# How certain do science believe the universe is expanding?

The continue and increased rate of expansion is backed up by red shift, but that can also be true for a slowing down universe. Can the acceleration of expansion be reducing, still causing red shift since it's accelerating and eventually starts decelerating and for a moment the entire universe stop?

• I don't understand the second sentence. Please clarify. – Allure Jun 12 '18 at 7:27
• It is unclear what you are asking about. Are we certain the universe is expanding? Yes, extremely certain. Are we certain, that expansion is accelerating? Pretty certain. Do we know anything certain about the second derivative of the expansion? Less so. – mmeent Jun 12 '18 at 7:27
• If you believe in the $\Lambda\text{-CDM}$ cosmology and that the supernovae observations are valid, then the universe accelerates. Otherwise it does not. – safesphere Jun 12 '18 at 8:43
• Scientists do not believe this. They have a model of the universe based on observation. The predictions of this model agree only with observation if the model states that that the universe expands. – my2cts Jun 12 '18 at 8:53
• I was asking if the only reason universe to say universe is expanding and will continue to be so is based on red shift, then there is a possibility that the acceleration I decreasing and yet it has a positive magnitude it may contribute to red shift. But in in a later time if the acceleration reaches zero and starts to decelerate then at some point the velocity will reach zero, can that be a possibility? – STAIN Jun 12 '18 at 11:37

In this context, the Hubble diagram can only be explained by accelerated expansion. This observation is backed up by theory. If you look at standard model of cosmology using Friedmann's equations, you can see that in a dark energy dominated universe like ours, you get exponential acceleration for the Robertson-Walker scale factor $a(t)$, which is a measure for the expansion of the universe. There is no hint in these equations that this acceleration will eventually stop.