# Why do we form cosmological theories based on old data?

Since the light we receive from distant galaxies may be between 7 and 14 billion light years away, the redshift we see indicates that the universe was expanding at that time (7 to 14 billion years ago). If the universe started collapsing today, we wouldn't know it for perhaps 7 billion years. Why are we so sure the universe is still expanding (based on that old information), especially since closer galaxies like the Andromeda Galaxy exhibit a blueshift and are assumed to be moving toward us?

• Um...do you have "newer" data available? – ACuriousMind Mar 29 '15 at 0:25
• There's a lot of data out there. "Minimal" is misleading. – HDE 226868 Mar 29 '15 at 0:54
• You use whatever data you've got. Do you have a better idea? – Hot Licks Mar 29 '15 at 1:15
• @CreigKronstedt We don't think we have it all figured out; we think we have one thing figured out, and there's a lot of evidence for that one thing. – HDE 226868 Mar 29 '15 at 1:50
• I just wonder where the strange thought "all input data into our cosmological models is from galaxies at least 7 billion light years away" comes from? – Johannes Mar 29 '15 at 3:22

The thing is, we don't completely base our understanding of the expansion of space on galaxies 7 to 14 billion light-years away.

For evidence that the universe is expanding, look at Edwin Hubble's original paper in which he confirmed what we now call Hubble's law. The galaxies he studied are on the order of millions of parsecs away. Multiply that by 3.26 to get the distance to the galaxies in light-years and you're still in the neighborhood of three orders of magnitude below the figures you cited.

As for the acceleration of the expansion of the universe . . . well, that was established by looking at supernovae that are at the most 5 billion light-years away (see Saul Perlmutter's overview of the whole thing, as well as a paper on the most distant one, SN1992bi). Another overview of the search is this one, and the High-Z team has a whole bunch of their results on their website.

You can look at some results from, too (for more, see this database). The team measured, among other things, properties of the CMB, as well as other key properties (see page 6 of the last paper) of the universe, which can then be used to determine what conditions should be like today.

Also, consider the Friedmann equations and, among other things, the density parameter, $\Omega$. WMAP determined some of the related quantities such that we can then figure out the current Hubble parameter (which was also determined). The measurements fit exceedingly well with the prior observations of an expanding universe.

The bottom line? We have newer measurements than those that are 7 to 14 billion years old. Sure, Planck and WMAP did peer into the early universe. But we have newer measurements, and they tell a story that goes against the "the universe might not be still expanding" idea.

The use of the Andromeda galaxy's motion towards us as an argument against the expansion of the universe is a common fallacy. It, along with the Milky Way, form the two dominant galaxies in the Local Group of galaxies, which are bound together by gravity (primarily the gravity of Andromeda and the Milky Way). The argument is fallacious because the expansion of the universe - and, of course, the acceleration of the expansion of the universe - is really only dominant on extremely large scales. In astronomy, the two galaxies are (relatively!) close together.

• Thank you for the very informative answer. I know that are years of research that went into making this determination. I used the 7 to 14 billion light years as the outer half of the universe. My main point was that information from this distance is so far in the past, that any change that might have occurred since 7 billion light years ago will not be available to us for billions of years in the future. That doesn't mean we shouldn't do research. It means that we need to be willing to admit that whatever we learn in the future will probably have little resemblance to what we know now. – Creig Kronstedt Mar 29 '15 at 13:18
• @CreigKronstedt You're welcome. We most likely won't get data that's any better anytime soon, and at any rate, once we do learn about the conditions today, things will have changed quite a bit. – HDE 226868 Mar 29 '15 at 15:24

As pointed out in the answer above, the observations that lead to the dark energy theory were not all distant (7-14 billion light years), but less so. Dark Energy expansion is observed throughout much of the observable universe - not just the very distant.

Also, consider the basic hubble discovery - galaxies 4 billion light years away were moving way from us twice as fast as galaxies 2 billion light years away. Now, if we apply the theory of gravity to this, we can assume that gravity would slow the expansion, so the galaxy 2 billion light years away should (apparently) be moving away at slightly less than twice as fast as the galaxy 4 billion light years away - now, they didn't just look at 2 galaxies, but they looked at many galaxies at many distances to see the extent of the slowing of the expansion due to gravity, and when they did this - using type 1A supernovas - which are excellent radar-guns for distant galaxies they found something quite surprising.

They found that the galaxies 2 billion light years away are traveling more than half the speed of the galaxies 4 billion light years away, and the galaxies 3 billion light years away are moving away at more than 1/2 the speed of galaxies 6 light years away - so something is operating to counteract gravity and it's very neatly consistent through the observable universe. Expansion happens everywhere they observed.

If you can find another explanation that can tell us why this is, I suggest you publish it right away. In the mean time, expansion or dark energy is what they're calling it. That makes more sense than, oh, the speed of light used to be slower, which would also explain the observations, but that's a much less popular theory.

• Small footnote, but it's perhaps more accurate to call type 1a supernova's "standard candles" - cause they are all very similar, so they give very good estimates of both distance and relative velocity. I just liked the way "radar-guns" sounded in that sentence. – userLTK Mar 29 '15 at 2:16
• In a decelerating universe, we'd expect objects twice as distant to be moving over twice as fast, because the early universe would be expanding faster, and the expansion would be slowing down. - I don't see how that could be otherwise. But if I'm missing something, I'm happy to hear what it is. – userLTK Mar 29 '15 at 9:23
• I think I'll withdraw my objection. I am confused between "velocity" and redshifts (which is nonlinear). The gist of your argument is certainly correct, that galaxies have smaller redshifts than might have been thought. – Rob Jeffries Mar 29 '15 at 10:49