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enioguedes
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30 Oct 2017 13:27

I would like to know if the atmosphere of a world similar to Titan with more methane would still be opaque (due to the production of the organic compounds in upper atmosphere) or if it would be more transparent (less hazy), since red dwarf stars produce much less UV light, except during the flares. I don't know, but maybe only during the flares the atmosphere of such worlds would turn to be opaque and orange (the UV and X-rays produced by the flare would break methane, with a descrease in surface temperature, due to the haze blocking sunlight), and when the flare period finish, the sky turn to be bluish and transparent again slowly, increasing surface temperature. Maybe in such worlds orbiting red dwarves the flare period plays an inportant role in both development and sustaining of exotic life.

Someone have any scenario suggestion?
 
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31 Oct 2017 14:54

If you were on the moon, and you held out your finger to block the sun, wouldn't you be able to see the corona because there is no bright atmosphere to block it?
"Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space." - Douglas Adams
 
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Watsisname
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31 Oct 2017 23:07

Hornblower, maybe, but I think it would be very difficult.  Not having the atmosphere certainly removes part of the challenge of seeing it, but another factor is dark adaption, which is difficult when you are standing on the brightly lit lunar surface and also have the sunlight shining in your helmet.  It's the same reason the astronauts had a hard time seeing stars.

So even on the airless Moon you'll still be combating two challenges -- that the brightest part of the corona is very close to the Sun itself, and the illumination by the surroundings will hinder your ability to see the dimmer extended corona.
 
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midtskogen
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01 Nov 2017 00:17

I think the space is in the way of making such an observation. But the corona was seen on Apollo 11, not from the surface, though.
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01 Nov 2017 00:31

Mouthwash, Doc and I have been talking about this a bit and we both conclude that a universe with a reversed second law would simply be equivalent to reversing time.  

A key example is that of a black hole, which is the maximal entropy state for any system.  For entropy to decrease, the black hole must be replaced by a white hole, which is a time reversal.

The second law says that a box filled with air quickly achieves an equilibrium where the air is uniformly distributed.  If you have the air all on one side blocked by a barrier, opening the barrier quickly results in the air filling the box.  It does this because there is a vastly greater number of ways that particles can be arranged uniformly through the whole box, rather than all bunched up on one side, so the system naturally evolves to the higher entropy state.

In a universe where entropy of a closed system decreases, then this implies a box filled with air will naturally evolve to having the air bunched up into highly structured states.  This would appear very strange, as if complex things arise for no reason.  This rise of complexity does occur in our universe with time going forwards, but it is allowed by an increase in entropy elsewhere.  For instance, the complexity of life on Earth is due to the hot Sun providing energy in the form of low-entropy sunlight, and a cold dark sky acting as a sink for the high-entropy energy to flow out.  Life is like an eddy in a stream, where some water can locally move upstream while the bulk of the flow moves downward.  

Reversing the increase of entropy for the Earth system would be like having infrared energy coming in from the surrounding space, driving processes on Earth, and then being shot out as low-entropy sunlight directly into the Sun, which then flows to the core to drive fusion processes in reverse.  Very strange.

Could a universe exist where the entropy stays constant globally?  Yes.  It would have to be completely lacking in evolution of structure.  No stars, no galaxies, no life.  All "irreversible" processes increase entropy by turning useful energy into waste heat.  To avoid that requires a universe with only purely reversible processes.  The expansion of the universe itself is isentropic (does not change the entropy), but not when you include matter which can collapse under gravity and do things.  Gravitational collapse of matter into structures increases entropy.

So in a nutshell: a universe where entropy decreases requires reversing time, and a universe with no entropy change would be very boring.
 
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midtskogen
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01 Nov 2017 01:00

Does a mirror inside a black hole reverse time (and entropy)? :)
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Watsisname
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01 Nov 2017 01:52

A stationary mirror cannot exist inside a black hole.  Or if you held out a mirror while falling inside of a black hole, hoping to reflect some light outward, the reflected light is still going inward. :)

Shorter answer: definitely not. :P
 
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midtskogen
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03 Nov 2017 10:34

Has it been investigated whether the acceleration of the expansion of the universe can be explained by an expanding bulk (of which spacetime is an intersection)?  Such a hypothesis may raise more questions than it gives answers and it may be even harder to test than dark energy, but, yes, I don't like dark energy and would like to see it go. :)
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Watsisname
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04 Nov 2017 04:48

A nice bit of trivia is that the evolution of a closed universe can be described as a point traced out by a rolling circle, i.e., it is a cycloid.  Not just superficially but exactly, as the mathematics works out to precisely the same form of equation.  So one might be inspired to investigate models of the evolution of universes by the motions of higher dimensional shapes.  Or as you propose, a higher dimensional bulk to explain the accelerating expansion.

