The Dawn of Time

! From our Galaxy it appears every galaxy is moving away. Does this mean we are at the center of the Universe?

! If the Universe is expanding, why don’t you and I feel it?! A galaxy is redshifted so that the Doppler shift implies it is

moving many times the speed of light. Is the galaxy really moving faster-than-light? What’s a cosmological redshift?

! What is cosmology? If an observational test (that different people can conduct) can not be conceived to determine whether a thing exists, then does that thing exist?

! How fast is the expansion of the Universe? If a galaxy is at a distance of a million parsecs (1 Megaparsec) from our Galaxy, about how fast does it appear to recede?

Learning Objectives

Cosmology! What is the Universe?

! All the matter, energy, and space-time that we can ever detect

! If there is no wayyou can ever detect something, then it doesn’t exist

! Cosmology is the study of the origin, structure, and evolution of the Universe

Hubble’s Law! Hubble noticed

a correlation between redshift and distance

! The farther a galaxy is from us, the faster it moves away

! This can be described by the equation v=Hod! Where v is velocity in

km/s, d is distance in millions of parsecs (Mpc), and Ho is the Hubble constant

! Current best estimate of Ho is about 70 km/s/Mpc

Common Misconception! Its common to think of the expansion

of the Universe like an explosion!Galaxies hurled away from each

other through space! This is incorrect! Einstein’s Theory of

Relativity tells us that space-time itself is expanding!The space in which the galaxies are

embedded is getting bigger and bigger!Like an inflating balloon

Analogy - Rubber Band! Space-time expands,

like stretching a rubber band

! Not only do distances grow…

! Even the photons’ wavelengths get stretched! longer wavelengths are a “shift” to the red,

called the cosmological redshift! The galaxies are not moving faster than light,

the light from galaxies is stretched by space

Wait a Second…! Why don’t we expand with the Universe?! Other forces hold us together

!Atoms - nuclear forces!Molecules, our bodies – electromagnetic forces!Planets, stars, and galaxies – gravity

! But gravity can’t hold superclusters together over the huge distances between them!The expansion of our Universe grows stronger

with distance (more expanding space)!But gravity grows much weaker with distance

(inverse square law)

! The bread expands when it is baked…! And the raisins

are carried with the bread as it expands

! Not a perfectgeometric analogy as the bread has an obvious edge, etc.

! Galaxy clusters = raisins

Analogy – Raisin Bread

Not an explosion through space, an expansion of space...

!Galaxies are not flying apart through space

!They are being carried along by the expansion of space

!Just as all of the raisins move apart as the bread rises, clusters of galaxies recede from each other as the Universe expands

A matter of perspective

! The expansion of the Universe means that all galaxies will appear to move away from the point of view of ANY observer in ANY galaxy

The expansion is slower than I’ve led you to believe

! Though in previous slides I’ve made it look like the Universe doubles in size while we’re watching, the expansion is far slower than that

! In fact, in the ~10,000 years of recorded human history, galaxies have only become 0.00001% more distant

Next Time

The Dawn of Time - II. A Cosmos is Born