Cowmic rays, similarly to ordinary photons, get "redshifted" as they travel through the expanding universe. Photons do redshift to lower frecuencies losing energy and "temperature" in the process. Meanwhile, cosmic rays lose momentum and thus velocity, so in that sense they are kind of redshifted as the universe expands.

However, why do they redshift? I've read that cosmic rays lose energy overtime by interacting with photon fields like the CMB. Is it because of this?

I have always been interested in multiple perspectives and ideas in regards in personal theoretical astrophysics and cosmology as the variety of hypothesis leaves a lot to learn, if you have a theory that hasn't been established but would like to explain I would be interested as I believe all perspectives have value, explaining the reasoning or observation that lead to you to your conclusions makes it easier to understand any concepts I made not be personally familiar with.

It seems to me that all alien life that exists must have some non-interference rule like the prime directive in Star Trek. Here's why:

• Very unlikely that we're the first intelligent life in the galaxy. In fact very unlikely that there's not multiple sets of intelligent life that have sprung up before us and don't have the entire galaxy and all potentially habitable planets mapped.
• If another alien species was aware of our existence and was hostile, it would seem very unlikely that they just haven't gotten to us yet. Possible, but with 1,000s, 10,000s or millions of years headstart, I think the idea that we just aren't up on their cosmic list yet seems doubtful.
• Given the above, if they have this level of headstart and we still haven't been contacted, then they must be purposefully avoiding interference.

Of course there's potential scenarios where any of the above isn't true, but it feels like the likelihood of one of those other potential scenarios being true is very very very low.

So it’s pretty well known that the Milky Way’s estimated diameter is 100,000 light years. But I’ve seen multiple sources (namely Wiki) stating that its isophotal diameter is 86,400 light years. That’s fine, close enough, different measurement techniques, whatever.

But what I don’t understand is the variability of these techniques. For example, M83 (the Southern Pinwheel galaxy) is estimated to have a diameter of 55,500 light years, just over half the diameter of the Milky Way. Yet apparently, its isophotal is 118,000 light years. At this point I’m just confused as to which is actually bigger, because they each destroy the other in different categories regarding diameter. Isophotal is apparently more light-focused, so does it just mean M83 is much brighter around the edges, but the Milky Way stretches out more, but in a dimmer way? HELP lol.

My understanding is that during Big Bang Nucleosynthesis, matter and antimatter was created in almost equal amounts. This has oft lead to questions about what happened to all the antimatter, as it certainly isn't around today.

As I understand, during Nucleosynthesis, matter and antimatter was being created from the abundant radiation energy, which in my mind would consist of something like ultra-high energy photons and other particles. Matter and antimatter is created from radiation, with most of it immediately annihilating and turning back into radiation. This would create a temporary system where matter/antimatter and radiation are rapidly interchanging. As very slightly more matter is created that antimatter, this means that as they interchange, matter begins to accumulate, until all the radiation energy has been turned into matter.

I did wonder if my understanding of this is correct, or if I've misunderstood something? Very happy to be corrected if this isn't right.

I’m an engineering major trying to learn about cosmology and astronomy more as it always intrigued me . I’ve not particularly studied or taken any course on it but I wanna learn more about it but not indulge much in the mathematics of it and mostly the theoretical aspects to read as a hobby ! Any book suggestions or courses I could take as a hobby?

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I’m interested in learning more about astronomy and cosmology. Does anyone know of any free education or maybe free online courses I could take?

I understand that the Big Bang started as a very small point outside of space and time. I cannot imagine there being no space. There is nothing, and what is nothing?

Is it likely that our current universe will implode on itself, resulting in another Big Bang? Are there any problems with such a theory? Is this the most likely explanation for our own Big Bang?

I was reading this writing (https://davidwoolsey.com/AttO/AttO_blog/Entries/2020/7/13_Black_Holes_and_Transverse_Tidal_Effects%2C_a_revised_essay_on_some_thoughts.html) about considering tidal effects in black hole models.

Outside of the main topic of the writing, there is a part that got my attention:

The authos indicates that in the context of Hawking radiation, only particles (like photons) with small enough orbital angular momentum will escape to infinity.

This made me think: could there be black holes with extremely large angular momentum that could transfer themselves part of it to escaping photons (even if they initially had small amounts of angular momentum upon escaping)? For example, I was thinking, if a black hole with an enormous spin emitted Hawking radiation and while escaping it made contact with the photon ring or the ergosphere (regions with high angular momentum), perhaps the photons could acquire quite a bit amount of angular momentum from these zones (which would be given by the black hole itself) trapping the photons forever, or even making them return to the black hole. Could this be possible? Is it possible that black holes trap their own Hawking radiation?

Formula 1: `H(t) = H₀ * (1 + (t/t₀) * ln(t/t₀))`
Where H(t) is the Hubble constant at a given time t, H₀ is the present-day Hubble constant, t is time, measured in billions of years, t_0 is a reference time, roughly the age of the universe (13.8 billion years), ln is the natural logarithm.

