Modern Physics

Question 1

Rutherford Nuclear Model

Answer 1

Dense positive nucleus surrounded by a swarm of negatively charged electeons

Determined through Rutherford gold foil experiment in which alpha particles were deflected by nucleus

Question 2

Particle-like Nature of Light Revealed by who?

Answer 2

The particle-like nature of light was revealed and studied through the work of Max Planck in 1900, and later Albert Einstein (who won the 1921 Nobel prize for work in this area)

Question 3

Quanta

Answer 3

Electromagnetic radiation is emitted and absorbed by matter as though it existed in individual bundles called quanta

• Discrete packet of energy

Question 4

Photon

Answer 4

A quantum of electromagnetic energy is known as a photon

• Light behaves like a stream of photons, which is illustrated by the photoelectric effect

Question 5

Photoelectrons

Answer 5

When a piece of metal is illuminated by electromagnetic radiation (visible light ultraviolet light, or X-rays), the energy absorbed by electrons near the surface of the metal can liberate them from their bound state, and these electrons can fly off

• The released electrons are known as photoelectrons

Question 6

Wave-only Theory of Light Prediction of photoelectrons

Answer 6

1. There would be siginifcant time delay betwen the moment of illumination and the ejection of photoelectrons
2. Increasing the intensity of light would cause the elctrons to leave the metal surface with greater kinetic energy
3. Photoelectrons would be emitted regardless of the frequency of the incident energy, as long as the intensity was high enough

None of these predictions was observed, something was wrong with the wave-only theory of light

Question 7

Inaccuracies of Wave-only Theory of Light Predictions

Answer 7

1. Photoelectrons were ejected within just a few billionths of a second after illumination
2. Increasing the intensity of light did not cause photoelectrons to leave the metal surface with greater kinetic energy; although more electrons were ejected as the intensity was increased, there was a certain threshold frequency, f₀
3. If light of frequency lower than f₀ were used to illuminate the metal surface, no photoelectrons were ejected regardless of how intense the incident radiation was

Question 8

Energy of Photon

Answer 8

E= hf

• h= Planck’s constant (about 6.63 x 10^-34 J•s)

Question 9

Photoelectric Effect Formula

Answer 9

K_max= hf - Ø

• A certain amount of energy had to be imparted to an electron on the metal surface in order to liberate it
  
  • This is known as the metal’s work function, Ø
    
    • If an electron absorbed a photon whose energy E was greater than Ø, it would leave the metal with a maximum kinetic energy equal to E - Ø or hf - Ø
    • This process could occur very quickly, which accounts for the rapidity with which photoelectrons are produced after illumination
  • Increasing the intensity of the incident energy means bombardment with more photons and results in the ejection of more photoelectrons
    
    • But since the energy of each incident photon is fixed by the equation E= hf, the value of K_max will still be E - Ø

Question 10

Threshold Frequency

Answer 10

E= hf

• Ø​= hf
  
  • Amount of needed energy to be imparted to an electron on the metal surface in order to liberate it
• Ø​/h= f, threshold frequency
  
  • If the incident energy had a frequency that was less than Ø​/h, the incident photons would each have an energy that was less than Ø​; this would not be enough energy to liberate photons
    
    • Blasting the metal surface with more photons (that is increasing the intensity of the incident beam) would also do nothing as none of these photons would have enough energy to eject electrons, so whether there were one or million wouldn’t make a difference
      
      • This accounts for a threshold frequency, Ø​/h

Question 11

Formula Relating Wavelength, Speed, and Frequency

Answer 11

c= λf

Question 12

Electronvolt

Answer 12

• The SI unit for energy is the joule, but it is too large to be used in instances relating to photoelectrons/ electrons
  
  • The electronvolt (eV) is equal to the energy gained (or lost) by an electron acclerated through a potential difference of one volt
    
    • Using the equation ΔU_E= qΔV
      
      • 1 eV= (1 e)(1 V)= (1.6 x 10^-19 C) (1 V)= 1.6 x 10^-19 J
• In terms of electronvolts, the value of Planck’s constant is 4.14 x 10^-15 eV•s

Question 13

Atomic Spectra

Answer 13

When atoms are excited they emit light of certain wavelengths which correspond to different colors. The emitted light can be observed as a series of colored lines with dark spaces in between; this series of colored lines is called a line or atomic spectra. Each element produces a unique set of spectral lines

