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This question already has an answer here:

After the big bang, everything expanded from a small mass. That expansion is said to be still happening.

Imagine if everything observable constantly grew in size. EG. Everything slowly doubled in size over a decade? Would we notice difference? Would it seem the same because everything we measure by grew as well? Assuming the speed of light adjusted as well.

The farther from the big bang the expodentially slower the expansion.

Imagine the expansion of the universe is happening within us. Is it possible?

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marked as duplicate by Qmechanic Jan 11 at 9:52

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The answer is either no or yes depending on what you have in mind.

In the SI system of units the unit of length is defined as that distance which is traveled by light in vacuum in a certain specified time. Then a standard atomic clock is used to determine how long to wait until a standard second (or other time interval) has passed. For example one could wait for about 9 billion oscillations of the caesium clock.

Now suppose someone came along and announced that after 9 billion oscillations of the caesium clock in fact two seconds have passed, not one. In this case the standard length (distance traveled by light in one second) would be the distance traveled during 4.5 billion oscillations of the clock, i.e. shorter. Consequently, when measured in this unit, everything in the universe has doubled in size. But nothing has really happened.

So this is one answer to your question. If you rescale all the distances just by redefining the units, then nothing happens. So if someone else is doing such a rescaling without telling you, then they would say that you and all the rest of the universe are getting larger (according to their units) but you would not be aware of it.

The cosmological expansion is NOT like this. In the cosmological expansion, the time scale is fixed at 9 billion oscillations of caesium per second, and the speed of light is constant. Equally, the cosmological expansion does not expand ordinary matter, it simply says that matter in free-fall far from other matter (e.g. galaxies) is growing in distance from other such matter. Each blob of matter meanwhile stays the same size.

Now let's come to another possible meaning of your question. Suppose that throughout spacetime there is an "expansion field". This is a field (unknown to present physics) which causes everything to expand: quarks move further away from each other, electrons clouds in atoms get larger, etc. etc. It is not possible to say whether such an effect would be noticeable until one adds more detail about the effect of this field. Most versions of such an expansion field would be detectable. But if you added enough effects of this field, then eventually you would be able to describe a field whose effect was not distinguishable from merely redefining the units, as in my first example, and in that case the field would not be detectable. What standard physics does with a case like that is to conclude that we may as well say that such a field does not exist. At least, it would have no role to play in physics so we might as well forget about it.

To finish then, if there was really some sort of physical effect, caused by a previously unknown kind of field, which made everything get larger, then most versions of such a field would do this in such a way that the effect would be easy to detect.

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  • $\begingroup$ My weighing scale would indicate $2^6$ times what does now, unless the gravitational constant and proton mass also double. Then it would be $2^{10}$. Nevertheless, it would not look bigger. $\endgroup$ – my2cts Jan 11 at 13:10
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We always measure size with relative to a known size. When we see in a microscope ,until we don't know that they are small ,they will look very much large.This is the first sense that is by seeing. The second sense is by touching.and the third thing is by a known speed of an object and it's known size.Usually we measure the size by these.

If everything starts to grow in size and speed of objects(as speed of light is used to measure length) also adjusted with respect to size Then all possibilities of knowing the difference in size is adjusted. You can't even know if you just doubled or tripled in size.This is similar to the case that time is stopped for 3hours and you can't know that it was stopped.

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