eli5-how do we know for sure how big space is? and how do we know we are right about everything we know in space?
In: Planetary Science
We don’t.
We can make educated guesses based on the data we have. Within the next few decades ( assuming no divine intervention and that we don’t have a nuclear war to wipe us all out ) science will continue to advance, technology will develop, and we will be able to discover more that will cause future scientists to laugh at what is accepted now.
Perhaps one day we WILL know how big the universe is.
From measurements and calculations.
The speed of light we can verify in labs and all over. The composition of stars.from spectrometry and the light they give off. The color of what stuff burns. The way that space expands and how the stars all fade off into infrared. From parallaxing telescope data at opposites sides of the year and seeing how things shift. From our knowledge of atoms and how they form. To calculations on how fast they can happen.
Science is a massive web of observations that fit with each other. All it really takes is one thing not fitting in which shows us that we are getting something wrong, at least a little. Before Newton, people weren’t entirely wrong when they said “thing fall down”. But Newton was less wrong when he figured out the planet also falls up to meet you. Just a little.
And we are wrong about something. Don’t let anyone fool you into thinking we’ve got this all figured out. Dark matter and dark energy and just why space expands are big ‘ol question marks where something doesn’t fit and we haven’t discovered it yet. Of the bits we know, we are very solidly positive that things fall down.
Short answer: we don’t. That’s why it’s called the “observable universe”.
They can make guesses to the size to the universe by observing things like gravity, distant stars, etc. But all that, at best, is a guess.
>and how do we know we are right about anything
That is the neat thing: we don’t
Science is not built to be right, but to be accurate. Physics is *the best available description of measurable phenomena*, it is not expected to be “right”
Of course some things are undoubtedly *not wrong*.
Astrophysics is quite a complex field of study, but we have solid evidence that:
1. light moves at a constant speed
2. the universe expands ever faster
3. at some point in the past, the universe was so small that we can assume it began its existence just then
4. The universe doesn’t have a geometrical center
This information is enough to estimate the size of the observable universe, any light further than that would have needed to be emitted *before* the start of the universe for us to see it.
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