
The dipole repeller - perseusprime11
http://www.nature.com/articles/s41550-016-0036
======
autocorr
Interestingly this is the first issue of Nature Astronomy, I hadn't known
until now that they were making a topical journal (similar to Nature
Geoscience, etc.) Looking through the different abstracts in the issue, I have
to be honest, they don't seem very different than normal papers you find
everyday on arXiv. I don't think it's a good thing to have another journal in
this case, since Nature is not open access friendly.

To be honest, I don't know too much about Nature Publishing Group, but what I
do know is that the standard astronomical journal in America, the
Astrophysical Journal (ApJ, "ap-jay"), is very well. It's owned by the
American Astronomical Society (AAS) and published by the Institute of Physics
(IOP) [1] which is itself a non-profit and does great work in education and
other fields. They have a lenient policy where articles become open-access
after 12-months.

My point is, they're not blood-suckers like Elsevier :) And I would be unhappy
to see Nature use it's prestige to muscle in a journal with more onerous
access policies.

[1]
[https://en.wikipedia.org/wiki/IOP_Publishing](https://en.wikipedia.org/wiki/IOP_Publishing)

~~~
timthorn
Nature was founded by Norman Lockyer, so it does have a good claim to
astronomical papers. :)

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codewiz
Also watch the "explanatory" video: [http://www.nature.com/article-
assets/npg/natastron/2017/s415...](http://www.nature.com/article-
assets/npg/natastron/2017/s41550-016-0036/extref/s41550-016-0036-s2.mp4)

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sliken
Isn't this playing a bit fast and loose with physics?

Isn't the the same thing as saying, wow, I'm heavy on the planet earth. The
sky is putting a force on me, by not having enough mass to counter the gravity
of the earth.

Our galaxy is being pushed by a lack of mass on one direction?

~~~
marcosdumay
It's being pushed by a lack of balance of the mass over the different
directions. That lack of balance is because there is a void in one direction.

Physics is full of those shortcuts, I don't think anybody would get anything
done without them.

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mentalpiracy
Is this perhaps evidence of a higher rate of cosmic expansion in low-density
"void" regions?

~~~
mirimir
That's what I think. But then, I didn't make it past Physics 2 ;)

------
mbenjaminsmith
Maybe a physicist can tell me why I'm wrong here:

We often visualize the effect of mass on spacetime by using a trampoline and a
bowling ball. The outer edges are flat and the closer you get to the bowling
ball the more spacetime is warped downward.

On the outer edges, the effect is almost nonexistent and the trampoline's
surface is nearly flat. What this suggests is that spacetime without mass is
flat and that mass warps spacetime in a downward direction. So no mass = flat,
mass = curved downward. So the effect is in the range of [0, black hole].

Obviously this is an very loose analogy but bear with me.

What if spacetime acts more like a waterbed? Not only does mass warp spacetime
"downward" but something (in our analogy the displaced water) pushes the rest
of spacetime upward.

To use a signal processing analogy, a filter will (nearly?) always affect the
passband to some degree, with very high Q filters creating a knee where
amplitude is increased significantly before the filter rolls off. That seems
counterintuitive (at least to me) but it's nonetheless true.

Using another analogy, curved spacetime creates regions of "downhill-ness".
What if the space between masses get a little bit of uphill curve as well?
That could explain e.g., the Pioneer anomaly and possibly at much larger
scales the effect in the article.

