> Read today (3/20) about the fact the Milky Way, and other galaxies
>in its local space are moving in a direction different than they way they
>are supposed to (i.e., expanding in the same outward direction as the rest
>of the universe). This comes from a study of the velocities, directions
>of galaxies in an area of about 500 million light years. New York Times
>science writer John Noble Wilford suggests that in order to explain this
>anamoly, we might have to do a little rewriting of our theories concerning
>universal expansion. I'm just a humble layman type, so I'm asking you guys:
>what does this all mean? Is a rewrite in order? Wilford's story should be
>in Monday's edition of the N.Y. Times for further perusal.
Yup, the story in in the main section of Monday's NY Times, around page
10 or 12.
Wilford describes the work of Tod Lauer and Marc Postman, who made
a large survey of brightest-galaxies-in-clusters all over the sky.
For a sample of galaxies with velocities of up to 15,000 km/sec, they
1. measured the redshift of the galaxy
2. performed careful photometry of the galaxy
The idea of their survey is to detect 'peculiar velocities', which
are simply the residuals left over after one subtracts the smooth
Hubble flow from a galaxy. An example: suppose you see a galaxy at
a distance of 10 Mpc, and you believe the Hubble constant is
100 km/s/Mpc. Then this galaxy ought to have a velocity away from us
of 1,000 km/s. Okay, now actually MEASURE the velocity; suppose you
find v = 9,800 km/s. The difference (-200 km/s, i.e. toward us)
is the 'peculiar velocity'.
So, what Lauer and Postman did was to use their photometry to
estimate the distance of each galaxy (don't ask me how -- you'll have
to read their papers), and then compute the difference between
the expected Hubble flow at that distance and the actual measured
If I remember correctly, what they found was that many galaxies,
over the entire area of their sample, had larger peculiar velocities
that we had thought; one implication is that there are large "bulk
flows" of many clusters of galaxies, all moving sort of in the same
direction. This is a big surprise, because everyone thought that
there would be no such "bulk flows" over such large distances: the
common wisdom was that flows over smaller areas were possible, but that
on the large scales, local motions would average out.
So, the bottom line is, if Postman and Lauer are correct, then
we have to think hard to explain the large-scale bulk flows. This
probably means thinking about the structure of the universe at early
times. Since that's not my specialty, I'll stop here.
Oh, and Postman and Lauer are both here at Princeton now, and tomorrow
(Tuesday, March 21) Marc will talk about these results. If you're in
the neighborhood, stop by Peyton Hall at 4:20 and go to the auditorium --
the talk starts at 4:30.
----- Michael Richmond
"This is the heart that broke my finger." rich...@astro.princeton.edu
/ ...the chances of getting picked up by another/ The Invincible \
/ ship within those thirty seconds are 2 to the / vin...@cs.jhu.edu \
\ power of 276709 to one against." - From The \ Computer Science Depart. /
\ Hitchhiker's Guide to the Galaxy - D. Adams \ Johns Hopkins University/
There are many, many problems with this. I will amplify when
I have more time - like tomorrow when it will be cloudy at
However, I do emphasize that the local Universe is not very quiescent
and there are large departures from Universal expansion. However, its
a bandwagon field now.
The expansion of the universe is only valid for distance scales much
larger than the the so-called "local group" consisting of our galaxy
and its neighbours. The theory describing the expansion is a special
solution of Einstein's Field equation requiring homogeneously distri-
Homogeneous distribution is only found on large distance scales, so
the global expansion is only true on large scales.
In fact Andromeda, our nearest Spiral Galaxy neighbour is moving towards
us. The Magellanic Clouds are orbiting our Galaxy.
> Bernd Lehle alias Wolfskin - Born to be wild * A supercomputer <
> Stuttgart University Computing Center Helpdesk * is a machine that <
> e-mail: le...@rus.uni-stuttgart.de * runs an endless <
> Tel:+49-711-685-4828, private: +49-711-6874428 * loop in 2 seconds. <
So it's moved from Centaurus-Hydra! (-: I'd treat claims for the Great
Attractor with a big pinch of salt. Even Donald Lynden-Bell (one of
the "Seven Samurai") wrote in a recent conference proceedings that it
could well be an artifact due to incorrect allowance for Malmquist
bias. Dark will probably explain in detail later. Put simply, in a
sample limited by flux (e.g. observed magnitude) objects with high
luminosities (i.e. intrinsically bright) will be over-represented
in the observed faint objects. The bias affects the slopes of the
distance-indicator correlations, so you get an incorrect distance,
and hence an erroneous peculiar velocity.
