Ken Tapping, July 24, 2013

In the sky this week…

  • Venus lies low in the west after sunset. Saturn lies in the Southwest overnight.
  • Mars and Jupiter lie close together in the dawn twilight, with Mercury lower down. Look for Mercury low in the dawn sky.
  • The Moon will reach Last Quarter on the 29th.

Our universe is expanding. By measuring the velocities and distances of distant galaxies as they move away from each other, we can calculate that just under 14 billion years ago everything we see around us in space was together in one small, hot lump. Then, in an event we call the Big Bang, that lump started to cool and expand, eventually becoming the universe we see around us today.

The discovery of the expansion is credited erroneously to Edwin Hubble. In fact the credit has to be shared among a number of observers. However, it is intriguing to consider that the idea of an expanding universe could have been arrived at in the 18th Century, with the work of Isaac Newton.

One of Newton’s great ideas was that every object in the universe attracts every other object with a force related to their respective masses (he called this force gravity), which decreases as the objects get further apart. His ideas made it possible to understand and predict the motions of planets, stars and galaxies, and to navigate our spacecraft around the Solar System.

Newton’s ideas suggest immediately that a static universe is impossible. If everything in the universe were attracted to everything else, the universe would collapse into one lump. If this is not happening, it can only be that the universe is expanding, with gravity gradually slowing it down. If the expansion is fast enough, it can continue forever. If it isn’t the expansion will slow, stop, and then everything will start to fall back. However, no one looked seriously at the ramifications of Newton’s ideas until centuries later.

Newton had only described what gravity does, and made no suggestion as to what it is. This question attracted the attention of Albert Einstein, and in the early years of the 20th Century, he attacked the problem. At the time, not everyone accepted the idea of an expanding universe, and Einstein was one of those who didn’t. He was therefore very unhappy when his calculations, in addition to accounting for gravity, also indicated the universe had to be expanding. So he added a fudge factor to make those equations describe a static, unchanging universe. When the observations of the expanding universe became irrefutable, he described his fudge factor, which he called the Cosmological Constant, as his biggest mistake. Actually, it turns out that he had added the right thing to the equations, but for the wrong reason.

If we ignore the fudge factor, both Newton’s and Einstein’s ideas predict an expanding universe, but one where the expansion is gradually slowing. The idea makes sense. If we throw a ball in the air, it gradually slows, stops and then falls back down. However, observations made over the last few years show something distinctly different; the expansion is speeding up.

This can only happen if something is pushing outward hard enough to more than counter the gravitational attraction trying to slow it down. This strange, outward force is now known as Dark Energy. Ironically, Einstein’s fudge factor is exactly what is needed to incorporate dark energy into our calculations.

Oversight led us not to realize the universe is expanding for a couple of centuries after Newton did his work. Then, Einstein’s attempt to deny this expansion gave us the means to incorporate dark energy into the expansion calculation. Science is a complicated process, which often wanders off course, with unpredictable results. It is rarely the orderly process they tell us about at school.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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