# Info on tests done in the past for expanding universe

1. Oct 5, 2013

### scott22

I am hoping that someone with broad knowledge of the physics literature can tell me when in the past tests were done on light coming from galaxies billions of light years away, tests that over time counted a decreasing number of photons from a single galaxy, or an increase in the wavelength of light from a single galaxy. It seems that an expanding universe would exhibit one or both of these characteristics, and I just assume these tests must have been done long ago. I am having no luck finding anything with the databases available to me and I was hoping someone could let me know where to look. Thanks.

2. Oct 5, 2013

### mathman

The origin of the notion is due to Hubble. He observed that there appeared to be a correlation between red shift of light from galaxies and how far away they were.

http://en.wikipedia.org/wiki/Edwin_Hubble

3. Oct 5, 2013

### Bandersnatch

The kind of evidence you're asking for doesn't exist, as it would require (currently)impossible accuracy to measure.

Consider a galaxy shining with light intensity I0 that was 7 billion ly away 100 years ago, and is receeding at 0.5c. Over 100 years of observation the intensity of light emitted by the galaxy would fall according to the inverse square law by:
$\frac{I_{100}}{I_0}=\frac{R^2}{(R+ΔR)^2}$

ΔR is 50 ly, so
$\frac{I_{100}}{I_0}=0,999999986$
or 0,0000014% lower intensity. Good luck measuring that, especially for an object already very faint.

Measuring the changing redshift of a single object is a similar problem. The time scale for observation is just not enough.

The evidence for the Big Bang is collected using other methods. Here's a review:
http://www.talkorigins.org/faqs/astronomy/bigbang.html

The closest to what you want is described in the "Tolman tests" section, but it compares standard candles, not the same object at different times.

4. Oct 13, 2013

### Jon B

Using the unshifted indicators in our local galaxy as a reference for the shifted spectrum of light from objects at the most extreme distances does not seem to take into consideration the affect that the gravity of our galaxy has on light. We can only observe what is in our local area and make assumptions based on that observation. Then we assume we are at the center of the universe and somewhere out there is an edge.