Question 14

The Bohr Model of the Atom

Answer 14

• According to Bohr model, electron orbits the nucleus at certain discrete radii (energy levels)
  
  • If electron aborbs photon or certain amount of energy it is excited to a higher orbit, which is known as its excited state
    
    • After spending a short time in its excited state, it returns to a lower orbit, emitting a photon in the process
• Since each energy level has a specific radius (and corresponding specific energy), the photons emitted in each jump also have specific energy and wavelengths
  
  • The electron’s energy levels are quantized

Question 15

Formula for Energy of Energy Level

Answer 15

The energy levels within the hydrogen atom are given by the formula: E_n= 1/n²(-13.6 eV)

• For other one-electron atoms, doubly-ionized lithium, the energy levels are:
  
  • E_n= Z²/n² (-13.6 eV)
    
    • Where Z is the number of protons in the atom’s nucleus
• When an excited electron drops from energy level n= j to a lower one, n= i, the transition causes a photon of energy to be emitted, and the energy of the photon is the difference between the two energy levels
  
  • E_{emitted photon}= |ΔE|= E_j - E_i

Question 16

Wave-Particle Duality

Answer 16

Light and other electromagnetic waves exhibit wave-like characteristics through interference and diffraction

• As seen in the photoelectric effect, light also behaves as if its energy were granular, composed of particles
  
  • Wave-particle duality: Electromagnetic radiation propagates like a wave but exchanges energy like a partcile

Question 17

de Broglie Wavelength

Answer 17

λ= h/p

• A particle of mass m and speed v, with linear momentum p- mv has an associated wavelength
  
  • Particles in motion can display wave characteristics and behave as if they had a wavelength
    
    • Since the value of h is so small, ordinary macroscopic objects do not display wavelike behavior, however, with subatomic particles, the wave nature is clearly evident

Question 18

Nucleons

Answer 18

• The nucleus of the atom is composed of particles called protons and neutrons, which are collectively called nucleons
  
  • The total number of nucleons, atomic number + nucleus number= the mass number (or nucleon number)
• Nuclide: term for a nucleus with specific numbers of protons and neutrons

Question 19

The Nuclear Force

Answer 19

The strong nuclear force, is a fundamental force which binds together neutrons and protons to form nuclei

• The coloumb force can be expressed by a smiple mathematical formula
  
  • The nuclear force is much more complicated; no simple formula can be written for the strength of the nuclear force

Question 20

Binding Energy

Answer 20

The difference between the mass of any bound nucleus and the sum of the mass of its constituent nucleons is called the mass defect Δm

Δm= (m_proton + m_neutron) - m_nucleus

• The missing mass is converted to energy when the nucleus is formed
  
  • The energy represent the amount of energy needed to break the nucleus into separate protons and neutrons, since this indicates how strongly a nucleus is bound, it is called the binding energy of the nucleus

Question 21

Atomic Mass Unit

Answer 21

• Mass of proton: 1.6726 x 10^-27 kg
• Mass of neutron: 1.6749 x 10^-27 kg
  
  • Because the masses are so tiny, a much smaller mass unit is used
    
    • The atomic mass unit amu, is defined as 1/12 the mass of a carbon-12 atom
    • The conversion between kg and u is 1 u-= 1.6605 x 10^-27 kg
      
      • Proton mass = 1.00728 u
      • Neutron mass= 1.00867 u

Question 22

Deuterium

Answer 22

Deterium is an isotope of hydrogen that contains 1 proton and 1 neutron

• Deuteron is the nucleus of deuterium
• The mass of deuteron is 2.01356 u

Question 23

Mass-Energy Equivalence

Answer 23

Conversion between mass and energy is given by Einstein’s mass-energy equivalence equation, E= mc²

E_{Binding Energy}= (mass defect)c²

• The energy equivalent of 1 atomic mass unit is about 931 MeV
• Highest binding energy per nucleon is 8.8 MeV for nickel-62
• When nucleus is split energy is released

Question 24

Radioactivity

Answer 24

The stability of a nucleus depends on the ability of the nuclear force to balance the repuslive Coulomb forces between the protons

• An unstable nucleus that will spontaneously change into a lower-energy configuration is said to be radioactive
• Modes of radioactive decay: alpha, beta, and gamma decay
• In nuclear reactions mass number and charge is conserved
  
