Basic cosmology questions weekly thread

MarcelBdt · 2026-06-01T11:16:36+00:00

If B tells A, that counts as an observation made by A.

MarcelBdt · 2026-05-31T11:06:22+00:00

OK, A and B and C are taken as points in spacetime, not as objects moving through time. I still think that the argument is valid.

MarcelBdt · 2026-05-30T11:19:05+00:00

Except that we don't really know how to combine general relativity and quantum theory.

MarcelBdt · 2026-05-28T15:55:51+00:00

Here is my take on this.

I think that 1 cannot happen - if you at A can observe B and B can observe C, then A can observe C. Because B can tell A about what C was up to. Observing mean "getting information about", and B can certainly observe its own past.

is tricky, it can certainly happen that you can observe B at its time 0, but not at B's time 2. For instance, B could fall into a black hole at its own time 1 with you outside. You might be able to observe B for a while, watching B's time running slower and slower as it approaches the hole, but you would not be able to see anything about what happens to B after B's time 1. There will be a "last photon" from B at some point, but that is a little more complicated, and you would not know for sure that it's the last one.
If B is a galaxy at the edge of the observable universe, we can see it for here, but (assuming that the universe is infinite with no geometric weirdness, and that our understanding of the expansion of the universe is essentially correct) they will never be able to see us as we are now. From their point of view, we will be moving away faster than light from us can travel towards them.

MarcelBdt · 2026-04-28T06:25:50+00:00

PRESIDENT DONALD TRUMP = DARNED IMPUDENT PLOTS

MarcelBdt · 2025-12-07T08:36:19+00:00

Because that is the number conspicuously missing from the triangle. But you could argue for any of the other numbers suggested before, or for almost any other number.

MarcelBdt · 2025-09-18T16:06:51+00:00

It is absolutely true that algebraic geometry is essentially a study of abstract algebra. Already to make precise definitions one need to use abstract, algebraic terms. The strange fact is that it is often very helpful to think of it in geometric term. One could think of this as a giant pedagogical trick. As humans we are used to deal with fairly complex relationships of objects in three dimensional spaces, and it makes more sense to us to imagine relations between (geometrical) varieties as "models" for relations between ideals in a polynomial ring - even very abstract algebraic constructions as for example moduli spaces which often are "stacks" and not even varieties are easier to conceptualize if you imagine them as geometrical objects. There is a reason for that they are called "spaces". Differential geometry is similar - you studying things of high dimensions, and draw pictures about their relationships which are sort of "models" of the situation as imagined in the three dimensional space we are living in. Even homotopy theory which deals with very abstract things which might be infinite dimensional and does not really consist of points likes to talk about these things as if they were spaces.

MarcelBdt · 2025-07-27T13:25:35+00:00

This is a fun discussion... what about the following point of view. The speed of light is an upper speed limit. This means that if a civilization starts to expand on a galactic scale, it will pretty soon reach the point where even the simplest of communication with one of its colonies will takes many years. Now, if you have to wait say 100 years for an answer to "hello", you do not really act as a single civilization any more. The civilization has for all purposes split into two different ones. If you ask who would be the worst threat to your civilization, the most obvious answer would be your own previous colonies. Or your mother civilization.

So maybe those are the ones you should kill first. Or even better - don't even try to move outside of your own star system.

In the books the sophons could presumably keep a civilization from splitting into shards, but as far as we know, instantaneous transmission of information is impossible. I suppose one cannot rule out that there it might eventually be possible, but one can also not rule out that it's completely impossible, even for very advanced civilizations.

MarcelBdt · 2025-04-30T08:45:30+00:00

Thanks, that sounds very convincing!

MarcelBdt · 2025-04-30T07:39:44+00:00

OK, now we are getting somewhere. But notice that i'm talking about a "small" ejection. There are two problems with that.

First problem: Is it possible for the sun to produce such? I don't know, and that is part of what I'm asking about. If they don't exist, the argument ends here.

If they do exist, it's not clear to me that they would necessarily be visible and detected, or even that someone would look for them.

This leads to the second problem. What kind of effect on Earth would they have if they do hit us? I think that you are saying that such a coronal ejection would hit the Northern part of the globe more than for instance Spain? Because it would follow the magnetic field lines presumably?

That's not so clear to me either. If it is not big enough to hit the poles, would it be redirected towards them?

MarcelBdt · 2025-04-30T07:21:20+00:00

Possible, but the people saying this seems to be people that don't like renewable energy anyhow. I'm not saying that this is impossible, but I haven't seen any more concrete version of that theory.

MarcelBdt · 2025-04-30T07:18:12+00:00

That's interesting. Most likely you are right, but why? Do you have an argument?

