Present condition of Universe
If we observe a distant galaxy, we are really looking into the past. But we don't know the present condition of the distant galaxy now because the light rays leaving the galaxy at present time have not yet reached the Earth.
How can we know the present condition of other galaxies and even what the Universe look like when our observation is subject to the limitation of speed of light ?
I mainly concern about what the Universe looks like at a particular moment. Since the images of distant galaxies at different distances from the Earth were their images in different epochs in the past, only God knows what the Universe looks like at this moment.
- ThomasLv 51 十年前最愛解答
You are right, the light rays leaving the galaxy at present time have not yet reached the Earth.
Therefore, we cannot know the present condition of anything, since no present information can reach the Earth.
As used by observational cosmologists, the Universe (upper case "U") most frequently refers to the finite part of space-time which is directly observable by making observations using telescopes and other detectors and using the methods of theoretical and empirical physics for studying the basic components of the Universe and their interactions. Physical cosmologists assume that the observable part of (comoving) space (also called: "our universe") corresponds to a part of a model of the whole of space, and usually not to the whole space. They frequently use the term the Universe to mean either the observable part of space, the observable part of space-time or the entire space-time.
A majority of cosmologists believe that the observable universe is an extremely tiny part of the "whole" (theoretical) Universe and that it is impossible to observe the whole of comoving space. It is presently unknown whether or not this is correct, since according to studies of the shape of the Universe, it is possible that the observable universe is of nearly the same size as the whole of space, but the question remains under debate. If a version of the cosmic inflation scenario is correct, then there is no known way to determine whether the (theoretical) universe is finite or infinite, in which case the observable Universe is just a tiny speck of the (theoretical) universe.