**Ask a monkey a physics question thread**

[MadMojoMonkey]

Well yeah. That centre is all around us in every direction... the past.

If everywhere is the center, then it wasn't rotating at the initial moment of the big bang (because this affirms the point-mass assumption).

We can't see where we're going, only calculate it.

This is called the "psychological arrow of time."
Rather the psychological arrow of time says we understand that time always flows in the same direction because we can remember the past, but not the future.

The further ahead we look, the further back in time we're seeing

The further away we look, the further back in time we're seeing.

, and the bigger the light cone required to calculate where we're going.

What?
What does the knowledge of the distant in both space and time have to do with...?

The vertex of a light cone is a spacetime event (in a 2-d plane, where one axis is distance and the other is time, so kind of a 1-D plane in time). The light cone describes that event's possible causal relationship with the rest of spacetime. The light cone is infinite in both directions, and contains all possible causal pasts in one direction and all possible causal futures in the other direction. Any spacetime coordinate not contained by the cone cannot have caused the event at the vertex or be affected by the event at the vertex, due to it being too far away in space and too close in time for the information from the event at the vertex to travel there at the speed of light.

How long before that light cone becomes the observable universe? At that point we are utterly blind to our future.

There will always be spacetime coordinates which are not causally related to other spacetime coordinates, based on the fact that the speed of light is finite.

Look at any straight line through the universe at a single moment in your spacetime frame. Pick any point on that line. It is not causally related to any other point on the line in that frozen instant, because it takes some time for a signal moving at finite speed to move away from it.

There is a unique light cone at every point in the universe which corresponds to that point in both space and time, which is the light cone's vertex.

If you're talking about the big bang's light cone, then it already encompasses everything we know, because we know we are causally linked to the big bang, so must be within its light cone, like the rest of the universe.

Dark energy is why we're told the universe's expansion is accelerating.

But it's a non-answer.

Why is the universe accelerating?
Dark energy.
What's dark energy?
It's what we're saying is making the universe accelerate in its expansion.
What is it beyond that, though?
Dunno.
Why are you even talking about it, then?
It's kind of a place-holder name while we look into it more.
... and it sounds cool.

That there is an outward motion from the centre, so why does dark energy not hint at a rotating universe with a definite centre of rotation? Because we see the acceleration in all directions? Of course we do! We're looking at the past, not the future!

SAME acceleration in all directions. Only a function of distance, not direction.

I don't understand your past/future argument, here.