Gravitational Waves and the Expansion of the Universe: Redshift or Not?

[TEFLing]

would gravitational waves redshift with the expansion of the universe ?

 

[phinds]

Well, I don't know if "redshift" is the right term, but they would certainly change frequency in that way since they propagate at the speed of light and light redshifts.

 

[marcus]

would gravitational waves redshift with the expansion of the universe ?

I agree with Phinds' conclusion, TEFLing. They would. Incoming wavelengths would be enlarged by the same factor as the distance between the source and us has grown while the waves were on their way here.
I.e. by the ratio a(now)/a(then)

 

[Chronos]

According to Keith Riles of the Michigan Gravitational Wave Group, the expected answer is yes - re: http://arxiv.org/abs/1209.0667. The answer is expected because we have yet to actually detect gravitational waves. The odds are, however, quite favorable given the outcome of the Hulse-Taylor study.

 

[Tanelorn]

What do people think the reason is for not detecting them yet? too weak or wrong frequency?

 

[phinds]

What do people think the reason is for not detecting them yet? too weak or wrong frequency?

Too weak, I think.

 

[Drakkith]

What do people think the reason is for not detecting them yet? too weak or wrong frequency?

From here: http://en.wikipedia.org/wiki/Gravitational_wave#Ground-based_interferometers

(Talking about the arms of the LIGO detectors, which have arms between 2 and 4 km long)
Even with such long arms, the strongest gravitational waves will only change the distance between the ends of the arms by at most roughly 10−18 meters.

10^-18 meters is one attometer. For comparison, the diameter of a proton is about 1700 times this distance (1.755 femtometers).

 

[TEFLing]

Could the GPS satellite network, or something else like that, be utilized as the arms of an interferometer? Are the onboard clocks accurate enough to detect tiny time delays ?

 

[TEFLing]

Didn't the Apollo astronauts leave a mirror on the moon? Could that comprise a nearly 400,000km arm?

 

[Drakkith]

Could the GPS satellite network, or something else like that, be utilized as the arms of an interferometer? Are the onboard clocks accurate enough to detect tiny time delays ?

I'm not sure.

Didn't the Apollo astronauts leave a mirror on the moon? Could that comprise a nearly 400,000km arm?

One arm isn't enough and the return signal would be extremely weak, so I doubt it's a feasible option.

 

[PeterDonis]

Could the GPS satellite network, or something else like that, be utilized as the arms of an interferometer?

Didn't the Apollo astronauts leave a mirror on the moon? Could that comprise a nearly 400,000km arm?

Neither of these will work, because the distance between the ends of each arm (even assuming we could set up two perpendicular arms) has to be constant unless a gravitational wave is passing. Obviously "arms" where one end is the Moon or a GPS satellite and the other end is the Earth don't meet that requirement.

There is a project to deploy a space-based GW detector, called LISA; see here:

http://lisa.nasa.gov/

Its arms are planned to be about 5 million km long.

 

[Chronos]

Unfortunately the Lunar Laser Ranging setup is nowhere near the accuracy of dedicated gravity wave detectors like LIGO. The exact distance to the moon uncertain at any given instant due to a variety of factors. Orbital eccentricities, tidal flexture of both bodies, and even the weather affect its true distance. LLR instrument sensitivity is also an issue. The best achievable accuracy with the LLR setup is in the 2-3 cm range. These combined uncertainties are huge compared to the threshold accuracy required to measure gravitational wave amplitude. Note that there is a satellite version of LISA recently proposed, and discussed here, that could have an effective arm length of around 8 light minutes.

 

[TEFLing]

Do gravity waves carry angular momentum / spin ?

 

[bapowell]

Yes, gravity waves are spin 2.

 

[Tanelorn]

thanks dude :)