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Old 13th January 2020, 11:11 AM   #1
Red Baron Farms
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Cool Resist the Accelerating Expansion of the Universe?

An Advanced Civilization Could Resist the Accelerating Expansion of the Universe And Earth-bound astronomers should be able to tell if someone is out there doing it
AKA giant fleets of Dyson sphere space ships powered by suns.

Wow, this one has all my BS detectors ringing at full alert. Did Massachusetts just legalize Marijuana? Or mushrooms? Or am I wrong? My astrophysics is just not strong enough to vet this, but I don't want to make the mistake of argumentum ad incredulum.

So are there any good physicists left on the board who can help vet this for me?
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Old 13th January 2020, 11:16 AM   #2
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What are the actual claims from the article?

What method does the researcher propose, for resisting expansion?

What method does the researcher propose, for detecting such a resistance?

The article requires login for me to read it, which isn't going to happen. I think it's entirely reasonable to expect you to provide at least some details of the claim, in the OP.
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Old 13th January 2020, 11:19 AM   #3
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Originally Posted by theprestige View Post
What are the actual claims from the article?

What method does the researcher propose, for resisting expansion?

What method does the researcher propose, for detecting such a resistance?

The article requires login for me to read it, which isn't going to happen. I think it's entirely reasonable to expect you to provide at least some details of the claim, in the OP.
Apparently it's some sort of Dyson sphere where the waste infrared heat is directed to provide thrust. It's from MIT but based on the work of Dan Hooper, a particle physicist at the Fermi National Accelerator Laboratory in Batavia, Illinois.
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Old 13th January 2020, 11:38 AM   #4
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Interesting.

My understanding is that the expansion is weaker than gravity, and that it's trivially resisted by any gravitationally-bound structure. Everything from solar systems to entire galaxies to galactic clusters all resist expansion. Only in the vast reaches of spacetime between gravity structures do we see the effects of expansion.

So, two points of contention I have about the claim (as summarized here):

First, it would only apply to gravitationally-discrete structures in an expanding region of spacetime. A star system that is already gravitationally unbound from the next nearest structure, and should be getting further away from that structure due to expansion.

But it seems weird to expect a Dyson-sphere-wielding civilization to find itself that many hundreds of thousands (millions?) of light years from the nearest blob of gravitationally-bound stuff. How did they get that far out? Were they separated from their home blob during the early moments after the Big Bang, and have been trying to get back there ever since? Are they on a billion-year journey to the next-nearest blob of stuff, and they're hoping to get there before expansion puts it out of reach forever?

I guess searching the vast empty reaches of spacetime for individual stars moving the wrong way might be easier than looking into the vast galactic clusters.

Two, assuming the most likely place to find Dyson civs is somewhere inside a mass-rich galactic cluster, anomalous star movement wouldn't be resisting expansion. It would be resisting the gravitational influences of the cluster.

Kind of like how galactic rotation curves don't contradict expansion. They contradict the gravitic influence of the visible matter in the galaxy. A Dyson sphere traversing a galactic limb in an anti-gravitic direction isn't resisting expansion. The overall gravity-binding of the structure (the galactic cluster) shields it from expansion effects. The thrust generated by the Dyson sphere is shielding it from gravity effects.
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Old 13th January 2020, 12:05 PM   #5
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Originally Posted by theprestige View Post
Interesting.

My understanding is that the expansion is weaker than gravity, and that it's trivially resisted by any gravitationally-bound structure. Everything from solar systems to entire galaxies to galactic clusters all resist expansion. Only in the vast reaches of spacetime between gravity structures do we see the effects of expansion.

So, two points of contention I have about the claim (as summarized here):

First, it would only apply to gravitationally-discrete structures in an expanding region of spacetime. A star system that is already gravitationally unbound from the next nearest structure, and should be getting further away from that structure due to expansion.

But it seems weird to expect a Dyson-sphere-wielding civilization to find itself that many hundreds of thousands (millions?) of light years from the nearest blob of gravitationally-bound stuff. How did they get that far out? Were they separated from their home blob during the early moments after the Big Bang, and have been trying to get back there ever since? Are they on a billion-year journey to the next-nearest blob of stuff, and they're hoping to get there before expansion puts it out of reach forever?

I guess searching the vast empty reaches of spacetime for individual stars moving the wrong way might be easier than looking into the vast galactic clusters.

Two, assuming the most likely place to find Dyson civs is somewhere inside a mass-rich galactic cluster, anomalous star movement wouldn't be resisting expansion. It would be resisting the gravitational influences of the cluster.

