# Gravitating reference points

1. Apr 8, 2012

### bill alsept

Last week I read this article in Science Daily about BOSS and its recent most accurate measurements of the universe to date. http://www.sciencedaily.com/releases/2012/03/120330081844.htm

In describing the experiment to measuring the accelerating expansion of the universe the author says “Baryon acoustic oscillation measures the angle across the sky of structures of known size, the peaks where galaxies cluster most densely in the network of filaments and voids that fill the universe. Since these density peaks recur regularly, the angle between appropriate pairs of galaxies as precisely measured from Earth reveals their distance -- the narrower the apparent angle, the farther away they are.”
My question is how can this be a viable test to base an accurate measurement if the target itself could be moving for other reasons than expansion?
I think I understand the procedure they are attempting which appears to be the opposite of the parallax effect and instead of measuring from two opposite points it is done from one point, that being the apex.
The way I see it these clusters of galaxies and their peak density points may be known distances (apart from each other) but what if the points are gravitating toward each other. I assume they would be hence the cluster.
If these points move toward each other (reducing the angle) wouldn’t BAO incorrectly interpret this as expansion?

2. Apr 9, 2012

### Chalnoth

The Baryon Acoustic Oscillation measurement is a statistical measurement. It does not get the distances of any given galaxy, but of a collection of them. In this case, hundreds of thousands of galaxies were used to make the measurement. Here's a plot of what, specifically, you're looking for:

http://www.nature.com/nature/journal/v440/n7088/fig_tab/nature04803_F4.html

This is a plot of the two-point correlation function versus typical separation. The two-point correlation function can be understood as a measure of how many galaxies you get separated by any given distance. So, you take a field with a whole bunch of galaxies and, using their redshifts and a cosmological model, estimate the typical distance between each pair of galaxies in the field. The standard cosmological model predicts that this correlation function will see a bump at a specific separation distance, which is exactly what we see.

3. Apr 9, 2012

### bill alsept

So their still useing redshift as the measuring stick and not the angle of separation between two object? The author of the article states "the angle between appropriate pairs of galaxies as precisely measured from Earth reveals their distance -- the narrower the apparent angle, the farther away they are.” Is there another way to explain it. Thanks

4. Apr 9, 2012

### Chalnoth

Both are used. In fact, you have to use both to extract any meaningful information from the data. Just having a list of redshifts doesn't give you any information about the absolute distances, and just having a list of absolute distances doesn't give you any information about the expansion. So the redshifts are used to set up relative distances, and then the typical separation is used to calibrate those relative distances to produce an absolute distance.

5. Apr 16, 2012

### bill alsept

Now I don't get it. Everything I have been reading claims the universe is expanding because redshift is observed. Measuring the angle of separation was not discussed in any of those claims. Like you say, If the typical separation is used to calibrate those relative distances to produce an absolute distance then I repeat my original question in post #1.
How can this be a viable test to base an accurate measurement if the target itself could be moving for other reasons than expansion? Example: if a cluster of galaxies where accumulating (closing the gap of their separation) then your calibration would be falsely interpreted as moving away.

6. Apr 16, 2012

### Chalnoth

Redshift alone doesn't get you expansion. You need redshift combined with an estimate of the distance. The original distance estimate was based upon the brightness of Cepheid variable stars. More famous recently is the brightness of Type Ia supernovae. Angular separation is another, independent measure of distance.

For two main reasons:
1. The BAO measurement does not give us anything for individual galaxies, but is a measurement of the typical separations between thousands (or more) of them.
2. The BAO peak is at a length scale much larger than clusters, and so isn't much affected by the clustering of galaxies.

7. Apr 16, 2012

### bill alsept

I would agree with that but most claims of expansion are based on it.

I realized that and questioned the accuracy of the measurement because the changing angle of separation (getting wider or narrower) could be interpreted two or more deferent ways.

That too I realize and as with everything else that is accumulating, large scale structures along with their densely packed peaks are also coming together. This too could be interpreted as expansion because the gap would be narrowing as the peaks moved closer together.

The densely packed peak spots in the large scale structure are what the article and I am talking about. The peak areas may be moving toward each other in the same way that clusters do. Therefore causing the angular measurement to be falsely interpreted as expansion.

Maybe I'm seeing it wrong. Can you explain how the angular measurement works and tell me why it could not be falsely interpreted as expansion?

8. Apr 16, 2012

### Chalnoth

Once again, it's the redshift-distance relation that is important. Redshift is easy to measure very accurately. Distance isn't so easy, and there are a number of different methods. But you do definitely need both in order to observe the expansion.

It really can't. There really isn't any other way to get the BAO peak.

Once again: the BAO peak is at much larger distance scales than gravitational collapse has a significant impact on.

This isn't what the BAO measurement is. The BAO measurement is that when you measure the average separation between large numbers of galaxies spread over billions of light years, you get a little bit of an excess at a little less than half a billion light years.

9. Apr 16, 2012

### TrickyDicky

In strict terms what is observed is the redshift-distance relation, not expansion. Expansion is deduced from this observation and its model-dependent interpretation. This is a useful distinction for avoiding confusion among beginners.