How do we know that the universe expands?

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In summary, redshift is not the only reason we believe that the universe is expanding. While it was a major factor in the 1930s, we now have multiple pieces of evidence such as the cosmic microwave background, galaxy evolution, and the Baryon Acoustic Oscillation peak that support this idea. These pieces of evidence were not developed on a whim, but rather through extensive research and observations. While the current model is the main contender, new evidence could potentially change our understanding of the universe.
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Is redshift the only reason? And why cannot redshift indicate whatever else than speed?
 
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Redshift isn't a reason why the universe expands; the fact that light is redshifted simply tells us that the universe is expanding.

What else do you suppose it can indicate?
 
  • #3
Maybe a bad formulation. I try again: Is redshift the only reason that we believe that the universe is expanding?
 
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In the 1930's yes this would be true. But we have many pieces of evidence today. These include:
* The temperature and anisotropy angular power spectrum of the cosmic microwave background
* The observation of galaxy evolution with redshift
* The observation of elemental abundances that agrees with Big Bang Nucleosynthesis
* The Lyman alpha forest (in Quasar spectra) evolution with redshift and the Gunn-Peterson test
* Time delays in gravitational lens systems (which give us a measure of the Hubble constant as a funciton of redshift)
* Observation of the Baryon Acoustic Oscillation peak seen in galaxy redshift surveys (evidence of the hot origin of the early universe)
* The observation of the evolution of structure in the Universe with redshift
* Agreement between the amplitude of structure in the CMB and the Universe today (When evolved forward with simple theory)

There are many more but that should keep you busy. It is not on a whim that the ideas of modern cosmology have been developed, they have been developed precisely because of the overwhelming evidence. Alternative ideas were reasonable when there was less evidence, for instance Fred Hoyle's steady state model, however new evidence ruled these out leaving the current model as the main contender at present, but that could all change with new evidence.
 

1. How do we measure the expansion of the universe?

Scientists use a variety of methods, such as the redshift of galaxies and the cosmic microwave background radiation, to measure the expansion of the universe. These methods involve observing the light emitted by distant objects and analyzing how it has changed over time, which provides information about the rate of expansion.

2. What evidence supports the theory of the expanding universe?

Aside from the measurements mentioned above, scientists also have evidence from the observation of the distribution of galaxies and the abundance of elements in the universe. These observations support the idea that the universe is expanding and has been doing so since the Big Bang.

3. How does the expansion of the universe affect the objects within it?

The expansion of the universe does not significantly affect objects on a local scale, such as within our own galaxy. However, on a larger scale, it can cause the distance between galaxies to increase over time. This is due to the stretching of space itself, rather than the movement of the objects within it.

4. How do we know that the expansion of the universe is accelerating?

Scientists have observed that the rate of expansion is increasing, rather than slowing down as expected. This was first discovered through observations of distant supernovae, which showed that they were moving away from us at a faster rate than predicted. This evidence, along with others such as measurements of the cosmic microwave background, supports the idea of an accelerating expansion.

5. Is there an end to the expansion of the universe?

It is currently unknown if there will be an end to the expansion of the universe. Some theories suggest that the expansion will continue indefinitely, while others propose that it may eventually slow down or even reverse. More research and observations are needed to better understand the ultimate fate of the universe.

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