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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

#31

Post by JohnBooty »

timthorn wrote: post #26
JohnBooty wrote: post #15

Earlier quoted context omitted.

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.

Thanks for the correction. I updated my post with a note pointing out that it's wrong.

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

#32

Post by nikdaheratik »

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…

We don't know much about what it does do, only what it doesn't do. Scientists have deduced its existence by measuring the speed of the expansion of the universe and determining that the mass that we can see based on the rotation of galaxies, etc. can not account for the speed of the expansion. There should be way more mass out there than there is.

We also know that it doesn't seem to react with light otherwise it would block out the stars from other galaxies as well as the cosmic background radiation. It's difficult to tell how it reacts with ordinary matter because we can't see it, but the assumption is that, since its got such a large gravitational effect, it must not react much at all otherwise it would dominate everything visible since it's 90% of the matter in the known universe.

It's difficult to make sense of what this means, but all of the other theories that explain the rotation of galaxies and the expansion of the universe don't fit very well either.

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

#33

Post by dragonwriter »

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…

> 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?

You can't. Because becoming a star (initiating nuclear fusion) requires nongravitational interaction between nucleons, which are normal, not dark, matter.

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

#34

Post by InclinedPlane »

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?

No, this is about baryonic (normal) matter entirely. We basically have different ways of determining what the matter makeup of the Universe is which are independent of direct observations. Those techniques actually indicated a higher proportion of atomic (baryonic) matter in the Universe than had been directly detected so far, while still indicating that the vast majority of the matter/energy balance of the Universe is in the form of dark energy (which we are pretty in the dark on its composition) and "cold dark matter" (which has been narrowed down to being primarily made up of "weakly interacting massive particles" of a type or types that are currently outside our understanding of particle physics). Specifically, those lines of evidence say that the expected balance of the density of matter/energy on cosmological scales comes out to 69% dark energy, 26% cold dark matter, and the remainder (actually a bit under 5%) being baryonic matter.

However, we've only detected about half of that expected 5% density of matter (in the form of stars & planets, black holes, giant interstellar gas and dust clouds, etc.) This recent discovery shows that much of the undetected baryonic matter is in the form of huge warm gas clouds surrounding galaxies.

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

#35

Post by throwaway613834 »

dragonwriter wrote: post #33
throwaway613834 wrote: post #25

Earlier quoted context omitted.

> 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…

> 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? You can't. Because becoming a star (initiating nuclear fusion) requires nongravitational interaction between nucleons, which are normal, not dark, matter.

I thought the claim was that dark matter doesn't interact with normal matter through other forces, not that it doesn't interact with itself?

But if so... then dark matter might be just bosons? The properties I'm reading seem to be pretty consistent with bosons.

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

#36

Post by brianpan »

nitrogen wrote: post #19
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.

Isn't a gas just a swarm of particles?

We like to say flock of particles.

Source: I am a smientist.

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

#37

Post by dragonwriter »

throwaway613834 wrote: post #35
dragonwriter wrote: post #33

Earlier quoted context omitted.

> 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? You can't. Because becoming a star (initiating nuclear fusion) requires nongravitational interaction between nucleons, which are normal, not dark, matter.

I thought the claim was that dark matter doesn't interact with normal matter through other forces, not that it doesn't interact with itself? But if so... then dark matter might be just bosons? The properties I'm reading seem to be pretty consistent with bosons.

IIUC, If it interacts with itself through other forces, they can't be through the forces that act on normal matter (which they would interact with if that was the case), including the forces involved in nuclear fusion. They'd have to be dark matter exclusive forces.

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

#38

Post by throwaway613834 »

dragonwriter wrote: post #37
throwaway613834 wrote: post #35

Earlier quoted context omitted.

I thought the claim was that dark matter doesn't interact with normal matter through other forces, not that it doesn't interact with itself? But if so... then dark matter might be just bosons? The properties I'm reading seem to be pretty consistent with bosons.

IIUC, If it interacts with itself through other forces, they can't be through the forces that act on normal matter (which they would interact with if that was the case), including the forces involved in nuclear fusion. They'd have to be dark matter exclusive forces.

Interesting, thanks. So what's preventing "dark matter" from simply being, say, lots of photons traveling through intergalactic space then? That seems like the next obvious candidate after ordinary matter.

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

#39

Post by subroutine »

lifeisstillgood wrote: post #9

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.

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”?

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