A New Map of the Universe, Painted With Cosmic Neutrinos

Neal Burns
Neal Burns
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Mike Hewson wrote: Well if

Mike Hewson wrote:

Well if neutrinos have mass, as observations appear to confirm, then yes you can catch up to one. You have to go faster and be in the band between a given neutrino's speed and that of light. You will gradually overtake it - if nothing else happens to you or it - though this may take quite some time to achieve ( but whose time ? ).

I had a thought: If it's possible to catch up to a neutrino, then there must be slower neutrinos around, because the expansion of the universe is accelerating, and so neutrinos should be traveling slower the longer ago they were emitted. But, how much they would have slowed probably depends on the age of the universe. So, there might not have been enough time for them to be "slow."

 

Mike Hewson
Mike Hewson
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Neal Burns wrote: Mike

Neal Burns wrote:

Mike Hewson wrote:

Well if neutrinos have mass, as observations appear to confirm, then yes you can catch up to one. You have to go faster and be in the band between a given neutrino's speed and that of light. You will gradually overtake it - if nothing else happens to you or it - though this may take quite some time to achieve ( but whose time ? ).

I had a thought: If it's possible to catch up to a neutrino, then there must be slower neutrinos around, because the expansion of the universe is accelerating, and so neutrinos should be traveling slower the longer ago they were emitted. But, how much they would have slowed probably depends on the age of the universe. So, there might not have been enough time for them to be "slow."

That's right because they (along with, say, the cosmic microwave background) have/are climbed/climbing out of the Universe's gravitational potential well. It would be easier to first say that they are losing kinetic energy, then solve for the velocity. And yes, all of this is dependent on one's model for universal evolution, assumed as Big Bang here (with lambda CDM expansion).

I recently asked an actual astrophysicist about the Big Bang etc and she replied that it was the best model they had to explain the current body of knowledge but she wouldn't be surprised if this was surpassed one day. She went on to say that it would depend on the behaviour of gravity at large densities ie. the melding of gravity with quantum mechanics. This is one reason why we care so much about neutron stars and pulsars : we want the equation of state of matter at such densities. Black holes don't help here because the horizon hides all. The squawks of radiation that pulsars give us are the best value for those questions.

Anyway General Relativity doesn't force you to accept any particular universe. GR is a differential scheme, meaning that one solves a (bloody complex) set of partial differential equations with boundary conditions in the usual way. It will only tell you what you have if you can say what you started with. Hence are there neutrinos about now that we can detect because they were formed during Big Bang processes? What about 'beforehand'?

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

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