Living between the Galaxies (Dark Energy)

[Jetcru]

In the series "The Universe" Michio Kaku made a statement at the close that, as time progresses and the Universe continues to expand, that our sky would grow dark and depopulated of stars (at least that was the video graphic that accompanied his statement about the lonely end of our universe). In fact, if I understood everything that I was hearing in the segment, wouldn't that be in error? That is, are not the galaxies themselves staying tight and fairly static (if not completely static), and isn't it that it is primarily the galaxies that are becoming more distant from one an other as dark energy expands the universe? Would we not see our skies staying relatively constant but with the continued fading in brightness of the other galaxies as seen in our skies?

Also If I were standing between galaxies and looking in a given direction would I not just see other galaxies with only black space between them, i.e. no individual stars.

 

[Chalnoth]

In the series "The Universe" Michio Kaku made a statement at the close that, as time progresses and the Universe continues to expand, that our sky would grow dark and depopulated of stars (at least that was the video graphic that accompanied his statement about the lonely end of our universe). In fact, if I understood everything that I was hearing in the segment, wouldn't that be in error? That is, are not the galaxies themselves staying tight and fairly static (if not completely static), and isn't it that it is primarily the galaxies that are becoming more distant from one an other as dark energy expands the universe? Would we not see our skies staying relatively constant but with the continued fading in brightness of the other galaxies as seen in our skies?

Also If I were standing between galaxies and looking in a given direction would I not just see other galaxies with only black space between them, i.e. no individual stars.

Yes, you are correct.

However, over periods of billions of years, our own galaxy will change significantly as it merges with other galaxies.

 

[PhanthomJay]

Also If I were standing between galaxies and looking in a given direction would I not just see other galaxies with only black space between them, i.e. no individual stars.

Well, when you look at say the Andromeda galaxy, you see stars of our own Milky Way galaxy in between. EDIT: That was silly, i didn't read your question correctly, sorrry.

Yes, you are correct.

However, over periods of billions of years, our own galaxy will change significantly as it merges with other galaxies.

Chalnoth, you are an expert on this, so let me ask a question: If our spacetime Universe is essentially flat or open and spacetime expands forever due to Dark Energy, then will not all other galaxies ultimately have receded beyond the observable universe, and we will see nothing except the stars of our own galaxy, which, ultimately 'ultimately', will also recede when dark energy overcomes the gravitational pull of our galaxy cluster, and we see only black space, void of everything except perhaps virtual particles? Is that correct?

 

[Chalnoth]

Chalnoth, you are an expert on this, so let me ask a question: If our spacetime Universe is essentially flat or open and spacetime expands forever due to Dark Energy, then will not all other galaxies ultimately have receded beyond the observable universe, and we will see nothing except the stars of our own galaxy, which, ultimately 'ultimately', will also recede when dark energy overcomes the gravitational pull of our galaxy cluster, and we see only black space, void of everything except perhaps virtual particles? Is that correct?

Well, any gravitationally-bound structure should remain gravitationally-bound. So things like clusters of galaxies also remain together, not just galaxies. I'm not very sure at the present time how much of the nearby universe we are gravitationally-bound to, or will be in the future.

That said, what we will observe from our perspective is galaxies getting more and more redshifted. None of the galaxies we can observe today will ever become completely invisible, but the light coming from them will get redder and redder with time. And the way in which the light from them redshifts will ensure that we will never obtain any light from them that was emitted after they passed our horizon. As time moves forward, we will just get the image from them as they got closer to that horizon.

In this way, the light coming from these far-away galaxies is rather similar to the light of an object falling into a black hole. An observer outside the black hole never sees the object enter the black hole. He just sees it getting closer and closer to the event horizon, redshifting more and more.

In the limit of future infinity, this light redshifts to non-existence, so in that limit the universe is empty except for vacuum fluctuations.

 

[PhanthomJay]

In the limit of future infinity, this light redshifts to non-existence, so in that limit the universe is empty except for vacuum fluctuations.

Thank you again for your always excellent responses! But regarding the above quote, it would appear that the universe would in the limit be empty, except, from our view, the gravitationally bound clusters of our galaxy and nearby galaxies, would make our view bright, not dark, unless our galaxy and our neighboring cluster galaxies condense into one supermassive black hole??

 

[Chalnoth]

Thank you again for your always excellent responses! But regarding the above quote, it would appear that the universe would in the limit be empty, except, from our view, the gravitationally bound clusters of our galaxy and nearby galaxies, would make our view bright, not dark, unless our galaxy and our neighboring cluster galaxies condense into one supermassive black hole??

