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Comment by lifeisstillgood | Hacker News Reader

Can I just check - my understanding is that we can only see 5% of the expected mass in the universe - so we have just found another 5%? meaning dark matter needs to account for 90%?

plus, how awesomely beautiful is the idea of tendrils of has connecting the galaxies through space.

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That's not it. We can figure out what the mass makeup of the Universe is through various independent means, and those are also independent of direct observation. Those lines of evidence tell us the Universe is a 69:26:5 split between dark energy, dark matter, and ordinary atomic (baryonic) matter. None of that has changed.

However, we've only been able to actually detect much less than that 5% contribution of the atomic matter in the Universe, meaning that we expect there's a lot more ordinary matter out there that we can't see. This research indicates that a big chunk of that missing ordinary matter is in the form of huge warm gas clouds on galactic scales.

> You have probably heard about the hunt for dark matter, a mysterious substance thought to permeate the universe, the effects of which we can see through its gravitational pull. But our models of the universe also say there should be about twice as much ordinary matter out there, compared with what we have observed so far.

My understanding is that, up until now, we could only observe half of the 5% expected mass, e.g. 2.5%, and we can now see the rest of it. The rest of the universe is still made up of 25% dark matter and 70% dark energy.

Like many of us, I'm no astrophysicist, but one of the linked papers [1] references that we've thus far been able to locate 10% of baryon content, with this discovery bringing this number up to 30%.

Assuming I'm interpreting this correctly, this means that there's still around 70% of expected baryonic matter unaccounted for.

1: https://arxiv.org/abs/1709.10378v1

Tendrils between galaxies is mind-blowing. Aren't galaxies very very far apart? And there is basically nothing in between? How are subatomic particles - baryons - able to survive the constant expansion? Do the tendrils keep getting thinner? Can they be a medium to transmit intergalactic information someday?

Also, to expand on that (I have no knowledge, just making assumptions), the space between galaxies would need to either create new matter, or the matter would get infinitely spread out over time?

So, if the stretching thing is true, would there be areas with absolutely no matter or anything at all?

In fact, our closest neighbour galaxy is just 25,000 light years away, while the milky way is 100,000 light years across.

I too love the idea of there being filaments of matter connecting distant galaxies. I can only imagine the artistic renderings will make the universe look like a neural network.

Can anyone clarify how these filaments remain “hot”?

>things in motion tend to stay in motion

comes to mind

Motion is relative though. And just because something is moving relative to another thing does not necessarily mean it is hot.

temperature, in my simple understanding, is brownian motion. What ever that means in the end, we know that vacuum is a pretty good thermal isolator, so the counter question should be, why should it cool down? Sure, heat can be given off as radiated photons which is messured coming from the filaments, but you probably don't even begin to understand why, ie. why the T is hot, I sure don't, so the question seems like stabbing in the dark (puns not intended).

Whee, the best thing to say about "... just because something is moving relative to another thing does not necessarily mean it is hot ..." is that this is unsettled. ;)

Ott 1963 shows T' = T / {\sqrt{1 - v^2 / c^2}}, or for short T' = \gamma T, where \gamma is the Lorentz factor, T' is the temperature calculated by an observer in a set of coordinates Lorentz boosted with respect to the coordinates in which the system is at rest with a temperature T.

Asymptotically one would expect that as \gamma -> 1 \RightArrow T = T' and \gamma -> \infty \RightArrow T = \infty assuming one is measuring temperature using photocalorimetry (consider a blackbody radiator...).

However, there are different proposals that depend on how and if one makes the first or second laws of thermodynamics Lorentz covariant, and how one goes about measuring a distant object's temperature. All of this is also in the context of Special Relativity; a general relativist could say that the whole matter comes down to a choice of gauge anyway, and temperatures are only directly comparable for two objects at the same point in spacetime (and even there you get a can of worms because they would then be "in contact" at the point p on the manifold, and even then there is a choice of gauge and some interesting cut-offs in which they could equilibriate (T_obj1 = T_obj2), dissipate excess stress-energy (T_{p} > T_obj1 + T_obj2), or collapse gravitationally).

Unfortunately the matter is not wholly settled and there is an absence of firm experimental evidence, and moreover the debate is confined to flat spacetime.

However, one has to do quite a bit of twisting to assume that T = T' (Landsberg did this in 1966 & 1967).

Here is a nice recent set of slides on temperature in GR: http://www.dpg-physik.de/dpg/pbh/aktuelles/pdf/Andersson.pdf

Q. Is a moving body hot/cold? A. Yes/no/maybe. The discussion is "pointless".

> I too love the idea of there being filaments of matter connecting distant galaxies. I can only imagine the artistic renderings will make the universe look like a neural network.

There are lots of "cosmic web" renderings from large-scale cosmological simulations of dark mater and the formation of structure in the universe. A major one, now over a decade old is the "Millennium Simulation": http://wwwmpa.mpa-garching.mpg.de/galform/millennium/

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