May be there more big bangs out there... while studding about space i some times think about a bacteria living on my body that may have a life span of 5 seconds if it has logical understating even then it has no time to know that a big human it is living on takes 9 months to get birth and than live for 60 years then die and he himself lives on an earth and the earth in solar system and then milky way galaxy then clusters of galaxies then etc....

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    $\begingroup$ A lot of the question looks like slightly unrelated philosophical musings. Could you perhaps edit it to reflect the actual physics which you want to see? $\endgroup$ – user191954 Oct 8 '18 at 12:35

Because light takes time to travel across the universe, old light from far away is also showing us an older epoch of the universe. So apparently we do directly view conditions from many stages in the history of the universe, and not just the current stage.

Similarly, although human civilization lasts for only one moment in the life of a star, we can see many stars at different stages in their existence, and this helps us put together theories of the whole stellar life cycle.

Theories of the life cycle of a star, galaxy, or universe, also have another check on them, in our theories of physical law. Those theories are formed mostly by immediate experiment on Earth (the major exception is gravity, where astronomical observation has also been important), but we can ask whether our theories of the long-term behavior of stars and galaxies and the universe, are what the physical laws would imply.

Nonetheless, it's absolutely true that we only observe what may be a small patch of the whole universe, and only a special cross-section of that small patch; and there may be important data that we miss because of that. (Just as there may be important data that we don't observe because it passes right through us, like neutrinos and gravitational waves passed through us unobserved, until recently.)

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