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Watsisname
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30 Aug 2017 00:40

A-L-E-X wrote:
Source of the post Wat, is there some limited evidence for quark stars and cosmic strings and other topological defects left over from the BB?

I think it depends on who you ask.  I'm not aware of any well-established evidence for them, but every now and then certain people or groups claim to find something that looks like them.
 
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30 Aug 2017 00:40

Watsisname wrote:
A-L-E-X wrote:
Source of the post I would like to think that lifebearing planets possess a similar variety.

Me, too. :)

A-L-E-X wrote:
Source of the post Wat, so it seems like the most basic model of the universe has it expanding into itself?  So basically speaking, the universe creates its own space and time is just a function of how quickly it creates that space?

I try not to think of it as the universe creating space -- that provokes a whole nest of difficult questions like "how does it create space?" or "where does that new space come from?".  There aren't meaningful answers to those questions, nor does describing it that way make the math any easier.

Instead, I like to think of the universe like a grid.  The grid is just an imaginary thing you slap onto the space to mark locations, just like street addresses.  Then to a very good approximation, the locations of things in the large scale universe (like clusters of galaxies) are fixed on that grid, since they don't move much through the space.  Now the question we're interested in is how does that grid evolve with time?  If the grid expands, then the physical distance between two locations gets larger, even though neither location is actually moving.

General relativity (GR) describes how the shape of space (and space-time) is affected by what the space contains -- the matter and energy.  Here's where things get a little weird, and possibly even surprising if you've heard some common descriptions about how GR works.

Most people have heard of the rubber sheet or trampoline analogy for gravitation in GR.  As it often goes "an empty space is like a flat rubber sheet.  Add a big mass to it, and the sheet gets curved."  That's sort of true, especially on a local scale, but the surprising bit is that in cosmology, empty space is actually not flat.  We call that a Milne Universe, which is totally empty and has negative curvature, meaning straight parallel lines will spread apart and the sum of angles in a triangle is less than 180°. 

That's probably pretty unintuitive.  The one thing about it that is intuitive is that such a universe will have a constant expansion rate.  There isn't anything trying to pull things apart or back together again, so there's no acceleration.  If it started out expanding it will stay expanding at the same rate forever.  

Now let's suppose we fill the space uniformly with matter.  A little bit at first, and then more and more.  This will add a gravitating effect, where everything is pulling on everything else and therefore the universe is pulled back in on itself.  If the universe started out static (a stationary grid), then it will start to shrink and collapse as a Big Crunch.  If it started out expanding, then the expansion rate will slow down.  The space also gets less and less curved -- closer and closer to "flat".  Eventually we'll reach a point called the "critical density" of the universe, which happens to be just a few hydrogen atoms per cubic meter, and this makes the geometry perfectly flat (straight parallel lines remain parallel, and sum of angles in a triangle is exactly 180°).  

A universe filled only with matter and exactly enough of it to make it "flat" is called an Einstein-de Sitter universe.  We don't live in such a universe.
The universe we do live in has some matter in it (obviously), but it isn't enough to make the space flat.  However, we also know that the space is close to flat.  So there must be something else.  Part of it is dark matter, which is just like adding more regular matter.  But the even bigger part is the dark energy.

The dark energy is kind of like adding a bit of something with antigravity everywhere.  It's a property of the space itself which makes it expand more.  A universe filled only with dark energy, and just enough of it to make it flat, is a de Sitter universe, and it expands exponentially faster.

The universe we live in seems to be somewhere between the de Sitter and Einstein-de Sitter universe type.  It is flat with a mixture of matter (regular and dark) and dark energy.  It started out expanding after the Big Bang, slowed down at first due to the matter, but now speeds up due to the dark energy, since the effect of matter weakens as it gets diluted while dark energy does not.

Okay, that's probably a lot to digest.  What's the take-home message?  The point is that in GR, matter and energy don't just cause gravity, but they also affect the shape of space itself -- the shape of the grid I was talking about.  It also says that the size of the grid can change, and the way it changes depends on what the space contains.  Matter will try to slow it down by pulling things together.  Dark energy will try to speed it up by spreading everything apart.

