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Scientific Inquiry


Asperion

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This will be a good place to place any question that is of a scientific nature. It will not matter how simple it seems (why is the sky blue?) or way more complex than that (explanation of black holes).

 

Let's get this started off with a good question:

What happens when a black hole is placed inside another black hole?

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Why, they merge into one, larger hole.

That is certainly the end result. There may be some interesting transient goings-on during the merger process, but those depend on details, like relative masses, how much angular momentum is in the system, whether one or both have accretion disks around them at the time and what the orientations (etc.) of those are with respect to the mutual-orbit plane of the black holes themselves, and so on.
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Wouldn't want it any other way! :thumbup:

 

Clearly you did not suffer through the same year of undergraduate physics that I did.

 

Putting a black hole in another black hole would create a more massive black hole.  However, a black hole in the closing stages of spiraling into another black hole creates a number of interesting effects, like intense bursts of radiation and gravity waves.  It would be an extremely exciting place to be.

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Clearly you did not suffer through the same year of undergraduate physics that I did.

 

Putting a black hole in another black hole would create a more massive black hole.  However, a black hole in the closing stages of spiraling into another black hole creates a number of interesting effects, like intense bursts of radiation and gravity waves.  It would be an extremely exciting place to be.

 

Don't forget the sunblock!

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Okay, I've got one. I've read of two astronomical discoveries in recent years that seem to contradict each other. One, the expansion of the universe was supposed to be slowing down post-Big Bang, as entropy overcame the initial energy of the explosion of the primal atom. But astronomers have discovered it's actually speeding up. However, they also discovered background radiation in the universe consistent with the aftermath of a Big Bang event, supporting that theory of the start of the universe.

 

Are both discoveries confirmed and accepted? Is the Big Bang still the prevailing theory? And has anyone reconciled it with the observed expansion phenomenon?

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Not a scientist, just an interested layman, but I know a bit about the basics.

 

The expected slowing is from gravity, not entropy. Everything in the universe is pulling on everything else in the universe, and this was expected to slow the expansion over time. Which led to the question: Did the Big Bang set the universe expanding faster than its own escape velocity?

 

The Big Bang is still the only game in town and astronomers regard the basic fact of its having happened as proven. After all, everything is in fact flying apart, and the background radiation is there. Every proposed alternative has failed. There is still much to be discovered and explained about the details of the Big Bang, though.

 

"Dark Energy" and the alleged acceleration of the expansion is another matter. From what I've read, when astronomers map out the rate of cosmic expansion through time, at first the rate seems to have decreased over time, as would happen because of gravity -- but then the rate starts speeding up again, as if some anti-gravity force were affecting everything. This is deeply mysterious. The name "Dark Energy" is astrophysicist shorthand for "We don't know what the heck is doing this."

 

Here my interested layman's qualifications end. I too would like to know how robust the observational evidence is for accelerated expansion. (I know about the supernova survey line of evidence. Are there others?)

 

 

Dean Shomshak 

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I black hole with an electrical charge would be easiest to move around.

Unlikely to get a large charge on one. If you put such a charge on one, it'll attract electrons or protons and end up neutral quite quickly.

 

The idea for moving large objects is the gravity tractor, which needs to be in the vicinity of the object to be moved for extended periods of time, but it will work.

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Yeah, whenever physicists start talking "dark-whatever," I become skeptical. :rolleyes:

"Dark X" just means it doesn't have radiation (and doesn't absorb radiation) that we've been able to find. The existence of dark matter in a non-cosmological but galactic-scale sense is just about bulletproof; perhaps its leading discoverer was Vera Rubin, who studied the rotation curves of galaxies, and they all rotate faster than you can understand if all that's there is the ordinary stars-gas-dust matter that you can see. The rotation is due to gravity, and if you have got gravity herking stuff around but you can't see what's making the gravity, "dark matter" is a plausible name for that unseen stuff that makes the gravity.

 

"Dark energy" is one name for the mysterious cause of the acceleration of the expansion of the Universe that was found by the two supernova teams back in the 1990s. It's "dark" because you don't see whatever is doing it. It's called "energy" because it's pushing stuff apart (the opposite of what matter does with its gravity). But we have no real idea what it is.

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It may be of interest that one form of dark matter is known: Neutrinos. The Big Ban is believed to have created lots of neutrinos; stars pump out more. But neutrinos' properties don't match the properties astronomers infer from how the stuff they can see is moving. Like, neutrinos can't condense into clouds whose gravity then bends light and affects the orbital speed of stars: Since neutrinos move at the speed of light, and they hardly interact with anything, they just spread out from where they formed and never stop. (At least that's my understanding.)

 

Dean Shomshak

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"Dark X" just means it doesn't have radiation (and doesn't absorb radiation) that we've been able to find. The existence of dark matter in a non-cosmological but galactic-scale sense is just about bulletproof; perhaps its leading discoverer was Vera Rubin, who studied the rotation curves of galaxies, and they all rotate faster than you can understand if all that's there is the ordinary stars-gas-dust matter that you can see. The rotation is due to gravity, and if you have got gravity herking stuff around but you can't see what's making the gravity, "dark matter" is a plausible name for that unseen stuff that makes the gravity.

 

"Dark energy" is one name for the mysterious cause of the acceleration of the expansion of the Universe that was found by the two supernova teams back in the 1990s. It's "dark" because you don't see whatever is doing it. It's called "energy" because it's pushing stuff apart (the opposite of what matter does with its gravity). But we have no real idea what it is.

 

Well, see, that's the thing: we have one theoretical explanation for an observed phenomenon, which can't be verified by any means except that phenomenon. Something else could be happening that we don't know how to perceive, or at least recognize. The theory's plausible based on our current understanding of how the universe works; but ultimately it's still a fictional peg crafted to fit an observed hole. That is,until we actually find dark matter, or dark energy, by some other means.

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Well, gravitational lensing by masses is known to happen, and it does work according to general relativity, as far as we are able to test that. The distribution of dark matter in galaxy clusters can be inferred by iterative inversion of the lensed images of background objects, again assuming that whatever dark matter is, it influences the curvature of space (and hence the distortion of images made by light moving through regions of space containing mass) in the same way ordinary matter does. (A recent popular-level book titled Einstein's Telescope spends some length discussing this.) So far what that gives us is consistent with the idea that dark matter is something that behaves like matter except it does not interact via the electromagnetic force.

 

As noted above, neutrinos do this -- they have mass but do not interact electromagnetically -- but the mechanisms that we know for making neutrinos in large quantities have problems creating them with the right "temperature". In this context, "temperature" means a spread in velocities for the particles in the population: hotter populations have larger spreads in velocities, and so when they collect together due to gravity they make big, extended, very fuzzy-edged blobs. Colder populations clump more tightly, they can have better-defined edges. If you assume that galaxy clusters and galaxies -- which you can observe -- formed in places where the dark matter clumped due to gravity, then you can get galaxies, galaxy clusters, and overall distributions of the clusters through the Universe that resemble what we now observe if the dark matter is "cold" in this sense. The problem is that the mechanisms for making neutrinos that we know about makes the neutrinos "hot", so that they don't make structures like what we observe. And the usual way of matter to cool off is to take excess energy and convert it into photons and radiate it away ... but that's an electromagnetic process, and neutrinos can't do that, so they don't cool off in times fast enough to make the structures we see in the Universe. So while lots of questions about formation of galaxies and structures in the early history of the Universe can be answered by large amounts of cold neutrinos, we don't know any way to make those, so for now that stuff is still referred to as "dark matter" understanding that we know something about how that stuff is distributed even if we don't know what it is.

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