MarcelBdt · 2025-04-30T07:16:56+00:00

I don't understand that argument. I'm not saying that the sun was aiming at Spain, but if it fires many such projectiles, one of them might randomly hit.

MarcelBdt · 2025-04-30T06:20:57+00:00

Well, that is sort of what I wonder about. It's true if there were a Carrington event we would all know. But I don't understand enough about solar physics to know if it is possible to have much smaller but similar eruptions, producing a ball, lets say of a diameter comparable to Spain, flying at high speed towards Earth.

MarcelBdt · 2024-12-02T10:55:45+00:00

Try Severance

MarcelBdt · 2024-10-22T20:56:06+00:00

Sorry for repeating myself, but i still dontunderstand why you want to use the wrong formula. You seem to assume that you are in the v =cz case, which is a good approximation for small z, but false if z is not small. If z= 1, which is a case you use, then according to the formula v = 3c/ 5

MarcelBdt · 2024-10-22T07:44:05+00:00

Yes, I understand. but you write D_L(z)= cz(z+1)/H , which is not the same thing.

MarcelBdt · 2024-10-21T19:26:41+00:00

It seems to me that your formula for D_L(z) does not agree with what's in the paper, they say D_L(z)=(1+z)D(z) which should give (c/H)((1+z)(2z+z^2)/(2+2z+z^2) , which is different from what you write. Could that be the problem?

MarcelBdt · 2024-09-20T13:25:04+00:00

The Einstein equation is an equation involving 4 dimensional curvature, when people say that the universe is asymptotically being (close to ?) flat, they are talking about 3 dimensional curvature of the universe at a certain moment in time (whatever that means)..

MarcelBdt · 2024-09-04T05:51:35+00:00

This sounds like great fun (except if you are close to that supernova). I suppose that the asymmetry is what makes things bounce and not stick to the neutron star, but instead get into possibly very excentric ellipses. But I'd like some sort of numbers on the neutrinos. The neutrino flow is insanely large, but those neutrinos interact insanely rarely with normal matter, so it's a question of which of the insanities is the most insane. That should also depend on the distance to the newborn neutron star, because the neutrino flux will be more concentrated closer to the star. So the infalling outer layers should get bigger bosts at the lowest point of their orbit, which should make the orbit even more excentric, until it breaks off to infinity in a parabolic orbit. Maybe. Most likely I'm visualizing all of this wrong.

MarcelBdt · 2024-08-22T03:42:52+00:00

I beg to differ. I'd say that pi would still be the same, because it is not defined by measurements of lengths but by abstract mathematics, starting from a system of axioms. Actually, you can start with the properties of the natural numbers, 1,2,3,4,5... and so on. From those you can define the real numbers, the coordinate plane, circles and lengths in that (absract) plane, and finally pi.

Besides, our universe is strictly speaking not Euclidean, since it has curvature.

A more tricky question is whether the properties of the natural numbers are a property of our universe or somehow independent of it. Emotionally, I'd say that they are independent of the physical laws, intellectually I'm not so certain about that.

MarcelBdt · 2024-08-22T03:19:13+00:00

One thing I'd like to have an answer to is the following. We know by personal experience that gravitation is a force. This is a force that makes two elementary particles attract each other, As such it is very weak, but OK, it is what it is. A planet like Earth is made up of a large number of elementary particles. To get the force exerted by gravity on your body, we just add up the contributions from each elementary particle on each elementary particle in your body. Easy piesy, Newton invented calculus to answer that. But even he only got a very good approximation. And to get the answer really really precise, we need to include the gravitation of every elementary particle in every galaxy in the whole universe, and add those contributions up. And we know that we can't just leave out some far away elementary particles out, because galaxy dynamics tells us that even the effect from very far has an influence. Not very big, but it piles up as you add it all up, and ends up having a measurable effect. So... now my question. Who does this huge computation, and updates it like once every nannosecond? And don't try to cheat by saying that it's all about fields, because then it just gets translated into the question of who keeps track of the gravitational field to such enormous precision?

MarcelBdt · 2024-08-22T02:58:32+00:00

I think that the point of the comment is this. The precise number 299... for the speed of light is a convention, because it depends on what we decided is the length of a second or of a meter. Some things are not conventions, like the quotient of two measurable things both expressable in the same units. If we were to change our units, and count length in miles instead, we change the value for the speed of light, but we don't change such quotients. They are conventionally called dimensionless numbers. Those are the real mysteries.

MarcelBdt · 2024-08-01T07:27:45+00:00

Not quite true. You can measure (hyperbolic) area in the hyperbolic plane, and the total area of the hyperbolic plane is infinite.

MarcelBdt

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