h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Well that is something!

My favourite is Aristoxenus, because he was a practical musician. He was passed over as Aristotle's successor to head the Lyceum.

He proposed the adoption of equal temperament, which was accepted some two thousand years later in the eighteenth century when Rameau re-proposed it and Bach accepted it.

Afterthought on gravity, Mach's Principle, after Ernst Mach, late 19th century pioneering physicist, cosmologist and polymath.

Mach conjectured that the inertia of an object, its resistance to changes of its state of motion, is a consequence of the sum of the actions of all the gravitational sources of the universe acting on the object.

There are some new theories that our understanding of gravity is incomplete, hence our imagining of large amounts of "dark matter" that don't actually exist. But don't ask me to explain, as it's all way over my head .

The understanding of gravity IS still incomplete.

Dark matter does really exist, just as surely as exoplanets discovered by means of observation of gravitational displacement of positions of stars produced by planets in orbit around the stellar primary do.

I think Hawking's primordial black holes* best fit the bill for dark matter under the MACHOS** model and there is some limited observational evidence for this, so far inconclusive.

Dark energy, postulated to account for the acceleration of cosmological expansion discovered by the type 1A supernova studies undertaken to better measure the size and scale of the universe is more controversial.

I personally don't think there's anymore to it than momentum of the expansion of the cosmos at the time of the collapse of the inflationary epoch being conserved, against a steadily declining inertial impedance under Mach's Principle produced by a steadily declining mass and gravitational flux density as a consequence of increase in volume of the cosmos produced by cosmological expansion while its mass remains constant.

Density is mass divided by volume, so mass of an expanding balloon or the cosmos remaining the same, its density declines as it expands.

If the inertial impedance under Mach's principle operating on an object decreases while its momentum remains constant, that object must accelerate, increase its speed over time.




*Hawking's primordial black holes, black holes of sub-stellar mass produced by means of the compressional forces of the big bang, rather than the gravitational collapse process.

**MAssive Compact (galactic) Halo Object

Surely the understanding of anything is still incomplete? What would full understanding look like? And how would you know when you attained it? How would you know you weren't missing something? The scientific answer is, you know by the confirmation of your predictions. That's the best there could be, but in the case of stuff like dark matter, how can you call it completeness?

I love von Neumann's saying: "... in mathematics you don't understand things. You just get used to them."

There is a terrific collection of von Neumann quotes here http://en.wikiquote.org/wiki/John_von_Neumann

I enjoyed ITIWBS's quote too:

"I personally don't think there's anymore to it than momentum of the expansion of the cosmos at the time of the collapse of the inflationary epoch being conserved, against a steadily declining inertial impedance under Mach's Principle produced by a steadily declining mass and gravitational flux density as a consequence of increase in volume of the cosmos produced by cosmological expansion while its mass remains constant."

Too long to tweet, unfortunately.

I thought that Hawking had proved that primordial black holes were impossible in his work with Roger Penrose on the quantum mechanics of black holes. He showed that black holes 'evaporate'. That is, they transfer small amounts of matter to the universe outside the hole. The smaller they are the faster they do this. Primordial black holes (which are only the mass of a mountain) do this so quickly that they explode in seconds.

Primordial black holes, formed by compressional forces of the big bang, can be of any size below that of stellar gravitational collapse black holes*, decay by means of loss of mass equivalency through gravitational radiation, decay times depending very sensitively on intrinsic mass equivalency, so that the more ancient the decay, the smaller the pbh and so that pbh decays at the same cosmological distance are of pbh objects with the same intrinsic mass equivalency.

There is some limited and incomplete observational evidence for this involving the most common type of gamma ray burster, the galactic halo burster, originating in the galactic halo region where the dark matter is.

The final decay process begins when the mass equivalency drops to a level where neutrinos are no longer confined by the event horizon.

A rapid neutrino burst follows, but for a pbh of only planetary size the neutrino burst is not energetic enough to be detected by the current generation neutrino observatories, though supernova and hypernova** explosions frequently are detected by the arrival of the neutrino burst somewhat in advance of the optical burst.***

As a consequence os the neutrino burst, intrinsic mass equivalency abruptly drops below that required for closure of the event horizon to optical discharges and is followed by an optical burst, rapidly escalating from comparatively low frequencies produced by gravitational red shift to the gamma ray maximum, after which the decay signature becomes a coventional thermodynamic decay signature.

The aftermath of the galactic halo gamma ray burster gamma ray maximum has been confirmed, but not, so far as I know the two earlier phases, neutrino burst and rapidly escalating frequency of the optical burst to the gamma ray maximum.