  • The decaying nuclide is known as the parent, and the resulting nuclide is known as the daughter

Question 25

Alpha Decay

Answer 25

When a nucleus undergoes alpha decay, it emits an alpha particle; a helium nucleus (consists of two porotns and two neutrons)

• Symbol: α or ⁴₂He
• Very large nuclei can shed nucleons quickly by emitting one or more alpha particles
  
  • Alpha decay most offen occurs in massive nuclei with too large a proton to neutron ratio
• Alpha decay decreases the mass number by 4 and the atomic number by 2

Question 26

Subcategories of Beta Decay

Answer 26

Three subcategories of beta decay: β^-, β⁺, and Electron Capture

Question 27

β^- Decay

Answer 27

When the neutron to proton ratio is too large, the nucleus undergoes β^- decay

• Most common form of beta decay
• Occurs when a neutron transforms into a proton and an electron and the electron is ejected from the nucleus
  
  • The expelled electron is called a beta particle
  • The transformation of a neutron into a proton and an electron (and another particle, the electron-antineutrino ^-v^-_e) is caused by the action of the weak nuclear force, another of nature’s fundamental forces
• Carbon-14 is a common example of a nuclide which undergoes beta decay

Question 28

β+ Decay

Answer 28

When the neutron to proton ratio is too small, the nucleus will undergo β+ decay

• Occurs when a proton is transformed into a neutron and a positron (the electron’s antiparticle), plus another particle (the electron-neutrino, v_e) which are both ejected from the nucleus

Question 29

Electron Capture

Answer 29

Another way in which a nucleus can increase its neutron to proton ratio is to capture an orbiting electron and then cause the transformation of a proton into a neutron

Question 30

Electron Capture of B eryllium- 7

Answer 30

⁷₄ Be + ⁰_-1e → ⁷₃ Li + v_e

Question 31

Gamma Decay

Answer 31

Gamma decay does not alter the idenity of the nucleus; it just allows the nucleus to relax and shed energy

• Asterisk indicates nucleus is left in a high energy, excited state
  
  • For this excited nucleus to drop to its ground state, it must emit a photon of energy, a gamma ray symmbolized by γ
    
    • ⁴²₂₀ Ca^*→ ⁴²₂₀ Ca + γ

Question 32

Radioactivity Decay Rate

Answer 32

As a radioactive sample disintegrates, the number of decays per second decreases, but the fraction of nuclei that decay per second- the decay constant- does not change

• The decay constant is determined by the identity of the radioisotope

Question 33

Activity of a Radioactive Sample

Answer 33

The activity of a radioactive sample is the number of disintegrations it undergoes per second; it decreases with time according to the equation:

A= A₀e^-λ​t (activity)

N= N₀e^-λ​t (Number of radioactive nuclei in a given sample)

m= m₀e^-λ​t (mass of the sample)

• A₀ is the activity (or number or mass depending on equation) at time t= 0 and λ is the decay constant (not to be confused with wavelength)
• Activity is expressed in disintegrations per second: 1 disintegration per second is one becquerel (Bq)
  
  • The greater the value of λ, the faster the sample decays

Question 34

Half-life

Answer 34

The time required for half of a given sample to decay

• Most common way to indicate the rapidity with which radioactive samples decay
• Half-life, T_1/2, is inversely proportional to the decay constant, λ, and in terms of the half-life, the exponential decay of a sample’s mass (or activity) can be written as:
  
  • m= m₀(1/2)^t/T_1/2
• A sample’s activity, or mass, or number of atoms, N can be graphed as a function of time resulting in the exponential decay curve

Question 35

Nuclear Reactions

Answer 35

• Nuclear radioactive decay
• Nuclear fission: bombardment of target nuclei with subatomic particles to artificially induce radioactivity
• Nuclear fussion: fusion of small nuclei at extremely high temperatures
• In all cases nucleon number and charge must be conserved
  
  • Gamma-ray photons can also be produced in nuclear reactions which have no charge or nucleon number

Question 36

Disintegration Energy

Answer 36

Nuclear reactions not only produce new nuclei and other subatomic particles, but also involve the absorption or emission of energy

• Nuclear reactions must conserve total energy, so changes in mass are accompanied by changes in energy acording to Einstein’s equation ΔE= (Δm)c²
  
  • General nuclear reaction is written as:
    
