How do we know for sure the universe is expanding?

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mike

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According to the Big Bang theory the very fabric of space is expanding, which is considered to be the reason for the red shift of the spectrum from distant stars.

But can we be absolutely sure that's the reason?

Is it possible to somehow measure the expansion of space on a local scale (e.g. between satellites in space)?

Is there any other theories (perhaps less likely) that can explain this phenomena?

For instance, how can we be absolutely sure it's not *we* (matter) who are shrinking?
(I don't really believe we are shrinking, but I'm just curious if there are any other theories about this, however unlikely...)
 
Interesting question. But the Big Bang is the only theory which explains not only observed cosmological motion but also background radiation, equivalent background temperature, and differential time flow.

Doug
 
Nothing in science is ever said to be known with absolute certainty. There are no dogmas in science; all findings about the natural world are subject to revision in the face of new evidence.

That said, we can be relatively certain that the universe is expanding due to a number of observations, most importantly the Doppler Effect - the wavelength of light emitted from an object moving away from you is lengthened, or red shifted. Likewise, light from an inbound object is blue-shifted - its wavelength is lengthened. Furthermore, the chemical composition of the light source can be determined by splitting its light through a prism, which reveals its spectral lines. These lines are equidistant from one another for each element, irregardless of what the Doppler shift is. Thus, we can confidently say that distant objects are moving away from us regardless of where we look in the sky, and that the universe is therefore expanding.

This has been confirmed through numerous other independent lines of inquiry. Examples include:

- The presence of a cosmological horizon.
- Background radiation.
- The abundance of primordial elements, which fits with the predictions of the Big Bang theory.
- The large-scale structure of matter in the universe, which likewise fits these predictions.
- Radiometric dating.
- Enormous numbers of mathematical inferences far too complicated to describe here that fit with the theory's predictions, and have been physically verified through astronomical observation.
 
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