Kind of like how galactic rotation curves don't contradict expansion. They contradict the gravitic influence of the visible matter in the galaxy. A Dyson sphere traversing a galactic limb in an anti-gravitic direction isn't resisting expansion. The overall gravity-binding of the structure (the galactic cluster) shields it from expansion effects. The thrust generated by the Dyson sphere is shielding it from gravity effects.
Quite. And why would advanced civilizations resist expansion? Really?

And how would we detect some system doing it? Does the person publishing this idea think we can track individual planet systems beyond a few dozen light-years?

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Old 13th January 2020, 12:23 PM   #6
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Originally Posted by MRC_Hans View Post
Quite. And why would advanced civilizations resist expansion? Really?

And how would we detect some system doing it? Does the person publishing this idea think we can track individual planet systems beyond a few dozen light-years?

Hans
We can track the trajectories of individual stars out to some ridiculous (trans-galactic) distance.

Presumably a Dyson Sphere would have black-body radiation on the order of the star it enclosed, and would be visible and trackable as such, out to however many light years we can track stars.

So it boils down to following a star-sized energy signature that moves counter to the direction of gravity or expansion.
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Old 13th January 2020, 12:28 PM   #7
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Originally Posted by MRC_Hans View Post
Quite. And why would advanced civilizations resist expansion? Really?

And how would we detect some system doing it? Does the person publishing this idea think we can track individual planet systems beyond a few dozen light-years?

Hans
Quote:
Consequently, the Local Group will be humanity’s home for the foreseeable future. Over billions of years, we might even colonize it, hopping from one star system to another and exploiting each sun’s energy along the way.
However, the accelerating expansion of the universe is sending galaxies over the horizon at a rate that is increasing. “As a result, over the next approximately 100 billion years, all stars residing beyond the Local Group will fall beyond the cosmic horizon and become not only unobservable, but entirely inaccessible,” says Hooper.
That’s a problem for an advanced civilization because it limits the number of new stars that are available to exploit.
So the question that Hooper investigates is whether there is anything an advanced civilization can do to mitigate the effects of this accelerating expansion.
and he claims we could detect it this way:
Quote:
There is a measurable prediction from Hooper’s work. If advanced civilizations have already begun this star-gathering process, it should be observable to astronomers. “Such a civilization could appear as a region up to tens of megaparsecs in radius in which most or all of the stars lighter than [two solar masses] are surrounded by Dyson Spheres,” he concludes.
So of course part of my BS meter goes off because we haven't even detected a single Dyson sphere and now he is speculating billions of them in some massive voyage as a group? Or would that be easier to spot than single ones?

Then of course is the stability problem of accelerating the Dyson sphere in any direction with waste heat thrust. It seems to me all that would do is crash the Dyson sphere into its sun, not actually move the sun.

and a whole lot of other problems that makes my head hurt even considering them... like would there be enough stars of this size capable of breaking away from their own Galaxies? or would this action drag the other stars in that galaxy along with them? And what about the group's super massive black holes at each galaxy's center?
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Old 13th January 2020, 12:49 PM   #8
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Originally Posted by theprestige View Post
First, it would only apply to gravitationally-discrete structures in an expanding region of spacetime.
Originally Posted by MRC_Hans View Post
Does the person publishing this idea think we can track individual planet systems beyond a few dozen light-years?

Note the mention of "tens of megaparsecs" in RBF's latest post. Apparently this about a civilization spanning a galactic cluster. (I haven't read the article itself either).
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Old 13th January 2020, 12:57 PM   #9
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This is getting almost tautological. And therefore uninteresting.

Yes, if we noticed that a substantial number of stars in a galactic cluster were moving together, counter to the gravity of the cluster, and counter to the flow of expansion, we'd be right to infer some artificial phenomenon. MIT, ladies and gentlemen!
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Old 13th January 2020, 01:11 PM   #10
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Originally Posted by theprestige View Post
We can track the trajectories of individual stars out to some ridiculous (trans-galactic) distance.
Only very prominent stars. Your plain vanilla yellow dwarf (=Sun-like) star fades into the crowd very quickly. But those are the ones likely to have inhabited planets.

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Old 13th January 2020, 01:21 PM   #11
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Originally Posted by MRC_Hans View Post
Only very prominent stars. Your plain vanilla yellow dwarf (=Sun-like) star fades into the crowd very quickly. But those are the ones likely to have inhabited planets.
I think we're talking past each other. Civilizations that can build Dyson spheres are on a level way beyond such petty concerns as habitable planets.
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Old 13th January 2020, 01:28 PM   #12
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Originally Posted by Red Baron Farms View Post
Then of course is the stability problem of accelerating the Dyson sphere in any direction with waste heat thrust. It seems to me all that would do is crash the Dyson sphere into its sun, not actually move the sun.