The Great Attractor was (is?) a splendid way to get lots of telescope
time to map out the three-dimensional structure of this part of the
sky. I just wish we'd thought of it for our telescope applications
over a decade ago. At that time only a few people seemed interested
in this area---mostly in the UK thanks to plates from the UK Schmidt
Telescope---and could see its importance (no northern-hemispheric
bias). The Great Attractor certainly has attracted researchers in
droves. There's a Great Attractor in them there superclusters.
: Malcolm Currie
: Starlink Project
Ok. Whoops! Sorry for my ignorance. As you can tell, I'm not too update
to date with the current research in the expansion of the universe.
I'll just huddle myself into a ignorance corner. :)
1. Measure dipole of CMB, interpret as due to velocity of Local
Group. Need a velocity of about 600 km/s in some direction,
call it vector "X".
2. Now, what Lauer and Postman did was to measure the motion of
the Local Group relative to a set of very distant galaxies,
in many different directions. They used brightest-cluster
galaxies (BCGs) to measure distances. Very tricky work, but they
have done (IMHO) a good job. Again we find a velocity vector
of approx 700 km/s, in some direction: call it vector "Y".
3. We would expect that vectors "X" and "Y" would point in the
same direction; that would mean that the large set of
distant galaxies is at rest w.r.t. to the CMB. Since the
distance scale is very large (15,000 km/s = 150-300 Mpc),
we expect local motions to be averaged out. But ...
4. It turns out that vectors "X" and "Y" are roughly ninety
degrees apart. Which can be interpreted many ways, one
of them being that the entire set of distant galaxies is
moving as a "unit" relatvie to the CMB at high velocities.
5. People are perplexed by this fact. In a few months, we'll
see a flood of papers offering solutions, nonetheless :-)
6. Postman and Lauer are working to extend their survey to more
distant galaxies, and to check their results with different
The Great Attractor, by the by, lies well within their sample, and
is completely unnoticable. They find no evidence that the Great
Attractor attracts, and their data doesn't really agree with smaller-scale
surveys on the subject of bulk flows.
By the way, a moderated sci.astro group would not suffer the slings
and arrows of outrageous Stapleton. Just a thought.
Fantastic folklore: Where should the MW move "outward" as seen from itself ?
(of course we all know that the motion is relative to the rest frame defined
by the background radiation [isotropy])
>: This comes from a study of the velocities, directions
>: of galaxies in an area of about 500 million light years. [..]
>The expansion of the universe is only valid for distance scales much
>larger than the the so-called "local group" consisting of our galaxy
>and its neighbours. The theory describing the expansion is a special
>solution of Einstein's Field equation requiring homogeneously distri-
It may be of interest that the observational results also permit a wide
variety of generalizations of Einstein's GR. Remarkably, even Newton's
theory of gravity would lead to essentially the same expansion law than
>Homogeneous distribution is only found on large distance scales, so
>the global expansion is only true on large scales.
>In fact Andromeda, our nearest Spiral Galaxy neighbour is moving towards
>us. The Magellanic Clouds are orbiting our Galaxy.
And there are some galaxies in the Virgo cluster (and in the direction
of the Virgo cluster) that are also approaching us. Incidently, the highest
blueshifts (some 500 km/s) were obtained for Virgo members which are 15-22
Mpc (45-70 Mly) away, not for our closest neighbors. The reason is of course
the high velocity dispersion in that large cluster of galaxies, which has an
overall redshift of some 1,100 km/s. There are also some galaxies moving at
some 2,600 km/s in the same direction, so probably members which happen to
move away from us, relative to Virgo's rest frame, so that the dispersion
is about 1,500 km/s.
The large mass of the Virgo cluster of galaxies gives rise to another major
velocity component present for all galaxies in our intergalactic
neighborhood, the Virgo-centric Flow, which amount to some 200 km/s for the
MW (it moves in that direction with this velocity, relative to the Virgo-
centered expanding universe). If it will fall into the Virgo cluster at last
is an open question still (at least at the current stage of my info).
Hartmut Frommert | Russia HAS a Space Station !
<phf...@nyx.uni-konstanz.de> | Mars Observer 2 would have survived.
|>Ok. Whoops! Sorry for my ignorance. As you can tell, I'm not too update
|>to date with the current research in the expansion of the universe.
|>I'll just huddle myself into a ignorance corner. :)
I wasn't making fun of you or mocking your ignorance. Rather I was
trying to be witty (and failing) at the expense of the Great-Attractor
model. Since the Centaurus direction doesn't seem to fit, I wondered if
they'd moved it to Hercules.
This peculiar-motion stuff is fascinating and as DrDarkMatter says
something is broken. It could lead to a major revision of our
understanding of the large-scale structure. So your original question
was worth asking as it's lead to other enlightening articles. So don't
be put off from further posts by my clumsy humour.