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Watsisname
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04 Dec 2019 00:46

midtskogen wrote:
Source of the post Indeed, yet it takes an extremely sensitive instrument to detect it!  How much energy could be "hidden" in frequencies we can't realistically measure? 


Less than than 1eV/cm3, or in perhaps more familiar units, less than 10-13 Joules per cubic meter, on average in the universe.  (The energy density locally can be a few orders of magnitude greater when there happens to be a strong wave passing by, like with the first LIGO detection.)


Doesn't sound like much?  Indeed!  This is only a few parts in 100,000 of the critical density of the universe (the critical density being about 5 proton masses or 10-9 Joules per cubic meter).  For comparison, matter makes up about 30% of the critical density, and dark energy makes up about 70%.


How do we know?  Gravitational waves contribute a form of energy density to the universe that we group under "radiation density".  That also includes electromagnetic radiation, neutrinos, and any particles moving at highly relativistic speeds.  We group these together because their energy densities dilute with expansion in the same way -- they all decrease according to the 4th power of the size of the universe, whereas matter density decreases with the 3rd power of universe size.

The individual densities of matter, radiation, and dark energy are all measurable from the Cosmic Microwave Background radiation.  Specifically, by the "CMB angular power spectrum", which is explained in more detail in the two PBS Space Time videos linked here.  The total density of matter, energy and radiation together also affect the curvature of the universe.  The total density is about 1.02 times the critical density (with an error of about .02), of which radiation is a few parts in 105 -- a tiny amount.  

Most of the radiation density comes from the CMB photons: about 0.25 electron volts per cubic centimeter.  The cosmic neutrino background also contributes some energy density.  So there is very little room left for gravitational wave energy in the universe.  This probably sounds surprising given all the reports of how much energy is carried in the waves detected by LIGO, and what I just said about supermassive black hole mergers radiating hundreds of thousands of solar masses worth of these waves.  But such strong gravitational wave sources are rare events, whereas the CMB and CNB uniformly fill every cubic meter of space.  

Another possible cause for confusion is that the energy carried in gravitational waves is usually compared to the energy of starlight, and specifically in terms of the peak power of the signal (which lasts less than a second) vs. total emission of starlight in the same amount of time.  But even all the combined starlight (over all time) in the universe is a tiny fraction of the energy of the CMB radiation, which again is only a tiny fraction of the total mass/energy density of the universe.

Another thing we can do is calculate what we expect the energy density of gravitational waves to be, based on our understanding of the sources.  There is a lot of research in that area.  Here's an example.  We also had some fascinating discussion of the contribution from exoplanets earlier:)


Is there any mechanism that can convert energy in the form of gravitational wave into another form of energy?


Yes, but not very well.  Gravitational waves cause spacetime to stretch and squeeze with their passage, and that energy can transfer over to vibrations in matter, or a wavelength shift of light waves in the case of the lasers in LIGO.  However, the "coupling" between the gravitational wave and matter is very weak.  Most of the wave energy goes right through, just like neutrinos passing through a planet, or a 100m radio wave passing around/through your body.

(Aside: the wavelength of the laser light in LIGO gets stretched or squeezed by the same amount that the distance between mirrors does, which might make us think it's impossible to measure the wave at all, but the trick is that the light enters and leaves the system much faster than the time it takes for the entire wave signal to pass by, because the gravitational waves LIGO measures have long wavelengths).


Great questions, by the way. :)
 
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04 Dec 2019 04:24

Watsisname wrote:
Source of the post  Most of the wave energy goes right through, just like neutrinos

So "good" ways to render energy pretty useless would be to turn it into gravitational waves or neutrinos. What is the most boring universe - a universe only containing neutrinos or a universe only containing gravitational waves?
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04 Dec 2019 17:47

midtskogen wrote:
Source of the post So "good" ways to render energy pretty useless would be to turn it into gravitational waves or neutrinos.

Definitely.  I think this is roughly equivalent to saying that radiating energy in these forms is a good way to increase the entropy of the universe.  Probably the best way to render energy useless is to collapse it (or matter, or whatever mass-energy you want) into a black hole (which is the maximal entropy state of any system), and then let the black hole evaporate (which increases the universal entropy even further by converting the mass into extremely long wavelength photons that spread out into the space.)

The real universe's evolution will do all of that for us, over the next ~10100 years, whether we like it or not.  In the shorter term matter may decay into lighter particles and radiation, and in the longer term, supermassive black holes will evaporate.  The universe will then be an ultra diffuse sea of low energy photons and gravitational waves, the total energy density will be almost solely due to dark energy, and the entropy of the universe will approach some theoretical maximum.  Heat death will be exceedingly boring. :P
 
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04 Dec 2019 21:30

Watsisname wrote:
Source of the post Probably the best way to render energy useless is to collapse it (or matter, or whatever mass-energy you want) into a black hole (which is the maximal entropy state of any system), and then let the black hole evaporate (which increases the universal entropy even further by converting the mass into extremely long wavelength photons that spread out into the space.)

