Is redshift the only reason? And why cannot redshift indicate whatever else than speed?
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.
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.
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.
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.
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.