Trying to figure out Dark Matter and Dark Energy

2 major cosmic problems

Imagine you have a bag containing a kilogram of potatoes, but when you put it on your kitchen scale, you find it registers as 10kg.

Small discrepancies are to be expected, but nothing of this size. Assuming your kitchen scale is accurate, maybe you would come up with the idea that lifting the potatoes onto the scale generated nine kilograms of "Dark Potatoes". These are otherwise totally invisible and undetectable, but explain the missing nine kilograms.

It is unlikely you would take such a possibility seriously, and would look deeper into the problem. That is basically where we are with trying to explain the structure and evolution of the universe.

Our current picture is that the universe started just under 14 billion years ago as something incredibly small, dense and hot. It started to expand and cool. Eventually it cooled enough for particles to form and there was light.

Then, as the expansion proceeded, stars and galaxies were born, and planets, and us.

It is when we started making precise measurements and comparing them with calculations that we ran into serious problems. For example, the speed a star orbits around the core of its galaxy depends on the mass of the galaxy and how it is distributed.

We can measure the orbital speeds of the stars, and total up the mass of all the material making up the stars and dust clouds in the galaxy. We can use the mass to calculate what the orbital speeds of the stars should be, and from orbital speeds of the stars calculate the mass of the galaxy.

All the numbers should agree, but they don't. The stars are moving far too quickly. They should be flying off into space rather than moving smoothly in their orbits.

The currently most accepted explanation is the masses of the galaxies containing those stars are far greater than we have estimated. That means there is a lot of mass that we cannot see—a lot of it.

The mass of material we can see makes up only 15% or so of the amount needed to explain the orbital speeds of the stars.

Where is the other 85%? If it is there, it is totally invisible. Apart from its gravitational influence, it is undetectable with any of the astronomical instruments we currently have at our disposal. This invisible material needed to get the right answer has come to be called "Dark Matter".

As in the case of the "Dark Potatoes", great efforts are being made to detect or explain this stuff, or through theoretical calculations, find another way to explain this massive discrepancy.

One suggestion is that gravity is a more complicated force than we thought. So far there is no solid evidence of that.

If our universe started as a compact, dense, hot object, which started to expand and cool, we would expect the gravitational attraction all those bits of material have for each other would gradually slow the expansion. Whether or not this would be enough to eventually bring the expansion to a halt is not clear, but it should be slowing down. It isn't. It is speeding up.

Imagine throwing a ball into the air and instead of seeing it slow down and then fall back to earth, it speeds up and vanishes into space. For this to happen, something has to be pushing it. If there is an outward force strong enough to overcome the gravitational forces trying to pull the universe back into one lump, as the expansion proceeds, that gravitational pulling will get weaker, allowing the expansion to proceed faster and faster.

This outward force has become known as "Dark Energy", which like its mysterious partner "Dark Matter", can be invoked to get the right answer, but is otherwise so far totally undetectable.

However, as in the case of the "Dark Potatoes", having to have Dark Matter and Dark Energy make up 95% of the mass/energy of the universe in order to explain the behaviour of the 5% making up the visible universe is not a comfortable solution. A great effort is going into clearing up this issue. It will take a while though.


• Venus and Mars lie very close together low in the west after sunset.

• Saturn rises around midnight and Jupiter appears low in the sky before dawn glow.

•The Moon will reach last quarter on July 9.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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