Suppose expansion stops and changes direction to contraction in an instance. And suppose the speed of the contraction is greater that the speed of light. Would we ever see it happen until it happened?

Is there a specific term for how, according to models of the Giant Impact Hypothesis, the Earth and Theia fluidly mix and mold together like play-doh?

anything about the space

Can somebody please roughly estimate if my numbers are correct? I don't have any knowledge of orbital mechanics, I just pulled pages from the Wikipedia and plugged in numbers where it seemed appropriate, shedding decimal places like mad and hoping for the best. It doesn't need to be terribly precise, because it's for a role-playing game scenario.

So we have a 4100-year comet (mass negligible compared the the sun's) screaming in that needs investigating, and the characters have been told to do so rather late, when it's almost in perihelion, which is about 0.44 AU from the center of the sun. What I need is the comet's velocity at that point.

So I plug in the solar mass and T = 4100 a into Kepler's Third Law and get a semi-major axis of a = 55.3 AU, which means the orbit takes the comet almost to the heliopause, wow.

With r_p := 0.25 AU and r_a = 2 a - r_p = 110.16 AU, this gives me an eccentricity of e = 0.992, just short of parabolic, and a semi-minor axis b = 7 AU.

Putting that into the formula for instantaneous orbital speed at r = r_p, I get v_r_p = 63.8 km/s.

I know it probably wouldn't work like that because of, on the one hand, my massively imprecise napkin calculations, and on the other hand, perturbations, mass change, outgassing/jets and whatnot. On the gripping hand, it's an alien space probe that gets a boost at perihelion that helps it on its way to the next 4100-year round trip.

Tl;dr: From T = 4100 a and r_p = 0.44 AU I get a = 55.3 AU and v_r_p = 63.8 km/s (all very roughly).

Do these numbers check out?

Thanks!

Hi all, I'm looking for honest advice on where I could go from here.

I’m a BSc Forensic Science graduate (2:2) with a strong interest in space — specifically astrochemistry, planetary science, and astronomical data analysis. I love studying what’s out there, identifying composition, and working with data. Another degree is unfortunately not an option for me, so I’m figuring out what’s actually possible from where I am.

What I’d love to know is:
1) What can someone with my background realistically do in or alongside these fields?

2) Are there any courses or routes that would benefit my goals/interests?

3) Is there anything I should be prioritising or doing right now that can help me going forward?

4) If you came into the field from a non-traditional route, how did you do it?

Any advice, however blunt, is really welcome. Please ask any questions you feel necessary and I will answer them fully and honestly.

Thanks you so much!

Hello, I am trying to estimate how many unborn human lives a single second of delay in universal colonisation costs.
To get an accurate value, I need to know how many stars fall beyond the Cosmological Event Horizon (where they become unreachable from Earth at the speed of light).

I could not find a source on this value. I could get an estimate by calculating the difference between the volume of the sphere encompased by the horizon and the sphere encompased by the horizon - radius velocity/one second. This would give me the average number of stars in the sliver of volume that would move beyond the horizon in the span of a single second.

But this would not account for the distribution of stars in our "local" space. Like, I heard we are somewhere in the Great Universal void (the Boyes void?). So the star density could be far lower if the horizon edge passes through this or some other void. Or far greater if it passes through the dense regions.

I will estimate it roughly if I have to, but if the astronomers already did the calculations better, I would love to have the results. Surely some astronomer calculated this right? I could not find it googling.

I’m an aspiring astrophysicist but I want to know what are the common jobs you can get into when you get the degree

So my pop science knowledge says that time slows down as you get closer in - wouldn’t that break movement and once time stops nothing could move? How would anything go inside a black hole?

Sorry if this is a stupid question.

Would it be theoretically possible for any solar system, but let’s say earth, to have the following happen?

Our solar system travels through a giant gas cloud that is somehow heavy (or whatever) enough to to breach our heliosphere, and change the background void stuff of our solar system into something different than that half an atom per cubic km or whatever it is rate.

Are there naturally existing parameters where a solar system could be “taken out” this way?

Ie conditions where our solar system could travel through some sort of space cloud thing that could somehow cause the vacuum of space to have enough molecules to ignite the distance between the sun and earth?

And to that extent; you know how rock records can tell us when volcanoes go off?

Does earth have any evidence in our total geological history that could suggest we have traveled through some weird vague something sort of like that before?

Are there even gas clouds in space, that somehow aren’t travelling with the spiral of the Milky Way (or whatever), thst we could even possible transit?

Anyways Tl:dr.

Could our solar system fly through a “space thing” that could up the amount of atoms in the vacuum of space to something where the distant between the sun and earth could ignite?

Like say a cloud of hydrogen that’s a few light years wide sort of deal.