    • A + B→ C + D + Q, where Q is the disintegration energy
      
      • If Q is positive, the reaction is exothermic and the reaction can occur spontaneously
      • If Q is negative, the reaction is endothermic and the reaction cannot occur spontaneously
        
        • Energy Q is calculated as Q= [(m_A + m_B) - (m_C + m_D)]c²
        • For spontaneous reactions- ones that liberate energy- most of the energy is revealed as kinetic energy of the least massive product nuclei

Question 37

Special Relativity Postulates

Answer 37

1. All the laws of physics are the same in all inertial reference frames
2. The speed of light in vacuum always has the same value (c= 3 x 10⁸ m/s), regardless of the motion of the source or the observer

Question 38

Inertial Reference Frame

Answer 38

An inertial reference frame is one in which Newton’s first law holds

• Given one inertial reference frame, any other reference frame that moves with constant velocity relative to the first one will also be inertial
  
  • E.G. a man on a platform considers himself to be in an inertial reference frame, placing an object next to him and exerting no force to it, the object will stay at rest
    
    • If a train moves past the platform traveling on a smooth track in a straight line at constant speed, a passenger on the train will consider himself to be in an interial reference frame as well, placing an object on the floor and exerting no force on it, it will stay at rest (relative to him)
      
      • If the train is moving at for example 40 mph, the man on the train will say the suitcase is at rest, however the man on the platform would say it’s moving at 40 mph
        
        • Though the two men will naturally disagree of velocity, they won’t disagree about physics laws such as conservation of momentum, which is the essence of postulate 1
          
          • Two men playing a game of darts on a smoothly moving train (one which travels in a straight line at constant speed) will not have to adjust their shots to account for the motion of the train

Question 39

Relativity of Velocity

Answer 39

• For the man on the train to throw a ball at 5mph, a speed measured by his frame of reference, on the train of 40mph parallel to the direction of motion of the train, the man on the platform will seemingly see the ball at a velocity of 40mph + 5mph
  
  • This simple addition of velocities does not extend to light or objects moving at speeds clsoe to that of light
    
    • E.G. for a person on a planet and a spaceship moving at (2/3)c toward the planet, if the spaceship emitted a light pulse toward the planet, the speed of that light pulse as measured by the man on the planet, would NOT be (2/3)c + c; as postulate 2 states that the speed of light would be c regardless of the motion of the spaceship
      
      • The correct relativistic formula for the “addition” of velocities, the one that follows the theory of relativity is:
        
        • Given reference frame, S_{on spaceship} moving with velocity u toward planet, if an object moves at velocity v (parallel to u), as measured with reference frame of the spaceship, then the speed as measured by the man on the planet would NOT be v_{on planet}= u + v but actually
          
          • v_planet= (u + v)/ (1 + uv/c²)
            
            • Does not have to be planet and spaceship example but any objects of relative reference frames such as train and platform
            • At normal, everyday speeds, u and v are so small in comparison with the speed of light that the fraction uv/c² is negligibly small, nearly zero creating the dominator= 1 becoming the fomula v_planet= u + v
              
              • Only when u or v is close to the speed of light that the difference becomes measurable

Question 40

The Relativity of Time

Answer 40

• On the spaceship traveling towards the planet at a speed of (2/3)c, if the pilot were to clap and then one second clap again, the measured time on the man on the planet would not be 1 second as time is relative, the time measured on the spaceship will not be the time measured by the man on the planet
  
  • The relation between these time frames, on the spaceship ΔT₁ and on the planet ΔT₂ with spaceship at velocity v, would be:
    
    • ΔT₂= γ•ΔT₁
      
      • γ is the relativistic factor which is:
        
        • γ= 1/ √ ( (1 - (v/c)²) )
          
          • Because the denominator of this fraction is never greater than 1, the value of γ is never less than 1
            
            • Therefore unless the spaceship was standing still relative to the planet, the time measured by the man on the planet would alwasy be longer than the time measured on the space ship
      • For all ordinary speeds, where v is very, very small compared to c, the value of γ is negligibly greater than 1, so the time frames are approximately equal, and a difference of ordinary time intervals is not noticed
        
        • As v gets closer and closer to c, γ gets bigger and bigger
          
          • E.G. on a spaceship moving at v= 0.99c relative to the earth, the value of γ is about 50, if a passenger on the ship says measured their time on the ship at 2 years then on earth the elapsed time would be 50 x 2= 100 years

