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First detection of the missing half of normal matter in our universe

newscientist.com

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Re: First detection of the missing half of normal matter in our universe

#21

Post by nikdaheratik »

nielsbot wrote: post #10

I am not an astrophysicist... but does this mean dark matter has finally been "seen"? That's a big deal, right?

Not the same as dark matter. This is normal matter that is just very hard to see because it is laid out in thin tendrils of dense subatomic particles that are stretched between galaxies.

Dark matter is supposed to only have weak interactions with other particles, basically only gravity and not EM or other forces. Which is why we haven't been able to really detect it and can only deduce its existence.

Re: First detection of the missing half of normal matter in our universe

#22

Post by Meegul »

wheresmyusern wrote: post #17

im confused about baryons. they say that it is a particle (presumably like an electron or photon or other particle) but then they go on to say that its a gas, not a particle. very confusing.

A baryon is a subatomic particle that is composed of three quarks, such as a proton or a neutron. An electron would be categorized as a lepton, rather than a baryon as it is not made of multiple quarks and is rather an elementary particle, for reference.

Here's a good reference for subatomic elementary particles, where you can see some familiar names like electron and photon: https://en.wikipedia.org/wiki/Baryon#/media/File:Standard_Mo...

Re: First detection of the missing half of normal matter in our universe

#23

Post by colechristensen »

wheresmyusern wrote: post #17

im confused about baryons. they say that it is a particle (presumably like an electron or photon or other particle) but then they go on to say that its a gas, not a particle. very confusing.

gasses are made of particles

neutrons and protons are baryons (each made of 3 quarks)

electrons are leptons

photons are bosons

There are more things in those categories that are much less a part of the human experience.

Re: First detection of the missing half of normal matter in our universe

#24

Post by colechristensen »

nikdaheratik wrote: post #21
nielsbot wrote: post #10

I am not an astrophysicist... but does this mean dark matter has finally been "seen"? That's a big deal, right?

Not the same as dark matter. This is normal matter that is just very hard to see because it is laid out in thin tendrils of dense subatomic particles that are stretched between galaxies. Dark matter is supposed to only have weak interactions with other particles, basically only gravity and not EM or other forces. Which is why we haven't been able to really detect it and can only deduce its existence.

"dark matter" is a blanket term for stuff we have had a hard time seeing but we very strongly believe is there

Some of the candidates for where all the missing mass is, is poorly interacting matter

Re: First detection of the missing half of normal matter in our universe

#25

Post by throwaway613834 »

nikdaheratik wrote: post #21
nielsbot wrote: post #10

I am not an astrophysicist... but does this mean dark matter has finally been "seen"? That's a big deal, right?

Not the same as dark matter. This is normal matter that is just very hard to see because it is laid out in thin tendrils of dense subatomic particles that are stretched between galaxies. Dark matter is supposed to only have weak interactions with other particles, basically only gravity and not EM or other forces. Which is why we haven't been able to really detect it and can only deduce its existence.

> basically only gravity and not EM or other forces

I don't understand how physicists make any sense of this in any kind of theory. If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? How does that work? Or would dark matter just somehow resist even interacting with itself to create friction, etc.?

Re: First detection of the missing half of normal matter in our universe

#26

Post by timthorn »

JohnBooty wrote: post #15
nielsbot wrote: post #10

I am not an astrophysicist... but does this mean dark matter has finally been "seen"? That's a big deal, right?

EDIT: My understanding is incorrect. See the replies to the post for a correction. No. They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see. The density of this stuff (filaments of gas stretching between galaxies) is so low that it is really, really, really hard to detect. "Dark matter" theories posit that the missing matter isn't just regular matter that's har…

> They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see.

No, the dark matter proportion hasn't changed. There was missing "regular" matter that has now been accounted for.

Re: First detection of the missing half of normal matter in our universe

#27

Post by nikdaheratik »

wheresmyusern wrote: post #17

im confused about baryons. they say that it is a particle (presumably like an electron or photon or other particle) but then they go on to say that its a gas, not a particle. very confusing.

Baryons are types of subatomic particles. They include protons and neutrons as well as other kinds that only exist briefly in an atom smasher. In this case, they are using a generic term because they have no way of knowing whether it's primarily protons/neutrons or what else it could be made of.

