I wish to ask you if the ISW test is useful to make prediction on f(R) models, because the cross-correlation function with f(R) models is too much similar from the standard one, given the galaxy distribution uncertainties and the error bars in the correlation between CMB and LSS.

Thank you very much

Thank you for your efforts, but I think I better give the subject a rest.

http://www.amazon.com/Introduction-Modern-Cosmology-Andrew-Liddle/dp/0470848359/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1213670159&sr=8-1

I do not intend to be rude, but we don’t want to turn every thread about cosmology into the same set of well-understood questions about the foundations.

In an expanding universe these boxes are growing in size. A resonance cavity with photons that expands in size will expand the wavelength of the photons inside.

I can certainly accept space that space can expand. As well as contract. Believe it or not, I tend to be a very credulous person. It’s easier that way. My problem has been that if space is expanding intergalactically and contracting intragalactically (or rather intragravitationally) and these two effects are in general equilibrium, how is it that the entire universe expands? I don’t want to go too far out on the limb of what seems to be my various hobby horses, since trying to explain how and why I differentiate between flat space and spacetime to Mark was deleted, but I do feel the basic foundation has some plastered over stress fractures running through it that those building on this foundation don’t seem able to explain away.
Inflation comes to mind.

But these potentials introduce some impedence varations along the transmission line. So the observer measures an incoming wave and attempts to deduce its “history” by its structure.

As with much of history, most of it doesn’t leave much evidence. Since photons of the same wave would seem as entangled as any such particles could be, is there any effect of those falling into gravity wells on those which continue their travels, such as incremental effects on their spectrum?

Pardon if I was not reading what you are saying correctly, but my understanding of the need for dark energy is based on explaining why standard candles appear to have a higher proportional redshift if they are closer, than those further away.

———————–

Think of the universe as made up of cells in space. Since on average a photon which enters a cell is equal in number and energy to those which leave we can think of these cells as perfectly reflecting boxes. In an expanding universe these boxes are growing in size. A resonance cavity with photons that expands in size will expand the wavelength of the photons inside. This has stretched out the wavelength of light which resulted after the deionization event to a microwave wavelength, which compose the CMB.

The physics here is to deduce how these boxes expanded in the past, particular prior to the deionization period. So reduce the problem of reflecting boxes or resonance cavities to one dimension. This is then a transmission line, or a string of them. If the couplers between each of these lines has “infinite” impedence then the model still holds. We can then consider the transmission line as adjusting the wavelength of a signal as it grows in length and remove the intermediate “couplers.” But these potentials introduce some impedence varations along the transmission line. So the observer measures an incoming wave and attempts to deduce its “history” by its structure. We know all about the “dark energy” accelerated expansion of the universe, but there might in addition be information in addition — analogous to these impedence variations or mismatches on the line due to these quirky potentials that vary in time and space.

Will this lead to modified gravity? Who knows!?

Lawrence B. Crowell

I didn’t understand the question at the end at all.

John, the ISW effect is in no way affecting the idea that we know acceleration is happening. It is a subtle effect on top of that (and very well understood). Einstein did not add a CC because gravity causes space to collapse – space can easily expand without one – but rather to allow a static solution. When you mention the balance in an Omega=1 universe, you are confusing a spatially flat universe (which we do seem to have) with a flat spacetime (which wouldn’t be expanding, but which we do not have). Omega=1 does not mean a flat spacetime.

Pardon if I was not reading what you are saying correctly, but my understanding of the need for dark energy is based on explaining why standard candles appear to have a higher proportional redshift if they are closer, than those further away. As you seemed to be describing it, this effect seemed due to the redshift of the further candles being somewhat reduced by blueshifting caused by intervening gravity fields, rather than a recent acceleration of the expansion.

The cmb does come from a single distance, approximately 13.7 billion lightyears away. My problem with the basic model is that gravity apparently causes space to collapse, which is why Einstein added a cosmological constant in the first place. According to Hawking and the general consensus, as well as tests on the cmb, the effects of expansion and this contraction of space are in apparent balance, Omega=1. What I don’t understand is how can the entire universe be expanding, if the effect of the expansion of space is currently neutralized by the collapse of gravity? Yes, space is expanding, but it would seem to be flowing into these gravity wells at an equal rate.

Since the only light we can measure is what actually manages to traverse these enormous distances and not what is otherwise absorbed and blocked, it is greatly redshifted, but the actual pressure of expansion would seem to be released by gravitational contraction and photon absorption, including what falls into astronomers telescopes. Otherwise it would not appear to be flat space.

I digress somewhat in order to ask a further question; Since the CMB originates at the general point where visible light ends, could there be a horizon line cause by this redshift, over which the visible spectrum cannot travel, but black body radiation can?

I guess the point I was making is that for a general scalar tensor theory, I think there may be many effects, but since those designed for cosmic acceleration are constructed to revert to GR where densities are high compared to the current cosmological one, there might be only a small effect for those.

Cheers,

That’s my point. As you describe it, given an even distribution of galaxies across the universe, the further light travels, the more gravity fields it passes through, so if these blueshift the light somewhat, this cancels that much redshift, so the proportion of redshift for closer sources is greater.

johnmerryman, the distribution of galaxies is taken into account in two ways here (this is very standard in cosmology): one is through averaging to get the background expansion and the other is through the influence of their potential wells, a small part of this I’ve described here.