If the pbh undergoing gravitational decay model is good, and I believe it is, one can think of it as a case of primodial matter rather belatedly entering the observable universe.

There are other kinds of dark matter than the galactic halo objects, for example, the great attractor of the Virgo cluster, or association of galaxies.

The Screaming Skull may be a more youthful and still active object in the same class as the great attractor.




*They can also be larger than coventional stellar collapse black holes, but above that critical size limit are usually not classed with pbh objects of substellar mass.

** Hypernova events are supernova events leaving behind a remnant black hole rather than the neutron star remnant characteristic of the next smaller class of nova events.

The term 'hypernova' was first introduced into the popular literature, so far as I know, by Arthur C. Clarke in context of a TV study of gamma ray bursters, coined by the researcher who put together the gravitational collapse model for gamma ray bursters.

The difference in arrival times for neutrino burst and optical bursts produced by nova events is on account of neutrinos being much less strongly impeded by local mass density in context of the object undergoing a nova than photons.

"I think Hawking's primordial black holes* best fit the bill for dark matter under the MACHOS** model and there is some limited observational evidence for this, so far inconclusive." [ITIWBS]

They're matter, by definition, and they're dark, also by definition. They aren't a different *kind* of matter, just hard to see. But would 85% of the matter in the universe consist of black holes? That seems like a very high proportion o to me.

Besides MACHOS, there are other kinds of dark matter, I mentioned the Great Attractor, a super-super-massive object which provides a gravitational anchor for the Virgo Cluster, the local association of galaxies the Milky Way galaxy belongs to.

Though its unquestionably there, its presence is detectable only by observation of its gravitational effect on other objects.

If its an object in the same class as the Screaming Skull, only no longer in the active phase, It would be reasonable to describe it as a cluster of supermassive black holes.

It may on the other hand be of primordial origin, dating from the time of the collapse of the inflationary epoch.

Many researchers think that every association of galaxies may contain an object in the class.




If one wants to take the question from a 'missing mass' perspective, one can go to particle decay processes.




For example, when mu and pi mesons decay, first they emit, respectively, an electron or an anti-electron, while the remainder is emitted in the form of a neutrino/antineutrino pair.

The neutrino/antineutrino pair, while nearly undetectable, have an aggregate mass-energy equivalency several hundred times that of the more readily observable electron or antielectron

Many researchers think neutrinos may account for some of the missing mass.




Another missing mass model is the interferometric losses model.

When a photon encounters another photon exactly out of phase, the two particle cancel, leaving behind a tiny speck of empty space.

The rate of interactions like this was at its maximum during the inflationary epoch and the inflationary model at least explains where space itself comes from.




I personally think that all the models above and several others besides contribute to the missing (or unobservable) mass question.




On reasons why understanding of gravity is still incomplete, current integration of gravitation and inertia still isn't up to a level comparable to Clark Maxwell's integration of electricity and magnetism, let alone his subsequent integration of electromagnetic theory with optics, work in the foundations of both quantum theory and relativity, but also unified field theory (the theory of everything).

Before Clark Maxwell, electricity and magnetism were still considered separate forces.

His theory unifying the two kinds of forces led serendipitously to the discovery that the two were unified in turn by means of the speed of light constant governing the relationship between them, which in turn allowed their unification with optics and development of a body of theory that allowed manipulation of each of the three forces by means of application the other two, which in turn allowed engineering development of electronic technology.

On the other hand, though its generally agreed the two forces are related, gravitation and inertia are still usually as separate kinds of force.

In the last line my post immediately above, 'treated' between "usually" and "as".

This is fascinating to a physics innocent such as myself.

What stops a cluster of massive black holes from coalescing? How are they kept separate?

Black holes are objects with a very large mass in a small volume. They will behave like any other objects in space, orbiting around each other. They will only coalesce if one scores a direct hit on another and since they are so small in volume, this is unlikely.

I appreciate the erudition. The popular media often fall far short of explaining science.

New to Gnomon's Guide today:

Entry A87853053 - A Canal Walk - the Corbally Naas Branch of Ireland's Grand Canal

I've rewritten the section on Gravity Assisted Slingshot in A87853747 Moving About in Space.

I hope it is more explanatory now.

another brilliant Entry! Unputdownable! I feel areas being refreshed that other reads don't reach.

Thank you, Gnomon.

Thanks Gnomon - I agree that's a good enough description for this entry.

Thank you both.

Vey enjoyable


- I won't say more now in case it gets into Peer Review.

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