Introduction to Astronomy

• Announcements– Project #2 due now

– HW #8 due Friday before final exam

CosmologyCosmology

Observations of the Universe

Evolution

Shape

Origin

Inflation

Observations of the UniverseObservations of the Universe

• Distribution of Galaxies– Looking out of the MW disk, we see about the

same number of galaxies in every direction• Isotropic

– Galaxies not evenly-distributed: tend to group together in loose associations

• Non-homogeneous

• Motion of Galaxies– Almost all galaxies are moving away from us– Further galaxies recede faster

• Hubble’s Law

• V = H0 D

Hubble Constant

• Careful here…all galaxies are moving away from all galaxies.– Result of expansion of the Universe– Just appears to us like they are moving away

from us

• Age of the Universe– If galaxies are now moving away from each

other, at some point in the past, they must have occupied the same space (Lemaitre, 1927)

• First notion of the Universe’s birth from an initial high-density, compressed state

– Measure how fast a nearby galaxy of known distance is moving away from us (Hubble Law)

– The galaxies must then have started moving apart a time t = D/V ago

– This works out to t = 1/H

– Rough estimate of age of Universe

• Works out to 4.3 x 1017 seconds = 1.4 x 1010 years

• Consistent with oldest stars ever observed (recall white dwarf cluster)

• Size of the Universe– Visible limit = “cosmic horizon”– The light from any object that lies beyond this

“boundary” has not had enough time to reach us since the birth of the Universe

– Presently, Rh ~ 14 billion ly

• George Gamow predicted a high temperature for early universe: would have emitted intense thermal radiation at short wavelengths. (1948)

• Ralph Alpher and Robert Hermann predicted present-day temperature of thermal radiation to be 5K (1950's)

Cosmic Microwave Background (CMB)

• 1960, Robert Dicke and James Peebles refined this estimate to near 3 K.

• 1964, Arno Penzias and Robert Wilson accidentally discover the 3 K radiation while trying to get rid of the "noise" in a large microwave horn antenna that they were developing for Bell Telephone. They win the 1978 Nobel Prize in Physics.

• Penzias & Wilson ruled out:– Urban radio interference (pointed horn at

NYC)– Radio radiation in or out of the plane of the

MW– Solar system source (noise was constant

between all 4 seasons)– Above-ground nuclear tests– Pigeons (and their droppings) roosting in the

horn

• Actual temperature of 2.73 K gives a peak wavelength of .001 m (1 mm) according to Wien's Law, which corresponds to a frequency of 2.8 x 1011 Hz.

• Best measurements (by COBE & WMAP satellites) show extreme uniformity once the Earth's motion is subtracted, with only very very small density variations

Difference between black and red: 0.0002 Kelvin

• In the early stages of the Universe, it was hot and dense– Wien’s Law: radiation in the early Universe

was very energetic (short wavelength)– As the Universe continued expanding, the

stretching of space stretches out this radiation• Longer wavelength = lower energy = lower

temperature = “cooling off” of the Universe

• Tspace = 2.7 K

Evolution of the UniverseEvolution of the Universe

• Expand forever or collapse?

• Depends on the total mass in the Universe– Or its density

• Add up all the mass you can see in some volume of space– Artificially add in dark matter (!)

• Critical density: the density required to just barely stop the expansion of the Universe

G

Hc

8

3 2

If measured density is less than ρc

the Universe continues expanding

If it is greater, the gravity of all that masswill eventually halt the expansion

Measurements have found that the actual density is about 3x less than ρc

• The Cosmological Constant– Einstein’s “biggest mistake”– The expansion of the Universe may actually

be accelerating, not slowing down!

Origin of the UniverseOrigin of the Universe

• Matter & radiation in the early Universe

• A mixture of gas and radiation (like the Sun, but much hotter & denser)– High temperatures = high-energy radiation– This radiation can give rise to particle pairs

(E = mc2)

• About 10-6 seconds after the Big Bang, temperatures = 10,000 billion K– Hot enough to create proton-antiproton and quark-

antiquark pairs

• Expansion continues, Universe cools down, no more particle-pairs created– Quark-antiquark annihilation– Asymmetric: one in every billion quarks survived

• Eventually recombined with other quarks to make protons and neutrons (giving rise to all matter we see today)

• These protons and neutrons combined to form the first atomic nuclei, as the expansion continued and the Universe cooled– 3 minutes after the big bang

• 300,000 years later, the Universe is cool enough to allow protons and electrons to combine to form the first Hydrogen and Helium atoms– Recombination– Space becomes transparent

• CMB originates at this time

– Beginning of “matter-dominated era”• Prior to this, matter and radiation were

interchangeable, now have separate roles

• Several billion years later, matter is cool enough for gravity to draw it into larger structures (protogalactic clouds)– When collapse begins, galaxy is forming– Collapse proceeds to form stars, galactic

disks, planetary systems, etc…

InflationInflation

• The Big Bang theory describes the Universe and its evolution starting from 10-6 seconds after the actual Big Bang…– When the entire Universe was the size of our

solar system

• Inflation Theory may tell us what happened even earlier– When the Universe was the size of a single

proton!

• At such high temperatures and densities, the laws of physics work differently– Gravity may become repulsive when working

in concert with the other 3 fundamental forces

• Between 10-35 and 10-32 seconds after the Big Bang, the Universe maybe expanded violently by a factor of 1025 - 1050

• Explains flatness of space– Expansion “flattens out” any curvature– Think about the Earth…locally, it appears flat to you

while you are walking, but it only appears this way because the Earth is a huge sphere.

• Explains uniformity of CMB– Cosmic horizon smaller in the past, so vastly-

separated regions of space could still “communicate” via radiation, leading to uniform CMB signatures