A-L-E-X wrote:
Source of the post So trying to ascribe a speed to that expansion is like asking where the "center" of the universe is- the answer is undefined because in a universe which creates its own space, that point no longer exists!

It's more like the problem of comparing velocity with acceleration.  The units don't work.  There is definitely a "rate" to the expansion of the universe (e.g. it seems to be about 70 km/s/Mpc), but it is not a "speed".  It is a "speed per distance".  Something twice as far away recedes twice as fast.  Because space itself expands, the farther away two things are, the faster they move away, so you can't ascribe a unique speed to the whole thing.  Instead you ascribe a speed per distance to the whole thing, but that's incomparable to plain "speed" as in speed of light.

What making the expansion rate faster will do is cause the cosmological horizon to shrink closer to you.  Objects may slip past the horizon and lose all further causal contact with you.  This is what happened with inflation, when nearby regions of space that had been able to "talk" to each other were suddenly driven apart so quickly that the causal connection was lost, and now they are no longer in the same observable universe as each other.  This is what resolves the Horizon Problem, which asked how distant parts of the universe could look so remarkably similar, as if they had actually been in contact with each other and established thermal equilibrium.

Thanks, Wat, that sounds great deal like the w~-1 we were talking about earlier.  Would anti-de sitter space be if w>0?  The Mline universe you mentioned sounds a lot like a universe governed by the rules of inertia and the flat universe would behave according to Euclidean geometry.
It sounds like we're moving away from the idea of space as a physical entity, and it's more like how two given parts of the universe relate with each other.  It interesting how the  rate of expansion has varied throughout the history of the universe- first we had the inflationary period of rapid expansion and then the expansion slowed down and then it sped up again- and what happens in the future isn't precisely known yet.
The horizon problem you mentioned also tells us that areas beyond the observable universe appear to recede from us faster than light, therefore we won't be able to observe them.  As the universe keeps expanding, our little island in the universe will grow more isolated.  Thanks for your great explanation, it also helps show what terms such as "comoving distance" mean in terms of relating one point in the universe to another.  The rate of expansion being a function of the distance between two objects rather than the actual speed itself.
Last edited by A-L-E-X on 30 Aug 2017 00:53, edited 1 time in total.
 
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30 Aug 2017 00:46

It really does help to picture the universe as an expanding balloon.  But instead of the two dimensional surface of the balloon, think of a three dimensional surface.  As the balloon expands, it also does not create more "space" but rather just gets spread out more.  And the center of the balloon also is undefined as the balloon expands, it's easy to visualize that there is no center to the surface of the balloon.  The fourth dimension (time) is represented by the rate of expansion.  The "center" of the universe may have existed right at the BB but as the universe/balloon expanded its surface moved away from it, therefore what we could hypothetically consider the "center" of the universe existed in the distant past, but does no longer.
 
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30 Aug 2017 01:01

Watsisname wrote:
Source of the post So I don't think the Moon could ever have done much to protect Earth from asteroids

I think Jupiter's influence is larger, and that the gravity influence in the solar system has more to say than apparent size as seen from Earth.  Jupiter gives some protection from long period comets, but on the other hand asteroids can also get a Jovial nudge towards Earth, so it seems a bit complicated.
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30 Aug 2017 02:19

Jupiter is important because it influences a huge part of the solar system through the size of its Hill Sphere, but the Moon's Hill sphere is very small.  So for the Moon affecting whether things impact Earth it's basically just a matter of whether it gets directly in the way or not.
 
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30 Aug 2017 04:03

Watsisname wrote:
Jupiter is important because it influences a huge part of the solar system through the size of its Hill Sphere, but the Moon's Hill sphere is very small.  So for the Moon affecting whether things impact Earth it's basically just a matter of whether it gets directly in the way or not.

Wat, do you think it's very important to have outer solar system gas giants for complex life to occur in a given solar system or do you think it's possible with a solar system with different characteristics too?
 
