# B Explanation of the asymmetry of the Universe

1. Jul 29, 2016

### Megnetto

Hi

My academic background is in philosophy, and I know very little physics. My attraction to the principle of sufficient reason motivates my question.

My understanding of one theory of the origin of the universe is that it started with a "singularity" which then exploded into our universe. It appears that our universe is far from being a collection of identical objects all evenly spread out in a perfect spatial symmetry. If the singularity itself were not symmetrical, this could explain the asymmetry of the universe. But what could then explain the asymmetry of the singularity?

2. Jul 29, 2016

### PeroK

A singularity is a mathematical concept and has no physical manifestation. Rather, it represents the limitation of a mathemetical model to describe the physical reality.

3. Jul 29, 2016

### Megnetto

That's interesting. So, assuming a mathematical concept can't go "bang" and produce a physical universe, what exactly did go bang?

4. Jul 29, 2016

### Bandersnatch

Hi Megnetto, welcome to PF.

Symmetrical doesn't really make sense with regard to the universe, since you don't have anything to be symmetrical around (like a point of origin - the singularity isn't a point in space as was mentioned by PeroK). But I think you really mean 'homogeneity' - i.e. looking the same everywhere (to be precise we should also include 'isotropy' - looking the same in every direction). This is known in cosmology as the 'cosmological principle'

The universe is actually very homogeneous at very large scales, that is, the cosmological principle is an observational fact, and the relic microwave background radiation indicates that it was very much so when the universe was young, even on small scales.
The current small-scale inhomogeneities like galaxies here versus voids there are well explained by gravitational attraction of the initially highly-homogeneous distribution of matter leading to its aggregation around any deviation from the hypothetical perfectly uniform distribution at the end of inflation.
Such deviations are expected on the fundamental level due to quantum-mechanical oscillations.

There's currently no compelling theory to describe the very early period in the universe's history - what the big bang theory does describe is the evolution from the early, hot and dense state to what we've got today and beyond. In other words, there is a limit to the domain of applicability of the theory, and it just doesn't include any 'beginning' - whatever that might be, and whether there was one or not (and not e.g. a change of state from some earlier form).

5. Jul 29, 2016

### Megnetto

Thanks for your replies. Just one more question for now: could you tell me what the relationship is between the "early, hot and dense state" you mention and the "singularity"?

6. Jul 29, 2016

### Bandersnatch

It becomes clearer when you consider that the BB is not a creation story that starts at some time t=0 and then progresses towards today, but an evolution model that starts with t=0 as today, and then extrapolates backwards and forwards from this point.
So, if you today see far away objects receding from each other, and try to extrapolate this recession back in time, you get everything becoming closer and closer together (and by extension the universe getting hotter and denser), until you reach the very hot and dense stage past which extrapolation starts to give you un-physical results in the form of infinite densities and temperatures - that's the singularity. The mathematics of the model are such that the time when the singularity happens can be unambiguously calculated (~13.7 billion years ago), but nobody takes this point in time to mean or represent anything more than the simple fact that the model used is just not a good tool to describe those early times.
Still, the model does work remarkably well for the remainder of time, and the singularity is often used as a convenient point of reference when describing its predictions, so you get e.g. expressions like 'five seconds after the BB singularity' this and that happened.
The time past the singularity at which the model starts to give testable predictions (some fractions of a second) is what is normally understood as the 'early hot and dense state' onward from which the BB theory is applicable.

7. Jul 29, 2016

### PeroK

I'm sure one of the experts can give you a better answer, but the "hot, dense state" is the universe as far back in time as the current theories can reliably take us. The singularity is the point-universe (of infinite density) that General Relativity (GR) predicts if we go back to a theoretical starting point. But, General Relativity alone is not sufficient to describe a universe on such a small scale. And there is, at present, no adequate theory to describe what happened further back in time, which essentially requires a unification of Quantum Mechanics and GR.

I found this, which may be interesting:

https://profmattstrassler.com/2014/03/21/did-the-universe-begin-with-a-singularity/

8. Jul 29, 2016

### PeroK

I thought I might add a note about mathematical singularities. The simplest example might be the function $1/x$, which has a singularity at $x = 0$. There are two things to note:

a) The value of $1/x$ at $x = 0$ is not infinite. It is undefined.

b) There is, however, no upper bound to the size of $1/x$ as $x$ gets smaller and closer to 0.

This is an example where "infinity" occurs in mathematics. But, it's more accurate to say that we have an "unbounded" function, as you never actually get "infinity".

If the function $1/t$ was used, say, to model the amount of money someone had at time $t$, then this model might work very well. But, the model would inevitably become inapplicable for small enough $t$. You can't have infinite money at time $t = 0$, so there must be some time $t = t_0$ before which the model (in reality) breaks down. You could still talk about the singularity at time $0$ but it can't actually exist economically.

9. Jul 29, 2016

### rootone

This way of looking at it seems to me very logical.
(although I had to reset a circuit breaker in my brain before agreeing with that conclusion)