1. Astrophysics and Cosmology
Chapter 33
Astrophysics and Cosmology
© 2006, B.J. Lieb
Some figures electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Giancoli, PHYSICS,6/E © 2004.
Ch 33

2. Stars and Galaxies Our sun is a very average star
Energy of sun is from fusion in center
Sun is located in Milky Way Galaxy
Milky Way has about 100 billion stars
About 100 billion galaxies in universe
Ch 33

3. Stars and Galaxies
Ch 33

4. General Theory of Relativity
concerned with accelerated reference frames
Principle of Equivalence: No observer can determine whether he or she is accelerating or is in a gravitational field.
Ch 33

5. Inertial and Gravitational Mass
Inertial Mass: resistance to force
Gravitational mass: gravitational force of attraction depends on the products of the two masses
Throughout this course we have assumed that these two “masses” are equal, but are they?
Experiments with an accuracy of a few parts in 1012 show no difference between inertial and gravitational mass.
Principle of Equivalence says they are equal.
Ch 33

6. Light beam in constantly moving elevator moves directly across because of special nature of light
In accelerated elevator, light beam is bent because of increasing velocity of elevator
Principle of Equivalence says that there is no difference between accelerated frame and frame in gravitational field
Ch 33

7. Bending Light Rays passing near the Sun
Consider three stars in the sky.
If the light from one of these stars passes very near the Sun, whose gravity bends the rays, the star will appear higher than it actually is.
Ch 33

8. Curved Space Light beam must travel by the shortest, most direct path
Can explain bending of light ray by assuming space is curved.
In Euclidian geometry, the sum of the angles in a triangle must be 1800
if sum >180 0 space is positively curved
If sum < 1800 space is negatively curved
Ch 33

9. Black Hole Black Hole is a massive collapsed star.
Gravity is so strong that photons can’t escape from its surface.
Can see X-Rays from matter being pulled into it.
Ch 33

10. Expanding Universe
Hubble measured the distance to galaxies by measuring the brightness of certain stars whose intrinsic brightness was known
the velocity of galaxies was determined by the Doppler effect
all galaxies showed a redshift (spectral lines were shifted toward lower frequency indicating they moved away from the earth.)
doesn’t mean we are at center of universe
Ch 33

11. Hubble’s Law
Hubble found a linear relationship between the velocity ( v ) of galaxies away from us and the distance to the galaxy ( d ).
The slope of the line is H, called the Hubble Constant.
Ch 33

12. So the light left the galaxy 1.4 billion years ago.
Example 33-1 If a galaxy is traveling away from us at 10.0% of the speed of light, roughly how far away is it?
So the light left the galaxy 1.4 billion years ago.
Ch 33

13. Hubble’s Law We can use this to estimate the age of the universe:
Assume that the universe a galaxy at distance d from has been moving away from us since the start of the universe and calculate how long that has been.
Ch 33

14. four forces unified, theory ≈ ???
Big Bang
0→10-43seconds
four forces unified, theory ≈ ???
10-43→10-35sec. Grand Unified Theory Era
gravity was a separate force
no distinction between quarks and leptons
Ch 33

15. strong force is separate force
→10-4 seconds Hadron Era
strong force is separate force
slight excess of quarks over anti-quarks results in excess of nucleons over anti-nucleons
10-4→ 10 seconds Lepton Era
slight excess of electrons over positrons results in only electrons after annihilation
Ch 33

16. radiation dominates (photons and neutrinos)
10-4→ 300,000 year Radiation Era
radiation dominates (photons and neutrinos)
300,000 year → now Matter Dominated Era
atoms formed
stars and galaxies formed
Universe is matter dominated
Ch 33

17. Future of Universe
Can the gravitational force reverse expansion of the universe? Answer depends on average density
critical density c = kg / m3
( a few nucleons per m3)
 < c curvature is negatively curved and universe expands forever
 > c curvature is positive and universe expansion is reversed .
Ch 33

18. Big Surprise!!
Recent evidence suggests that the universe is expanding at an accelerating rate. .
Ch 33

19. Dark Energy
There is now evidence that the expansion of the universe is accelerating
This would imply the existence of an unknown force which has been given the name “Dark Energy”. Dark implies that astronomers can’t see it.
It is interesting that Einstein had included a fudge factor in the General Theory of Relativity called the “Cosmological Constant” that explained the stability of the universe.
When it was discovered that the universe is expanding then the Cosmological Constant was not necessary so Einstein considered it his biggest mistake.
Now we find that the Cosmological Constant may explain the acceleration that has been recently observed.  .
Ch 33

20. Best Estimate of Mass-Energy in the Universe
73 % dark energy
27% subject to the known gravitational force
23% is dark matter (matter that astronomers can’t detect)
4% is baryons and leptons
Ch 33