Can someone explain their distance units to me? I'd read "km s-1" as kilometers per second. which can't be right and isn't just a typo as the magnitude is way to small for "km".
"It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s−1 away contributes significantly to the observed flow"
To get the distance that you'd understand you have to divide the number presented with the H0 which is still estimated somewhat differently by the different observations (but the future projects should improve the estimate).
If we take the approximate value of 70 km/s/Mpc (the last unit is mega parsec) we get that the 15000 km/s corresponds to 214 Mpc, which multiplied with 3.26 gives you the number of millions of light years: around 700 million light years. Or, for even easier "rule of thumb" a little less than 22000 km/s is a billion light years using the current approximate constants.
The Milky Way is estimated to be at least 0.1 million light years across, so the 15000 km/s which you quoted is up to 7000 diameters of the milky way away from us.
The reason to use the speeds and not the distances in the papers is that we actually measure the speeds (redshifts, to be precise), and the distance estimates will improve.
> The reason to use the speeds and not the distances in the papers is that we actually measure the speeds (redshifts, to be precise)
Comoving coordinates are a more important consideration. The coordinate distance (which is the comoving distance) between galaxies is fixed in the standard cosmological model.
Standard candles like supernova light curves do indeed rely on redshift, and it is straightforward to convert cosmological redshift into a recession velocity.
However, standard rulers include angle-diameter-surface brightness relations which do not measure redshift; one calculates the redshift for these objects through e.g. the Mattig or Sigma-D relation.
Extragalactic distance ladders combine these and other classes of observables.
Using redshift and a scale factor for cosmological distances rather than Gly or Gpc also defocuses one from distracting differences between cosmological proper distance and relativistic proper length, for example, while bringing into focus peculiar velocity. It's also handy when dealing with different systems of units, such as geometrized ones in which G=c=1, as is common practice in cosmology.
Just by way of example, starting with your H_0 value, and being slightly sloppy (especially wrt sigfigs), for ~1 Gly, I plug in H_0 = 70, z = 0.076 , \Omega_{m} = 0.31, and the standard values for the other densities, and get four useful distances: comoving = 750 h^-1 / Mly, proper distance at z=0 = 1043 Mly, proper distance at z=0.078(a=0.928) = 969.4 Mly, time interval = 1.004 Gy.
Is it really useful (rather than simply nifty) to think of a galaxy at a slightly greater redshift moving away at ~ 2x Earth's diameter every second?
Thanks. You, of course, have much deeper knowledge of the subject, I hoped somebody with more knowledge will eventually comment and add some more details.
For those who want to know what you talk about (at the start) and I've avoided for simplicity, lacking some more exact link, I hope wikipedia is good enough:
If I may ask, not doing astrophysics professionally, but being interested, I don't understand why you started with z = 0.076 and then calculated proper distance at z=0.078. I also don't know what (a=0.928) there stands for.
The two proper distances can be thought of as, for the larger, what a notional instantaneous (>> FTL) analogue of radar measurement would decide the distance to the galaxy we are observing is, and for the smaller, what we would read if we had optical telescopes that could read the display of such a measurement device showing a counterpart in the distant galaxy what the instantaneous distance to us is.
(Or instead of an instant FTL analogue of radar, you could use a pair extremely long measuring tapes with the 0 end anchored on the observed party; if we look at a telescope at their tape's indicator, it would read 969.4 Mly, but looking at our own tape would read 1043 Mly.)
The numbers are close because ~1 Gly is near enough that the metric expansion of space hasn't carried our galaxy cluster and theirs very far apart in the past billion years.
They would see similar numbers looking at a galaxy at z=0.076 in a different part of their sky than where the Milky Way is; we would see a different and almost certainly higher redshift when looking at that galaxy from here.
Since the universe is expanding and the relative velocity is greater for further objects, you can express the distance from Earth as the relative velocity.
Seems like cheating. The expansion of the universe is a correlation between distance to things and their red shift. You still need to have a proper measure of the distance to something rather than using it's velocity as a stand-in for distance. Particularly if you're looking at things like flow fields.
It's not. It's honesty, and keeping the measurements in the form that will remain more usable. What we measure directly is the redshift. The estimation of the distance (that is, the exact value of the H0, the Hubble constant) is done using various methods and the results aren't bad, just not precise enough to use it combined with the redshift. That's what makes us preferring the redshift now, as the speed can be exactly calculated from it: https://en.wikipedia.org/wiki/Redshift
Knowing that the Universe is "flat" ( https://en.wikipedia.org/wiki/Shape_of_the_universe )
allows us also to do good calculations, even if we still don't have the exact value of H0. In fact, it's also not so bad: the estimates based on the Planck satellite, measuring the state of the Universe 13.7 billion years ago as the galaxies and stars haven't existed, and the ones based on the Hubble satellite data (measuring the state of the Universe much more recently) mismatch only some 9%. It is amazing enough for me. This difference, after more research, can produce some new physics discoveries or make us improve some engineering (and we surely should invest in more research) but it's quite good for everybody else: even if the "emptiness" from this article affecting the Milky Way is not 700 million but 730 million or 670 million light years, it won't affect our plans: for example, the Andromeda galaxy is just some 2.5 million light years from us and we estimate it will collide with the Milky Way in some 4 billion years, give or take. Should we care much if the "emptiness" is 268 or 292 times farther than the Andromeda is at this moment?
See my other response here for the way you can calculate the distance from the speed, based on your preferred estimate of the H0, it's simple, just less precise at the moment than simply stating the speed (or the redshift).
"It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s−1 away contributes significantly to the observed flow"