Question 41

Dilated Time and Proper Time

Answer 41

ΔT₁ on the spaceship can be referred to as the proper time, which is the time measure in the frame at rest with respect to an event (the clapping is the event)

• The proper time is measured by a clock at rest with respect to the person clapping
  
  • At ΔT₂, a time measured in a different inertial frame, can be referred to as the dilated time

Question 42

The Relativity of Length

Answer 42

• On a spaceship traveling towards a planet at a speed of (2/3)c, the measured length of the spaceship is 100 meters. An observer on the planet watching the ship fly by, would NOT measure its length to be 100 m as length is relative, the two measured lengths would not be in agreement
  
  • In the reference frame of the spaceship, the length of the spaceship is called L₁, if its velocity toward the planet is v, then the length measured by the observer on the planet would be:
    
    • L₂= L₁/γ
      
      • γ is the relativistic fator, because it is greater than 1, the length measrued on the observer of the planet would be shorter than the length measured by a passenger on the spaceship
        
        • This is known as length contraction; lengths that are parallel to the velocity v are shortened by a factor of γ

Question 43

Contracted Length and Proper Length

Answer 43

• L₁ can be referred to as the proper length, which is the length measured in the frame at rest with respect to the object
  
  • In the example of the spaceship, the ship is the object, so the proper length is measured by a ruler at rest with respect to the ship; the length L₂ measured in a different inertial referrence frame can be referred to as the contracted length

Question 44

The Relativity of Energy

Answer 44

• E= mc² states how much energy is equivalent to a given amount of mass
  
  • This amount of energy is called rest energy because an object resting, on for example a desk, has energy and, in fact is energy, simply by virtue of the fact that it exists and has mass
    
    • Because c², the square of the speed of light, is such a big number, a smal amount of mass is equivalent to a huge amount of energy
      
      • When an exothermic nuclear reaction takes place, the total mass of the product nuclei is always less than the total mass of the original nuclei the “missing mass” Δm, has been converted to energy, in accordance to the equation ΔE= (Δm)c²
        
        • KE= 1/2mv² used in calculating kinetic energy of an object of mass m, moving with speed v
          
          • If v gets close to c (in which case the object is moving at a relativistic speed) then the 1/2mv² formula won’t work; the correct formula is
            
            • KE= (γ - 1)mc² γ is the relativistic factor
            • An object’s total energy, E_total is now defined as the sum of its rest energy, E_rest= mc² and its kinetic energy
              
              • E_total= mc² + (γ - 1)mc² = γmc²

Question 45

Approaching Speed of Light

Answer 45

As v approaches c, the object’s kinetic energy approaches infinity

• This shows why it’s impossible for a massive particle to move at the speed of light (or at any speed greater than c)
  
  • The force accelerating a particle would have to do more and more work to increase the particle’s kinetic energy, and it would take an infinite amount of work to push a particle to the speed of light (where its kinetic energy would be infinite)
    
    • The universe has a speed limit: Light moves at speed c, and everything else moves at a speed less than c

Question 46

The Equivalence Principle

Answer 46

• After publishing on special relativity, Einstein published the theory of general relativity which could account for accelerated motion and for motion in the strong gravitational fields near large masses
  
  • The guiding principle of general relativity is that it is impossible to differentiate between an accelerating reference frame and a reference frame in a gravitational field; such frames are equivalent
    
    • In an elevator in space accelerating upwards it would feel as though you’re pressed against the floor as would in a gravitational field in a stationary elvator
      
      • Likewise, in both instances light would bend in the elevator due to the acceleration of the elevator in the first case and the strong gravitational field in the second case
        
        • Light bends because space itself bends- the space near large amounts of mass is bent, and it is this bending that causes all the gravitational effects that is seen
          
          • It is not simply space that bends- four-dimensional spacetime bends, with the result that time dilates in a gravitational field just as it does for an object moving at a very high speed
• General relativity has had particularly special importance in astrophysics, as astronomical objects have ery large amounts of mass, the bending of spacetime is significant in the vicinity of these objects

Question 47

Black Holes

Answer 47

Very massive objects may have so much gravitational pull that the velocity required to escape from their attraction is greater than the speed of light

• In general relativity, because light is subject to the curvature of space that affects matter as well, light cannot escape from these objects either, and as a result, they are called black holes
  
