Light Speed II
Professor Tao has kindly made available some colourful slides of his Cosmic Distance Ladder talk, which he gave in Australia last year. The slides nicely summarise some high points in the history of distance measurements across the cosmos.
At a seminar here yesterday we heard that as recently as the 1920s people still believed that the Milky Way was the entire universe, with the sun sitting at its centre. A mapping of globular clusters in the Milky Way soon indicated that the sun lay on the periphery of the galaxy, and it was finally recognised that the strange spiral nebulae were in fact distant galaxies, not unlike our own. The same seminar concluded, on the observation that accelerated publication rates had not produced as many major breakthroughs in the last two decades, that technology had finally caught up with observations over the electromagnetic spectrum and that we may well have seen most of what there is to see. Naturally there was some dissent. To believe that in a mere 80 years humanity can go from a relatively trivial understanding of the cosmos to complete comprehension is hubris.
Today we are just beginning to observe the so-called Dark Matter in the skies. The luminous baryonic matter is a small fraction ($\frac{\pi - 3}{\pi}$) of the matter we imagine is out there. No doubt this is only another small step in the evolving vision of our mind. A Theory of Everything can never be more than a theory of what little we have known before.
At a seminar here yesterday we heard that as recently as the 1920s people still believed that the Milky Way was the entire universe, with the sun sitting at its centre. A mapping of globular clusters in the Milky Way soon indicated that the sun lay on the periphery of the galaxy, and it was finally recognised that the strange spiral nebulae were in fact distant galaxies, not unlike our own. The same seminar concluded, on the observation that accelerated publication rates had not produced as many major breakthroughs in the last two decades, that technology had finally caught up with observations over the electromagnetic spectrum and that we may well have seen most of what there is to see. Naturally there was some dissent. To believe that in a mere 80 years humanity can go from a relatively trivial understanding of the cosmos to complete comprehension is hubris.
Today we are just beginning to observe the so-called Dark Matter in the skies. The luminous baryonic matter is a small fraction ($\frac{\pi - 3}{\pi}$) of the matter we imagine is out there. No doubt this is only another small step in the evolving vision of our mind. A Theory of Everything can never be more than a theory of what little we have known before.
posted by Kea | 9:56 AM
6 Comments:
We (with me included;-) have enormous need to believe that we understand everything. Some amount of historical and biological perspective should make it obvious that the belief that a final theory has been found is comparable to the belief that human kind has achieved Omega Point in the evolution of consciousness and chimpanzees with almost same genome are quite near to it.
...have enormous need to believe that we understand everything.
LOL. Well, I certainly don't. I certainly feel a terrible need to try and understand as much as possible, but I have no illusions about getting very far!
Nice allusion to Tipler's singularity work. ;) Even though a final theory may not be upon us, we are certainly on the verge of a theoretical milestone.
Hi kneemo, yes I do not deny that from our perspective the milestone is looming out of the mist!
"... the strange spiral nebulae were in fact distant galaxies, not unlike our own. The same seminar concluded, on the observation that accelerated publication rates had not produced as many major breakthroughs in the last two decades, that technology had finally caught up with observations over the electromagnetic spectrum and that we may well have seen most of what there is to see. Naturally there was some dissent. To believe that in a mere 80 years humanity can go from a relatively trivial understanding of the cosmos to complete comprehension is hubris. ..."
When I did a cosmology course in 1996, one thing that worried me was that the galaxies are back in time. If I were Hubble in 1929, I wouldn't have just written recession velocity/distance = constant with units of [time]^-1. Because of spacetime, I'd have considered velocity/time past = constant with units of [acceleration].
This is the major issue. I think it is a fact that the matter is accelerating outward, simply by Hubble's law
v = dx/dt = Hx (Hubble's law)
Hence, dt = dx/v
Now, acceleration
a = dv/dt = dv/(dx/v)
= v*dv/dx
= (Hx)*d(Hx)/dx
= (H^2)x
Because the universe isn't decelerating due to gravity, H = 1/t where t is the age of the universe [if the universe were decelerating like a critical density universe, then the relationship would be H = (2/3)/t].
Hence, a = (H^2)x
= x/t^2
= c/t
= c/(x/c)
= (c^2)/x
So there's cosmic acceleration.
Lee Smolin on page 209 of his book The Trouble with Physics:
"The next simplest thing [by guesswork dimensional analysis] is (c^2)/R [where R is radius of universe]. This turns out to be an acceleration. It is in fact the acceleration by which the rate of expansion of the universe is increasing - that is, the acceleration produced by the cosmological constant."
You can see my problem. The empirically verified Hubble law acceleration happens to be similar to the metaphysical dark energy acceleration. Result: endless confusion about the acceleration of the universe.
It would be useless to try to discuss it with Smolin or anyone else who believes that the two things are the same thing. The real acceleration of the universe is just an expression of Hubble's law in terms of velocity/time past.
The assumed dark energy is completely different, not a real acceleration but a fictional outward acceleration which is supposed to cancel out the (also fictional!) inward acceleration due to gravity over great distances.
In fact the vital inward acceleration due to gravity which was supposed (by the mainstream) to be slowing down distant supernovas until this effect was disproved in 1998 by Perlmutter's group, isn't real because the enormous redshift of light from such great distances doesn't just apply to visible light but also to the gauge bosons of gravity. For this reason, the gravity coupling constant is weakened because the exchanged energy arrives in a very severely redshifted (very low energy) condition.
So the cosmological constant (outward acceleration fiddled to be just enough to cancel out inward gravity for Perlmutter's supernovae, so that the observations fit the lambda-CDM model) is actually spurious.
There is really no long-range gravity over distances where redshift of light is extreme, because the masses are receding and gravitons are redshifted. Hence, there is no cosmological constant, because the role of the cosmological constant is to cancel out gravitational acceleration, not to accelerate the universe.
In virtually all the popular accounts of dark energy, popularisers lie and say that the role of dark energy is to accelerate the universe.
Actually, it's not real, and even if the dark energy theory were correct, it is a fictional acceleration made up to cancel out gravity's effect, just like the way the Coulomb force between your bum and your chair stop you from accelerating downward at 9.8ms^{-2}.
You don't hear people describe the normal reaction force of a chair as an upward acceleration.
So why do people refer to dark energy as the cause of cosmic acceleration? It's really pathetic. There is an acceleration as big as the fictional dark energy acceleration, but it isn't due to a cosmological constant or dark energy. It's just the normal expansion of the universe, due to vector bosons. There's too much disinformation and confusion out there for anybody to listen. My case is that you take the real acceleration, use Newton's second law and calculate outward force F=ma, and then the 3rd law to give the inward reaction force carried by vector bosons, and you get gravity after simple calculations.
If anything is accelerating in the Universe it is the publication of speculative ideas. Inflation and "dark energy" lead to a divergence of ideas rather than a solution. Great posts.
Post a Comment
<< Home