Graph showing changing rate of expansion over time Credit: Zosia Rostomian, LBNL, and Nic Ross, BOSS Lyman-alpha team, LBNL
Accelerating Expansion
The Anomaly
In 1998 Riess, Perlmutter and Schmidt announced to the astonishment of the cosmological community that, based upon their detailed observations of the velocities of type 1A supernova, the rate of the universe's expansion was increasing, not decreasing as had been assumed. Their data, when plotted on the graph to the left, best fit the line (shown in yellow) that corresponded to a universe whose expansion was accelerating with time. Up until this point the model for the flat universe had assumed there was enough mass and energy in the universe such that the expansion would simply continue to slow down for ever, never quite stopping. There did not appear to be a sufficient amount of matter to reverse its direction into a contraction mode. The current flat model of the universe, however, provides no explanation as to why the rate of expansion is speeding up.
In 1998 Riess, Perlmutter and Schmidt announced to the astonishment of the cosmological community that, based upon their detailed observations of the velocities of type 1A supernova, the rate of the universe's expansion was increasing, not decreasing as had been assumed. Their data, when plotted on the graph to the left, best fit the line (shown in yellow) that corresponded to a universe whose expansion was accelerating with time. Up until this point the model for the flat universe had assumed there was enough mass and energy in the universe such that the expansion would simply continue to slow down for ever, never quite stopping. There did not appear to be a sufficient amount of matter to reverse its direction into a contraction mode. The current flat model of the universe, however, provides no explanation as to why the rate of expansion is speeding up.
The Argument
To address the apparent accelerating expansion of the universe, the current flat model requires that some force must be acting on matter at the largest scales of the universe and that there must be some form of energy pervading all space. Because we are unable to detect it scientists have given it the name dark energy. It is assumed that at intergalactic distances greater than galactic super clusters, the force exerted by dark energy becomes stronger than gravity and is the underlying factor causing the universe's expansion to accelerate. Dark energy is assumed to make up 68.3% of the universe's mass/energy. However, as it continues to elude detection, dark energy remains a mystery.
On the 4th of October 2011 the Nobel prize was awarded to Riess, Perimutter and Schmidt for their discovery of the accelerating expansion. Many of those who followed this story, since their 1998 discovery might condense the trio’s find down to those two words. Dark Energy. However, following their win Riess stated:
“Now, that (the accelerating expansion) seems to be the smoking gun of dark energy, but we still don't understand what dark energy is or even - there's the outside possibility that we don't quite understand the laws of gravity and that there really isn't this dark energy. So the fact that we see the universe accelerating is really the tip-off that something interesting is going on, either on the gravity side or the content side of the universe”.
Above - Dr. Adam G. Riess is a Professor of
Astronomy and Physics at the Johns Hopkins
University and a Senior member of the Science
Staff at the Space Telescope Science Institute
To address the apparent accelerating expansion of the universe, the current flat model requires that some force must be acting on matter at the largest scales of the universe and that there must be some form of energy pervading all space. Because we are unable to detect it scientists have given it the name dark energy. It is assumed that at intergalactic distances greater than galactic super clusters, the force exerted by dark energy becomes stronger than gravity and is the underlying factor causing the universe's expansion to accelerate. Dark energy is assumed to make up 68.3% of the universe's mass/energy. However, as it continues to elude detection, dark energy remains a mystery.
On the 4th of October 2011 the Nobel prize was awarded to Riess, Perimutter and Schmidt for their discovery of the accelerating expansion. Many of those who followed this story, since their 1998 discovery might condense the trio’s find down to those two words. Dark Energy. However, following their win Riess stated:
“Now, that (the accelerating expansion) seems to be the smoking gun of dark energy, but we still don't understand what dark energy is or even - there's the outside possibility that we don't quite understand the laws of gravity and that there really isn't this dark energy. So the fact that we see the universe accelerating is really the tip-off that something interesting is going on, either on the gravity side or the content side of the universe”.
Above - Dr. Adam G. Riess is a Professor of
Astronomy and Physics at the Johns Hopkins
University and a Senior member of the Science
Staff at the Space Telescope Science Institute