The Invaluable Thing Called Internet
Well it takes special moments to realise the importance and value of internet. Really special moments because at any other time internet seems more like something that has existed since time immemorial and we don’t really value it as much as we should. SOPA would be a nice example for such moments, but no, I’m talking about some personal ones that leave you with a warm feeling for the internet.
Back in 9th grade, I remember asking my Science teacher a question. A particular question. For the record, she was a damn good teacher, always ready to answer doubts, even the strange ones. But this question bamboozled her. She did not give a wrong answer, just left it unanswered with the promise that she’ll get back to me, if she finds the answer that is. Do I even need to tell you that she never did? Well, what was the question?
Q: Its a widely popular theory that the universe is expanding. Great, universe is expanding. So am I gaining inches at my waist every passing day because of it? No? Ok, so the boundaries are expanding. But what are they expanding into? Just like a balloon expands into the surrounding space, the universe has to expand into something! Or atleast so I believed. So what is it expanding into really?
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A: My cooked up theory
I had heard of some twin universe theory. I had no clue what that theory proposed but I worked on the title: “twin”, “universe”, “theory”. And then the 9th grader in me proposed that there are two universe. While one expands, the other contracts. Simple.
And I happily dusted my hands off this conundrum, never to give it a thought again. Remember, “google it” wasn’t a very common phrase back then. I discussed it with quite a few people before truly giving up on the search. Actually I never gave up the search, that’s probably why I still remember having this doubt and I’m thrilled on finding an acceptable answer! An answer you said? Yeah an answer. Where? On reddit: What is ‘Space’ expanding into? Finally. Thank you internet. The reddit page has racked up quite a lot of upvotes on hackernews, that’s where I found it first. Looks like when people are told a question, many get curious about the answer.
EDIT: Since the reddit thread keeps updating with new answers and comments, I’m posting the answer that I found the most useful.
By adamsolomon It’s not expanding “into” anything. Like all of the curved spacetimes we talk about in general relativity, the spacetime describing an expanding universe isn’t embedded in some higher-dimensional space. Its curvature is an intrinsic property.
To be specific, it’s the property describing how we measure distances in spacetime. Think about the simplest example of a curved space: the surface of a sphere. If I give you the longitudes of two points and tell you they’re at the same latitude (same distance from the equator) and I ask you to tell me how far apart they are, can you do it? Not without more information: those two points will be much further separated if they’re near the equator than if they’re near the North or South Pole. The curvature of this space means that distances are measured differently at different points in space, particularly, at different latitudes.
An expanding universe is also a curved space(time), but in this case the curvature doesn’t mean that distances are measured differently at different points in space, but at different points in time. The expansion of the Universe means quite simply that the distances we measure between two points which are otherwise stationary grows over time. In effect, the statement that “space” is expanding is really a statement that our cosmic rulers are growing.