From Robert P. Kirshner, “The Universe, Dark Energy and Us” (NYT op-ed piece of October 7th):
Because light takes time to get to us, we can see into the past by observing distant objects. In the recent past (say, the last five billion years) we see acceleration. But if we could look far enough into the past, then the balance should tip — the dark matter should be denser when the universe was a smaller place, while the dark energy, if it resembles the cosmological constant, should hold steady. This would make the universe slow down.
Note “the recent past (say, the last five billion years)”. Not what most of us would think of as the recent past. But then Kirshner is a professor of astronomy at Harvard, and astronomers take a longer view — and they aren’t above making a little academic joke about that.
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