A gas in this sense is really a very loose collection of light particles, mainly hydrogen/helium or just empty separate protons and neutrons. As opposed to heavier atoms that are the product of stellar fusion and form more solid matter.

If you think about this in terms of cosmology, the universe was originally a solid uniform blob of high energy particles. As it cooled, some of those particles grouped together to form stars and galaxies. However, there are parts in between galaxies that clumped together a bit but didn't really have enough mass to form anything. So it's really just made of tendrils of basic atomic building blocks.

Re: First detection of the missing half of normal matter in our universe

#28

Post by iheartmemcache »

Dang I read the first paragraph of the article and immediately went searching for the real papers since I didn't expect any media outlet to include them at the bottom, but here they are for anyone who made the same mistake I did! https://arxiv.org/abs/1709.05024 https://arxiv.org/abs/1709.10378

Not a cosmologist but here's my go at the de Graff paper. (Let's get this out of the way, the title is click-bait and the paper/researchers makes no such claims as to anything near 50%. New Scientist is trolling for hits with the word "half" or the journalist is fundamentally misunderstanding the work.) In de Graff, et al, they claim 30% of "90% of the missing baryonic matter [that composes the ~25% of our total universe observable from within our light cone]" has been found in the CMB structured as filaments between galaxies. They claim there's effectively a planar network layered on top of Minkowski space composed this baryonic matter. The temperature was at this "Goldilocks" midrange no one had previously analyzed (ranging from 10^5-10^7K). This wasn't previously found because people were searching "only the lower and higher temperature end of the warm-hot baryons, leaving the majority of the baryons still unobserved(9)". [See "Warm-hot baryons comprise 5-10 percent of filaments in the cosmic web.", Nature, Eckert et al for more about baryons of this composition.]

Additionally, these baryons have 10x the density of what we observe (so this could potentially be evidence for the first stable baryonic matter composed of second generation quarks, or more likely the binding energies are different from our standard uud/udd nucleon quarks) permeating the universe, and where the roads in the network meet ("dark matter haloes"), you have embedded galaxies and galaxy clusters. They continue with their analytic methods of the CMASS data, and claim within the framework 30% of the total baryonic content (which, again, all analytical methods put this into no more than ~25%) is composed of this form of this matter. I skimmed their methods and it seemed to at least logically hold -- they are using the appropriate data (SDSS 12) and didn't cherry-pick their galaxy pairs (so, no p-hacking here!).

Re: First detection of the missing half of normal matter in our universe

#29

Post by justin_vanw »

JohnBooty wrote: post #15
nielsbot wrote: post #10

I am not an astrophysicist... but does this mean dark matter has finally been "seen"? That's a big deal, right?

EDIT: My understanding is incorrect. See the replies to the post for a correction. No. They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see. The density of this stuff (filaments of gas stretching between galaxies) is so low that it is really, really, really hard to detect. "Dark matter" theories posit that the missing matter isn't just regular matter that's har…

No, this has nothing to do with dark matter. Dark matter has to be in galaxies because the reason we think it exists at all is because galaxies spin 'too fast', so there is more mass in them and therefore more gravity than can be accounted for. This article is talking about gas between galaxies.

Re: First detection of the missing half of normal matter in our universe

#30

Post by justin_vanw »

throwaway613834 wrote: post #25
nikdaheratik wrote: post #21

Earlier quoted context omitted.

Not the same as dark matter. This is normal matter that is just very hard to see because it is laid out in thin tendrils of dense subatomic particles that are stretched between galaxies. Dark matter is supposed to only have weak interactions with other particles, basically only gravity and not EM or other forces. Which is why we haven't been able to really detect it and can only deduce its existence.

> basically only gravity and not EM or other forces I don't understand how physicists make any sense of this in any kind of theory. If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? How does that work? Or would dark matter just somehow resist…

Friction, and really any kind of 'contact' between things is just electrons repelling. Some ideas for dark matter don't interact in any way other than gravity, so your idea is pretty mainstream on that, but there's basically no evidence that dark matter exists in some specific form, and in fact every experiment seems to constrain the possible forms it could have more and more. It's still possible it exists, but I would suspect that whatever the explanation is for the 'missing' matter will be pretty revolutionary.
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