Press J to jump to the feed. Press question mark to learn the rest of the keyboard shortcuts
1

Hey I'm not sure if this is the place to post for this but I'm getting very desperate. As a sort of trial problem with my advisor, I was asked to solve for the back-reaction of the helical breather cosmic string, which I've never seen done out but is sometimes used as a test case for other stuff. I keep getting divergences in some of my back-reaction forces though... even though they agree with all of our other predictions! Is there some simple physical phenomena I am neglecting, like "Infinity oscillatory objects' back reactions are infinite" or something? It's been five months and I'm out of ideas.

1
comment
5

Hi ! I am a second year physics undergraduate and have some background in introductory cosmology. I've read Andrew Liddle's introductory cosmology book. Currently I am reading Observational Cosmology by Stephen Sergeant. I couldn't understand the part where he discusses the CMB power spectrum , CMB anisotropies and Baryonic Acoustic Oscillations. Can you please suggest some resources which will help me understand CMB power spectrum in a better way .

5
2 comments
10

I have read that eventually, the andreomeda galaxy will merge and collide with ours because it is moving toward us. Is this true? And if so, why is andromeda moving towards us if universal expansion is causing further separation between us and basically everything else?

10
28 comments
5
  1. It is said that universe is X billion YEARS old.
  2. YEAR is measured from Earth rotating around Sun (both of them formed long after big bang)
  3. Beginning of universe was coincident with beginning of time itself, which means time is not necessarily a linear thing (debatable - meaning the rate that time passed was not necessarily constant).
  4. Then what does it mean to say universe is X billion "YEARS" old?

Can some educated mind shine some light on this matter?

PS: Keep in mind time is also relative to the observer, which just makes this question much more complicated than it reads I guess...

5
11 comments
0

I recently read an article talking about the cryovolcanism on Ceres, the article mentioned that Pluto is colder than Ceres. How is this possible if Pluto is closer to the sun?

0
7 comments
12

I know "force" isn't the right word but I don't know what else to call it. Dark energy seems more akin to gravity than matter. But we can't exactly say the universe is "made" of gravity, can we? So how can scientists say the universe is "made" of dark energy?

12
34 comments
22

Is there any evidence that can prove or disprove the theory that only our part of the universe is expanding while other parts may be sitting relatively still or even getting smaller? Maybe we are in what is kind of like a bubble in a boiling pot where we are in one of the expanding bubbles and we cant see the other parts that are not because this part is too big for us to see the end of. Sorry this is so long but I just want to make sure that i am not misunderstood.

22
22 comments
10

I've been wondering about how relativity is taken into account when calculating the expected diameter of a galaxy based on its angular velocity, as you often see in these articles about Dark Matter. I am by no means an expert, but understand the basics and am looking for someone who knows their stuff to answer this.

So here's my question: If a galaxy is say, tens of thousands of light years wide, that means a point on the edge of the galaxy will feel the gravitational effect of distant parts of the galaxy as they were tens of thousands of years ago. I would expect this effect to be greater than the present gravitational influence of those distant parts because the stars there have converted some mass to energy during the intervening time.

However, an observer from Earth would see light from all parts of the galaxy at approximately the same time, so the excess gravitation due to mass 'from the past' would appear to us as an anomaly. In addition, the stars in the galaxy are actively rotating and possibly moving through space, so I would imagine that the distribution of gravitational influence on a point mass across the disk of the galaxy would be interesting when taking relativity into account.

I'm wondering how the above point about mass 'from the past' exerting force on a point mass is taken into account in current calculations, if at all? I'm not sure if it is a significant phenomena, or even how to search for studies on this. Like I said, I'm not an expert on this subject matter but this has piqued my curiosity recently. Thanks!

10
7 comments
6

I've been thinking about inflation and I'm not fully grasping why it is so necessary.

I'm hoping someone with some knowledge and patients can bring me up to speed.

If the universe could still plausibly be far far larger than what is visible to us, Is it un-useful to think about everything being the size of a grain of sand?

Is it better to think of an amazingly dense state that becomes less dense?

Is it misleading to think of it expanding from a point?

Does the universe have to expand amazingly quickly PURELY because of the uniform temperature of the cosmic microwave background, or is there another reason?

6
14 comments
17

Any suggestions? Personal favorites?

17
21 comments
Community Details

23.4k

Subscribers

37

Online

**/r/cosmology** - a community for questions, discussions, and articles about cosmology.

Create Post
Cookies help us deliver our Services. By using our Services or clicking I agree, you agree to our use of cookies. Learn More.