Why we believe

[Alex K]

OK cool I don't understand that, but I am confident you know your stuff on this subject and I do not. Do you think my overall point is valid, in your opinion? That the Big Bang results were inherently exotic? (Trying to scrape some credibility back with that obscure word. Unless it's the wrong word. In which case, pretend it was a really cool word.)

Ok, for full disclosure I must mention that I'm technically not a cosmologist but a high energy theorist, so this is not my exact field of expertise, but let's say a closely related field in which I've dabbled from time to time. Imagine a Guitar player playing Base. As far as my historical understanding goes, the notion of a singularity came from the fact that, if you take the universe as is now with matter, radiation and a cosmological constant and you evolve the so-called Friedman-Equations backwards, you end up with a universe that becomes hotter and hotter, at some point dominated by radiation. The radiation dominated solution has the strange property that the scale factor, which roughly speaking measures the size of spatial distances, reaches zero in the finite past. This is the singularity of classical cosmology. However, we know that this picture can't be quite correct, because it would not accomodate the relative uniformity of the cosmic microwave background, and having a universe as flat as we observe it would be very unnatural in this scenario. Therefore, people have proposed to have a short era of ultrarapid expansion called inflation at some point in time in the very early universe - the universe then keeps evolving as usual after inflation ends, but the naive backwards extrapolation of course becomes invalid at that point. The question is now, what happens before inflation. Does the universe after all further go towards a singularity before that? What happens there is model-dependent and highly unclear, but from what I've seen, there is no reason to think that an ordinary singularity occurs as would in a radiation dominated universe without inflationary epoch. Of course a singularity of classical relativity would be called into question by quantum gravity, but I'm arguing that there isn't one even in the classical non-quantum picture.

The world is however quantum mechanical, and this would also concern the fluctuations of the gravity field and inflation field - these quantum fluctuations are then what's visible in the microwave background,

and what ultimately gives rise to the large scale filament structure of the universe.



Every dot in this picture is a galaxy, and the more narrow filaments are a result of a few billion years of gravitational clumping, as is nicely illustrated in the millenium simulation.

So in this picture, the large scale development of the Universe is directly subject to quantum randomness either way.

Ok now I'm done hijacking the thread with physics again. But you asked for it


p.s.

one more thing: whether the laws of physics themselves were selected randomly is not clear. In a multiverse scenario that would be the case, but that's speculation.