Old Idea - New Idea
When I first read about the Hubble expansion many years ago I wondered if it would be possible to build an experiment that directly measured the effect. I pictured some kind of orbiting laser interferometer that measured the relative redshift between two widely spaced points.
Now, for some reason, I remember this today and I think, "well here is LISA, which is exactly that". Where does the Hubble expansion lie on one of those signal vs frequency graphs that gravity telescope people are so fond of showing.
If H_o is approximately 75 km/s/kpc than the path length difference between two LISA endpoints is 4x10^-20 meters. Now I just have to understand the language of the science, do they talk about strain (h_+ etc) or path length difference, something even more obscure?
More: (20/4/05)
The LISA system will supposedly be able to measure a phase difference between two arms to one part in 10^20. The hubble redshift over the length of one of LISA's 10^9 meters will be 10^-15. Does this mean we will see a strong DC signal from the expansion?
The hubble expansion would represent a DC (not counting cosmological acceleration) signal in the redshift and a linear signal in the distance. I dont really know what the output of a time delay interferometer (TDI) is. Does it integrate for distance with some analog/optical device or do we get a stream of phase differences over time? They talk about strain spectral density, which has units of 1/(Hz)^1/2, thus total strain might have units of Hz^1/2....
Now, for some reason, I remember this today and I think, "well here is LISA, which is exactly that". Where does the Hubble expansion lie on one of those signal vs frequency graphs that gravity telescope people are so fond of showing.
If H_o is approximately 75 km/s/kpc than the path length difference between two LISA endpoints is 4x10^-20 meters. Now I just have to understand the language of the science, do they talk about strain (h_+ etc) or path length difference, something even more obscure?
More: (20/4/05)
The LISA system will supposedly be able to measure a phase difference between two arms to one part in 10^20. The hubble redshift over the length of one of LISA's 10^9 meters will be 10^-15. Does this mean we will see a strong DC signal from the expansion?
The hubble expansion would represent a DC (not counting cosmological acceleration) signal in the redshift and a linear signal in the distance. I dont really know what the output of a time delay interferometer (TDI) is. Does it integrate for distance with some analog/optical device or do we get a stream of phase differences over time? They talk about strain spectral density, which has units of 1/(Hz)^1/2, thus total strain might have units of Hz^1/2....
posted by Danny at 9:18 AM
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