How do scientists know that distant parts of the universe obey the physical laws exactly as we observe around us? How do scientists know that distant parts of the universe obey the physical laws exactly as we observe around us?
The question might look a bit odd but I am really stuck on my head. We know, scientists (with tools) explored physically only our solar system and some parts of our galaxy which is really a tiny part of the observable universe. And now they are constantly using these knowledge along with 'tested physical laws' to measure the properties of distant parts of our universe.
For example, we tested and found the speed of the light is constant within our local periphery many times (within our Earth and Space around Earth). But yet we presume that the speed of the light is constant even at the farthest part of our universe. Certainly we did not test it in the other distant part of the universe because we have no way until now. Not only light but also the other physical properties like, luminosity, gravity, and etc related physical laws are agreed upon based on tests within our solar system. And based on these laws we deduced the properties of other parts of the universe (i.e. age, distance, mass, luminosity of stars in millions/billions light years away).
My question is, how do we know that these physical laws which we tested within a tiny area of the universe are consistently working in the distant parts of it? Is there any probability that the distant part of our universe obeys physical laws differently and our prediction based on applied physical laws gave us an unreal illusion of the actual reality, yet consistently?
We don't know in general but to the extent we can measure, the laws seem to be the same, even if conditions are not.
For example radioactive decay: We know how fast various elements decay, and we can observe the results of radioactive decay in distant supernovae.  The conclusion is that, for at least some elements, the rate of radioactive decay is the same on Earth as it is in distant supernovae.
After accounting for redshift, spectral emission lines remain unchanged by distance. This implies that the fine-structure constant is indeed constant.
Distant galaxies have gravitational fields, and interactions between galaxies proceeds in the same way in distant galaxies as it does in local ones. Eventually, the justification is philosophical:  There is no observational reason to believe gravity behaves differently in distant parts of the universe, and so we believe that it does not,
In the extreme conditions of the early universe, some physical laws were different.  For example, instead of distinct electromagnetic and weak fields, there was a single Electroweak field.  But this can be described as single &quot;law&quot; with the electromagnetic and weak interactions being just the low energy approximation of the electroweak interaction.
So if it were discovered that Gravity (for example) was working differently in distant parts of the universe, but that there was a consistent pattern or rule for how it varied, then that would simply become the new theory of gravity (with general relativity becoming only the local approximation to this new law).
There is a more fundamental assumption: that the behaviour of matter and energy in the universe can be modelled by &quot;laws&quot;.  There are no angels dancing on pinheads. The justification for this is strictly in the realm of philosophy.