I realize the further we look into the universe the further into the past we are seeing and thus if we attempted a complete map of that universe we would be seeing it as a patchwork of different points in its history.
In the universe of Star Trek they clearly have maps of the universe that exists in their present. It would be impossible to run a galactic federation without such maps. My question is will it ever be feasible to produce such a map in the future, showing the universe as it is now rather than as we see it?
I’m no expert but from what I’ve heard about general relativity, there is no well defined concept of ‘now’. I think ‘now’ depends on your relative velocity to other points in space somehow. Never really understood this so I’m looking forward to a response from an expert.
Sent from my iPhone using Tapatalk
On a galactic scale such a map would not be particularly problematic, in the sense that cosmic expansion does really come into play on the distance scale of the galaxy. Though you’d still have to contend with the fact that in the absence of some possibly relativity-busting warp technology that communication from the edge to the centre of the galaxy takes about 50,000 years.
The hard boundary to the volume of space that we could draw a map up of its current state in the future (without using extrapolation of its past state) is the cosmological event horizon, which is a sphere of about 15 billion light years radius (slightly larger than the age of the Universe times c). This is is significantly less than the volume of our current observable Universe, which is a sphere of radius of about 45 billion light years.
You’re right: there is no absolute definition of now. If we view spacetime as being somewhat analogous to a joint of beef, each “now” would be a thin slice of beef. However there are multiple ways in which we can slice the joint of beef to make up all our “nows” and in fact general relativity allows the joint of beef to have such weird shapes that sometimes we can’t even slice it into “nows” at all!
However many spacetimes and particularly cosmological spacetime have nice symmetrical “shapes” that lend and suggest a preferred way of slicing them, so “now” can be given a perfectly logical meaning in cosmology.
There is no local definition of “now” for points separated from us spatially, but on the scale of the whole Universe, it’s fairly simple. Take the time since the Big Bang, right here (if you want to be nitpicky, it’s the proper time). Now take all other points in spacetime for which the proper time since the Big Bang is the same as it is here. You can with fairness call that set of points in spacetime “the current Universe”.
Really, though, what the OP is asking about is our ability to predict evolution of astronomical structures like galaxies. If we look billions of lightyears out and see a galaxy as it was billions of years ago, how well can we extrapolate what it would look like billions of years later? We can make models of this, but the tricky part is that it’s very difficult to test these models. The best we can do is to test them statistically: We can look at the full set of, say, ten-billion-year-past galaxies, run our models five billion years into the future, and then compare what we see to the full set of five-billion-year-past galaxies we can see.
To clarify, my proposed scheme would make a map of the Universe as it looks in 2016, but this map would not be available until the (very) far future. You could imagine maybe lots of very localized alien civilizations who have maps of their immediate surroundings in 2016 “phoning in” their maps to a collator at a central point who combines them to form a map of the Universe. The hard limit of such a map wold be a sphere centred on the collator with a radius of about 15 billion light years. In fact the maps from the alien civilizations on the very edge of the sphere would take an arbitrarily long time to arrive at the collator, due to the expansion of the Universe.
You could in fact possibly improve on this by maybe taking a map from say 500,000,000 BC from outside the 15 bn ly radius sphere and extrapolating it forward to 2016, which would allow you to map a slightly bigger volume of the Universe as it looked in 2016. If you allow this extrapolation back to its logical conclusion then your limit would be the observable Universe (i.e. a sphere of approximately 45 billion ly).
On the other hand if you wanted to make a map of the Universe in 2016 using this scheme, with no extrapolation, but at some definite point in the future (rather than at some arbitrary far point in the future), that would depend on where your past light cone intersected with “2016”