# How do we know that the Universe is still expanding now?

Yes, I know that most galaxies have a red shift and that means they are moving away from us.

The problem is that the farthest galaxies are 13.8 billion light years away. That means that the info is 13.8 billion years old.

What has been puzzling me for years is that the red shift observation shows that the galaxies were receding 13.8 billion years ago. We don't have direct data on what they are doing now. It is natural that the universe was expanding with a tremendous speed at that time because it was near to the big bang, but how do we know that it still is?

If the furthest galaxies are moving with higher speed than the nearer, that means to me that their speed is reducing with the time.

• you probably meant 13.7 Jan 2, 2016 at 12:14
• Now is a tricky concept when there's a speed limit for information. Jan 2, 2016 at 16:05
• @VojtaKlimes: Do you really think we know the distances to three significant figures (relative distances maybe, but absolute distances?) Anyway we are talking about galaxies, the most distant of which has a nominal distance of ~13.1 Gly as of May 2015. Jan 2, 2016 at 16:06
• Cosmological models based on our best data and physical theories are used to try and understand what distant galaxies will be observed doing Gyears hence, assuming there will be anyone here to do the observing. and that there will be a here. Jan 2, 2016 at 16:10
• Jan 2, 2016 at 16:50

The expansion rate is currently — and hence locally — around $70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$. Due to the aforementioned velocity scatter, we cannot probe expansion below ~1 Mpc, but at this scale, space doesn't really expand anyway due to the mutual gravitational attraction of the Local Group. This means that in principle, the expansion of the Universe could have come to a halt within the last few million years, and still be in accord with the cosmological priciple. However, this would mean that the evolution of expansion velocity as a function of time would have a sharp kink. Again, this is not impossible, but we know of no physical mechanism which would be able to cause this.