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30 Aug 2017 07:41

JackDole wrote:
XBrain130 wrote:
Source of the post Are yousure? The console tells me "unknown comand"

'LandTo' only works up to SE 0.980. In SE 0.981, the commands are very different.
In SE 0.981 you have to enter something like this: 'Goto {Time 2.0 Height 200 Lat 63.069167 Lon -151.007778}'

Jack, does this apply to the patch "e" too?  I couldn't get it to recognize the LandTo function and with the patch installed all my saved locations are off :-( I need to redo the lat/long.  What is "Time"? and I assume "Height" is elevation above the surface in meters?

Is the FOV function changed too?  What about the single key functions like pressing <END> to straighten the horizon?
 
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30 Aug 2017 08:58

A-L-E-X wrote:
Source of the post does this apply to the patch "e" too?  I couldn't get it to recognize the LandTo function and with the patch installed all my saved locations are off :-( I need to redo the lat/long.  What is "Time"? and I assume "Height" is elevation above the surface in meters?

Is the FOV function changed too?  What about the single key functions like pressing <END> to straighten the horizon?

For me, LandTo and Fov work exactly as in SE 0.980. The END button also.
 
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30 Aug 2017 10:11

What if Jupiter formed in the place of Mars?
 
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30 Aug 2017 11:11

Starlight Glimmer wrote:
What if Jupiter formed in the place of Mars?

First that went through my head was that Earth would have much more meteors passing by. You know how Jupiter is just 'pulling' objects more than any other planet in our solar system. So, you want to say if Jupiter switched places with Mars at the beginning of the formation? Well, Jupiter would be much warmer than it is now. As it has the largest atmosphere than any other planet, it would possibly be even more warm than Mars on the same spot. Since Mars doesn't have such great atmosphere to keep the heat in.
Also, it would have big impact on Earth's history and formation. As I mentioned at first line, meteors would be passing by Earth much more frequently as Jupiter would sometimes 'slingshot' the passing objects into the other parts of Space. I am not sure what would happen if Jupiter comes closest to Earth. But, it would look big on the sky. Maybe it would be even second brightest object seen from Earth. But, it wouldn't be only bright at that time. It would be even at other time, I guess. But, there is SE to experiment with that, of course. :)
Anyway, maybe our orbit would change a little bit when Jupiter comes close to us? And not to mention that it would pull objects from Asteroid belt. Which would be even bigger problem for Earth

Oh, and I almost forgot about the Jupiter rings! Well, I don't have good answer about that. I think they would be pretty much the same as they aren't icy, but rather dusty.

In overall, it would have big impact on our Earth and Solar System. I hope I represented it good enough. If I missed something, tell me. :)
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30 Aug 2017 11:23

Marko S. wrote:
Starlight Glimmer wrote:
What if Jupiter formed in the place of Mars?

First that went through my head was that Earth would have much more meteors passing by. You know how Jupiter is just 'pulling' objects more than any other planet in our solar system. So, you want to say if Jupiter switched places with Mars at the beginning of the formation? Well, Jupiter would be much warmer than it is now. As it has the largest atmosphere than any other planet, it would possibly be even more warm than Mars on the same spot. Since Mars doesn't have such great atmosphere to keep the heat in.
Also, it would have big impact on Earth's history and formation. As I mentioned at first line, meteors would be passing by Earth much more frequently as Jupiter would sometimes 'slingshot' the passing objects into the other parts of Space. I am not sure what would happen if Jupiter comes closest to Earth. But, it would look big on the sky. Maybe it would be even second brightest object seen from Earth. But, it wouldn't be only bright at that time. It would be even at other time, I guess. But, there is SE to experiment with that, of course. :)
Anyway, maybe our orbit would change a little bit when Jupiter comes close to us? And not to mention that it would pull objects from Asteroid belt. Which would be even bigger problem for Earth

Oh, and I almost forgot about the Jupiter rings! Well, I don't have good answer about that. I think they would be pretty much the same as they aren't icy, but rather dusty.

In overall, it would have big impact on our Earth and Solar System. I hope I represented it good enough. If I missed something, tell me. :)

I assume Jupiters moons would be warmer too. I doubt they'd have life though. I'd assume it'd have more violent and active weather due to increased heat.
How big would Jupiter be in the sky at Opposition? (When Earth and it are closest together)
 
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30 Aug 2017 11:40

Starlight Glimmer wrote:
Marko S. wrote:
Starlight Glimmer wrote:
What if Jupiter formed in the place of Mars?

