Gravitating reference points

In summary, the article in Science Daily discusses how the Baryon Acoustic Oscillation (BAO) experiment uses the angle between appropriate pairs of galaxies to precisely measure their distance and determine the accelerating expansion of the universe. This is a statistical measurement that takes into account hundreds of thousands of galaxies and uses both redshift and angular separation to calibrate the relative distances and produce an absolute distance. The BAO peak is at a length scale much larger than clusters of galaxies, reducing the impact of their clustering on the measurement.
  • #1
bill alsept
124
0
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?
 
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  • #2
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
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
bill alsept said:
So their still useing redshift as the measuring stick and not the angle of separation between two object?
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
Chalnoth said:
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.

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
bill alsept said:
Now I don't get it. Everything I have been reading claims the universe is expanding because redshift is observed.
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.

bill alsept said:
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.
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
Chalnoth said:
Redshift alone doesn't get you expansion.
I would agree with that but most claims of expansion are based on it.

Chalnoth said:
Angular separation is another, independent measure of distance.
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.


Chalnoth said:
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.
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.

Chalnoth said:
2. The BAO peak is at a length scale much larger than clusters, and so isn't much affected by the clustering of galaxies.
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
bill alsept said:
I would agree with that but most claims of expansion are based on it.
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.

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

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

bill alsept said:
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.
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
Chalnoth said:
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.

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.
 

1. What are gravitating reference points?

Gravitating reference points are points in space where the gravitational forces of two or more massive objects are equal, resulting in a stable equilibrium.

2. How are gravitating reference points used in space exploration?

Gravitating reference points are often used as stable positions for spacecraft, allowing them to conserve fuel and energy while remaining in a specific location in space.

3. What are the five Lagrange points in the Earth-Sun system?

The five Lagrange points in the Earth-Sun system are L1, L2, L3, L4, and L5. L1 is located between the Earth and the Sun, L2 is located on the opposite side of the Earth from the Sun, L3 is located on the opposite side of the Sun from the Earth, and L4 and L5 are located in the Earth's orbit around the Sun.

4. Can gravitating reference points be found in other systems besides the Earth-Sun system?

Yes, gravitating reference points can be found in any system where two or more massive objects interact with each other, such as the Earth-Moon system or the Sun-Jupiter system.

5. How do scientists study and map gravitating reference points?

Scientists use mathematical models and simulations to study and map gravitating reference points. They also use data from spacecraft and telescopes to observe and measure the effects of gravity at these points.

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