It is certainly possible, and there is no shortage of people proposing ideas.  But I'm not aware of any bulk hypothesis developed into a working cosmological model.

A working model must of course make specific predictions which are consistent with observations. This can occur naturally, as like how general relativity naturally predicts the perihelion precession of Mercury's orbit.  Or by design, as like how epicycles were designed to predict retrograde motion.  The current Lambda-CDM cosmological model predicts acceleration by design, and goes on to naturally produce other observational tests as well, which have been successful across a wide range of scales -- and furthermore continues to be favored with more data.  The challenge for any new model is to meet or exceed Lambda-CDM's predictive power.  Predict the same observations to the same or greater precision, and better yet predict new observations better than Lambda-CDM.
 
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midtskogen
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04 Nov 2017 15:25

When there's no shortage of ideas, it gets more likely that some of them turn out to be consistent with observations, yet they may be wrong.  But even if it turns out that dark energy is wrong, we might get stuck with the term.  Just as we keep talking about gravity as a force, because it's easier to think of it that way.  And dark energy might be easier to think about than the more correct cause.
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04 Nov 2017 23:59

Just so.  I think Newtonian gravity is a good analogy.  Newton's gravitational force is a magic -- a magic that acts instantly at a distance.  It makes predictions consistent with many observations, but it is physically unsatisfying.  It explains what the magic does, but not what it is or why it acts that way.  Einstein's relativity has better predictive skill and also explains the origin of gravitation, but it's more computationally and conceptually challenging.  We're all happy to still use Newton in most cases.

Similarly, using the cosmological constant to model the accelerating expansion provides no physical explanation.  It's a mathematical fudge that causes acceleration because of magic.  We can interpret it as a negative pressure, or vacuum energy, but nobody really knows and the best attempts at deriving its energy density from quantum field theory lead to enormously wrong predictions.  

So there is definitely some serious gap in our knowledge begging to be filled.  Someday a new model may arise which not only makes better predictions but also provides a satisfying physical explanation.  If so, Lambda-CDM will still be a good model of cosmology, and many may still use it if is less challenging than the newer model.
 
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Salvo
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07 Nov 2017 07:47

The thing that makes everything harder is time. I hate time, it's just a giant mess!
I mean... why is everything moving trough the dimension of "time" all the time? Why does everything seems to go in the same direction?

It seems like more you're inside a curvature of space more you move faster inside time. Actually, you also move faster inside space, so it makes sense...? Well, if you was in complete void inside intergalactic space and no gravity source from any direction you should stay still in time too, right? Probably not, instead the illusion is that parts of the universe that contains no visible matter are the ones that expands faster, and they expand faster and faster more is the void that they contain. Probably because they mainly contain dark energy so there is not an opposite force from baryonic matter's curvature of space that opposes the "negative pressure" of dark energy.

It is just so complicated, but I think it's just a matter of time. As soon as we'll fully understand how time works (if we'll ever will) I'm quite sure we'll understand what's wrong with expansion of the universe.

Sorry, I just finished reading this. I hope it's a normal consequence.
The universe is not required to be in perfect harmony with human ambition.

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Watsisname
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07 Nov 2017 18:24

I don't think time itself is too complicated.   It's just counter-intuitive! :)  Here is essentially all you need to know about how time works:

► The Relativity of Time



Salvo wrote:
Source of the post I mean... why is everything moving trough the dimension of "time" all the time? Why does everything seems to go in the same direction?

This is a surprisingly deep question, often called "the arrow of time".

► The Arrow of Time
 
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the photo guy
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13 Nov 2017 20:01

Hey this may be a strange question but is there anyway of telling where the location of the big bang started?
I don't know a whole lot about the big bang but if there was supposedly a universal size of darkness and nothing ness before the big bang, then where is it believed to have come from? the center? behind you? Thank you and have a great day :D
HI! I would like to say that I'm just a friendly teen who loves talking and helping people out :) Space engine is truly a wonderful program and the website is also very cool! Many great people here. I hope to make some good friends here.
 
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Watsisname
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13 Nov 2017 21:39

A deep cosmological principle is that the universe is the same density everywhere (on sufficiently large scales), and this was also true at the moment of the Big Bang.  This means at the Big Bang, the density was infinite not at just one point within space, but at every point everywhere in space, so literally every point in space was at the Big Bang.  

In other words, the Big Bang happened everywhere. :)

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