The outskirts of Elko are wonderful places to view the Milky Way. This dim band of light that spans our dark skies is really a continuum of stars that visibly blend together and represent a central looking view of our galaxy.
First described by Galileo to be composed of actual individual stars, the Milky Way points out how many other solar systems there can be within our home galaxy, each with their own sun and orbiting planets. Estimates of 200 to 300 billion active stars going around in a spiral are pretty common nowadays, although this is constantly changing with every new telescope and space probe.
But, by taking detailed telescopic measurements over a hundred years, we know our position within this structure pretty well. Our Sun, like all other stars within our galaxy, orbits the center in a nearly circular path that takes approximately 225 million years to complete one revolution. This is quite long compared to human time frames and during the entire life of the Earth our Sun and solar system have made only 20 orbits around. Although we travel at the fantastic speed of 140 miles per second in our journey, we have only gone a fraction of a full orbit since humans first appeared and looked up into the night sky.
When looking at a photograph of a typical spiral galaxy (and there are millions out past our own), a compelling question would be: why do we stay in this assembly and move around in a circle? Obviously we must be attracted to something in the center that provides the centripetal force necessary to keep us moving in this whirlpool of sorts.
Experience tells us that you need to hold on tight when standing on the edge of a merry-go-round, otherwise you will be thrown outward. The same effect occurs in outer space where our Sun and its collection of nine planets must be pulled by some form of gravity towards the central region 50,000 light years away.
This all makes sense and you can perform a simple experiment to check a similar circular pulling mechanism. Lay a bicycle down on its side so that the front wheel is still free to rotate. Attach a tiny magnet onto the metal rim. If you spin the wheel the magnet holds fast because the force required to move the magnet in a circular path is less than that developed by the magnet’s attraction to the iron rim. There comes a point, however, where if you increase the spinning the magnet will fly off because the attraction to the iron wasn’t strong enough to keep it moving around in a circle.
Since our Sun and solar system is not flying off — as far as we know — the gravitational force attracting us to the center must be large enough to hold us steady at a constant radius just like the magnet on the rim. Where does this force come from? It has to be the pull exerted by the millions of stars in the center of our galaxy. By adding up all of their tugging force we have enough.
There is only one BIG problem with this picture and it has left astronomers scratching their heads for over thirty years: There just aren’t enough stars in the center of our galaxy to hold onto us. We can only account for 10 percent of the necessary stars.
Well, no worry. It’s easy to make up explanations. Within the last twenty years astronomers have conjectured that there must be some extra matter at the center of our galaxy that we simply cannot see. Because it does not shine as normal stars do they have coined the term for it: Dark Matter.
Remember, whatever it is the amount has to be really huge because this is where 90 percent of the mass of our galaxy is! Being that large you would think it would periodically block the light from other central stars but that has never been seen, thus the elusive dark matter is invisible too.
Shades of the aether!
You may recall this was the theoretical material used in several 19th century theories to explain a medium through which light could travel at such a high speed in a vacuum. It had to be a solid to allow light to move along, yet at the same time it had to be pliable enough to allow the planets to orbit the Sun without slowing down. Doomed from the start, the luminiferous aether theory was finally laid to rest by the zero result of the Michelson–Morley experiment of 1887.
Some scientists speculate that there could be a tremendous black hole in the center of our galaxy providing 90 percent of the pull, and for one brief shining moment after the 2015 detection of gravitational waves from colliding black holes, astronomers held out hope that the universe’s mysterious dark matter might consist of millions of black holes sprinkled throughout the universe. Unfortunately, because of a paper in the Physical Review Letters published this month by Uroš Seljak, a UC Berkeley professor of physics and astronomy, these hopes have now been dashed.
Based on his team’s statistical analysis of 740 of the brightest supernovas discovered as of 2014, and the fact that none of them appear to be magnified or brightened by hidden black hole “gravitational lenses,” the researchers concluded that primordial black holes can make up no more than about 40 percent of the dark matter in the universe.
The results suggest that none of the universe’s dark matter consists of heavy black holes, or any similar objects.
So we push on. Dark matter is one of astronomy’s most embarrassing mysteries and still, to this day, no one can find it.