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So this is a rather simplistic understanding but I want to know if it's fundamentally correct:

We have nothing which we'll represent as $0$.

Then we need a Universe which we can represent as any number of parts that sum to $0$

$$ 0 = -1 + 1 $$

In this way we can take $0$ and make a "Universe" that is now equal parts $1$ and $-1$. Nothing in this Universe could be created or destroyed because then it wouldn't equal 0 and yet we've now taken $0$ and made "something" of it by expressing it as different parts.

Is this a valid, albeit simplistic understanding? Or am I missing something here?

This would even mean that an infinite number of Universes could exist, $0$ would just be expressed in different ways, or repeatedly in the same way.

  1. $0 = -1 + 1$
  2. $ 0 = -2 + 2$
Anne O'Nyme
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wooobie
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1 Answers1

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Although I completely agrees that the question itself doesn't physically make sense, here is somewhat of a answer for the question:

Can you interpret the geometry of the universe in terms of a the potential energy cuonterbalancing its kinetic energy?

The Friedmann equations, which is a isotropic and homogeneous solution of the Einstein field equations of general relativity, describes the evolution of the (scale of the $R$ of the) universe.

$$\left(\frac{\dot{R}}{R}\right)^2=\frac{8\pi G\rho_\text{tot}}{3}-\frac{k c^2}{R^2} $$ The first term represents contains the energy density due to the matter, radiation, dark matter and dark energy in the universe. The last term is related to the curvature of the universe. For $k=+1$ you have a closed universe, $k=-1$ an open en $k=0$ a flat one.

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From observation, we believe to be living in a flat universe, i.e. $k=0$.

Although this equation is a solution of the (difficult to solve) field equations of Einstein, it can be classically derived.

Imagine a point mass $m$ being accelerated by gravity at the surface of a sphere of radius $R$, density $\rho$ and mass $=4\pi D^3\rho/3$. So the force equation is $$m\ddot{D} = -\frac{mMG}{D^2}$$ Some simple calculus give you and using that $D=rR$ (that is not really important: $R$ is not the real distance) $$\frac{m\dot{R}^2}{2} - \frac{mMG}{R} = \text{constant} = \frac{kc^2}{R^2}$$ where you chose the integration constant to match to one in the Friedmann equation.

And here you see it: on the left hand side you have the total energy, kinetic plus potential.

The Friedmann equation can be interpreted (!) as saying the following

  • a universe that is spatially closed (with k = +1) has negative total ``energy'': the expansion will eventually be halted by gravity, and the universe will recollapse.

  • In an unbound model, where k = -1, it will expand forever.

  • However for a given rate of expansion there is a critical density that will bring the expansion asymptotically to a halt. This is the case when the knietic energy is exactly counterbalancing the potential term, yielding $k=0$.

References: D. Perkins, Particle Astrophysics, Chapter 5.3

or on the web you also have this.

Anne O'Nyme
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