Question about accelerated expanding universe theory.

[Ramon163]

Hey,

After watching a show on TV about expanding universe and dark matter&energy, I wondered the following.

In the program, it was mentioned that the universe is expanding at an accelerated rate. This was measured using spectroscopy, comparing Type 1A supernovaes at different distances. It was discovered that Type 1A supernovas at a greater distance were moving away from us at a greater speed then the supernova's closer to us. So it was concluded that the universe is expanding at an accelerated rate.

That got me wondering... Because it takes millions of years for that light to reach us, doesn't that mean that: The supernova that is farther away is what is actually was going on for instance 100 million years ago and a supernova closer to us for instance 30 million years ago, which automatically means that of-course it seems that the supernova farther away is moving faster because what we are seeing is closer to when the big bang happened then the supernova closer to us.

Am I missing something here, or is my data just wrong?

 

[George Jones]

See

https://www.physicsforums.com/showthread.php?p=1183126#post1183126

and the post#51 in the same thread, though these posts might be confusing.

 

[Ramon163]

Thanks for your reply, and the posts are indeed a little confusing.
But I did a little research into some of the terminology you used, and I think I understand a little.

If I understand it correctly, my problem with this is that what I mentioned in my earlier post: They concluded an accelerated expanding universe by measuring the same phenomenon at 2 different distances (Super novae). But to conclude that something is accelerating, shouldn't you measure the same object at 2 different time intervals to make that conclusion? Or is that impossible because the redshift doesn't change measurably in our lifetimes to conclude anything like that?

 

[Chronos]

We apply physics as we understand it to observational evidence and make deductions. Anything more [or less] is purely speculative.

 

[Chalnoth]

Thanks for your reply, and the posts are indeed a little confusing.
But I did a little research into some of the terminology you used, and I think I understand a little.

If I understand it correctly, my problem with this is that what I mentioned in my earlier post: They concluded an accelerated expanding universe by measuring the same phenomenon at 2 different distances (Super novae). But to conclude that something is accelerating, shouldn't you measure the same object at 2 different time intervals to make that conclusion? Or is that impossible because the redshift doesn't change measurably in our lifetimes to conclude anything like that?

Well, that would be really really nice, but I don't think we'll ever have detailed enough measurements to observe the accelerations directly. However, what I will say is that the supernovae are not, by far, our only evidence for dark energy.

The CMB, for instance, provides an extremely accurate measure of the geometry and matter density of our universe. From this we basically know that our universe is very nearly flat, but also that the matter density (both normal and dark) is insufficient to explain this flatness.

From the CMB observations we can also compute the expected typical separation of galaxies today, through a technique called Baryon Acoustic Oscillations (BAO). Basically, we look at the typical separation between the hot and cold spots on the CMB, do a little bit of physical modeling, and are able to compute from this the typical separation between galaxies later on. We can then measure how far apart galaxies appear to be at different redshifts, and we can say how far away they must be based upon how far apart from one another they are. This, it turns out, is also a highly accurate measure of matter density and geometry, but in a rather different way than the CMB itself, making the combination of the two vastly more accurate.

And these three types of measurements, supernovae, CMB, BAO, all converge on the same results for cosmological parameters, to within measurement errors.

 

[George Jones]

Well, that would be really really nice, but I don't think we'll ever have detailed enough measurements to observe the accelerations directly.

We are tantalizingly close to being able to do this, but, for economic and other reasons, such a project won't start for several decades. Once started, the project would take a couple of decades to start to get good results.

http://arxiv.org/abs/0802.1532

 

[Chalnoth]

We are tantalizingly close to being able to do this, but, for economic and other reasons, such a project won't start for several decades. Once started, the project would take a couple of decades to start to get good results.

http://arxiv.org/abs/0802.1532

Yeah, sorry, you're right. For some odd reason I had been thinking that this sort of experiment wouldn't be measuring acceleration. Whoops.