# Expansion of the Universe

Expansion of the Universe

Hubble’s Law, written by Edwin Hubble in the 1920s, describes the expansion of the Universe.

Edwin Hubble & Expansion of the Universe – Timeline

1917: Vesto Slipher discovered that the spectra of galaxies were almost always red shift (moving away). Infact, most galaxies are moving away and 2 out of 15 spirals moving at over 2 million miles per hour.

1929: Edwin Hubble derived distances to these galaxies and showed that implied recessional speed, v1, is proportional to its current distance from us

• Hubble’s Law: V – H0d, where  H0 is Hubble’s constant (71 km·s –¹/Mpc), v is velocity, and d is distance
• The value of Hubble’s constant is how fast the Universe is expanding now; if Hubble’s constant is bigger, the Universe is expanding faster

1927: Belgian astronomer, G. Lamaitre, had a similar result, proving that the Universe is expanding

•  Combined Einstein’s theory of relativity with the redshifts of spiral galaxies
• Published a paper on mathematical  super structure connecting redshifts and expanding Universe of general relativity, but nobody noticed since he was only an obscure Belgian priest and mathematician
• Universe began as a single pinpoint, a primordial soup

1998: Acceleration of the expansion of the Universe is caused by “cosmic anti-gravity” or “dark energy” (still unexplained)

Measuring Velocities of Red Shift

• Light of a galaxy moving away from us will be “red-shifted,” or the wavelength gets longer
• Light of a galaxy moving toward us will be “blue-shifted,” or the wavelength gets shorter
• The faster the speed galaxies travel, the more the “red-shift”
• Objects at the edges of galaxies tend to move faster than objects in the centers

Understanding the Expansion

• Galaxies are all moving away from us: Does that mean we are at the center of the Universe?
• No. There is “no” center. All points in space claim to be the center
• e.g. Raisin bread rising: raising don’t expand, the space between them expands

Olbers’ Paradox— Why is the night sky dark?

• In the 19th century, astronomer Wilhelm Olbers asked: If the Universe is finite, why isn’t the sky bright from starlight?
• The solution is not that stars are increasing far away, but that the apparent brightness of a star decreases (1/d²), the area of shells of stars surrounding the Earth increases like d², so the effects cancel out
• Another solution was that the Universe has finite size, so that not all of the light from all the stars has had time to reach us (Universe expanding); the Universe is 14 billion years old, and we only see “out” 14 billion light years distance from us