Scientists have recently discounted one alternative hypothesis to the theory of dark energy http://www.physorg.com/news/2011-03-hubble-alternative-dark-energy.html .
The Hubble telescope was used to more accurately measure the expansion of the universe. It was determined that the farther you look away from Earth the faster objects are moving by a factor of 73.8 km/s for every Mpc. This accuracy allowed scientist to determine that Earth does not reside in a bubble of relatively empty space which would cause us to perceive that objects are moving faster the further we look. This would require that Earth be near the center of one of these regions of empty space.
Measuring distance was difficult but astronomers measured the apparent brightness of Type 1A supernovae and pulsating Cepheid stars. These apparent brightness compared to the actual brightness allowed for calculation of distance. This is very similar to what we did on Problem Set #2. The article mentions that a new space-based telescope the Webb telescope is scheduled to be launched in a few years and will be able to further refine our calculations of the Hubble constant.
The idea is that the more accurately we can define the Hubble constant the more that we can determine is the cause of the acceleration instead of deceleration of the expanding universe.
Jeff Chandler
Very interesting. The primary method by which the acceleration of the universe's expansion was discovered, and the method used to measure it here, is exactly the same method in we've seen in the problem sets: if you know how bright something is meant to be, and you observe how bright it actually is, you can measure its distance. Of course, the difficult thing here is knowing how bright an object is supposed to be. For dark energy, the primary evidence comes from looking at the brightness of Type Ia supernovae. We know the distance to the closest Type Ia supernovae by independent methods, so we can measure their brightness. The assumption is then that all Type Ia supernovae, wherever they occur in the universe, are always the same brightness. There are physical reasons why this is plausible, but it is still up for some debate. However, the consensus now is that the basic result that lead to the need for dark energy is sound.
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