Why wasn't the Big Bang an isotropic explosion?

Main Question or Discussion Point

I'm sure I'm making a plethora of naive assumptions in this question, but I was just thinking that the Big Bang, or the birth of our universe should logically be isotropic if space and time is assumed to be homogenous and infinite in every direction.

mfb
Mentor
Isotropic up to quantum fluctuations. Yes, that is the usual assumption (and observations agree very well with that). Note that the big bang was not an "explosion" in space. Space itself expanded (and continues to expand).
Space does not have to be infinite, but it is probably without a boundary. This is not a contradiction, see the surface of earth for example: finite area, but no boundary.

Thanks for the reply. I did actually think explosion was a poor lexical choice, so thanks for correcting me. How were stars and galaxies formed then if the expansion is isotropic? Is this explained by quantum fluctuation alone?, there should surely be a homogenous gravitational field, which would mean matter shouldn't cluster? My knowledge of cosmology, as you can probably tell, is very limited.

UltrafastPED
Gold Member
Gravitational equilibrium of an isotropic distribution of matter is unstable; Newton understood this and so proposed an infinite distribution of stars.

This the slightest imbalance, anywhere, or at anytime will start the gravitational agglomeration of matter.

But such a slight imbalance at any time during the Big Bang, early or late, will result in inhomogeneity at some level - this is where detailed models must be compared to observational evidence in order to refine our understanding.

1 person
How come gravitational equilibrium of an isotropic distribution is unstable? Is there a good book you would recommend reading? I don't know if I'm verging on some seriously difficult physics here...

UltrafastPED
Gold Member
"Philosophiæ Naturalis Principia Mathematica" by Isaac Newton - if you read Latin!
http://en.wikipedia.org/wiki/Philosophiæ_Naturalis_Principia_Mathematica

But the argument is simple:

One object alone exerts gravity on nothing but itself.

Two objects exert gravity on each other, and are attracted. If they were originally still they will collide; if they had their own motions they will follow a conic section: parabolic (a bullet), hyperbolic (some comets), elliptical (planets)

What if you arrange 3 in a line? Then maybe you can cancel the net gravity on the middle one, but the other two are still going to move.

Keep adding one more, and every arrangement is unstable.

In the limit you can generate an infinite array - like a giant salt crystal, but with stars instead of atoms of Cl and Na - with an equal number of stationary attractors in each direction.

But once the perfect alignment is broken, you will have worlds in collision!

Chronos
Gold Member
The basic idea is quantum fluctuations resulted in slightly overdense regions in the early universe. These over dense regions show up as temperature fluctuations in the CMB. So, thanks to quantum fluctuations, the universe was never truly homogenous.

Awesome. Thanks for the reply. Definitely going to read up on some cosmology

Isotropic up to quantum fluctuations. Yes, that is the usual assumption (and observations agree very well with that). Note that the big bang was not an "explosion" in space. Space itself expanded (and continues to expand).
Space does not have to be infinite, but it is probably without a boundary. This is not a contradiction, see the surface of earth for example: finite area, but no boundary.
sooo if you get to the edge of the universe you go to the beginning?

D H
Staff Emeritus
There is no "edge of the universe".

I thought space-time was finite

UltrafastPED
Gold Member
The surface area of a sphere is finite ... but there is no edge.

George Jones
Staff Emeritus
Gold Member
I thought space-time was finite
In what sense?

There are models of the universe for which space finite, but I know of no seriously considered models of the universe for which spacetime is finite. In fact, all spacetimes that satisfy a certain finiteness condition have closed timelike curves.

bapowell