First that went through my head was that Earth would have much more meteors passing by. You know how Jupiter is just 'pulling' objects more than any other planet in our solar system. So, you want to say if Jupiter switched places with Mars at the beginning of the formation? Well, Jupiter would be much warmer than it is now. As it has the largest atmosphere than any other planet, it would possibly be even more warm than Mars on the same spot. Since Mars doesn't have such great atmosphere to keep the heat in.
Also, it would have big impact on Earth's history and formation. As I mentioned at first line, meteors would be passing by Earth much more frequently as Jupiter would sometimes 'slingshot' the passing objects into the other parts of Space. I am not sure what would happen if Jupiter comes closest to Earth. But, it would look big on the sky. Maybe it would be even second brightest object seen from Earth. But, it wouldn't be only bright at that time. It would be even at other time, I guess. But, there is SE to experiment with that, of course. :)
Anyway, maybe our orbit would change a little bit when Jupiter comes close to us? And not to mention that it would pull objects from Asteroid belt. Which would be even bigger problem for Earth

Oh, and I almost forgot about the Jupiter rings! Well, I don't have good answer about that. I think they would be pretty much the same as they aren't icy, but rather dusty.

In overall, it would have big impact on our Earth and Solar System. I hope I represented it good enough. If I missed something, tell me. :)

I assume Jupiters moons would be warmer too. I doubt they'd have life though. I'd assume it'd have more violent and active weather due to increased heat.
How big would Jupiter be in the sky at Opposition? (When Earth and it are closest together)

You are right! I forgot about the moons. Maybe life would be even more possible! Like, Europa would be interesting to explore! Plus it is much closer!
And, I mentioned in the post that it would probably be 2nd brightest object seen from Earth. But, if I would have to imagine it. I think it would look much, much bigger than just one dot. Maybe it would look like this with naked eye: Image
I am not sure, but probably bigger or smaller. As that is from 588 million kilometers away, and you still can see it clearly with amateur telescope, I have no doubts that it would look bigger than the picture I provided. And Mars is just 54.6 million kilometers away from us with the closest approach.
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30 Aug 2017 14:04

Yeah, I'd say that looks about right.  Maybe a little smaller.

Jupiter is about 139800km across, so its angular size on the sky if it were at 54.6 million km would be 8.8 arcminutes (compared to current maximum size of about 50 arcseconds).  In other words, it would get about 10 times larger than it does now.  

8.8 arcminutes is about 30% the size of the full moon at apogee (micromoon).  So compared to my shot of the last micromoon it would look like this:

Image

Pretty amazing... would be possible to see the bands on it with the unaided eye.  The Moons would still seem star-like points though.  Ganymede would have the largest angular diameter of 19.9 arcseconds, which is similar to Saturn.  The best the human eye can resolve is about 1 arcminute.
 
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30 Aug 2017 14:14

That is indeed amazing. I could imagine that in this alternate timeline that Jupiter or whatever the humans on that alternate earth my call it, will take interest in it. Incorporate it into their religions. I can imagine that humanity could possibly reach orbit and beyond more sooner due to enlightened interest. Ganymede could be similar to mars. 
 
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30 Aug 2017 19:26

Bit more accurate representation for how big it would look on the sky, in the sense that the Moon is like a pea held at arms length, and you can't quite see the craters on it.  But you could still (barely) spot the bands on Jupiter.

Image

Other interesting thing to remark on is its magnitude.  Jupiter appears about magnitude -2.7 at its brightest.  If we brought it 10 times closer to Earth, it would be 102 = 100 times brighter just by being larger on the sky.  But its surface brightness would also increase by a factor of 13, by being 3.6 times closer to the Sun.  So together this would change its magnitude by -2.5log10(100*13) = -7.8 magnitudes, or down to about -10.5 magnitude at its brightest.  That's pretty bright -- brighter than any Iridium flare and approaching that of the full moon! (Jupiter's clouds make it more reflective than the Moon which is why it is still so bright despite being apparently smaller and farther from the Sun).  

It would be a commanding presence in the night sky when the Moon isn't around, and plainly visible in broad daylight as well.

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