Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

An ignorant question about cosmology

  1. Mar 12, 2004 #1
    so the farther we look the more redshift we see in distant galaxies, but the farther we look the farther in time we look as well. so in other words, the farther back in time we go the faster galaxies appear to be moving away from us - the closer in time the slower...wouldnt' that implicate a deceleration?
     
  2. jcsd
  3. Mar 12, 2004 #2
    The relative recession speed of a galaxy depends on the distance, not time. Because of the expansion of the universe, the farther away an object is, the faster it will appear to recede.
     
  4. Mar 12, 2004 #3

    russ_watters

    User Avatar

    Staff: Mentor

    Graphically, draw 3 lines starting from the same point and separated by 45 degrees each. The starting point is the Big Bang. The lines are analagous (1 dimentional) to the expansion of the universe with time. As you can see, the two lines separated by 90 degrees separate faster than two separated by 45.
     
  5. Mar 12, 2004 #4
    yeah, i understand that distance in a uniformly expanding mass will create relative acceleration (rising bread dough and all that) i just don't understand how we get that from our observations. you're telling me that time is not an important factor? if we take the bread dough analogy it's sort of like only having a picture of that bread dough at certain times - at 2:00 the farthest dough was moving away from us more quickly than nearer dough was moving away from us at 2:15, but it is now 3:00 - hasn't the rising actually slowed down?
     
  6. Mar 12, 2004 #5

    LURCH

    User Avatar
    Science Advisor

    I think the replies so far have overlooked BillyBoy's point. The further away we look, the faster things are receding. and, the further away we look, the furthwer back through time we are seeing. So, the further back in time we look, the faster things are receding. And when we look at things closer to our present time, thigns are receding more slowly, and all of thias sugests that expansion is decelerating. That is your point, is it not, Billy?

    In response I would say that the rate of recession astronomers setr out to look for took these factors into account, and the rate they observed did not match prediction. So the more distant objects are moving away faster than closer objects, but not faster enough to match the speed they should be going if the universe were expanding at a deceleratying rate, or even a constant rate.

    Myself, I keep wondering what role time dilation might play, and if it has also been accounted for? But I think it certainly must have been figured into the original prediction; cosmologists are nothing if not maticulous! :smile:
     
  7. Mar 12, 2004 #6
    yes, thank you so much lurch (and everyone for taking time to reply), i am also assuming cosmologists are a maticulous bunch and i am only trying to get my head around things that i hope are explainable.

    "...In response I would say that the rate of recession astronomers set out to look for took these factors into account, and the rate they observed did not match prediction..."

    okay. but what was our prediction of distance/time to relative acceleration based on? uniform rate of expansion continuous through time? i thought it was now widely held that the universe's expansion had undergone successive changes in acceleration since the big bang - slowing down and speeding up? in other words, we cannot use the presumption of a uniform acceleration curve to dictate acceleration relative to time. no?

    "So the more distant objects are moving away faster than closer objects, but not faster enough to match the speed they should be going if the universe were expanding at a deceleratying rate, or even a constant rate."

    yes, okay i understand this (it seems counterintuitive but then we are going back in time, are we not? :smile: ) but does this imply that the more redshift we see on distant radiation sources the lower our acceleration curve? (or higher deceleration)?
     
  8. Mar 13, 2004 #7

    LURCH

    User Avatar
    Science Advisor

    Not directly, no. In fact, the red shift of any individual object at any given time tells us nothing about its acceleration/deceleration. Instead, cosmologists must derive these values in an indirect manner. For example, suppose he made observations of a large number of objects, all of which were roughly the same distance from earth. Let's consider the distance to be 1 million light-years. If all of these objects have similar degrees of red shift, we could conclude that they are are moving away from you at roughly the same speed. From this, we might infer that that is the speed at which things were receding from earth 1 million years ago. Of course, this value would only apply to objects one million light-years distant. You probably recognize this concept as the "Hubble constant", so we will label the value H.

    For an object to million light-years distant, we would expect the speed at which it is retreating to be 2H. That is the rate at which it would be receding, if the universe was expanding at the same speed two million years ago as it was one million years ago. When astronomers began their survey, they expected to find the more distant objects to have a recession speed >2H, indicating that the rate of expansion to million years ago was greater than it was one million years ago. Instead, they absurd value <2H, indicating that the universe is expanding more rapidly in the more recent measurement. That is when cosmologists began speculating models that conform to the description you mention here:

    These models are a direct result of observations that varied from prediction. Using a uniform rate of expansion continuous through time" as a baseline ( - ), cosmologists expected to see a larger value in the past and a smaller value in the present ( \ ), but they found that the value was less in the past and greater more recently ( / ). This necessitated a new model for the expansion.

    AFAIK, there is no observation to support the decelrating-then-accelerating model. It's just that this is the least deviation from the original Big Bang model. Maybe somebody with more recent info can confirm or deny this, has anybody heard of an observation of more rapid expansion in the older universe?
     
  9. Mar 14, 2004 #8
    thanks a million lurch, i really appreciate someone taking the time to explain this stuff. so now i can see that the acceleration curve can easily be based on the assumption of uniform expansion as when you chart a curve you can assume a straight line and compensate for deviations. and what you're telling me is that we are in fact seeing them slowing down (relatively of course) and nearer objects are more redshifted and we are not, in fact, seeing greater redshift on distant radiation sources - that is, of course compensating for uniform expansion creating relative acceleration, not as great a shift as it would need to be indicate older-faster acceleration if hubble expansion was a constant. of course! i get it now :smile:

    also, on the topic of 'change in volition/acceleration of expansion', i don't know how recent you consider this but -

    http://seti.sentry.net/archive/public/2003/Oct/0029.html

    this 'cosmic jerk' is reputed to have taken place about 5 billion years ago and it seems as if it has been at least initially verified. i find this to be a personal vindication of sorts as i recall running across several 'cosmic jerks' in high school...
     
  10. Mar 16, 2004 #9

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This may be repeating stuff that's already in another thread (e.g. the 'dark energy one'), but ...

    Adam Riess, the leading astronomer mentioned in the link billy boy posted, is also the guy quoted in this Feb 2004 HST press release, about his team's analysis of supernovae found as part of the GOODS project.

    While the data are still rather too sparse, IMHO, to make strong claims about the extent of deceleration, that there are deviations from a flat, matter-filled Big Bang universe is fairly well established.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: An ignorant question about cosmology
Loading...