[Lewis Thomson (OP)]

Vítězslav has sent in this question to us at TNS office,

"I'm wondering how Is it possible, that we're making conclusions about the expansion of the universe, based on observation of processes, which actually occurred more than 13 billion years ago, at the times of the very beginning of universe's existence and therefore soon after the Big bang?

The red spectrum shows us history long gone. And we're not able to observe the processes taking their place now. ( There's no " Now " in universal measure anyway.)"

Tell us what you think in the comments below...

[Eternal Student]

Hi.

From the title:  Is the universe being observed 'in the past'?
    Yes, that's pretty much the idea.  The further away you observe, the longer light has taken to travel etc.

I'm wondering how Is it possible, that we're making conclusions about the expansion of the universe, based on observation of processes, which actually occurred more than 13 billion years ago

    Basically, we don't have to look that far away.   Actually, the universe was quite opaque that long ago and we can't look that far away even if we wanted to.
     If you are looking through a telescope at something that is only 100 light years away, then you're seeing it as it was just 100 years ago,  not 13 billion years ago.
     So, just to be clear, the expansion of space may have started 13 billion years ago but it is still happening now   (or  almost "now"):  We can see evidence for it at a range of distances from us, or equivalently at a range of times.
     Also, half the conclusions and interesting results can be obtained by studying the theory,  i.e. using a pencil and a piece of paper.   The expansion of space isn't something that is based solely on empirical observations.     

Best Wishes.

[Janus]

To expand a bit on the previous post.
Yes, when we look out at the universe, we are looking into the past. However, how far "into the past" we are looking depends on how far we are looking away.  We see a galaxy 1billion ly away as it was 1 billion yrs ago, and one 5 billion ly away as it was 5 billion years ago.  So,for example, the fact that quasars show up in very distant galaxies, and not nearby ones, indicates that they are a feature of the early universe.

In terms of the universal expansion, we see that red-shift increases with distance, this, in of itself, indicates that the universe is expanding.  Going further than that, exactly how the red-shift changes with distance indicates how the rate has changed over time.  If you plotted distance over red-shift and it resulted in a straight-line plot, it would mean that the universe expanded at a constant rate over time.  If the plot curves away from the straight-line in one direction, it shows that the universe was expanding faster in the past than it is now, and that the expansion is slowing over time. If it curves away in the other direction, it shows that it expanded slower in the past, and that the rate has increased over time.
In the 1990's a study was done which produced the last result, indicating that the universe has increased its rate of expansion over time (This was contrary to what they expected to find.  They had assumed that the expansion would have slowed over time, and were trying to determine by how much.)