New figure for speed of solar system relative to the ancient light

[marcus]

I just noticed an item in Ned Wright's "News of the Universe" about a new determination of the solar system's speed. It is roughly what we've been saying, around 370 km/s in the direction of constellation Leo, but this paper determines it with increased precision.

http://arxiv.org/pdf/0911.1955

They need precision because data is adjusted to get rid of the effect of the solar system's speed. The CMB dipole is subtracted out---the doppler hotspot in the Leo "heading" direction and doppler coldspot in the opposite "tailwards" direction.

The author finds that v/c = 0.0012338 which is way more precise than we need (or I need anyway.) He also gives the coordinates of the hotspot, the Leo-ish direction we are going, if anyone is curious. Multiplying that by the speed of light and throwing away some precision, I get 369.88 rounded to 369.9 km/s. I'll just try to remember 370 for next time someone asks.

The coordinates are halfway down the righthand column of page 1 of the paper. The velocity is of the solar system barycenter. There's always going to be other stuff like the Earth's orbital speed of 30 km/s to be counted in but its direction varies seasonally and the main thing is the 370.

Here's the Ned Wright link:
http://www.astro.ucla.edu/~wright/cosmolog.htm#News
For some reason I missed this when it first came out, last year. If you want to find it it's the 7th item in Wright's "News of the Universe".

 

[marcus]

I got a message indicating interest in this general topic of velocities of the various entities we belong to. Since it might be of general interest, I'll respond here rather than privately.

Since we know the motion of the solar system around the Milkyway core* AND we know the motion of the solar system relative to the primordial light we can do a simple VECTOR SUBTRACTION to find out the speed and direction of the galaxy itself relative to the ancient light background. Or what is nearly the same, that of our LOCAL GROUP of galaxies including Andromeda which is on essentially the same course as ours and slowly catching up with us.

This is basically kind of trivial and I don't know the earliest paper that has the figure. But there is one from 2002.
http://arxiv.org/abs/astro-ph/0210165

The authors are prominent/reputable, e.g. Mark Kamionkowski (Caltech) is one. The figure is not likely to be off by much.

For the Local Group, they say 627 km/s (plusminus some 20 or so) in a direction which as I recall if you marked it by stars instead of coordinate numbers would be aimed at the Southern Hemisphere constellation Crater, near Corvus the crow. I often have seen Corvus which is a small diamond of stars. You can find it easily. Crater (the wine-cup) is too dim for me to see over the glow of Oakland, a city south of here. Anyway that's the direction our galaxy is going.

On a night when Corvus diamond is on meridian south of you, you can picture our galaxy like a sailing Frisbee that you threw with your right hand towards Corvus with enough spin so that the center-of-Frisbee speed is 600 something and the backward spin speed on the righthand edge (relative to center-of-Fris) is 250 something.

You will see that on the righthand edge of the sailing Frisbee (which is where "we" are) has an airspeed which is LESS than the forwards 600. Because the backwards 250 CANCELS some. Actually the directions of motion are not all aligned and so on. If there were perfect cancelation then our airspeed would be 350. But actually it is 370.

If someone wants to work it out with sines and cosines using galactic coordinates, they give the coordinates in the paper. I'm satisfied with this intuitive picture of the Frisbee.

==quote Kamionski and Knox==
The simplest explanation for the dipole is a Local Group velocity v = 627 ± 22 km/sec
toward (l, b) = (276º ± 3º , 33º ± 3º) with respect to the CMB rest frame.
==endquote==

*Many already know the orbital speed of the solar system around Milky's core is 250 km/s.