Expert: Courtney Seligman - 11/15/2010

Question
I have been reading a lot on the origin of the universe and often come across the reference to the acceleration of the galaxies(measured by the red shift)and hence the expansion of the universe. I have never come across reference to the fact that the accel. of the furtherest galaxies, while being the greatest(accel.) are also the oldest. In other words the red shift is surely that of a long time ago and hence we have no clue as to what a distant galaxy is doing right now. How can we therefor know that the universe is still expanding. (even the closest galaxy's condition is seriously far away in time.)
Can we possibly have reached a point of maximum expansion already and now be decelerating?

Answer
The redshift is caused by the expansion of the space the light is passing through, on its way here. The further the light has to go to get here (that is, the farther away the galaxy from which it came was at the start of its journey), the more space it has to go through. The more space it has to go through, the longer it takes to pass through it, and the more the space expands during that time. As noted at the start, it is that expansion that causes the wavelength of the light to "expand", or "shift" toward the red end of the spectrum.

In other words, if a galaxy is far away, the light will take a long time to get here, and during that time it will suffer an expansion of its wavelength corresponding to the expansion of the space it moved through during that time. As a result, the farther away the galaxy is, the more the light is "redshifted" by the expansion of space, on the way here. The redshift is not caused by the speed at which the galaxy was moving away from us at the time the light left it; but by the expansion of the space between the galaxy and us, during the time it took to get here. That doesn't mean that the galaxy isn't moving away from us now, or even in the past; it just means that what we are measuring is an apparent motion away from us, caused by the expansion of the Universe over the time it took the light to reach us.

Now suppose that expansion was not uniform, but gradually changing in one way or another. Then for galaxies at larger and larger distances, the expansion of space wouldn't quite match their distance. If the Universe were slowing down, it would have expanded faster in the past, and galaxies whose light took most of the age of the Universe to get here would show a bigger redshift than we would expect. If the Universe were speeding up, it would have expanded slower in the past, and galaxies whose light took most of the age of the Universe to get here would show a smaller reshift than we would expect.

As a result of this, if we knew how far away different galaxies were (at the time their light left them), and the Universe was always expanding at the same rate as now, we would find that for galaxies at larger and larger distances, the redshift would increase uniformly. Whereas if the Universe expanded faster in the past, we would find that the redshift is bigger than expected for very distant galaxies, and if the Universe expanded slower in the past, we would find that the redshift is smaller than expected for very different galaxies. (This "faster" or "slower" expansion refers to the average rate of expansion of a unit of empty space. Over time, as the Universe gets bigger and the galaxies get farther away, their motion away from us gets larger, even if the rate of expansion per unit of distance stays the same.)

Until the 1990's, astronomers were divided about which of the three possibilities mentioned above might be correct, as there was no reliable way to measure the distance of very distant galaxies. In fact, their distances were generally estimated by using their redshifts. Galaxies with larger redshifts were presumed to be farther away, but their actual distances couldn't be measured. However, in the 1990's it was discovered that a certain type of supernova has a characteristic brightness which can be used to establish a probable distance for galaxies at any distance. That allowed us to compare the redshift to the distance, and see whether the Universe is slowing in a way which would eventually lead to the collapse you mention at the end of your question, or slowing a little but not enough to ever stop expanding.

The answer was a big surprise (although actually, it is predicted by Einstein's Theory of General Relativity, so it shouldn't have been a complete surprise): For about the first half of its current age, the Universe WAS slowing (although not enough to ever reverse the expansion), but over time the slowing became less and less, until it disappeared completely; and for about the last half of the current age of the Universe, the expansion has actually been going faster and faster. So it will never stop expanding, but will instead expand at a gradually increasing rate, for the rest of eternity.

The easiest way to understand this is to realize that when the Universe was young, all of its mass was relatively close together (compared to now), so the gravity of that mass could fight the expansion, and slow it down. But as the Universe got bigger, the gravitational effect of its mass was reduced, while its tendency to expand remained the same. Eventually (several billion years ago), the two effects became the same, and the slowing disappeared. Later (including now, and into the far future), the larger and larger size of the Universe made and will continue to make the effect of gravity smaller and smaller, so that the natural tendency of the space between the galaxies to expand no longer has any significant opposition, and the expansion is approaching a maximum rate, equal to the rate at which it would expand if the Universe had no mass at all. In fact, we are already 70 to 80 percent of the way to that point. That is, the expansion rate is already about 70 to 80 percent of the maximum value it can ever have; but it will gradually continue to increase its rate of expansion, until it is 100% of the maximum rate.

With the above as "background", the answer to your question is that although we can't SEE what the most distant galaxies are doing now, we CAN tell how the expansion of the Universe has changed over time. So we can CALCULATE how far away they must have been at that time, and how far away they must be right now. The result of that calculation is that they are much farther away now than they were at the time their light left them, because (1) they have been carried away from us by the expansion of the intervening space ever since their light left them and (2) that expansion is even faster now than it was in the past.

NOTE: There are actually three distances involved in discussing how far away distant galaxies are: (a) the distance their light had to travel, through expanding space, to reach us (that's the distance usually quoted); (b) the distance they had at the time their light left them (which is smaller than the distance the light had to travel, because the space the light traveled through expanded while the light was on its way here); and (c) the current distance of the galaxy (which is larger than the distance the light had to travel, because of the expansion of the Universe during that time). There is a brief discussion of this triplet of distances at http://cseligman.com/text/galaxies/lighttravel.htm

At this point I want to apologize for sending an answer which could use some editing -- paring it down in places, and explaining things in more detail in other places -- to make it clearer. But I have to leave in a few minutes and won't be back until late tonight, and I didn't want you to have to wait until then (or more likely, tomorrow) for an answer. If there is any part of this which you would like better explained, let me know and I'll be happy to do what I can to improve on this answer.