THis is probably something covered in astronomy 201 or something but hey I never got that far.
This is something that has been bothering me since my first college astronomy class. It has to do with judging the current distance between us and various other objects in the universe. When astronomers do this are they simply trying to describe the universe as we see it or are they taking into account the events that must have occurred since the light we are seeing left the objects we are now observing?
My particular question has to do with the speed at which distant objects seem to be moving away from the earth. There are several factors that are included in this speed such as the classic “raisons in a pudding” example where all objects appear to be moving away from each other with the most distant having the greatest velocities as the volume increases. There is one factor that I have never heard discussed though. I even asked my college astronomy prof. but he was unable to answer the question. (But then this was also the guy who took class time to tell us that evolution was statistically impossible.)
The factor I mean probably has a name but not knowing what it is I will have to simply describe it.
Assumption 1: Current theory states that at some point in the past the matter in the universe had closer proximity that it does now. All matter was confined to a much smaller volume.
Assumption 2: Since the time described in Assumption 1 matter has been becoming more diffuse. Things have been moving further apart.
Assumption 3: During the process described in Assumption 2 the matter in the universe has been having an effect on all other matter via gravity, slowing its velocity.
Therefore the further you go back in time the less things have decelerated.
So when you look at a light source of a very distant object you are seeing the light it emitted at some point in its deceleration. You are therefore seeing the object moving away from you not at its current rate but at the rate it was going prior to millions of years of gravitic deceleration.
How does astronomy and particularly cosmology account for this factor?
What, if anything is this factor called?
I have tried to research this myself from time to time (not terribly seriously) but not knowing what to call it I was never able to pin it down.
My understanding is the measurements you take of distant galaxies and such are indeed as they were far in the past. You measure how far away they were then, what sort of light they gave off then, how fast they were moving then, etc.
You don’t know anything about how they in our current time. The standard example is the Sun may have blown up 5 minutes ago, and we don’t know anything about it yet, because the light hasn’t gotten here yet.
Yes, that was more or less the premise for my post. The question is how do they account for this when they want to figure out things like how fast the universe is expanding? If no two data points are based in the same time frame then how can you compute a collective velocity that has any meaning?
I would have thought that comparing the accelerations of nearer objects with those of further ones gives a picture of the change in acceleration across time throughout the universe (assuming that everything is accelerating away from us, with Andromeda being the exception that proves that rule). I guess astronomers and cosmologists can check that assumption more rigorously or may use methods that don’t rely on it.
However, judging the distance of a celestial object from us in the first place is often a nightmare and there is considerable uncertainty involved.
Don’t red shift measurements cover this (as is possible)? If super-distant-quasar “A” is moving away from earth, and earth is moving away from (or towards) super-distant-quasar “A”, wouldn’t the amount that the light from the quasar is shifted towards the red (or blue) indicate the relative velocity in something closer to “real time”?
Is this making any sense? Thinking about it, it’s making less sense to me, now, it seemed a lot clearer in my head :).
Basically, what I’m thinking is:
Quasar emits light
Quasar is moving away from earth
Earth is moving away from quasar while light is heading towards it
the amount of doppler shift detected in the light indicates that earth was moving away from quasar at a certain relative velocity
Ugh. I haven’t had enough coffee in the morning for this, and I’ve probably misunderstood your question entirely. But I suppose my brain needed the exercise.
Astronomers have only recently pinned down how far away some quasars are and hence, how old they were because they are fairly recent discoveries and there were no reference points.
Physics/Astronomy grad here (although with muddled recollection of school). This isn’t directly answering anything, just hopefully clarifying some weirdness
"Don’t red shift measurements cover this (as is possible)? If super-distant-quasar “A” is moving away from earth, and earth is moving away from (or towards) super-distant-quasar “A”, wouldn’t the amount that the light from the quasar is shifted towards the red (or blue) indicate the relative velocity in something closer to “real time”? "
Not exactly. The proper frame of reference isn’t to think of the quasar moving away from us added to us moving away from it. Relativistically, space is expanding such that earth is standing still, while all of those quasars (and every other point in space) is moving away from us (from our perspective). This is also dependent upon how far away something is (the further away, the higher the velocity), so there isn’t really any “real time” velocity that can be measured along the path. Trying to treat things as Earth-velocity added to Quasar-velocity turns the mathematics and theory into an unworkable mess.
Sad isn’t it?