Dark energy

Ken Tapping, June 3rd, 2015

In the sky this week…

  • The western sky after sunset is dominated by Venus (lower and brightest) and Jupiter (higher and less bright).
  • Saturn lies low in the southeast.
  • The Moon will be Full on the 2nd.

Imagine you could throw a ball straight up at 150 km/h. After a little over four seconds it will reach the peak of its trajectory, around 88 metres above the ground, and then start to fall back down. This is what we would expect to happen. Something you would not expect is for the ball to continue accelerating after it left your hand, and vanishing into the clouds. This is why astronomers were very surprised to find that the expansion of our universe is not slowing; it is getting faster and faster.

Just under 14 billion years ago our universe was extremely small, very hot and unbelievably dense. Then something made it start to expand. That event is often referred to as the Big Bang. As the young universe expanded, it became less dense and cooler, until galaxies and stars started to form. Since every piece of matter in the universe is exerting a gravitational attraction on all other pieces of matter, there is a force resisting the expansion, and which should be slowing it down.

Until very recently one of the big questions in astronomy was whether the expansion of the universe will continue for ever, or whether it would come to a stop and everything would start to fall back, leading to a Big Crunch sometime in the future. Is there enough matter in the universe to exert enough gravity to stop the expansion? People counted stars, galaxies, gas clouds and so on, to see how much mass there is. The conclusion was that there is not enough material in the universe and that it will expand, slowing but never stopping. This logic and the conclusion it led to have been proved very wrong by more recent measurements. There is far more mass in the universe than we thought, most of it being Dark Matter, which we looked at last time. However, the expansion of the universe is accelerating, not slowing down. High School physics tells us that a thing will only accelerate if there is a force acting on it. In the universe we know of gravity pulling inwards, but for the expansion to accelerate there has to be a strong outward force. What is it?

The concept of "Dark Matter" was introduced as an extra factor to make the physics fit the observations. This is not satisfactory science and there are great efforts being made to get more solid evidence of its existence. In a similar way, the suggestion was made that the accelerating expansion of the universe is due to something called "Dark Energy". As before, it makes the physics work, but what is it really? There are several theories being tested through additional observations. However, the most intriguing one dates back to the work of Albert Einstein.

When Einstein was trying to calculate what the universe is like, he kept getting one that is expanding or collapsing. However at the time scientists thought the universe was static. If a universe is static, it will immediately start to collapse under its own gravity. To stop this from happening Einstein introduced an outward force, which he called the Cosmological Constant. He chose the force to be just strong enough to stop the collapse. Then, when the universe was discovered to be expanding, the need for Einstein's outward force vanished.

Then, in the 1990's we found that not only is the universe expanding, it is doing so faster and faster. One proposal is to bring back Einstein's outward force. Instead of making the force just strong enough to stop the collapse, we make it far stronger. This would cause the expansion of the universe to accelerate. Fortunately, today we know how to test this idea through independent means. We are doing experiments to see if this force exists. If it does, we'll measure it and see if it is strong enough to explain how the universe is expanding. Otherwise we will need a different idea.

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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