Data Showing Dark Matter Is Not Cold Neutrinos?

[Geonaut]

TL;DR

It seems to me that the only way to prove that dark matter is not simply cold neutrinos would be to measure the density of cold neutrinos in the universe, calculate the corresponding energy density, and then compare that to the dark matter energy density required to explain phenomena x, y and z, but can we actually do that?

How do we know that cold neutrinos do not make up 100% or a large percentage of the dark matter content in the universe? In my mind, the only way to prove that dark matter is not simply cold neutrinos would be to measure the density of cold neutrinos in the universe and then calculate the corresponding energy density, but can we actually do that? If hot neutrinos are difficult to detect then I'd imagine that we might not even have reliable data on the cold neutrino density. Can someone point me to the experimental data if it exists?

 

[Vanadium 50]

Where do all these cold neutrinos come from? And how do they stay so cold?

 

[Geonaut]

Where do all these cold neutrinos come from? And how do they stay so cold?

Those are good questions, but they're not an answer to my question. My question is essentially: Have we measured the cold neutrino density in the universe in an experiment?

Whether or not these cold neutrinos are present in a large quantity in some reasonable model that currently exists isn't what I'm asking.

 

[berkeman]

Those are good questions, but they're not an answer to my question.

Sometimes good questions can cut to the chase, and avoid a lot of wasted time...

 

[Orodruin]

Whether or not these cold neutrinos are present in some reasonable model that currently exists isn't what I'm asking.

It is however the question you should be asking because

Summary: It seems to me that the only way to prove that dark matter is not simply cold neutrinos would be to measure the density of cold neutrinos in the universe,

is not really true. There is nothing ”simple” about having cold neutrinos as dark matter if you cannot provide a production mechanism. I could posit that dark matter consists of invisible pink unicorns and tell you that the only way to disprove this would be to measure the invisible pink unicorn density.

Physics is not about proving things. It is about testing the predictions of models against reality and models with just cold neutrinos fail for various reasons.

 

[Geonaut]

Sometimes good questions can cut to the chase, and avoid a lot of wasted time...

That's true, and sort of the point of my question. Whether or not a good model exists where there's lots of cold neutrinos is not what I'm asking, and it's not what I'm asking for a lot of reasons. I don't want to get off topic though so I won't dive into that.

 

[berkeman]

That's true, and sort of the point of my question. Whether or not a good model exists where there's lots of cold neutrinos is not what I'm asking, and it's not what I'm asking for a lot of reasons. I don't want to get off topic though so I won't dive into that.

And what did you think of the reply by the university professor above who is quite familar with neutrinos? Nuff said?

 

[Geonaut]

It is however the question you should be asking because

is not really true. There is nothing ”simple” about having cold neutrinos as dark matter if you cannot provide a production mechanism. I could posit that dark matter consists of invisible pink unicorns and tell you that the only way to disprove this would be to measure the invisible pink unicorn density.

Physics is not about proving things. It is about testing the predictions of models against reality and models with just cold neutrinos fail for various reasons.

Of course there's nothing simple about anything in theoretical physics, and that's not what I'm saying. I'm not even claiming that dark matter is composed of cold neutrinos, or pink unicorns, I'm simply asking if we've measured the cold neutrino density in the universe. I don't know how to make that question any clearer than that. I think I know the answer to my question, if I had to guess, I'd say the answer is no we don't have that data. Even if the idea conflicts with your favorite model that doesn't mean a better unknown model doesn't exist where you have a large amount of cold neutrinos. Indeed, physics is about proving things as you say, but you can't prove anything without experimental evidence which is exactly why I'm interested in whether or not we have it.

 

[Orodruin]

Indeed, physics is about proving things as you say

I explicitly said the opposite of that.

Physics is not about proving things.

Even if the idea conflicts with your favorite model that doesn't mean a better unknown model doesn't exist where you have a large amount of cold neutrinos.

This is ignorant. ”Cold neutrinos are dark matter” imply certain properties of said dark matter. Those properties are necessarily in conflict with what we know about the history of the Universe. It is not about having a favourite model, it is about having the properties that would make the dark matter cold neutrinos in the first place.

 

[Geonaut]

I explicitly said the opposite of that.
This is ignorant. ”Cold neutrinos are dark matter” imply certain properties of said dark matter. Those properties are necessarily in conflict with what we know about the history of the Universe. It is not about having a favourite model, it is about having the properties that would make the dark matter cold neutrinos in the first place.

Ok, I read part of your comment wrong, that's my mistake, but your own personal take on what physics is all about doesn't actually answer my question so I don't think we should focus on that. I never said "cold neutrinos are dark matter" in fact I said the opposite, I said "I'm not even claiming that dark matter is composed of cold neutrinos". Perhaps I shouldn't have mentioned dark matter in the question in the first place. Once again, my question is simply: Have we measured the cold neutrino density in the universe?

Ok sure, that would be the next question: Do they have the same properties as dark matter? I would guess yes, but without a deeper personal investigation I wouldn't conclude that that's true. Perhaps the answer is no they don't for whatever reason that's not immediately obvious.

The reason why the data for the cold neutrino density question is my first question is because all of these other questions are a waste of time if we've already measured the cold neutrino density and used it to rule that out the idea that that's what dark matter is composed of. It may be true that cold neutrinos can't be dark matter for whatever reason, but answering that question for myself would almost certainly take a lot of time. With that in mind, it's clear that the first question to ask is: Have we measured the cold neutrino density in the universe?

There's no need to speak in a condescending manner by comparing ideas to unicorns and calling people ignorant, if I've offended you somehow, I apologize, although I'm not sure how I could have. There's nothing ignorant about recognizing the fact that models are theoretical, and that better models could exist. I'm not claiming that any specific idea about dark matter is true. Once again, I'm simply asking: Have we measured the cold neutrino density in the universe?

 

[Geonaut]

It is however the question you should be asking because

is not really true. There is nothing ”simple” about having cold neutrinos as dark matter if you cannot provide a production mechanism. I could posit that dark matter consists of invisible pink unicorns and tell you that the only way to disprove this would be to measure the invisible pink unicorn density.

Physics is not about proving things. It is about testing the predictions of models against reality and models with just cold neutrinos fail for various reasons.

The difference between the idea that dark matter consists of pink unicorns versus cold neutrinos is that we know that neutrinos actually exist, unlike invisible pink unicorns or any other dark matter candidate, which makes this question an important one. They are neutral, weakly interacting particles, it's only natural to ask yourself if they might be what dark matter consists of. It could be that the idea doesn't work in the most effective models that we have, but that's not what I'm asking.

 

[Vanadium 50]

The difference between the idea that dark matter consists of pink unicorns versus cold neutrinos is that we know that neutrinos actually exist

But they don't have the necessary properties. Calling "invisible pink unicorns" "cold neutrinos" doesn't change anything.

 

[Drakkith]

Once again, I'm simply asking: Have we measured the cold neutrino density in the universe?

Probably not. Low-energy neutrinos are virtually impossible to directly detect.

 

[PeterDonis]

I'm simply asking if we've measured the cold neutrino density in the universe.

Then that's what you should have asked in the OP, instead of asking this:

How do we know that cold neutrinos do not make up 100% or a large percentage of the dark matter content in the universe?

Also, you should have given the thread a different title, something like, oh, I dunno, "Have we measured the cold neutrino density in the universe?"

That said, since you have now repeatedly said you are not asking about whether we have viable models of dark matter as cold neutrinos (we don't), but only about whether we have measured the cold neutrino density, @Drakkith's response in post #13 addresses your question. And that means this thread can now be closed.