The Big Bang theory states that the universe exploded from a single point known as a singularity and has expanded over the course of 13.8 billion years. Three scientists provide different explanations for the observed expansion rate of the universe.

Scientist 1

The universe is expanding at an increasing rate. Galaxies move away from one another as the universe expands. The light they emit is stretched by the expansion as it travels through an expanding space. The result is a shift towards the red end of the color spectrum, known as the galactic redshift. The further away from the earth a galaxy is, the greater the redshift.

Supernovae are explosions caused when massive stars collapse under their own gravity. Type 1a supernovae always release light with the same amount of intensity, which makes them useful for measuring cosmic distances because intensity is lost as their light travels through an expanding space. Comparing the color of light measured when it reaches Earth with the color at the time of the supernova explosion gives the amount of redshift and thus the distance. Distances determined by observing the redshift of type 1a supernovae confirm that the expansion of the universe is accelerating. To explain this, cosmological models need some new kind of accelerating energy—a dark energy that accelerates the expansion of the universe.

Scientist 2

The universe is not expanding at all. In our own galaxy, distant stars appear fainter and smaller, but their surface brightnesses remain constant. In contrast, the Big Bang theory tells us that, in an expanding universe, surface brightness decreases with distance. Therefore, the most distant galaxies should have much dimmer surface brightnesses than similar nearby galaxies.

But this is not supported by observations. Contrary to the predictions of the Big Bang theory, the surface brightnesses of distant galaxies are identical to nearby ones. Supernovae data confirm that galaxy distance is proportional to the redshift at all distances. The predictions of this simple formula do not need to include complex corrections for hypothetical dark matter and dark energy. The redshift of light over increasing distance must be caused by another phenomenon, one that causes the intensity of light itself to decrease as it travels through space.

Scientist 3

The supernovae data do not imply that the universe is expanding at an accelerating rate. The standard Big Bang theory is a mathematical model, not a physical portrayal of how light released from a supernova explosion disperses into an expanding universe. When a supernova explodes, its energy begins to disperse. By the time we observe the flash, the universe has become larger and more dilute. Accordingly, the intensity of light has decreased proportionally to the distance traveled. This physical portrayal explains the observed redshift values without need for a mysterious dark energy that increases the rate of expansion of the universe.

_Sources:

_{Perlmutter, S. Supernovae, Dark Energy, and the Accelerating Universe.
http://supernova.lbl.gov/PDFs/PhysicsTodayArticle.pdf
Zyga, L. A second look at supernovae light: Universe's expansion may be understood without dark energy.
http://phys.org/news/2011-10-supernovae-universe-expansion-understood-dark.html
Universe is Not Expanding After All, Controversial Study Suggests
http://www.sci-news.com/astronomy/science-universe-not-expanding-01940.html}