is too vague for a precise answer.