More Friedman-esque - This formula suggests a gradual decrease in the Hubble constant over time, with a slower rate of change at later times. This implies a decelerating expansion in the early universe, followed by a period of accelerating expansion.

Formula 2: `H(t) = H₀ / (1 + α * ln(t/t₀))`
Where H(t) is the Hubble constant at a given time t, H₀ is the present-day Hubble constant, t is time, t₀ is the present age of the universe, and α is a dimensionless parameter characterizing the rate of change of the Hubble constant.

More ΛCDM-esque - This formula implies a more rapid decrease in the Hubble constant at early times, followed by a slower decrease at later times. This suggests a rapid expansion in the very early universe, followed by a period of slower expansion.

Could be a dumb idea either way. Just looking for insights.

I’ve heard that the most commonly accepted theory for how the universe will end is heat death. But what happens afterwards? Will there be true “nothingness,” or will matter still exist? Is there any chance at another Big Bang happening again?

Once again, please forgive me if this is a stupid question.

In principle neutrons have a very short time when they are isolated (around 10 minutes) and they suffer beta decay, but because of electron degeneracy pressure, it is heavily supressed in a neutron star (https://physics.stackexchange.com/questions/63383/what-stabilizes-neutrons-against-beta-decay-in-a-neutron-star).

So, if this happens, then, shouldn't neutron stars be "safe" from the decay of neutrons and protons (in case there is proton decay, as there are almost no protons but mainly neutrons)?

I would imagine that dark matter would be attracted to areas of higher mass density like star systems, as the stronger gravity would pull them in. Would this mean that the solar system would have a mini dark matter halo? If so, would this have an observable effect on the orbits of the planets?

I've been thinking about all this recently. I've been playing with the idea that our universe is just a black hole to a host galaxy in a host universe. It seems that we can account for about 5% of stuff in our universe and we have an issue with all this missing stuff. Since gravity can communicate between a galaxy and its black hole, and since a black hole can be about 5% of the galaxy, wouldn't this explain it? And CBR is just stuff falling into the black hole (our universe)?

Hey!
As an indie developer and astronomy lover i have decided to create an app for learning interesting facts on astronomy and space exploration in the format of quiz. It is called Space Academy and available in the App Store. I hope you will have fun with it! I will also appreciate your thoughts on my app.

From what I understand within galaxies and inside the space between galaxies that are close enough, gravity is strong enough to overpower the effects of the universes expansion. My question is does expansion occur within these spaces and gravity just keeps cosmological structures intact? Or does the presence of a strong gravitational field stop the expansion from actually occurring?

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For example, how far apart would the milky way need to be from Andromeda for space between them to increase from dark energy, vs their gravity keeping them at the same distance / coming together (excluding gravity impact of other galaxies in our cluster, if the 2 galaxies were in a void).

I have heard that the expansion of space does not apply within gravitationally bound structures, such as between the stars in a galaxy.

If this is true, why does space expand only when there's nothing around? Or does space technically expand at the same rate everywhere, but within a galaxy it's just so little as to be negligible?

I am very new in the study of cosmology so please forgive me and be patient. I’ve been incredibly curious about black holes and how they form, work, and die. My current topic I’m looking into is hawking radiation, but the seemingly basic principles of “virtual particles” really stumps me. How are there particles, or anything for that matter, within space? Isn’t space literally just “space” with nothing in it? What are these particles and how do they exist, let alone react with each other? Where do they come from? What makes them virtual? Why have I never heard about them in other areas of cosmology? How does a black hole “lose virtual particles” and energy if nothing can escape it? Obviously I have lots of questions about this so any input or recommendations for readings or videos is highly appreciated. Thank you all for reading.

Hi, looking for a serious cosmologist with an even more serious sense of humor. Working on a film script that requires s a factual (miss) representation of the elements and history of the universe as backdrop for the main story. I need help making the milieu/ world building rock solidly accurate before I take my comedic liberties. DM me if interested.

Since hearing about this meeting of professors to discuss the standard cosmological model, I have been trying unsuccessfully, to get access to some of the meeting to listen to the discussions.

First, I wanted to learn from some of you if any of you who are also interested or have access and hear your thoughts on a potential re-evaluation of the standard cosmological model.

Second, in your opinion, what ideas or individuals involved do you find interesting or worth delving further into their research or theories?

Third, if there is potentially an error in the standard cosmological model, what or where do you think is the highest probability area of that error?

A summary of the questions the meeting poses:

"Is the universe simple enough to be adequately described by the standard ΛCDM cosmological model which assumes the isotropic and homogeneous Friedmann-Lemaître-Robertson-Walker metric? Tensions have emerged between the values of cosmological parameters estimated in different ways. Do these tensions signal that our model is too simple? Could a more sophisticated model account for the data without invoking a Cosmological Constant?"

Royal Society Meeting link

Guardian Article on Meeting

Hello! I’m a rising senior in astrophysics at UF. For my program, I have to take one more physics class to graduate and I’m debating which one I should do if i want to go into cosmology. The choices I’m between are quantum mechanics, thermal physics (thermodynamics), or particle physics. What would be the smartest to take?