The ever-present danger of global variables.

Toby Ord's chart shows 4 things:

• Everything we can observe now is the "observable universe".

• Everything we can ever observe if we stay here is the "eventually observable universe".

• Everything we can ever observe if we send spacecraft out in every direction at all speeds less than light is the "ultimately observable universe".

• Everything those spacecraft can ever affect is the "affectable universe".

His chart is drawn in funny coordinates where a galaxy at rest moves straight up the page and light moves at 45° angles. The Big Bang is the horizontal line at the bottom, and the infinite future is the horizontal line at top. The expansion of the universe is hidden in these coordinates!

How big are these 4 things?

• When we observe distant galaxies we see what they were like long ago, when they were closer. Those galaxies *now* form a ball of radius 46 billion light years in diameter. So people say the radius of the observable universe is 46 billion light years. But beware: we can't see what the galaxies in the observable universe look like *now*.

• The galaxies in the eventually observable universe *now* form a ball of radius 63 billion light years.

• The galaxies in the ultimately observable universe *now* form a ball of radius 80 billion light years.

• The galaxies in the affectable universe *now* form a ball of radius 16 billion light years.

These figures change with time. For example, shortly after the Big Bang the radius of the affectable universe was 63 billion light years. It has now shrunk to 16 billion light years. 90% of the galaxies we could in theory once reach - if we could have started right away - are lost to us now!

Your two choices when ordering dry whole spices:

(1) A shrink-wrapped glass jar containing enough to make 0.3x curry

(2) "Great, where do we back up the truck? ... what do you mean your house doesn't have a cardamom bunker?"