4.3 21 st century cosmology

1. 4.321st century cosmology

2. 21st Century Cosmology • By 1980, accumulating evidence had made the Big Bang theory widely accepted by cosmologists, but it faced two problems leading to the development of a new theory  a “revised” Big Bang with an important addition. • Flatness Problem: the properties of the universe are close to the dividing line between an open and closed universe … meaning the geometry seems nearly flat (ρ ~ ρc). • Horizon Problem: related to the observed isotropy of the CMB … how did every part of the entire Big Bang universe get to be so nearly the same temperature at the time of recombination?

3. 21st Century Cosmology • The key to these problems seems to be found in the hypothesis called the inflationary universe. • It is believed there was a period of sudden expansion during the early evolution of the universe known as inflation. • Triggered by the sudden energy release from the decoupling of the strong and electroweak forces. • New theories unifying the electroweak force with the strong force at high energies are called grand unified theories(GUTs).

5. The Cosmological Constant • In 1916, to balance the attractive force of gravity, Einstein added a constant to his equations called the cosmological constant, represented by an uppercase lambda (Λ). • This represents a force of repulsion balancing the gravitational attraction between galaxies so the universe would not contract or expand. • 13 years later, Edwin Hubble announced his observations the universe was expanding. • Einstein said introducing the cosmological constant was his biggest blunder • Modern cosmologists think he may have been right after all!

6. The Cosmological Constant • One explanation for the acceleration of the universe is that there is, after all, a cosmological constant representing a type of antigravity force comprising part of the fabric of space-time. • Causes a continuing acceleration in the expansion of the universe. • Cosmological CONSTANT  the universe would have experience this acceleration throughout history. • Another solution is to suppose totally empty space, the vacuum, contains energy driving the acceleration. • Cosmologists label a universal vacuum energy as quintessence. • Unlike the cosmological constant, quintessence would not necessarily remain constant over time.

7. Dark Energy • The observed acceleration of the universe’s expansion is evidence that a type of unknown energy is spread throughout space, referred to as dark energy. • However, it does not contribute to the formation of starlight or the Cosmic Microwave Background (CMB) radiation.