There's all kinds of ways to couple the "sphere" to the sun, such as "reflecting" the solar wind on side of the star. And keep in mind that most people who actually think about this stuff don't mean the "sphere" concept that shows up a lot in science fiction, they usually mean a Dyson Swarm.
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Old 13th January 2020, 01:42 PM   #13
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Originally Posted by theprestige View Post
Interesting.

My understanding is that the expansion is weaker than gravity, and that it's trivially resisted by any gravitationally-bound structure. Everything from solar systems to entire galaxies to galactic clusters all resist expansion. Only in the vast reaches of spacetime between gravity structures do we see the effects of expansion.

So, two points of contention I have about the claim (as summarized here):

First, it would only apply to gravitationally-discrete structures in an expanding region of spacetime. A star system that is already gravitationally unbound from the next nearest structure, and should be getting further away from that structure due to expansion.

But it seems weird to expect a Dyson-sphere-wielding civilization to find itself that many hundreds of thousands (millions?) of light years from the nearest blob of gravitationally-bound stuff. How did they get that far out? Were they separated from their home blob during the early moments after the Big Bang, and have been trying to get back there ever since? Are they on a billion-year journey to the next-nearest blob of stuff, and they're hoping to get there before expansion puts it out of reach forever?

I guess searching the vast empty reaches of spacetime for individual stars moving the wrong way might be easier than looking into the vast galactic clusters.

Two, assuming the most likely place to find Dyson civs is somewhere inside a mass-rich galactic cluster, anomalous star movement wouldn't be resisting expansion. It would be resisting the gravitational influences of the cluster.

Kind of like how galactic rotation curves don't contradict expansion. They contradict the gravitic influence of the visible matter in the galaxy. A Dyson sphere traversing a galactic limb in an anti-gravitic direction isn't resisting expansion. The overall gravity-binding of the structure (the galactic cluster) shields it from expansion effects. The thrust generated by the Dyson sphere is shielding it from gravity effects.
We're going to need a bigger boat, right?
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Old 13th January 2020, 02:10 PM   #14
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Or a smaller one.
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Old 13th January 2020, 08:08 PM   #15
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Originally Posted by Red Baron Farms View Post
A small issue is that the 13 Jun 2018 pre-print with this idea from Dan Hooper has not been published after 18 months. It is listed as "Report number: FERMILAB-PUB-18-254-A". That hints that he, colleagues or peer reviewers have found a problem with the idea.
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Old 13th January 2020, 08:24 PM   #16
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Originally Posted by Reality Check View Post
A small issue is that the 13 Jun 2018 pre-print with this idea from Dan Hooper has not been published after 18 months. It is listed as "Report number: FERMILAB-PUB-18-254-A". That hints that he, colleagues or peer reviewers have found a problem with the idea.
Really? I would have guessed this was just low priority.
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Old 13th January 2020, 09:50 PM   #17
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Boy will they have red faces when they discover that Dark Energy might not even be real in the first place!

https://www.space.com/dark-energy-not-debunked.html

Quote:
"Taken at face values, our result suggests that ~100% of the evidence for dark energy simply goes away. This will be confirmed by future observations," Lee and co-author Yijung Kang told Space.com in an email.

To come to such a bold conclusion, the researchers observed the spectra — bands of colors that can be produced when matter interacts with or emits electromagnetic radiation — of the stars in nearby galaxies that host these supernovas. Studying the light coming from these galaxies helped them to determine the ages of the stars in those galaxies.

The team found what they reported as a significant correlation between the luminosity of these supernovas and the ages of the stars in these galaxies. They found that supernovas in younger galaxies are fainter than in older galaxies, which would upend the assumption that supernova luminosity doesn't evolve over cosmic time. So, because they found fault with one of the main pieces of evidence for dark energy, they concluded that there is a likelihood that dark energy may not exist at all.
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Old 14th January 2020, 12:25 AM   #18
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Originally Posted by theprestige View Post
This is getting almost tautological. And therefore uninteresting.

Yes, if we noticed that a substantial number of stars in a galactic cluster were moving together, counter to the gravity of the cluster, and counter to the flow of expansion, we'd be right to infer some artificial phenomenon. MIT, ladies and gentlemen!
I think the point is that you wouldn't notice these galactic clusters failing to move apart if you didn't look.
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Old 14th January 2020, 01:08 PM   #19
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Originally Posted by Puppycow View Post
Boy will they have red faces when they discover that Dark Energy might not even be real in the first place!

https://www.space.com/dark-energy-not-debunked.html
Has Dark Energy Been Debunked? Probably Not.
Quote:
"Taken at face values, our result suggests that ~100% of the evidence for dark energy simply goes away. This will be confirmed by future observations," Lee and co-author Yijung Kang told Space.com in an email.
The first evidence for dark energy was from type 1a supernova which their paper looks at. But we have 4 other lines of evidence, e.g. fitting the CMB with models gives a cosmological constant that matches dark energy. The article lists 2 of these lines of evidence.

