I am confused when to use 'in' and 'across' when I am saying something like

This food is the most expensive in the world.


Japan has the biggest aquarium in the world.

I think "in the world" is correct for the sentences I wrote. But when should I use "across"? Would the sentence still be correct if I said

Japan has the biggest aquarium across the world?

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    Do you want the last sentence in your question to have in the world or across the world? – green_ideas May 27 '17 at 0:29

If you are making a comparison (which includes superlatives), "in" refers to the population from which you are making the comparison. The biggest X in Y refers to the biggest X of the examples found in Y.

"Across" refers to dispersion; M-W defines it as meaning "throughout". For example, French-speaking people "across" Europe refers to French-speaking people wherever they might be throughout Europe.

So to your examples, you might say, "This food is the most expensive in the world" (correct), or "This food has become expensive across Asia", meaning throughout Asia. When you talk about "across the world", the term "worldwide" is more typically used.

"Japan has the biggest aquarium in the world" would be at least grammatically correct. "Aquariums have become popular across the world" or more idiomatic, "Aquariums have become popular worldwide" would be an example of usage for "across".

  • Great answer, everyone seem to always forget about worldwide, I know I do sometimes. What can you say about all over the ... and anywhere/everywhere in the ...? – SovereignSun May 27 '17 at 7:49
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    @SovereignSun, thanks. Thinking about these terms for this usage, they seem to be pretty interchangeable to me--across, throughout, all over, anywhere/everywhere in. There is probably some nuance of difference, but they convey essentially the same meaning. – fixer1234 May 27 '17 at 8:01

Japan has the biggest aquarium in the world.

is the correct sentence, if you wanted to use across, you might say

If you search across the world, you will find Japan has the biggest aquarium.

Usually for superlatives

in the world

is used.

He is the fastest in the world.
They are the tallest in the world.
London is the most expensive city in the world.

Maybe you can think of it as

across the world

describes an expanse, whereas

in the world

describes a location, but sometimes they can be interchanged

Ray's pizza is the best pizza in NYC.
Ray's pizza is the best pizza across NYC.


Today's events happen to provide an excellent example

British Airways is the only airlines in the world, to have cancelled all of their flights across the world.

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