i mean how can they really know? and do they know what is beyond the known UNIVERSE??
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Space is expanding. This is actually an observation (it is not a "theory").
In fact, we can measure the rate of expansion of the universe. Over a distance of one million parsecs (3.26 million light-years), the rate of expansion is around 70 km/s (if you measure the distance between two objects separated by a million parsecs, the distance between them grows by 70 kilometres every second).
Put the two values in the same units (the easy way is to chang light-years into kilometres), then divide one by the other: this gives you the time when the distance between these two objects was... zero.
If you push the equation beyond that value, then the distance would have been... negative. If you understand what a negative distance is, good for you. The rest of us don't understand negative distances so we say that this moment was the "beginning" of the observable universe.
The question as to what is "beyond" the universe is linked to the implied question above: what existed "before" all distances were zero?
We also do not know the answer to that.
The moment when "all distances were zero" is the Planck Time, roughly 13,700,000,000 years ago (give or take a few).
When we look at things which are a million light-years away, we see them as they were a million years ago. Ten million light-years away? Ten million years ago.
We cannot see further away than 13,700,000,000 light-years away and we cannot see anything older than 13,700,000,000 years in the past.
Whatever comes "before" that (if there really was a before) is whatever comes "before" our Observable Universe. Whatever is further away than 13,700,000,000 light-years away is "beyond" our Observable Universe.
In the first case, we do NOT even understand (yet) what came before the Planck Time (because we do not understand what happens "before" the moment when all distances are zero). In the second case, we assume that if the entire universe is bigger than 13,700,000,000 across (and there are good reasons to think it is), then it continues on much as it is around us.
In fact, we can measure the rate of expansion of the universe. Over a distance of one million parsecs (3.26 million light-years), the rate of expansion is around 70 km/s (if you measure the distance between two objects separated by a million parsecs, the distance between them grows by 70 kilometres every second).
Put the two values in the same units (the easy way is to chang light-years into kilometres), then divide one by the other: this gives you the time when the distance between these two objects was... zero.
If you push the equation beyond that value, then the distance would have been... negative. If you understand what a negative distance is, good for you. The rest of us don't understand negative distances so we say that this moment was the "beginning" of the observable universe.
The question as to what is "beyond" the universe is linked to the implied question above: what existed "before" all distances were zero?
We also do not know the answer to that.
The moment when "all distances were zero" is the Planck Time, roughly 13,700,000,000 years ago (give or take a few).
When we look at things which are a million light-years away, we see them as they were a million years ago. Ten million light-years away? Ten million years ago.
We cannot see further away than 13,700,000,000 light-years away and we cannot see anything older than 13,700,000,000 years in the past.
Whatever comes "before" that (if there really was a before) is whatever comes "before" our Observable Universe. Whatever is further away than 13,700,000,000 light-years away is "beyond" our Observable Universe.
In the first case, we do NOT even understand (yet) what came before the Planck Time (because we do not understand what happens "before" the moment when all distances are zero). In the second case, we assume that if the entire universe is bigger than 13,700,000,000 across (and there are good reasons to think it is), then it continues on much as it is around us.
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