**Posted: ****January 23, 2017, 9:35pm**

The discovery of the expansion of the universe was made by Edwin Hubble in 1929. Today, using the Hubble Space Telescope, the phenomenon of the accelerating expansion of the universe is being studied by measuring distant supernova. This discovery was made by Dr. Adam Riess and his High-Z Supernova Search Team. Riess is a renowned cosmologist and received the 2011 Nobel Prize in Physics along with Dr. Brian Paul Schmidt and Dr. Saul Perlmutter for providing evidence that supported the phenomenon of the acceleration. I attended the USC Dornsife’s Irene McCullouch Distinguished Lecture featuring Dr. Adam Riess at the Bovard Auditorium on Monday evening.

Albert Einstein was the first to state that dark energy existed. Though the origin of the accelerating universe effect is unknown, it is being recognized as a type of dark energy that is dominating the universe. And per Riess, one of the most important unsolved problems in cosmology and astrophysics is understanding the unexplained and mysterious nature of dark energy.

On talking about how this discovery of the accelerating universe was made, Riess discussed the fact that the farther away a galaxy is from us, the faster it will appear to receive from us. He said that measuring distance is the hardest thing to do in space. The distances on earth are measured using various methods such as parallax, lighthouses, fog horns and objects of known size. However, the problem with these methods is that they involve human creations and so in space, distance measurement uses the standard candle approach, which is one of the tools of cosmology. A galaxy has millions of stars that have known luminosity and can be used as standard candles. When one of these stars explodes, it could be as bright as four billion times the luminosity of the sun. In addition, the inverse square law states that intensity is inversely proportional to the square of the distance from the source of that physical quantity.

The other property that Riess and his team needed to measure was the speed at which the galaxies were rushing away from the earth. They did so by looking at the nature of the light coming from distant objects. A supernova emits light at known wavelengths and because of the expansion of space the wavelengths are stretched, i.e., it gets redder. This redshift measures the apparent recession velocity of the galaxies. The Hubble’s constant, which is the measured slope or the linear relation between the velocity and distance, gives the present expansion rate of the universe. If the universe would contract instead of expanding, then this slope would be negative. Riess’s thesis work used supernovae to measure greater distances and the rate at which the universe is expanding. According to Riess, we can observe the universe at some time with the galaxies all flying apart from each other and then measure that expansion rate using the Hubble’s constant. Then it can be seen that by moving backward in time all the galaxies are contracting and over time they would all be on top of each other. On taking the inverse of this rate, the approximate age of the universe can be calculated.

When Hubble discovered that the universe is expanding, Einstein retracted the cosmological constant that he had introduced 12 years prior to the discovery made by Hubble. However, Riess’s discovery used a non-zero value for the cosmological constant. He decided to consider the cosmological constant because in the fall of 1997, while neglecting the cosmological constant, Riess had calculated the mass of the universe, which turned out to be a negative value. That value indicated a negative deceleration, which meant acceleration. That was the first indication of his discovery.

Initially, in the 1990’s two models of the expanding universe were considered. One was a heavyweight model, which was dense and meant a rapidly decelerating universe. The other was a lightweight model, which was sparse and meant a slowly decelerating universe. Riess and his team looked at distant Type 1A supernovae to decide which model defined our universe. Riess studied four supernovae in his thesis. He discovered the accelerating expansion and dark energy in 1998 and confirmed it with more distant supernovae from Hubble. In 2009, they could find tens of supernovae at greater distances. The supernovae showed Riess that the universe was decelerating before it slowly started accelerating. So they removed both models that they thought they were choosing between and ended up with this new model when the universe began to accelerate five billion years ago.

As the reason for the acceleration of the universe remains unknown, Riess and his team have guessed vacuum energy also known as the cosmological constant, dynamical dark energy or the modified gravity to be the reason. They also expect to learn more about the nature of the dark energy in the next decade from a spate of different measurements, including a refurbished Hubble Space Telescope.

Published on July 26th, 2017

Last updated on August 10th, 2017