
The Last of the Universe’s Ordinary Matter Has Been Found - amaks
https://www.quantamagazine.org/the-last-of-the-universes-ordinary-matter-has-been-found-20180910/
======
gcbw2
The hypothesis is they should see CMB distortion between galaxies, which is
hidden by the distortion caused by the galaxies themselves. The solution was
to assume they were removing the distortion caused by the galaxy halo, and
what was left was the original distortion they were looking for.

sounds like the work to rule out false-positives would be huge. This is
putting a lot of weight on a technique that is not fully described in the
paper (i might have missed, just glanced at them for now, and i am an amateur
that just like to fiddle with similar data).

~~~
nfc
I'm not really sure of what you mean by false positives.

After my understanding of the stacking technique as used in other contexts in
astrophysics and without going into too much details :

The problem they were facing is that the signal to noise of the images of the
filaments was too low to say that they had detected anything in any individual
images. However by stacking (adding) images they were able to detect it
because the signal grows roughly with N (N being the number of images) and the
noise grows with sqrt(N). So by stacking enough images you'll get the signal
to noise necessary to say you've detected sth.

~~~
gcbw2
Think like that: there are two bright lights in front of you. There will
always be a halo between those two bright lights.

Now, my hypothesis is that "between all two bright lights, there is third,
dimmer one, hidding". And then i prove it by filtering X from the two bright
light halo, and prove that Y is left proving that the third light is there.

Now, how can i be sure Y is really a third dimmer light? and not just noise on
the function i used to _try_ to clean up the halo of the two bright light?

~~~
nfc
You are right that this could be the case, and it's mentioned in at least one
of the papers [1] where they say that a better understanding of the physical
state of this gas is needed to estimate its contributions to the baryonic
mass.

[1] [https://arxiv.org/abs/1805.04555](https://arxiv.org/abs/1805.04555)

Now we can get to discuss the third paper ;)

------
TomMckenny
Slightly related, I wonder if anyone has figured out the density of
interstellar comet or asteroid like objects.

I notice that Oumuamua happened to pass within some 20 million km of earth
within a decade or so of having systems in place to spot it. Wouldn't this
imply there are an awful lot of them?

~~~
coldtea
> _I notice that Oumuamua happened to pass within some 20 million km of earth
> within a decade or so of having systems in place to spot it. Wouldn 't this
> imply there are an awful lot of them?_

Finding one in a decade's span within 20 million km would imply there are "an
awful lot of them"?

~~~
Baeocystin
Practically instantaneous on the lengthy time scales the universe operates in,
don't you think? And our observations have far from complete coverage of the
sky.

------
gmailsyncer
There must be regions of the universe where this "not very dense" gas is dense
enough to use a ramscoop

~~~
stephengillie
Probably more in high-gravity locations, such as planets and stars. Thinking
about the Juno satellite orbit, would it be better/easier/cheaper to hang a
mining station in orbit around Jupiter, or have a satellite/ship dip into
Jupiter's atmosphere every orbit?

~~~
sandworm101
Counterintuitively, the energy costs are basically the same. Whether you dive
in to pick it up, or run into it out in space, you will still have to
accelerate the gas to orbital velocities before you can cram it into a tank.
Some of the gas in space may already be moving very fast, may even be in an
orbit, but it is probably not aligned with the orbit of your tank.

Either way, the energy required to maintain operations anywhere near Jupiter
means you probably want to find your hydrogen somewhere else.

------
yalogin
I know and understand nothing about this topic but given the fact that the
universe is infinite by definition makes the conclusive/definitive statement
very surprising.

~~~
sandworm101
Who is telling you that the universe is infinite? Astronomers/cosmologists
generally all agree that our current universe (the matter-containing 4
dimensions bit we live in) had a defined beginning from which is has grown.
There is an outer edge,. defined by the rate of expansion starting from the
big bang.

~~~
aikah
Then what do physicists call what is beyond the edge of the universe?

edit: excuse my french ;)

~~~
jeremyjh
Imagine you are in inside of a giant beach ball, walking on the surface. You
will never find an edge, and it is meaningless to talk about such a thing. Yet
the space is finite.

~~~
jaipilot747
A beach ball has no "edge", but it definitely has a boundary surface, and
everything on the other side of this surface is "not beach ball".

Now what lies beyond the universe's boundary surface? Is such a surface even
present?

~~~
jeremyjh
That is indeed where the analogy starts to crumble. The reason is that space
itself is curved. It is curved by the matter inside it. Still the property
holds that if you travelled far enough fast enough you’d come back to where
you started.