Best way perhaps, if time is not an issue.  Converting mass into gravitational waves via collapse into merging black holes could happen in a very short time.
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06 Dec 2019 05:14

Watsisname wrote:
A-L-E-X, it's no secret or surprise that black hole mergers release a lot of energy in gravitational waves.  Detailed simulations of mergers and the gravitational waves they emit have been performed for about a decade, largely so that scientists can interpret the results of LIGO detections and test general relativity's predictions.  But that's not what you said you read, and the radiated energy has nothing to do with the idea that they would form a new universe inside.  There are no simulations showing what happens inside the merged black hole.  If there were, they would be wrong anyway, because we do not know the initial conditions for the spacetime geometry in the original black holes.  The Kerr metric does not correctly describe the interior of a real black hole. 

In fact you don't need to do anything as fancy as numerical relativity simulations to compute how much gravitational wave energy can be released.  As I described here, the minimum energy that can be radiated is zero, if the combined mass equals the direct sum of the initial masses, while the maximum happens if the combined mass is equal to the square root of the sum of the squares of the initial masses (just like the Pythagorean theorem).  That limit comes from the laws of black hole thermodynamics, and specifically that the horizon area, being proportional to the entropy, cannot decrease in the merger.

Example:  Combine two 1-million solar mass black holes, and the resulting mass can be anywhere between 1.414 million solar masses and 2 million solar masses, meaning anything up to about 586,000 solar masses worth of gravitational waves can be emitted.  That is a huge amount of energy.  But again it says nothing about what happens inside.  All it says is the mass of the final black hole was less than you'd get by directly adding them.  Mass-energy is conserved, so the difference is what was radiated.

A-L-E-X wrote:
Source of the post I think we discussed it before- It was an observation about early universe black holes being much more massive than they should be, considering how young the universe was back then Roger Penrose used to back up his conjecture that those black holes must've been a holdover from a previous universe.

Yes, I said back then that this conjecture was absurd, like proposing that anything unexplained must be due to aliens.  And thanks to newer research, we do have a good understanding of how those black holes grew so large so quickly, without being remnants of a previous universe.

I forgot to mention the Penrose diagrams for black holes that you posted a few months back.  The interesting thing about them is they dovetail nicely into the Poplawski conjecture that unifies quantum mechanics with relativity by positing a black hole cosmology model.  I remember you stating back then you believed that it was a very elegant and intriguing conjecture.  If we discovered a fifth force, that would further complicate matters.  One of the reasons Einstein was frustrated in developing a GUT was that two of the four forces we currently know of were discovered while he was trying to unify them (the original ones he worked with were EM and G.)  My view of a black hole and the reason why we wont know exactly what happens "inside" a black hole until we have a fully functional theory of quantum gravity is that the black hole is a quantum object on a macro scale and represents the full unification of all the forces, relativity, and quantum mechanics.  A fully functional theory of quantum gravity will be able to prove whether the black hole cosmology theory is correct.
Wat, out of these possibilities, which would you rank as having the highest probability of being true 1) a previous universe creating this one 2) parallel timeverses (my view of them as being created after the Big Bang during Inflation) 3) black hole cosmology, a la Poplawski 4) String Theory Landscape (10^500 bubble universes)  5)Tegmark's multilayer multiverse model 6) cyclic model combined with loop quantum cosmology (collapse stops at 10 planck lengths therefore there are no infinite densities or singularities.) or 7) CCC

Perhaps LHC or some future even stronger particle collider will actually be able to confirm quantum gravity and create microblack holes (and micro-universes?)
Last edited by A-L-E-X on 06 Dec 2019 05:20, edited 1 time in total.
 
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06 Dec 2019 05:16

Watsisname wrote:
midtskogen wrote:
Source of the post So "good" ways to render energy pretty useless would be to turn it into gravitational waves or neutrinos.

Definitely.  I think this is roughly equivalent to saying that radiating energy in these forms is a good way to increase the entropy of the universe.  Probably the best way to render energy useless is to collapse it (or matter, or whatever mass-energy you want) into a black hole (which is the maximal entropy state of any system), and then let the black hole evaporate (which increases the universal entropy even further by converting the mass into extremely long wavelength photons that spread out into the space.)

The real universe's evolution will do all of that for us, over the next ~10100 years, whether we like it or not.  In the shorter term matter may decay into lighter particles and radiation, and in the longer term, supermassive black holes will evaporate.  The universe will then be an ultra diffuse sea of low energy photons and gravitational waves, the total energy density will be almost solely due to dark energy, and the entropy of the universe will approach some theoretical maximum.  Heat death will be exceedingly boring. :P

If the black hole evaporates and black hole cosmology is correct, would that be akin to a baby losing its connection to the mother when the cord is cut?

And it wont be so boring if the phantom energy models are right :P  And even if heat death is what will happen, there will still be some comfort if black hole cosmology is correct and some of the DNA of our universe will survive in some "baby" universe :-P  Perhaps this universe has some cosmic DNA of its parent also.
 
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07 Dec 2019 00:38

A-L-E-X wrote:
Source of the post Wat, out of these possibilities, which would you rank as having the highest probability of being true


I'm an observational cosmologist, not a theorist.  :)

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