Kurt Ringwalt
The Prairie School
Capstone Project
​
The Most Powerful Force in the Universe
The blue oval represents Supernova Cosmology Project data, which has solidified the current model of dark energy and constrained the theory's uncertainty significantly.
Move One: Discovery and Background Information
When discussing the large-scale structure and behavior of the universe, it’s important to have some context. For example, units like kilometers become almost useless as one leaves the inner solar system. A more useful unit of distance within our galaxy is the light year, or cyr (c is the constant used to express the speed of light). For reference, it takes light one about second to travel to the moon and back and eight minutes to reach Earth from the sun. Dark energy, however, is only really observable on a much larger scale through intergalactic interactions. On this scale, the most useful unit is the megaparsec, or Mpc, which is equal to about three million light years. For reference, the nearest galaxy to our own is the Andromeda galaxy, which is just under one Mpc away.
The name ‘dark energy’ is simply a way of saying ‘force we don’t entirely understand.’ What we do understand about dark energy has come primarily through the Supernova Cosmology Project. My professor, Shane Burns, actually worked on this project as a graduate student. Two basic concepts are crucial for understanding how this project proves the accelerated expansion of the universe: the behavior of type 1A supernovae and the redshift of light. A supernova occurs when certain classifications of stars ‘die.’ The term supernova refers to the resulting explosion. Type 1A supernovae are the brightest and most powerful. It turns out that despite variations in the masses of stars, type 1A supernovae always have almost exactly the same peak luminosity (brightness). This knowledge was instrumental in proving the existence of dark energy.
A graph of brightness of type 1A supernovae versus time
Understanding redshift was also a crucial part of this project. First, a bit more background: the different colors of light that we see have correspondingly varying wavelengths. Red light is on one end of the visible spectrum near infrared light and has a longer wavelength than blue, which is on the opposite end of the spectrum near ultraviolet light. Now, imagine an object at rest begins rapidly accelerating away from you. If it was emitting blue light before it began moving, the wavelength of this light would be elongated more and more towards the red end of the spectrum the faster this source of light was moving; it would be redshifted. Combining this knowledge allowed SCP physicists to observe type 1A supernovae, measure their redshift and luminosity, and calculate their distance and the velocity at which they were moving relative to the earth. Basic physics formulas that relate redshift to velocity and observed luminosity (since the peak luminosity at a distance of 0 is constant) to distance allowed the correlation between distance and velocity to be studied. The conclusion was surprising to everyone: the original calculation of the Hubble constant was 71 km/s/Mpc (kilometers per second per megaparsec). This means that for every megaparsec away from the Earth one moves, on average the objects at this distance are moving away from the Earth at 71 km/s faster. These findings do not apply over relatively short distances, like between planets or stars of the same galaxy. This is because dark energy is spread evenly throughout the universe while matter is not. Essentially, in areas highly concentrated with matter, like single galaxies, gravitational attraction between closely positioned stars overcomes the repulsive force of dark energy.
An explanation of how type 1A supernovae occur. Type 1A supernovae were used to prove that the universe is expanding at an accelerating rate.
A table of omportant units of distance in cosmology.