[Harri (OP)]

I posted a question regarding neutrinos a few days ago and one of the reasons I wanted to explore the way neutrinos travel through space is because of an article I read about the Deep Underground Neutrino Experiment otherwise known as DUNE. In this article I read that  supernova neutrinos 'reach Earth hours before the light produced in the explosion'. Of course my question is going to be how do these neutrinos travel faster than the speed of light and reach Earth 'hours' ahead of the light?

[Halc]

I read that  supernova neutrinos 'reach Earth hours before the light produced in the explosion'. Of course my question is going to be how do these neutrinos travel faster than the speed of light and reach Earth 'hours' ahead of the light?

The neutrinos begin their journey to Earth immediately upon the core explosion, since neutrinos have almost no interaction with matter.  Light cannot do this, and the light you see is from the explosion shock wave reaching the surface of the star hours later, which generates the light you eventually see.  The light from the initial explosion is long since absorbed by the material of the star.

[Harri (OP)]

Hi Halc.  That explains that then!  It's a typical knee jerk reaction from a newbie like myself when he thinks, hey something DOES travel faster than light! I guess that if the article was aimed at a non scientist like myself then it would have said the neutrinos get to Earth earlier than the light 'because ...'.

[chiralSPO]

Just to reiterate what Halc said (and hopefully add something):

Neutrinos don't travel faster in a vacuum than light does through a vacuum, but they do usually travel at near-light speed because they have so little mass. The key is that most neutrinos can pass right through long expanses of dense matter without any interactions (they can interact through the weak force, so some will get absorbed or scattered)--so neutrinos effectively don't care what medium they are traveling through. Light, on the other hand, interacts quite strongly with matter through electromagnetic forces (so the dense ionic plasma of a star is really going to interact with light). Light will likely get absorbed and re-emitted or scattered billions of billions times before leaving the star.

According to this reference, light at the core of our sun has a mean free path of about 1cm, meaning it likely takes thousands of years (on average) to reach the surface: https://image.gsfc.nasa.gov/poetry/ask/a11354.html

In contrast, this website claims to calculate the mean free path of a neutrino through starlike medium is on the order of 100 lightyears! (I cannot confirm their calculations, but it looks reasonable): http://burro.case.edu/Academics/Astr221/StarPhys/neutrinos.html

[Harri (OP)]

What developed first after the big bang , neutrino's or light?

[Halc]

What developed first after the big bang , neutrino's or light?

I cannot see how there would not be both from pretty much the earliest times, but the universe was completely opaque, so none of the early light got anywhere before being absorbed.  Only 380,000 years later did hydrogen form, turning the universe transparent and allowing the light from that time to reach us today as the CMB.  The cosmic neutrino background we see today is, well, 380,000 years older than that.

[Petrochemicals]

Hi Halc.  That explains that then!  It's a typical knee jerk reaction from a newbie like myself when he thinks, hey something DOES travel faster than light! I guess that if the article was aimed at a non scientist like myself then it would have said the neutrinos get to Earth earlier than the light 'because ...'.

You too can travel faster than the speed of light, quite easily actually.

https://abcnews.go.com/Technology/story?id=99111&page=1

[chiralSPO]

Hi Halc.  That explains that then!  It's a typical knee jerk reaction from a newbie like myself when he thinks, hey something DOES travel faster than light! I guess that if the article was aimed at a non scientist like myself then it would have said the neutrinos get to Earth earlier than the light 'because ...'.

You too can travel faster than the speed of light, quite easily actually.

https://abcnews.go.com/Technology/story?id=99111&page=1

I would expect that, unless otherwise specified, "the speed of light" is typically meant to mean "the speed of light in a vacuum" ie the constant, c.

Yes, light can get slowed down by interactions with matter. It doesn't even have to be particularly special matter for significant differences (even liquid water can slow light down to about 75% of c)

[Harri (OP)]

Is it likely/possible that neutrinos then are travelling faster than the universe is expanding? If a neutrino were to reach the edge of the universe (let's just imagine there is one) what would be the outcome? My basic understanding is that nothing could leave the universe as the amount of energy in the universe remains constant.

[evan_au]

Is it likely/possible that neutrinos then are travelling faster than the universe is expanding?

Quite the opposite.

It is thought that parts of the universe are expanding away from each other faster than the speed of light.
- So light (or neutrinos) emitted from a star in that part of the universe will never reach us.
- We must infer their existence, since we can't see them

In principle, neutrinos emitted by the big bang would still be detectable, since the big bang is all around us.
- It is estimated that these Big Bang neutrinos were emitted about 1 second after the big bang (compared to 400,000 years for Big Bang microwaves)
- That means that the neutrinos have been red-shifted far more than the microwave background radiation
- Since neutrinos are hard to detect even when they are "fresh", we can't detect these big bang neutrinos with current techniques..

See: https://en.wikipedia.org/wiki/Cosmic_neutrino_background

[Halc]

Is it likely/possible that neutrinos then are travelling faster than the universe is expanding?

The rate of expansion is not a speed.  Speed (of a neutrino, or light, or whatever) is measured in km/sec, while space expansion is measured in km/sec/mpc which reduces to sec^-1
Thus you cannot compare the speed of one thing to a not-speed of another.

If a neutrino were to reach the edge of the universe (let's just imagine there is one) what would be the outcome?

You tell me. You imagined it. Maybe it could stick its head through and peer at the wonders beyond.

My basic understanding is that nothing could leave the universe as the amount of energy in the universe remains constant.

There exist models of the universe that are bounded on at least one side, but it is never reachable since it is always in the past.

In a model that covers the entire universe and not just the part that exists in our inertial frame, or just the parts we see, things do not maintain their speed over time, and hence neutrinos eventually coast down to arbitrarily slow speeds at the heat death.  Any neutrino created momentarily after the big bang should already have done this, assuming it has managed to avoid being absorbed along the way.

[Harri (OP)]

So in simplistic terms is it fair to say that the expanding universe isn't expanding at a particular speed but rather, everything is receding away from everything else at an increasing rate ? This rate depends on certain universal factors.

Ha ha you threw it right back at me, what would happen if a neutrino reached the edge of the universe? I'm guessing there might be an edge to the universe but not an edge to space. Everything that reaches the edge of the universe just expands the universe into space. I'd love to know if that makes sense to you or not.

[Halc]

So in simplistic terms is it fair to say that the expanding universe isn't expanding at a particular speed but rather, everything is receding away from everything else at an increasing rate ?

Everything is receding from any given thing at a rate proportional to the distance between the two things.

This rate depends on certain universal factors.

Mostly only on the age of the universe.  I said the rate was the reciprocal of time: In particular, it is the reciprocal of the age of the universe.  Not exactly because the expansion rate is not constant, but accelerating, which mucks up the simplicity of it.

Ha ha you threw it right back at me, what would happen if a neutrino reached the edge of the universe? I'm guessing there might be an edge to the universe but not an edge to space.

That sort of conjures an image of the big bang happening at a particular location in space, which isn't the model at all.  No, I know of no model where there is an edge to the stuff, and your view might have stars only on one side.

Everything that reaches the edge of the universe just expands the universe into space. I'd love to know if that makes sense to you or not.

No, the model is that the big bang happened literally everywhere, but there was no separation between any two places at the event, so time and space really have no meaning at such a singularity. Space, measured along lines of equal proper age, has no finite boundary.