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Author Topic:   Cosmic Background Radiation in Big Bang cosmology
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 1 of 2 (192836)
03-20-2005 3:40 PM


In Message 165, Lyndon Ashmore asks about inflation and spacetime geometry. Reminder; I spell my name with a "y". It's not important, but spelling it right helps me find posts I need to deal with.
He says:
I am sure that Silas is aware that he has dug himself into a hole here. For inflation (which Silas rightly says no one knows how it started they are only grateful that it explains the effects) one needs a ‘flat’ universe and indeed most now believe this to be the case.
However to explain where the energy went I believe that he needs space to be curved.
The two replies are mutually exclusive. One must be wrong!
The resolution is simple. Ashmore is wrong that curvature is required or has anything to do with an energy problem. Expansion explains the CMBR; not curvature.
Which one is it Silas? Can we have consistent answers to the two questions of the Horizon problem and the where did the energy go problem?
Already given in the post Message 153 in the other thread. It was off topic there; so I'll say it again here.
There are essentially two answers for where the energy goes to compensate for a loss of energy associated with the cosmological redshift. You can think of the loss as being compensated by a kind of gravitational potential energy bound up in the expansion of space. This means considering a kind of kinetic energy bound up the rate of expansion and a kind of potential energy in how far it has proceeded.
However, for a full understanding of the matter one really has to get to grips with the fact the energy conservation is not quite the same in general relativity and in special relativity.
In general relativity, you don't actually have a good handle on energy conservation except as a localized phenomenon. Put another way, it is crudely analogous to the energy loss in a Doppler shift. The energy "loss" in regular Doppler shift is simply a consequence of a Lorentz transformation between frames. Imagine then a non-inertial frame. There is no conservation of energy in such a frame.
GR does away with frames altogether. There is no single inertial frame for the universe; just local approximations at every point in spacetime. So you can think of the energy "loss" as a kind of artifact arising as a consequence of transport from one frame to another. But I'd love to get a more informed comment from others on this.
This matter of energy conservation is something that really worries people. But energy conservation is simply an abstraction to capture certain invariants in physics. We relate all kinds of diverse phenomena ... motions, placement in a field, mass, radiation, etc; with an quality called energy, and use the connection to express an invariant property. The invariant works out in a locally Euclidean geometry, but it becomes poorly defined over a non-Euclidean geometry.
A discussion of the issue in a lot more detail is available at Is Energy Conserved in General Relativity? by Michael Weiss and John Baez.
The CMBR in BB cosmology is radiation from the "surface of last scattering", which was some 380,000 years after the singularity, and long after any inflationary epoch. The homogeneity of the hot plasma from this epoch is explained in conventional cosmology by an inflationary epoch very very early in the universe's development, which homogenized the universe to well beyond the cosmological horizon. It also drove the universe to the point of a flat geometry, or at least incredibly close to it, to within many orders of magnitude.
Cheers -- Sylas

AdminJar
Inactive Member


Message 2 of 2 (192838)
03-20-2005 3:47 PM


Thread copied to the Cosmic Background Radiation in Big Bang cosmology thread in the Big Bang and Cosmology forum, this copy of the thread has been closed.

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