# Tired light vs. expanding universe

## Main Question or Discussion Point

I was wondering if anybody knows of any experiments that differentiates between tired light and an expanding universe?

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## Answers and Replies

Garth
Gold Member
Welcome to these Forums ntdg634!! Keep asking questions.

I was wondering if anybody knows of any experiments that differentiates between tired light and an expanding universe?
The time dilation of cosmological objects such as the light curves of super novae.

The observation of such is predicted by the expanding universe but not the tired light hypothesis.

Garth

This is a good page to read concerning tired light problems.
http://www.astro.ucla.edu/~wright/tiredlit.htm

The only steady state model I know that works involves gauging out the expansion is Weyl models of cosmology. Also called Einstein-Weyl models of cosmology. It is an extension to Riemannian differential geometry. The Weyl geometric approach has gauge freedom so the choice of gauge can actually model either interpretation of expansion. The extension is "conservative" meaning that not only is General Relativity Theory (GRT) maintained as a special case but within that special case GRT is exact.

Essentially in the Weyl model using the Hubble gauge the expansion is occurring in some sense but our measure of space covaries with the expansion such that no change in proper distance between emitters occurs with expansion.

This can be modeled with a thought experiment.

The gravitational curvature alone is not all that determines gravitational time dilation. Imagine a large hollow massive sphere. As you approach this sphere the gravitational time dilation will increase as you approach this sphere. If you pass inside this sphere then space-time will be flat inside, yet the time dilation will remain slowed to that of the surface anywhere inside the sphere. Under GRT the depth of field determines relative time dilation not the curvature.

Now imagine two observers separated inside this sphere and the mass of the sphere is steadily increasing. Inside the time dilation will steadily increase compared to a far removed observer even though the space-time inside remains flat. Now when one of our observers sends a light signal to another the signal will be redshifted because of the finite value of C (relativity of simultaneity). The second observer will receive this signal at a later time when the space-time interval has changed. This leads us to a time dependent rather than velocity dependent Hubble expansion.

This gets around the usual problems with photon energy loss tired light postulates because it is not the energy but our measurement of it that changes.