Cosmology
Cover photo
Zoom in on Hubble Deep Field
1,000 galaxies in this field
Definition of Cosmology
Cosmology- The study of the structure and evolution of the Universe
Scale of Structure in Universe
Voids and walls have sizes of 100-200 Mpc
23,000 galaxies are found in two 80° x 4.5° wedges
On scales bigger than 300 Mpc, the Universe is roughly homogeneous (the same everywhere- the number of galaxies is the same) and isotropic (the same in all direction)
Cosmological Principle
The assumptions of homogeneity and isotropy are known as the cosmological principle
The assumption is that if we look at large enough scales, the Universe will appear homogeneous and isotropic
We also assume that the laws of physics are the same everywhere
This implies that there is no center or edge to the Universe!
Obler's Paradox
Stars appear fainter due to inverse-square law, but the number of stars increases as the square of the distance
So, if one looks hard enough in any direction, one will see a star (some in far-away galaxies)- the sky should be white!
But this is not observed- hence the paradox
Why?: Either Universe is finite or Universe looks different in time
Age of Universe
Hubble's Law: v=H0 d
Time=distance/velocity=d/v=d/H0 d=1/H0
Here time is the age of the Universe
Age= 1/H0 =1/65/km/s/Mpc =15 billion years
Assumes H0 is correct and that galaxies move with constant speed for all times
This is the resolution of Obler's paradox: We are only seeing a finite part of the Universe- 15 billion l.y.
Big Bang
At the beginning of the universe, an explosion called the Big Bang occurred.
The big bang happened everywhere in the Universe, but the Universe was collapsed to a single point
Space itself is expanding
There is no center to the Universe
Cosmological redshift occurs when a photon's wavelength is stretched by the expansion of the universe.
Expansion of the Universe
Fig. 26-4
No center of Universe
Distance between each and all galaxies increases
If balloon deflates, time runs backward
Expansion of the Universe
Fig. 26-3, visually why there is no center of the Universe
Cosmological Redshift
Fig. 26-5
Fate of the Universe
Universe is expanding
Will it expand forever?
Can think of this just using Newton's laws, without including relativity
The escape speed of the Universe determines whether it will escape forever
Escape speed depends on mass and distance
Escape Speed of the Universe
Fig. 26-7
Model Universe
Fig. 26-8
The amount of mass in a certain volume determines the fate
Critical Density
Notice that the early history of the Universe also depends on the mass
Future of the Universe
Fig. 26-9
The Big Crunch- critical density is great enough to pull matter back into central point = Closed
Oscillating universe
Low density universe expands forever (purple line is critical density)= Open
Critical Density
Critical density is the density of the Universe that is just great enough to allow gravity to slow the expansion of the Universe and reverse the flow
Ð0 is the ratio of the measured density over the critical density
Ð0 =1 the Universe has enough mass to stop expanding and just start to contract at time= infinity
Ð0 < 1, Universe is Open
Ð0 >1, Universe is Closed
Critical Density
If you add up all mass in the known volume of the Universe, is it greater then, equal to, or less than critical density?
What is we add in dark matter?
Supernovae
Fig. 26-10
Attempt to measure Hubble constant and curvature of Universe
Decelerating curve- red and purple
Hubble law is black curve
Einstein's Cosmological Constant
Fig. Discovery 26-2
Einstein thought Universe was static
Constant has no physical meaning
Problem 26-6
According to the Big Bang theory, without a cosmological constant, what is the maximum age of the Universe if H0 =50, 65, or 80 km/s/Mpc?
Cosmic Microwave Background
Penzias and Wilson in 1964 found a background hiss at microwave wavelengths
What questions can you ask to try to ascertain what and where the hiss is?
Cosmic Microwave Background (CMB)
The background hiss originates from every point in the sky
The background signal never varies in intensity
After conversations with theorists at nearby Princeton University, they realized they had detected the signal of fiery creation of the Universe- the big bang
Microwave background is the thermal remnant of the hot, fiery explosion that has been redshifted (due to cosmic expansion) to microwaves.
CMB-Temp
Microwave background is the thermal remnant of the hot, fiery explosion that has been redshifted (due to cosmic expansion) to microwaves.
Initially the explosion was very hot, but as the Universe expanded and cooled the primeval fireball would currently be only 2.7 K.
COBE satellite measured the spectrum and temperature in 1989
Cosmic Microwave Background
If the microwave background is at rest and Earth is traveling through it, the background will appear slightly red and blueshifted
The amount of the shift is very small
Implies a small shift in the temperature measured for different regions of sky
Summary
State and explain cosmological principle
Age of Universe
How is it determined
Understand the uncertainties involved in this measurement
Expansion of Universe
Cosmic composition of early Universe
Relation between the future of the Universe and geometry of space
Explain and understand importance of cosmic microwave background