# Gravitational collapse

Discussion in 'Physics & Math' started by arfa brane, Jun 18, 2013.

1. ### hansdaValued Senior Member

Messages:
2,219

As there is no "time-dilation" with cosmological expansion, it(cosmological expansion) can be considered as Newtonian.

3. ### TachBannedBanned

Messages:
5,265
Few ideas but fixed. There is no cosmological expansion in Newtonian mechanics. Cosmological expansion is one of the GR predictions that differentiates it from Newtonian mechanics, like the advancement of Mercury perihelion, for example. Take a break from posting crank ideas and read a book, take a class.

5. ### hansdaValued Senior Member

Messages:
2,219
Do you know, initially Einstein was opposing this idea of "expanding universe"; but later on he agreed to it. See this link.

7. ### TachBannedBanned

Messages:
5,265
Being a very smart man , Einstein corrected his views.

8. ### arfa branecall me arfValued Senior Member

Messages:
5,373
More about that video (sorry I can't post images from it, so you will need to follow the link).

Near the end of the vid, both the Newtonian and Einsteinian spacetime 'slices' are rolled up. The caption notes that this transformation doesn't change any distances (it preserves them). It also preserves the right-angles between the coordinate 'lines' of both surfaces. So what?

Well, one detail is that if the cylinder is placed inside the truncated cone, they then intersect at one 'place' which is (isomorphic to) a circle = radius of the cylinder
Otherwise all the remaining circular sections through the cylinder (flat spacetime) don't intersect; if you want any vertical intersection, you can align a vertical coordinate-line on the cylinder with a vertical coordinate-line on the truncated cone, by 'skewing' the cylinder at an angle. To align more than a single line, you can't roll the cylinder along the surface of the cone, you have to lift it away and reposition it (with 'skew') each time. So you can't parallel transport any vectors from one slice to the other . . .

9. ### hansdaValued Senior Member

Messages:
2,219
It is not exactly Einstein's smartness but Hubble's discovery, which changed Einstein's view about expanding universe.

Last edited: Jul 28, 2013
10. ### TachBannedBanned

Messages:
5,265
The point is that Einstein was smart and he changed his point of view. The same cannot be said about the cranks populating this forum.

11. ### hansdaValued Senior Member

Messages:
2,219
So, you dont want to consider Hubble's discovery.

12. ### TachBannedBanned

Messages:
5,265
What gives you this bright idea?

13. ### hansdaValued Senior Member

Messages:
2,219
See this quote of Einstein from my earlier link.

"New observations by Hubble and Humason … concerning the redshift of light in distant nebulae make the presumptions near that the general structure of the Universe is not static."
—Einstein

14. ### TachBannedBanned

Messages:
5,265
Very good, so, I'll ask you again: what gives you the bright idea that I disagree with the fact that Einstein did not consider with Hubble's discovery in changing his position relative to the expanding universe?

15. ### hansdaValued Senior Member

Messages:
2,219
Einstein developed his GR long before Hubble's discovery. GR predicts expanding universe. But Einstein did not accept this fact of expanding universe till Hubble's discovery, though he(Einstein) developed his GR long before.

16. ### TachBannedBanned

Messages:
5,265
Actually, you got it wrong (like most everything you do). The original EFEs predicted cosmological expansion. Einstein was uncomfortable with the idea and added a constant (the cosmological constant) to the equations in order to counter the expansion effect. Later on, he realized his error and reverted to the ORIGINAL EFEs.

17. ### hansdaValued Senior Member

Messages:
2,219
Again you are wrong. It is only Hubble's discovery which changed Einstein's idea of "static universe". See this NASA site.

18. ### TachBannedBanned

Messages:
5,265
From the very site: "In its simplest form, general relativity predicted that the universe must either expand or contract."
Give it a rest, crank.

19. ### arfa branecall me arfValued Senior Member

Messages:
5,373
Ok, well obviously this thread is really about what Einstein said, when he said it, and what he meant.

So my 10c worth is: he realised that in order for the equations to "predict a static universe", he had to add something to counteract the effect of gravity, so he assumed spacetime wasn't expanding (or it didn't occur to him), and that gravity would then imply all the matter and energy attracting together (i.e. gravitational collapse), unless he added a term to counteract it.

Ed: that's also pretty much what Wikipedia says (although they could of course be misquoting the man).

Last edited: Aug 1, 2013
20. ### hansdaValued Senior Member

Messages:
2,219
" Einstein first proposed the cosmological constant (not to be confused with the Hubble Constant) usually symbolized by the greek letter "lambda" (Λ), as a mathematical fix to the theory of general relativity. In its simplest form, general relativity predicted that the universe must either expand or contract. Einstein thought the universe was static, so he added this new term to stop the expansion. Friedmann, a Russian mathematician, realized that this was an unstable fix, like balancing a pencil on its point, and proposed an expanding universe model, now called the Big Bang theory. When Hubble's study of nearby galaxies showed that the universe was in fact expanding, Einstein regretted modifying his elegant theory and viewed the cosmological constant term as his "greatest mistake". "

Q1) Einstein developed GR. GR predicts universe is either expanding or contracting. WHY Einstein still thought that the universe is static(though it was his mistake)?

Q2) How Einstein realized his mistake that the universe is not static?

21. ### TachBannedBanned

Messages:
5,265
Give it a rest , crank. I bolded the part that contradicts your claims.
BTW, GR was introduced by Einstein in 1916 , no $\Lambda$.
$\Lambda$ was introduced by Einstein in 1917, as an addition to his EFE's.
$\Lambda$ was retracted by Einstein in 1922 and was resurrected just recently by Saul Perlmutter.

22. ### hansdaValued Senior Member

Messages:
2,219
But that was Einstein's mistake and later on he regretted it.

23. ### TachBannedBanned

Messages:
5,265
Well, crackpot, this was your angle all along, to "prove that Einstein was wrong".
Turns out that he was right after all: $\Lambda$ was retracted by Einstein in 1922 and was resurrected just recently by Saul Perlmutter.