Empirical measurement of the expansion of the universe

[BernieM]

Doppler shift of light shows distant galaxies moving away from us at high speed. This of course has been taken to prove that the universe itself is expanding and it has been theorized that the expansion is accelerating.

To actually empirically measure and prove the theorized acceleration, one would need to take at least 2 snapshots of the same object at two different times and compare the speed of the object to know what the actual acceleration rate is. (Why do i get a sneaking suspicion that this statement will become one of debate? )

Using the best and most sensitive (i.e., 1 part per million, per billion, trillion, etc ...) technologies currently available today, how much time is required between two snapshots of the object is required to actually be able to detect enough difference in doppler shift between the two snapshots to empirically measure the acceleration of the object?

 

[mathman]

The acceleration of expansion was deduced from observations of Ia supernovae at different distances (therefore different ages), not from different time snapshots as you describe.

 

[BernieM]

I was asking if you DID want to take two measurements of the same galaxy or supernova or whatever object in the universe at two different times to 'prove beyond a shadow of a doubt' the relative increase in speed of the object, how much time would be required between measurements given our current technology to measure doppler shift.

 

[mathman]

I was asking if you DID want to take two measurements of the same galaxy or supernova or whatever object in the universe at two different times to 'prove beyond a shadow of a doubt' the relative increase in speed of the object, how much time would be required between measurements given our current technology to measure doppler shift.

I won't try to calculate an answer, but my guess it could take thousands (??) of years just to see expansion from one object by direct measurement (as opposed to Doppler shift). For acceleration, many thousands - also you would need several measurements to observe acceleration.

 

[jimgraber]

Very crude guess/calculation: The hardest part is measuring the distance. That is why the uncertainty in the Hubble constant is on the order of ten percent. Thus if a "direct" measure requires looking at a single object after its distance has changed by more than ten percent, it would require more than ten percent of the age of the universe, or more than a billion years.

Jim Graber

 

[BernieM]

The calculation of the distance to an object may vary by 10 percent, but wouldn't the doppler shift for that same object taken over many measurements in a short period be exactly the same? So if I waited say 10,000 years, would I be able to notice any detectable and measurable shift in the doppler shift in comparison from my original measurements? Or would I need to wait a billion years to see any detectable difference in doppler shift from my original measurements of the same object?

 

[Chronos]

For a statistically significant result - about 7-10 million years between snapshots should suffice given current technological capabilities.

 

[George Jones]

I was asking if you DID want to take two measurements of the same galaxy or supernova or whatever object in the universe at two different times to 'prove beyond a shadow of a doubt' the relative increase in speed of the object, how much time would be required between measurements given our current technology to measure doppler shift.

We are 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. See

http://arxiv.org/abs/0802.1532

 

[BernieM]

I think that answers my question perfectly. Thanks.