Adam Riess pointed out some possible problems with the paper. It has a small sample of galaxies. A pivotal figure plotting brightness and stellar population age of that sample has a lesser correlation between them when more galaxies are added. Having some stellar population ages older than the age of the universe needs justification (Lee and Kang just say that "Some models overestimate ages for older galaxies,"). A data point from the paper should not exist according to the study "Think Global, Act Local: The Influence of Environment Age and Host Mass on Type Ia Supernova Light Curves".
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Old 14th January 2020, 04:20 PM   #20
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Originally Posted by Red Baron Farms View Post
Quote:
over the next approximately 100 billion years, all stars residing beyond the Local Group will fall beyond the cosmic horizon and become not only unobservable, but entirely inaccessible,” says Hooper.
That’s a problem for an advanced civilization because it limits the number of new stars that are available to exploit.
But it's not a problem. Or more specifically, it's not a problem you can solve.

OK, so imagine you're some super-advanced civilization with the power to travel between galaxies and colonies them, and you can plan out 100 billion years in the future. You know that galaxies beyond the local group will expand away from you forever, but you want those resources. What do you do?

Nothing. Because you can't get those resources, not really. Sure, you could send someone over there, and THEY could get those resources. But you can't bring them back. You can't even talk to them again. From your perspective, you might as well have sent them into a black hole, so why bother?
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Old 14th January 2020, 04:51 PM   #21
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Originally Posted by Red Baron Farms View Post
Did Massachusetts just legalize Marijuana?
In 2016, by ballot initiative, following a successful decriminalization initiative in 2008 and an initiative that legalized medical marijuana in 2012. The politicians delayed legal sales of recreational marijuana until 2018.
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Old 14th January 2020, 05:55 PM   #22
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Originally Posted by Ziggurat View Post
But it's not a problem. Or more specifically, it's not a problem you can solve.

OK, so imagine you're some super-advanced civilization with the power to travel between galaxies and colonies them, and you can plan out 100 billion years in the future. You know that galaxies beyond the local group will expand away from you forever, but you want those resources. What do you do?

Nothing. Because you can't get those resources, not really. Sure, you could send someone over there, and THEY could get those resources. But you can't bring them back. You can't even talk to them again. From your perspective, you might as well have sent them into a black hole, so why bother?
If someone can get to that galaxy and start accelerating it toward you before it recedes beyond the horizon, then you can do something about it.
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Old 14th January 2020, 07:02 PM   #23
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Originally Posted by Roboramma View Post
If someone can get to that galaxy and start accelerating it toward you before it recedes beyond the horizon, then you can do something about it.
I'm pretty sure we'd have noticed that kind of motion by now, if it were happening within our horizon.

What would be really wild would be if we saw a galaxy near the edge of our horizon, moving towards it.

That would imply a galaxy-harvesting civilization just beyond our horizon, that we would never meet. Though we might meet some of their remnants, cosmological shipwrecks, forever stuck on our side.

Might make for an interesting Sci Fi epic. We'd know they exist. We'd know they have a huge head start on galaxy harvesting know how. But we'd know it was possible. And we'd know that we had maybe a million years or so to figure it out and catch up, before they got to us. Maybe not catch all the way up, but enough to hold them off and force a truce.

Assuming we could stay on task that long.
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Old 14th January 2020, 09:15 PM   #24
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Originally Posted by theprestige View Post
I'm pretty sure we'd have noticed that kind of motion by now, if it were happening within our horizon.
I don't think we would. There are billions of galaxies, all moving with respect to each other. If a few of them have velocities toward each other that are higher than expected statistically we wouldn't necessarily have noticed, particularly since I don't think we're anywhere near cataloguing the motions of all them.

Quote:
What would be really wild would be if we saw a galaxy near the edge of our horizon, moving towards it.
You mean moving toward the horizon faster that expected I guess? From our perspective almost everything is moving toward the horizon. But, yeah, if it were at a higher rate than expected then that would be very interesting!

Quote:
That would imply a galaxy-harvesting civilization just beyond our horizon, that we would never meet. Though we might meet some of their remnants, cosmological shipwrecks, forever stuck on our side.

Might make for an interesting Sci Fi epic. We'd know they exist. We'd know they have a huge head start on galaxy harvesting know how. But we'd know it was possible. And we'd know that we had maybe a million years or so to figure it out and catch up, before they got to us. Maybe not catch all the way up, but enough to hold them off and force a truce.

Assuming we could stay on task that long.
Fun scenario, but if they're beyond our horizon they're not going to be able to harvest our galaxy, as we are beyond their horizon as well.
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