A dark puzzle

How would you feel if this happened to you? After spending a while in the yard tossing a ball to and fro, you threw the ball straight up. On the basis of previous experience, you would expect the ball to slow down as it got higher, stop rising, and then accelerate downwards again, thanks to the force of gravity. However, what happened instead is that the ball accelerated upwards, going faster and faster, until it vanished somewhere in the sky.

Of course you don’t expect that to happen. After the ball leaves your hand, the only force acting on it is gravity, which is trying to pull it back down. To make it accelerate upwards there has to be another force, pushing upwards, and stronger than gravity ís downward pull. That’s exactly how a rocket rises; the upward thrust of its engines exceeds the downward pull of gravity. However, the ball has no engines and should not accelerate upwards. If this were to happen, it would be a great puzzle. However, over the last few years astronomers have been running into exactly the same sort of puzzle.

In the 1920’s Milton Humason and Edwin Hubble showed our universe to be expanding, with a velocity that increases with distance. This is exactly what we would see if we lived on a raisin in an expanding cake, and measured the distances of the other raisins and how fast the rising dough was carrying them away from us. In the same decade, Monseigneur Georges LemaÓtre, a senior Jesuit, examined their data and concluded that billions of years ago, the universe was concentrated in one, very small, extremely dense lump. In an event now often referred to as the Big Bang, that lump started to expand, eventually developing into the universe we see around us today. Improving instruments have yielded much larger quantities of data, and of much higher quality, and today it is generally accepted that LemaÓtre was right. This leads us back to the situation we experienced in the backyard. Since all the bits of universe are pulling at each other, their mutual gravitational attraction resists their moving further apart. We would therefore expect the expansion to be gradually slowing down. For many years the main science question was whether the bits of the universe are moving too quickly for gravity to stop the expansion completely, or whether the expansion would slow, stop and reverse, leading to everything falling back together in a Big Crunch, perhaps followed by another Big Bang. This idea of a series of Big Bangs and Big Crunches avoided the need to explain what was going on before the Big Bang.

However, all that improved data is telling us something bizarrely different. The expansion of the universe is not slowing; it is speeding up. That means there is an outward force pushing it, and as the everything gets more spread out, gravity weakens, the outward force has less resisting it, and the expansion accelerates. We can calculate the strength of this outward force, but we have no idea what it is. We have however given it a name, “Dark Energy”. A lot of “scientific energy” is going into finding out exactly what this strange force is.

Jupiter is low in the southwestern sky after sunset. Saturn rises around 8 p.m; Venus is very low in the dawn twilight. The Moon will be full on the 19th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.