Talk:Hubble's law
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1 Hubble's parameter vs. Hubble's constant |
H(t)
Hubble's constant does indeed decrease over time, even in the presence of a non-zero cosmological constant. The cosmological constant causes Hubble's constant to decrease slower (resulting in an accelerating expansion), but a constant or even increasing Hubble's constant is not contained in any of the current models, and would have extremely strange consequences. --AxelBoldt
You are right, sorry. I forgot that hubble parameter is (da/dt)/a, not just da/dt.--AN
I removed this:
- although some cosmologists have wondered if the expansion of universe is uniform throughout the whole universe. These speculations have been driven in part by the observation that the expansion of the universe seems to be accelerating at least in our section of it.
Have people seriously conjectured that the universe is not homogeneous? Why would the acceleration lead to that conjecture? AxelBoldt
- My understanding is that on large scales, it's been observed to be remarkably homogeneous. (I'm thinking of the cosmic microwave background here.) Fluctuations in the microwave backgrounds are on scales of thousandths (or less!) of the mean value. The distribution of matter is of course not homogenous, but I can't say I've ever run across serious suggestions that the expansion of spacetime might be unequally distributed. Comments on
the accelerating universe and cosmological constant seem to cover this. -- April
- I might add that the whole apparatus of cosmology now rest on the cosmological principle: the statement (assumed true for the construction of the theory) that the Universe is isotropic and (at least at large scales), homogeneous. AstroNomer
- But this is almost a universal assumption by science, isn't it? That there are no special points in spacetime. Physicists assume that an experiment done on Friday in Berkeley will come out the same if done on Monday in Munich; biologists assume that the laws of chemistry were valid millions of year ago; astronomers assume that the nuclear reactions in distant stars are the same as the ones observed on Earth etc. Of course, this is not just an arbitrary assumption, because many things make eminent sense when interpreted with this assumption. AxelBoldt
When objects reaches a speed near the speed of light, due to the hubble effect, relativly to an observer, wouldnt they appear to smother out into some wierd form of "inverse event horrizon" or something??? :b! (that would in turn lead to a conclusion that there is a end of the universe, that it is not infinitly large.. That if something reach too far away from you, it would end up at the *edge* of the universe, where it flattens out and bla bla bla.. //Noone
Hubbles Constant
Hubbles Law gave us the picture of an expanding universe. The expanding universe is the result of the big bang. Everything in the universe is expanding away fom this source. Simple calculation can esimate the minimum velocity of expnsion at 0.5c. Because of the expansion from a central point, there can be no constant that will identify the rate of expansion for all conditions. Assuming the minimum velocity away from the big bang is 0.5c, the velocity of recession of two groups of galaxies would be:
Vr=Vo+[Vx(Cos Angle)]
Where: Vr= Velocity of recession for galaxies o and x, Vo is the velocity of galaxy o from the big bang, Vx is the velocity of galaxy x from the big bang and Cos Angle is the angle the light from galaxy x intersects galaxy o.
This formula can be simplified to:
Vr=0.5c(1+Cos Angle).
J. Chwastyk@comcast.net
- There is no central point in the Big Bang model. There is no "source." It is not an explosion, as such. Chris Mid 23:59, 22 Jun 2005 (UTC)
Please elaborate
"Furthermore, if one takes Hubble's original observations and then use the most accurate distances and velocities currently known, one ends up with a random scatter plot with no discernable relationship between redshift and distance. Nevertheless the relationship was confirmed by observations after Hubble."
- Confirmed by what? Please elaborate. - Omegatron 21:15, Aug 4, 2004 (UTC)
Hubble's parameter vs. Hubble's constant
IMHO what this article is talking about, is "Hubble's parameter". "Hubble's constant" is the dimensionless parameter <math>h = \sqrt{2(\omega_b + \omega_{cdm})/(\Omega_t+\Omega_d)}<math>. OTOH it's rather late local time, and I may be confused. --Pjacobi 00:32, 2005 Jan 24 (UTC)
Correction: Red sunset is not a redshift.
Deleted: "this is also why the sun appears redder at sunset than when high in the sky)"
This isn't true. Red sunset is due to preferential scattering of blue light off oxygen, nitrogen, and dust. Known as Rayleigh scattering. (More correctly Tyndall effect, John Tyndall, 1859)
Reference at Physics FAQ (http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html) Chris Mid 23:07, 22 Jun 2005 (UTC)