# B Speed of gravitational waves

1. Feb 13, 2017

### wolram

It occurred to me that if the speed of gravity is the speed of light, then the universe must be accelerating due to Dark Energy, What my question is do we know the frequency of gravitational radiation, If we do then any compression or expansion of these waves will change the cosmological constant.

Thank you for your replies.

2. Feb 13, 2017

### Staff: Mentor

What does "the universe must be accelerating" mean, and why do you expect this as consequence?
Different sources emit radiation at different frequency. That's like asking about "the mass of an animal" - there is no such value. There are typical values for some types of animals, but that is a completely different thing.
That doesn't make sense at all.

3. Feb 13, 2017

### 1oldman2

This would be a typical representation of the signal frequency of a BH merger.

4. Feb 13, 2017

### wolram

I mean if we take a known source like black hole merger and take the red shift of it , one could compare it to the gravitational waves, if the GW's frequency shortens or lengthens one could determine if the universe is accelerating or not.

5. Feb 13, 2017

### Chalnoth

First, I don't think the second half of that first sentence follows at all. Why do you think the speed of gravity has anything at all to do with the accelerated expansion? After all, you can get the exact same expansion from a Newtonian model of an expanding universe with a cosmological constant, and Newtonian gravity has infinite propagation speed. The speed of gravity just isn't a factor in the Friedmann equations at all.

Second, why would you think that gravitational waves would have any impact on the cosmological constant?

6. Feb 13, 2017

### Chalnoth

Huh? The gravity waves are simply redshifted along with the expansion.

7. Feb 13, 2017

### wolram

That is what I mean we can test expansion by comparing light travel time with gravitational radiation travel time.

8. Feb 13, 2017

### Staff: Mentor

How will that help? They are the same.

9. Feb 13, 2017

### Staff: Mentor

In principle yes, in practice no.
The frequency depends on the black hole masses, in general those are not known in advance - the masses are determined by the frequency spectrum. This is different from electromagnetic radiation, where we can use spectral lines with well-defined frequency.
Even if we get lucky enough to find a gravitational wave source where we have an independent way to measure a mass, the precision won't be very good.
And to make it even worse, the current sensors can only measure mergers up to ~1 billion light years away. That might be sufficient to see the overall expansion, but measuring the acceleration of that expansion would need larger distances.

The acceleration of the expansion has been measured with electromagnetic radiation. A much less sensitive measurement via gravitational waves would be nice, but wouldn't improve our understanding of the universe.

10. Feb 14, 2017

### wolram

Thanks for explaining mfb.

11. Feb 20, 2017

### Agni101

Actually you r quite right. If you try to calculate gravitational wave intensity at points nearer to a black hole it will be observed that as one moves away from the black hole the intensity decreases.

Now if you try to measure the gravitational wave frequency you will get WRONG answers as there are more black holes and so by superposition principle the waves will superpose to give resultant freq with arbitrary values.

12. Feb 20, 2017

### Staff: Mentor

Black hole mergers have a luminosity exceeding all other sources in the observable universe by a huge margin.
For periodic sources, you can do a Fourier transformation - it will give you the right frequencies. In addition, you can take the data from multiple observatories (2 LIGO, soon 1 VIRGO, more in the next years) to locate them in the sky and separate sources even if they have a similar frequency.