# Why does velocity not vary with distance within 10 Mpc?

1. Apr 17, 2012

### em370

For a science project I used Harvard's zcat data to construct a graph in excel of the velocity and distance of distant objects. I expected velocity to increase with distance due to the expansion of space but it seemed to only begin once the object was 10 Mpc from Earth. I included the graph but it is very small due to file size restrictions.

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• ###### Velocity graph.bmp
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2. Apr 17, 2012

### Steely Dan

Well, all galaxies have a relative motion through space relative to the Milky Way. In the field this is called a peculiar velocity. In particular, it's meant to be distinguished from the speed inferred from cosmological expansion. For objects close to home, the peculiar velocity may be similar in magnitude to the speed inferred from Hubble's law. Only once we get to cosmological distance scales does this stop being true (since most galaxies move at most at speeds of several hundred km/s in clusters).

3. Apr 17, 2012

### Staff: Mentor

While the response from Steely Dan is true, the actual data looks really weird.
- at the linear part, velocity is not proportional to distance
- in our universe there is nothing magic at 10 Mpc, the region at 11Mpc is similar to the region at 9Mpc. However, the graph shows two regions which are completely different.
- compared to the small deviations of most of the data points, there are a few points which are really off, up to 3-4% c (as deviation to the line).

4. Apr 17, 2012

### zhermes

Both responses are good ones above. Just wanted to add that there seems to be lots wrong with your plot---the velocities are also way too large. Remember Hubble's law:
$$v \approx H_0 d \approx 70 \textrm{ km/s} \left( \frac{d}{1 \textrm{ Mpc} } \right)$$

5. Apr 17, 2012

### em370

I took all of the data from this website https://www.cfa.harvard.edu/~dfabricant/huchra/zcat/ so I can't vouch for how the testing was done but harvard is a reputable university. I did notice that the line was shifted upwards quite a bit but I thought whatever may be forcing objects within 10 Mpc to not vary with distance may also be responsible for that. Also, the section of data outside 10 Mpc gives hubbles constant as about 72 km/s per Mpc which is fairly accurate along with a high initial velocity at 0 Mpc, which according to posters is not accurate. The velocity measurements are recorded as observed at Earth based on redshift. I feel like I must be missing something because the data come straight from the zcat and I doubt a distinguished college would publish something if the velocity data was off by a large amount.

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