Wednesday, June 9, 2010

Cosmology Corner: Feel the Power of the Dark Side

Imagine you are lying on your back at the end of a dock at 2 am in the Tahitian islands in the South Pacific. Your feet are dangling in the warm water. There is no light pollution and not a cloud in the sky.

There is so much to see. You pick out the Centaur, the Southern Cross, and the Coal Sack. The Milky Way is a bright band of light across the sky.

Even if you are not an astronomy buff, you cannot help but be astounded by the wonders of the night sky above you.

The universe as we know it contains hundreds of billions of galaxies, innumerable nebulae, and a number of stars and other objects beyond our ability to count in one lifetime, let alone a thousand lifetimes.

What if I told you that everything that we can see up there is only 4% of everything in the universe? As incredible as that statement may be, I would not be wrong in stating this.

So what is the rest of it? Think about it. That is a whole lot of nothing compared to a little bit of something

It is currently thought that Dark Matter makes up about 22% and Dark Energy is the remaining 74%. Given what the night sky looks like under ideal viewing conditions, that is a whole lot of darkness that is the rest of that great expanse.

Considering that 96% of the universe is not visible at any wavelength, how do we know anything about it?

Dark Matter was first inferred by Fritz Zwicky in 1933. By studying the rotational curves of galaxies considered to be like our own Milky Way, he deduced that if there were not some form of invisible matter, all the stars towards the outer edges of the galaxies should be shooting off into space given their observed rotational speed. Something had to be holding all of those visible objects to their galactic plane.

Over the past 77 years more and more evidence towards the existence of dark matter has been found.

For instance, from the imaging of the Hubble Space Telescope we have another strong indicator for the presence of dark matter: Gravitational Lensing. This was observed when the HST imaged the galactic cluster Abell 2029. The mass of this galactic cluster and its associated dark matter, make it possible to ‘see’ a galaxy that is directly behind the cluster. This is because the mass of the cluster bends the light of the hidden galaxy around it. When imaged, the HST picked up the lensing arcs, and it was determined that the cluster had to contain approximately 10^14 Suns worth of dark matter to make the light from the galaxy lens in that manner.

The gentleman for whom the HST was named, Edwin Hubble, came up with what is known as Hubble’s Law, which can be used to determine how fast an object (usually a galaxy) is moving away from us. Hubble’s Law, and observations made by his namesake, shows us that the universe is expanding. This is one of the single greatest points of evidence to support the Big Bang Theory.

However, recent observations and other experiments have given us an anomaly. Evidence has arisen that shows that the expansion of the universe is accelerating.

Dark Energy is the label given by scientists to the energy that has to exist to explain this acceleration. There is some form of energy that scientists have not yet fully quantified that is making the universe expand faster with each passing moment.

Given the rate of accelerating expansion and other observations, that energy has to make up approximately 74% of the universe. Exactly what is it and how it behaves is the subject of much speculation.

Even with everything known to date about the state of the universe and its contents, we still have a lot to learn about its composition and its future.

But we do know that darkness will lead the way.


If I'm lucky this will show up in the next issue of Focus, the Valley of the Moon Observatory Association's newsletter.

1 comment:

AlisonH said...

WOW. Very, very cool--thank you.