The analogy is often made between galactic distances increasing and the distances between raisins in a rising loaf of raisin bread. Every raisin is increasing in distance from every other raisin no matter if the raisin is near the surface of the loaf or buried deep within.

This Just In: The universe expands as the Flying Spaghetti Monster counts the parsecs from the "center" of the universe to the edge

And I always imagined Evert's avatar came from an early illustrated Alice in Wonderland.

It's impossible to understand the concept of "the edge of the Universe" with "common sense". With "common sense" I may be thinking of what my brother does right now in Helsinki, 90 km from me. I understand the distance. I understand what time it is. He might be listening to the radio time signal at the same time as me. If I go visit him, I need an hour to get there, but if I phone him, the feeling of right now will be more obvious, despite the distance.

But if my brother were at the edge of the Universe, the distance would be so huge that the idea of right now doesn't work anymore. There simply isn't any absolute right now. If I'd go there with the speed of light, I'd be there right now, only to discover that my brother had died 13.7E9 years ago. And determining whether he was alive at the time when I started my travel is probably difficult, too. There's simply no way to picture the Universe "from above" at one moment, at least not using "common sense".

This is a common Relativity Theory misunderstanding. We can measure the time in the edge of the universe relative to us. What we can not have is a global time for all the universe.

As we speak, there are events taking place in, let's say, 5 billion light years away from us. What we can't have is a common reference frame, and therefore a common time measurement.

Would it be fair to say that 5 billion light-years away there are 10 billion years' worth of events happening right now?

I'm pretty sure the distance that the Sun travels in eight minutes is negligible.

Ultimate Fate of the Universe

Particularly fun read is the "Theories about the end of universe" section of the article.

That does it, I'm moving to the suburbs now!

You're right, of course, but it doesn't hurt to try to think out of context for a minute.

But the general theory of relativity doesn't need to predict that the Earth would float off into the black beyond. As you said, it is not confirmed that the speed of gravity is C. Therefore, it may be that the theory of relativity correctly predicts things like Mercury's orbit, but for the wrong reasons. I.e. it is a formula that that has produced correct results so far, just like Newton's theory of gravity produced correct results until problems like the orbit of Mercury showed up.

There is a number of problems that the theory or relativity seems not to be able to explain. For example, the positions of the spacecraft leaving the solar system - they are off from calculations by a tiny amount, but scientists can't say why. Physicists had to introduce dark matter, dark energy and the dark force to explain the many mysteries of the cosmos. And then there is the quantum world: quantum entanglement escapes relativity, and although it can not be used to transmit information (which I don't really believe it can't, as transmission of information faster than light does not really violate causality), it shows that there is spooky action at a distance, indeed. Which means that the very fabric of the universe is interconnected in ways unknown to us. Einstein himself could not believe it.

And then there is the problem of black holes, which have never been observed, and can not exist for Ric = 0. For a black hole to contain an infinite amount of matter in a single point, it means that energy is infinite, which means matter can get the speed of light, which the theory of special relativity does not allow!

Crazy stuff...

Yes. The idea of a donut might lead us astray. It's not like our 3D world would be inside the donut, but on the surface. They are talking about a hyper donut. The Eukleidean infinite space would be a hyper plane. The curved Universe would be a hyper sphere or a hyper saddle (hyperbolic paraboloid). Or a hyper donut. "Hyper" just adds one dimension to it.

Yes. That's why there's a problem reconciling quantum mechanics and general relativity. The idea that the universe has a well-defined local metric breaks down.

In the analogy you're a flatlander living on the surface of the doughnut. For you, there is no direction "into" or "through" it.

While possible, it is more likely that the answer would be grossly wrong than that it would happen to come out to be correct by accident. General relativity is a far more complicated theory than Newtonian gravity is; any theory that replaces it will need to have it as a special case.

It's not clear that that is a problem that is due to a break down of general relativity. Some people like to think so, but it's by no means clear. It could well be something else entirely.

Dark matter is inferred from galactic rotation curves and gravitational lensing. In a way it's an "intrinsic" curving of space time. General relativity deals with it just fine.
Dark energy enters in the form of the cosmological constant, which is just an extra term appearing in the gravitational field equations. It's manifestly dealt with naturally in the context of general relativity.

It doesn't, because...

Yes. It. Does.
Do a search on the forums, this has been done to death. If you construct Minkovski diagrams for the situation where two observers in relative motion exchange information faster than with the speed of light, you'll see that one of the possibilities has the reply arriving before the question is asked. Someone (SiegeLord?) posted the relevant diagrams in the previous thread where this point came up.
This is apparently a hard point to grasp, since I've had discussions with other scientists who seemed equally unaware that this is ruled out.

What Einstein could or could not believe is as irrelevant to modern physics as whether Darwin believed in evolution^[1] or not is to modern biology.
He did not have some magical insight into the nature of the universe that others lack.

I know a great many people who would disagree with that sentiment.
There are X-ray sources in the sky that can only be explained by accretion disks around black holes. There are regions in the inner part of galaxies where matter is observed to be concentrated within a Schwarzschild radius.
I guess it depends on what you would consider a "black hole observation". Gravitational wave detectors will be very interesting in this sense.

No black hole contains an infinite amount of matter. Either way, whatever happens inside the event horizon of a black hole is causally disconnected from what happens outside. Our physical laws may not apply there at all - but it doesn't matter.

I really want to get high with Evert.

You do know of the Kepler's Laws and the Law of Gravitation? Both allow us to solve the the mass of black holes if they are surrounded by something that orbits them. We've seen things(stars usually) orbit invisible things that are presumed to be black holes, and figured out their mass via those laws.

That's of course beside the other bits of math that describe how the singularity actually seems to work (which is rather unlike the way you are describing it).

No. The cosmological constant is a possible term in the field equations. It empirically gives rise to dark energy. There is a difference between those statements.

It's not open to agreement or disagreement any more than "1+1=2" is open to agreement or disagreement.

B sees O's past, which is B's future. Who says the observer at O and B cannot communicate? The transmission of the message to A is in B's future lightcone. He can influence events there having knowledge from the future. Which part of "causality is violated" didn't you get?

Dark matter isn't required to explain gravitational lensing, dark matter changes the prediction of gravitational lensing by general relativity <i>in a way that can be and has been verified<i>.
I happen to not like dark matter. What I like and don't like doesn't change what's there or not there though.

Einstein's many great contributions not withstanding, it is irrelevant whether he could personally believe in (say) quantum mechanics or not.^[1] Einstein had many great ideas, but that doesn't mean all of his ideas were. In fact, many (most) of them turned out to be wrong.^[2]

Yes, sure.
We have a theoretical prediction saying how an accretion disk around a black hole would behave. We observer said phenomenon (many times under many different circumstances). But it doesn't actually mean anything, it's just a leap of faith to say that theory and observations agree.

No, it doesn't. How does a "10 solar mass black hole" have "infinite mass"?

can be does not mean the same thing as is.

The other way around, actually. Mass is a form of energy.

You're confusing total energy, potential energy and kinetic energy.

There is no reason they have to because the inside of an event horizon is shielded from the outside universe. It's irrelevant either way because you cannot get information out of a black hole.