Well, for that sort of stuff, this Wikipedia article is good:
http://en.wikipedia.org/wiki/Future_of_an_expanding_universe

I'd have to run the numbers to see whether or not the light redshifting to the point it's basically undetectable happens before or after the black hole era. I don't know.

 

[PhanthomJay]

Chalnoth:

Very interesting article, thank you once again! The Universe is sure in its infancy when I look at those ridiculously high number of years it will take for the Universe to 'die'.

I have always speculated that the only way that this universe, or others, makes any sense at all, is that if all that is, and all that is not, are one and the same... Everything and nothing, one and the same... Infinity and Zero, one and the same.... But this is pure conjecture on my part...is there anything in the literature that gives any credence to this ?(I apologize if I have over speculated, so that is why I am putting this in the form of a question).

 

[Chalnoth]

Chalnoth:

Very interesting article, thank you once again! The Universe is sure in its infancy when I look at those ridiculously high number of years it will take for the Universe to 'die'.

I have always speculated that the only way that this universe, or others, makes any sense at all, is that if all that is, and all that is not, are one and the same... Everything and nothing, one and the same... Infinity and Zero, one and the same.... But this is pure conjecture on my part...is there anything in the literature that gives any credence to this ?(I apologize if I have over speculated, so that is why I am putting this in the form of a question).

Well, infinity and zero are definitely not the same thing. At the very least, there is only one zero, but there are two infinities (positive and negative) on the extended real number line. And there are also different classifications of infinities (countable and uncountable), but still just one type of zero.

As for everything and nothing being the same thing, well, maybe kinda sorta, but only if you are very careful about what you mean. If the laws of physics are unitary, after all, then our universe at one time is just a different configuration of the same physical system at a different time. So the future completely-empty state is just a different configuration of the current state with lots of stuff. If the laws of physics are unitary.

 

[PhanthomJay]

As for everything and nothing being the same thing, well, maybe kinda sorta, but only if you are very careful about what you mean. If the laws of physics are unitary, after all, then our universe at one time is just a different configuration of the same physical system at a different time. So the future completely-empty state is just a different configuration of the current state with lots of stuff. If the laws of physics are unitary.

Awesome! Many many thanks again for your expertise in this area and for your well written and perfectly clear responses!

 

[Jetcru]

Thank you Chalnoth for the reply! You cleared up my question and added a new bit of knowledge, i.e. bound galaxies, which I did not know about or think about. It would seem though that if there are galazies are gravitationally bound then wouldn't the distance to that galazy be shrinking and not expanding. Have they measured such shinking distances between galaxies? It certainly makes sense that there could beand that the speed of closing between that galazy and ours would have to be greater than the rate of expansion. But that would mean blue shifted galaxies, No? I am going to have to read up on this!

Anyway thank you again for sharing your knowledge on this subject.

 

[Chalnoth]

Thank you Chalnoth for the reply! You cleared up my question and added a new bit of knowledge, i.e. bound galaxies, which I did not know about or think about. It would seem though that if there are galazies are gravitationally bound then wouldn't the distance to that galazy be shrinking and not expanding. Have they measured such shinking distances between galaxies?

Well, not necessarily shrinking. Gravitationally-bound objects are in mutual orbits. At any given time, two gravitationally-bound objects may be moving towards or away, but don't have a tendency over time to do either (provided we can neglect friction).

But yes, some nearby galaxies are blue-shifted. Such as Andromeda, which will collide with our galaxy in a few billion years. As you go further away, though, the overall expansion effect becomes much greater than local velocities, and all galaxies are redshifted. It's more difficult to measure the relative velocities between far away galaxies, but when we see objects like the coma cluster, we're pretty confident that those galaxies are in mutual orbits:
http://www.astronet.ru/db/xware/msg/1222246/comacluster_spitzer_0.jpg.html

This view is backed up by weak lensing of such objects that demonstrate that these collections galaxies are associated with massive concentrations of dark matter.

 

[Chronos]

Under the big rip scenario, dark energy will eventually overwhelm gravity. The local group will become gravitationally unbound at that point.

 

[Chalnoth]

Under the big rip scenario, dark energy will eventually overwhelm gravity. The local group will become gravitationally unbound at that point.

Yes, but that is manifestly unlikely, as it violates the weak energy condition of General Relativity (which states that the matter density is non-negative).