I am a bit confused about Universal Expansion. How can we measure the rate of acceleration that a particular object is accelerating away from us? OR is it all ust positional?
Order of Kilopi
I am a bit confused about Universal Expansion. How can we measure the rate of acceleration that a particular object is accelerating away from us? OR is it all ust positional?
Order of Kilopi
We don't measure the acceleration of a given object, we only measure the distance and redshift of every object (in principle). That only gives us a snapshot of each region, so then we must apply the cosmological principle to "flesh out" the history of those motions, and that's where we can infer the acceleration. It's a bit like interviewing 100 people at various different ages about what their lives are like, and then assuming that everyone lives a similar kind of life, so that the interviews represent not single snapshots from 100 lives, but rather one complete description of a human life. If you do that, you never know if the assumption is correct, but you can at least tell if it is able to tell a consistent story. Since science is all about telling consistent, quantitative stories, using the minimal number of complicating assumptions, the cosmological principle is an appropriate scientific principle so long as it remains consistent with our observations-- and they say the expansion is accelerating.
Established Member
We use Type Ia supernovas. This category of supernovae produces consistent peak luminosity because of the uniform mass of white dwarfs that explode via the accretion mechanism. The stability of this value allows these explosions to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.
The further out they are, the faster they are moving away from us.
Order of Kilopi
These snapshots of velocity show that at some point in time, the rate of expansion started accelerating (about 6B years ago?). And before that, the rate of expansion was decreasing due to gravity.
The rapid expansion at the very early stage (inflation) is only deduced indirectly from the observed properties of the universe (homogeneity, etc).
Established Member
Which is true whether the rate of expansion is decelerating, accelerating or constant. In an expanding universe recession velocity always increases with distance.Originally Posted by Roobydo
What tells us the rate of expansion is accelerating is the relationship between those supernova "light curves" and their redshifts, across the range of redshifts, when compared to what would be expected in a decelerating universe. We found that with increasing redshifts the supernovae were dimmer and their peaks lasted longer than would be expected in a decelerating universe. The universe had expanded more than it should have if it was still decelerating. It must have, at some point, started to accelerate.
Order of Kilopi
Yes I get this concept ... kind of like looking backwards in time. So as we look backwards in time do we see galaxies moving away from each other at a slower rate than they are now?
Order of Kilopi
OK ... but how does this infer acceleration?
Order of Kilopi
Would you be able to show this in say a 2D figure?
Like if E is Earth .... how do we see galaxies at different distances from us:
E----------------G1---------------------------------G2-------------------------------------------G3--------G4----G5--G6-G7 ?
Assuming that between each is an equal amount of time:
E = t0
G1 = unit x 1
G2 = unit x 2
etc
Order of Kilopi
So we see less Redshift differences between galaxies the farther out we go?
Order of Kilopi
Yes, until we get to the really far galaxies, and then we see more redshift the farther out we go (because then we are seeing back to the epoch of deceleration).
Established Member
No, it is not nearly that simple, unfortunately, sorry if I misled you with my rather garbled explanation! The redshift of the light from the most distant galaxies is also affected by the accelerating rate of expansion - cosmological redshift merely represents the overall change in the size of the universe between the emission and detection of light.
What we have to do is find ways to ascribe a distance or time to a given redshift. This is where "standard candles" help us, but we found our standard candles were a little further away than expected. That meant the rate of expansion must have been accelerating rather than still decelerating, so distances were increasing a little quicker than expected, so a certain redshift represented less distance back in time than we thought! Phew! In a stroke we solved the riddle of how we seemed to be finding galaxies that were older than the universe!
I'm not sure if I am helping here, or making things worse!
Order of Kilopi
But less so, fractionally speaking, than for lesser redshifts. So it is still true that acceleration creates a smaller redshift per the next step in distance, relative to the average value over that full distance, than lack of acceleration would. I think that's what tommac was asking.
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Do you mean change in relative velocity of galaxies in general as we look to further distances, increase in the velocity of a specific galaxy in a given period of time, or an increase in the rate of acceleration at a given distance from Earth?
Order of Kilopi
Rae of acceleration of the expansion of the universe ...
Order of Kilopi
"When astronomers succeed in observing supernovae with redshifts and distances much larger than those of the supernovae with redshifts between 0.4 and 0.7, the Hubble diagram for the universe actually reverts toward the original line describing a cosmos with no acceleration produced by a cosmological contant. This reversion occurs because as we look farther out in space, we look further back in time, to eras when the cosmological constant had produced a cumulative effect much smaller than at the present time or at times 'only' 4 to 7 billion years ago. We can effectively recapture the Hubble diagram for a universe without a cosmological constant by looking so far back in time that we observe epochs when the cosmological constant had produced negligible results." -- Donald Goldsmith
Everyone is entitled to his own opinion, but not his own facts.
Order of Kilopi
Yes, that's what I was trying to say in post #10, if I didn't succeed!
Established Member
Heh, it is also what I was trying to say, and I know I didn't succeed! The graph curves upwards as we approach the most distant galaxies.
Order of Kilopi
I got your meaning but more descriptions of the same thing are always helpful because you never know which set of words will click with the lurkers.
Order of Kilopi
This is a good graph but really the last little bit should have an upward curve
Order of Kilopi
Yes I agree, you never know what will make it "click."
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