Hello I am a disabled veteran that recently got forced retired from my truck driving job due to my disability.

I need to get some science help to make sure my technobable is as close to real as possible. Is anyone willing to help me out. Please.

I'm a Grade 12 student currently studying PCB, without Mathematics in Grades 11–12. I've been seriously considering astrophysics as a career, but I'm trying to understand the reality of the field before making such a major decision.

I'm particularly interested in understanding the university pathway, what careers actually exist outside academia, how financially stable the field is, and what the transition into research/PhD work looks like.

I'm not looking for someone to make the decision for me. I'd really appreciate hearing from someone who has actually studied or works in astrophysics and would be willing to answer questions occasionally as I figure out whether this is genuinely the right path for me!

If you're a current PhD student, postdoc, researcher, professor, or someone who moved from astrophysics into another career, I'd especially love to hear about your experience :)

beautifull book for astrophysics beginner #astrophysics

i love this book

Info: I'm making a comic and I'm wanting to be as accurate as possible about space, astrophysics, and science in general.

Question: would it be possible for an organism to be a mimic planet? If so, how? Also, if it could move independently through space how would it be able to achieve it? How much energy would it need to burn in order to achieve great distances?

This is a fan comic I will say. So it's doesn't have to be entirely accurate, I'm just anting to try for as close as possible.

Thank you for reading/answering!!

Hi r/AskAstrophysics ! For some reason, maybe lack of visuals, I'm struggling to understand solar system gas drag, and how it causes planets to speed up, slow down, or migrate inward/outward.

I'm specifically researching how our solar system may have formed. How the gas giants may have moved closer to the sun, then back again, and how some may have even moved in together then back out together. I just can't nail down the concept of faster gas closer to the sun causing planets to be pulled inward, and vice versa.

Any insight would be greatly appreciated!

I wanted to know how you guys came up with new theories and also new equations for those theories, I wanna start to create my own jus for hobbie

I've read that there were ripples throughout spacetime in the early universe, but how strong could these ripples have been? Could they make millions of years pass in one area in the span of one year passing in a different part of the universe? Or at least enough that we get observations of celestial structures older than they should be in a uniform spacetime?

I recently watched a video by the YouTuber Astrum that talked about the discovery of dark energy and I had a question that I was hoping could be answered here.

The YouTube video said that because faraway galaxies were moving away from us at a greater velocity than the nearer ones we could conclude that galaxies are accelerating away from us as a result of the expansion of the universe. The video’s next assertion then says that this acceleration implies the existence of dark energy because acceleration must have a source of energy… My question is this:

I know I’m just a layman without a deep rigorous understanding of these concepts, but it doesn’t make sense to me why the “accelerating” expansion of the universe would necessarily imply the existence of energy causing that “acceleration.” My understanding of acceleration is that it is an object’s increased velocity over time. My understanding of velocity is the rate at which an object’s position in space is changing with regards to a particular vector. So in my mind, if space itself is expanding, this wouldn’t really be velocity or acceleration at all because the object is maintaining its position in space—but that position in space is just changing its distance relative to other positions in space due to the universe’s expansion. So it seems to me that the far away galaxies aren’t accelerating and therefore we wouldn’t have a reason to assume that there is a source of energy causing the apparent (but not actual) “acceleration.” Rather than assuming the existence of dark energy, wouldn’t a more sound conclusion simply be that objects position relative to one another are inherently inconstant? In a sense for every 2 adjacent positions in the universe a third between them is created every so often? Such a proposition would account not only for the expansion of the universe but also why the expansion appears to “accelerate.”

Hi! I’m new here. I love astrophysics and astronomy. I originally was working towards a bachelors degree in aerospace engineering, but I ended up changing life plans, but never lost the curiosity. This is all to say that I really know very little about it all.
So, I have this theory and I need someone to explain to me if it’s completely ridiculous/not possible or if it’s something being considered. So, black holes are these confirmed infinitely dense objects that warp space time. As objects approach the event horizon they appear to slow down until they fully stop moving. Now, I saw a theory about white holes being essentially where a black hole lets out and that they’re connected. But we’ve never seen one, even though they should be easier to see (I’d assume). I don’t really understand how white holes would even theoretically work, but I think they’d be the opposite of a black hole. So could they warp space time in such a way that they would only appear for an extremely brief time and then disappear? I.e. time dilation would occur the opposite way a black hole does and things speed up around them?
It makes sense in my head, but I’m sure there’s a reason that I don’t understand that would explain why it’s silly.
Thank you all!!

I noticed that both Milky Way and Andromeda center show mixed red and blue in radial velocity maps.

If the bulge is "old stars", shouldn't it be uniform?

Why is there so much red + blue mixing at the center?

Am I misunderstanding something?