Hubbles Cosmology A New View of Our Universe

Hubble’s Cosmology A “New” View of Our Universe

What is cosmology? • The study of the structure and evolution of the entire universe NASA • (NOT cosmetology – the study and application of beauty treatments)

Cosmological Principle • Homogenous – the same over large regions • Isotropic – the same in all directions

NASA Hubble Ultra Deep Field

Olbers’ Paradox • Assumptions: – Homogenous – Isotropic – Infinite – Unchanging • One or more of these assumptions must be wrong. Creative Commons • Why is the night sky dark?

Conclusions from Hubble’s Law • The universe is expanding • => universe was smaller in the past • => must have started from a single point which then expanded • This is called the Big Bang Theory

• Hubble’s estimate: H 0 ~ 500 km/s/Mpc • Current estimates: H 0 ~ 70 km/s/Mpc PNAS March 15, 1929 vol. 15 no. 3 168 -173 Hubble’s constant – Badly wrong – Very hard to measure distance in space NASA • Why so far off…?

The Hubble Time • Look at the UNITS of the Hubble constant… km/s/Mpc • Can cancel out the two distances with simple unit conversions. • The reciprocal of what remains has units of time. This is called the Hubble time. t = 1/H 0

Age of the Universe • Since Hubble’s Law tells us about how fast the universe is expanding, Hubble’s Time gives us the approximate age of the universe. – Well-known value – Constant rate of expansion • Neither of these assumptions are valid! Creative Commons • Assumptions in Hubble’s Law method:

Why We Knew Hubble’s Constant Was Wrong • Hubble’s original value for the Hubble constant (500 km/s/Mpc) gave a Hubble time age of the universe of a couple billion years. • HOWEVER, we know of objects in the universe that are several billion years old, such as stars in globular clusters. • Therefore, Hubble’s value for Hubble’s constant could not be correct. NASA/ESA Creative Commons

Resolving Olbers’ Paradox • Remember, it takes light time to travel. • Lookback time – a measure of how old an image is based on its distance away • The universe has a finite age, so only so much light has NASA/A. Field had time to reach us here on Earth.

Hubble’s Cosmology Activity