~~~
pif
Question: why don't you try and learn the foundations of the current theories
(it's incredible how many man-centuries of high quality intellectual work you
can find condensed into a book) before launching into inventing new theories
out of nothing? Newton thanked the giants whose shoulders he was standing on.
You are pretending you are a giant on your own. Do you sincerely believe it?

~~~
mbenjaminsmith
> Do you sincerely believe it?

Do I believe in myself? Yes. I'm a genius (by I.Q. score) and work hard every
day. That's why I own a software company and have a beautiful family, money, a
passport full of stamps, etc.

Do I think I just solved one of the biggest mysteries physics? No. I think
you'd have to be a moron to have read it that way as I went out of my way to
say I wasn't qualified, that it was just an idea, etc. If I were a physicist
and working on the idea in earnest I wouldn't have presented it here.

I don't want to make this too much of a personal attack because it's less
about you and more about HN (and the internet) in general:

People like you really (actually, as in for real) make me sad. I'm 100%
confident that you're not bold enough to call me an idiot in person but you're
happy to do it via a textarea on a web page. That's cowardice. That's a lack
of character.

Why are you like this? Because you're a loser. You don't have the courage to
carve out a real life for yourself so you attack anyone you can -- especially
people who seem to have the qualities you lack. That's what losers do.

Let's turn this around:

You're obviously lacking something. I'm 40 and successful and violently
independent but I have had some help along the way -- people who were willing
to give me advice, insight into the life of a successful person. If I can help
you by way of advice or mentoring to turn your life in a positive direction,
reach out to me m@lattejed.com. I'm 100% serious.

Having said that, this will be my last contribution to Hacker News. This used
to be a great place for intellectual discussion. Not so anymore. Leaving has
nothing to do with someone not liking my waterbed analogy -- I've been
thinking about leaving for a long time. I waste too much time here and get
nothing in return.

~~~
sperglord
I think you're taking this too personally. The burden of proof is on you, the
person suggesting a new idea to support it somehow. You're asking other people
to do the work involved which is not fair to them.

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vpribish
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"

~~~
acqq
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).

[https://en.wikipedia.org/wiki/Hubble's_law#/media/File:Recen...](https://en.wikipedia.org/wiki/Hubble's_law#/media/File:Recent_Hubble%27s_Constant_Values.png)

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.

~~~
raattgift
> 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?

~~~
acqq
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:

[https://en.wikipedia.org/wiki/Comoving_distance](https://en.wikipedia.org/wiki/Comoving_distance)

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.

~~~
raattgift
Sorry about the delay, I didn't notice the reply initially.

> 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

a here is the FLRW scale factor:
[https://www.wikiwand.com/en/Scale_factor_(cosmology)](https://www.wikiwand.com/en/Scale_factor_\(cosmology\))

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.

------
michaelsbradley
Maybe Biot–Savart operating at intergalactic scale?

cf.
[http://plasmauniverse.info/Perattpdf/PerattEvidenceCosmic.pd...](http://plasmauniverse.info/Perattpdf/PerattEvidenceCosmic.pdf)

------
pmoriarty
Invisible force? Aren't all forces invisible?

Oh, I see. The title of the HN submission has been creatively edited and is
not the title of the original Nature article.

Clickbait.

~~~
mirimir
Yes, clickbait. The "invisible force" here is gravity.

This is the key bit of context:

> When describing the gravitational dynamics in co-moving coordinates, by
> which the expansion of the Universe is factored out, underdensities repel
> and overdensities attract.

So it's basically a coordinates transformation thing.

~~~
mirimir
Upon reflection, Hoffman et al. (2017) itself may arguably be clickbait. To
the extent that I understand it, they're looking at inhomogeneity in cosmic
expansion. It seems equivalent to turbulence in fluid dynamics. That is, non-
uniform flow.

And in considering airflow, for example, one doesn't say that underdensities
repel air, or that overdensities attract air. Rather, it's the non-uniform
flows that create the underdensities and overdensities.

Anyway, it seems to me that Hoffman and coauthors should have stuck to
describing correlations, because that's all they have. Using the language of
causality is confusing. They don't mention it, but maybe they have an agenda
to promote varieties of dark matter with positive and negative gravity.

~~~
mannykannot
I don't think the analogy holds, because the gravitational force between
molecules is not significant in airflow.

~~~
mirimir
Fair enough. But what about the causality issue?

