Alright I have no idea about anything, but I read your post and my first thought was “good question, but wouldn’t local interactions be identically impacted by expansion, thus negating expansion as a variable?” In other words: would it not be a constant applied to both masses?

Unless the local bodies are 2 different galaxies, in which case now I want to know too.

> Q. How do we know that the universe expansion is really the explaination for the observed redshift of light from stars.

There is no other mechanism that can lead to the observed redshift/distance relation.

> As i understand it, space is a quantum soup of virtual particles.

It is not, this is just a myth in popular science descriptions.

> is it possible the magnetic and electric permeability of free space is changing over time?

If the chance has an effect on the universe then it also changes the ratio of wavelengths from different spectral lines, something we would observe.

> Q. If one has crossed the event horizon of a black hole, is one traveling faster than light and thus going backward in time wrt the outside universe on the other side of the event horizon?

How you describe the motion depends on your (largely arbitrary) choice of coordinates for the inside, but it's not time travel in any of them.

Where would the difference in energy come from?

Bullshit.

Redshift comes from the expansion of space, not from collisions.

If it came from collisions then we couldn't make out any individual redshifted objects because all the light would be scattered into random directions.

Total layman jumping in here, but in the past I've wondered why the expanding space factor doesn't need to be included in calculating local mass-mass interactions. Even though the expansion is something exceedingly small (like 60 km/3 million light years every second or so?), it seemed that it should be included for precision in calculating how masses will move with respect to each other.

The typical answer (summarized) is that "local mass interaction totally overcomes spatial expansion, so only the gravitional effect exists in local systems", but it still seems that there would still have to be some accounting that some of the gravitional "pull" is having to be "used up" to counteract the expansion.

Your explanation above appears to make this even more necessary, since if we think of the expansion as negative curvature (which is in fact really the case), then even local space is, however minutely, curved in a negative way due to expansion. Therefore, any positive curvature of space is being exerted on that already negatively curved background, and hence the positive curvature of space would necessarily have to be minus whatever that negative curvature was (however miniscule).

Unless I should have been interpreting the typical answer to mean "local mass-mass interactions are of course affected by the expansion of space, but the local mass interaction is simply so large with respect to local spatial expansion, that the local effect of spatial expansion, while not zero, can be ignored for calculations". Or something to that effect.

It's not reversing directions. There is some discussion if it is rotating slightly slower or faster than the rest of Earth, and if that changed recently. "Slightly" here means something like 0.001%.

You can also produce it with accelerators on Earth (far too expensive to make it interesting commercially) and traces will be produced in natural high energy collisions (but some iron nuclei will be destroyed by these, too).

> The tilt is decreasing, so we should be seeing less extreme seasonal changes.

From 23.4 degrees to 22.5 degrees in 10000 years. Here is a plot. It's a very small effect. Around 0.01 degrees (or 1/2000 of its value) per century.

Ground squirrels do

Radiation is all about bodies emitting photons, where the amount of energy depends upon how hot the body is. That's why fires feel warm, infrared heaters feel warm, and the sun feels warm.

Conduction is about direct heat transfer. Heat is just thermal movement of the atoms in a body, so put that in contact with a colder body and the hot atoms run into the colder atoms and make them move faster, transferring heat.

Convection is the same as conduction, except that the transfer is done through air.

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I argue that under normal conditions the velocity in the shallow flood plain is zero and during a flood it is non-zero. :)

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This blows my mind a bit, thanks to the steady diet of documentaries I've had for years.

If I'm reading Diatribe correctly, there would be no "big rip" scenario because local gravity would overcome large scale expansion. And there are lots of similar implications... Am I getting that right?

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Did the recent discovery of ancient galaxies mean that the Big Bang theory is now disproven?

Yeah I haven't thought much about this since college but I do remember curved geometries being introduced and the idea that the issue with whether or not the universe would eventually collapse in on itself was a function of which way the universe was curved.

Probably not remembering it right, but your comment jogged my memory.

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Relative velocities of distant objects aren't well defined in curved spacetimes. It's often said that distant objects are receding faster than light, and there are standard ways of writing down their distance such that the distance grows faster than the speed of light. However, there is no relativistically meaningful sense in which these objects are moving faster than light in relation to us. Also, the distance isn't uniquely defined either.

In intuitive terms, the relative velocity is the angle between two vectors in spacetime. Imagine drawing two arrows on a sheet. If those arrows are in the same place, you can measure the angle between them. If they are in different places, but the sheet is flat, you can also define the angle between them uniquely. However, if they are in different places and the sheet is not flat, the angle between the arrows is not uniquely defined.

Cosmic expansion really does just mean that things are moving apart in a uniform way. There is nothing fundamentally physical about the idea that space itself is expanding; that's just a mathematical convention that is convenient in some contexts. (It's a coordinate choice.)

Thus, the gravitational attraction of the matter in the universe slows the expansion precisely as it would slow the expansion of a distribution of matter inside the universe. Indeed, Newtonian gravity predicts exactly the correct expansion dynamics for a matter-dominated universe. Similarly, the gravitational repulsion of the dark energy accelerates the expansion (although there is some subtlety to this).

Further reading on expanding space not being a physically real phenomenon:

• A diatribe on expanding space

• The kinematic origin of the cosmological redshift

• On The Relativity of Redshifts: Does Space Really "Expand"?

Further reading on cosmological dynamics with Newtonian gravity:

• The dynamics of Newtonian cosmology

• or more generally, just search for "Newtonian cosmology"

It's a direct consequence of general relativity. The same equations that tell you how orbits around the Sun work also predict that matter slows down the expansion of the universe. Applied to cosmology you get the Friedmann equations.

It's interesting that Newtonian physics predicts the same thing here if you use a finite mass distribution and then consider the limit to infinite size, but that's not how it was derived of course.

Because of the way the expansion rate changed over time, as I discussed in my initial comment. The Hubble rate ("how much the universe expands in percent per year") decreased over time. A good analogy is the ant on a rubber rope problem where the rubber rope expands much faster than the speed of the ant, too - but the ant still makes progress on the rope and over time is less affected by its expansion. For the universe, this would be a perfect match in a scenario of constant expansion. That's not what we have, but it's reasonably close for the last 10 billion years.