6.1MODERN COSMOLOGY

Olber’s Paradox

Why is the sky dark at night? Most astronomers believed the

universe was infinite. If that was so, then every line of

sight should end up on the surface of a star at some point.

The night sky should be as bright as the surface of stars.

The fact it is dark at night is known as Olber’s Paradox. Solution: If the universe had a

beginning, we can only see light from galaxies that has had time to travel to us; the visible universe if finite!

Hubble’s Law

In 1929, Edwin Hubble published his discovery that the sized of galaxy red-shifts are proportional to galaxy distances. Nearby galaxies have small

red-shifts and more distant galaxies have large red-shifts.

You may recall the Hubble Law. Implies the universe is

expanding.

The Big Bang Theory

If you had a video of the expanding universe and ran it backwards, you would see the matter and energy of the universe compressed into a high-density, high-temperature state.

Cosmologists believe the universe must have begun from this moment of extreme conditions and refer to this idea as the Big Bang Theory.

Age of the Universe

Knowing the current rate of expansion of the universe, we can estimate the time it took for galaxies to move apart as they are today: Time = distance/velocity

T = 1/H0

Hubble Time

Ht = 1012/H0

H0 = Hubble constant ~ 70 km/s/Mpc

~ 14 billion years

The more distant the objects we observe, the further back into the past of the universe

we are looking!

Cosmic Microwave Background Radiation (CMB)

Discovery of radiation from the time of the Big Bang began in the mid 1960s when two Bell Laboratories physicists, Arno Penzias and Robert Wilson, were measuring the brightness of the sky at radio wavelengths.

With further developments, they recognized the mysterious extra signal they had detected was radiation from the Big Bang known as cosmic microwave background radiation, or CMB.

In 1990, satellite measurements confirmed the background radiation has a temperature of about 2.73 K (1100 times cooler than it actually was at the time of the Big Bang due to the extreme red-shift).

Cosmic MicrowaveBackground Radiation (CMB)

The Wilkinson Microwave Anisotropy Probe (WMAP) satellite mapped the all-sky structure of the Cosmic Microwave Background (CMB) radiation.

Dark Matter

Combined mass of all “visible” matter (i.e. emitting any kind of radiation) in the universe adds up to much less than the critical density.

Can dark matter be composed of normal matter? If so, then its mass would mostly

come from protons and neutrons (baryons)

Density of baryonic matter is only 4% of critical density.

Most dark matter must be non-baryonic!

21st Century Cosmology

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.

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!

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.