  • They are black because they emit no light, nor can they even reflect light
    
    • They cannot be seen directly, but their gravitational effects on nearby objects can be quite dramatic and they can be detected thereby

Question 48

Quasars

Answer 48

• Quasars are extremely bright but extremely distant objects which can be explained through black holes
  
  • Caused by black holes that pull matter in at extremely high speeds
    
    • The matter rubs against other falling matter, and the frictional effects generate light, after attracting all nearby matter the source of light essentially “runs out of fuel”
      
      • This is the reason ones that are very distant can be seen (very distant in astronomy means that it takes a light a very long time to reach earth, astronomers are seeing light emitted a long time ago giving picture of the quasar in the very distance past)
        
        • Won’t “wink” out of the sky for years because light they emitted long ago is still travelling to earth

Question 49

Edwin Hubble Discovery

Answer 49

Galaxies outside of our own send light to us that appears redshifted: by the Doppler Effect for light, they must be moving away from us (When an object moves away from us, its light waves are stretched into lower frequencies or longer wavelengths which is called reshifted)

• This is evidence that the universe is expanding as in an expanding universe all galaxies would appear to be moving away from each other
• Equations of general relativity have been used to explain this

Question 50

Electron Microscope

Answer 50

de Brogolie’s allowed for electrons to be studied by wave theory

• Electron microscopes bombard very tiny objects with magnetically-focused electrons and can get extremely high resolution because of the very small wavelengths of the electrons

Question 51

Quantum Mechanics

Answer 51

• Erwin Schrodinger introduced the equation that described the way that waves propagated in space and time
• Werner Heisenbergdeveloped an equivalent formulation of quantum mechanics using matrix algebra, showed that the new quantum mechanics predicted that the degree to which one knew the position of a particle was inversely proportional to the dgree which one could known the momentum of a particle
  
  • Heisenberg’s uncertainty principle: one cannot simultaneously know both where a particle is and where it is going to arbitrary accuracy
• Wolfgang Pauli, stated the pauli exclusion principle that certain types of particles, such as electrons, cannot be in the same quantum states (no two electrons can have identical quantum numbers)

Question 52

Superconductivity

Answer 52

• The resistivity of a material is slightly temperature dependent:
  
  • An object will conduct better at low temperatures and at very low temperatures (often only 20 or 30 degrees above absolute zero), the resisitivity of some substances will drop to zero
    
    • A material with zero resistivity and therefore zero resistance to electric current is a superconductor
      
      • With quantum mechanics superconductivity can be explained (cannot be explained by classical electromagnetic theory)
        *

Question 53

Quantum Field Theory and The Standard Model of Particle Physics

Answer 53

• Unification of quantum mechanics and relativity
• Quantum field theory describes three of the four fundamental forces of nature
  
  • The electromagnetic force is responsible for almost all of the forces seen in daily life, including friction, the normal force, and many others
  • The strong nuclear force is what holds the nucleus of an atom together (as protons are electrically repelled to protons, kept bunched in nucleus by strong nuclear force)
  • The weak nuclear force mediates radioactive decay
  • General relativity and the force of gravity have remained difficult to relate to quantum mechanical efforts
    
    • For general relativity to be significant a great deal of mass is necessary and for quantum mechanics small enough to describe atomic structure and atomic interactions in quantum mechanics
      
      • What has the mass of the sun but size of an atom or smaller, only a black hole has such densities
        
        • As a result physicists investigate black holes and what can be learned about them

Question 54

Cosmic Microwave Background

Answer 54

• The very early universe, before expanding very much, involved extraordinarily high densities
  
  • Light emitted extremely long ago that is just now reaching us (as it was emitted very far away) can help in investigating current problems in physics
    
    • In the very early universe, a great deal of matter and a great deal of energy (including light) were in a very small space, and as a result, photons that were emitted from one source just hit particles of matter and were absorbed
      
      • As the universe expanded , the density of the universe decreased as the space between particles increased (meaning light could travel farther before hitting matter and absorbed)
        
        • At a certain point, light became able to travel almost freely without bumping into matter anymore, and that light, which happened to be in the microwave area of the universe at the time, is still moving through the universe today
          
          • Since the light was everywhere in the universe at the time, it is everywhere today as well, forming a background to everything in the cosmos
            
            • Measurements of the CMB is also evidene of the expanding universe, more precise measurements of the CMB will yield answers to questions about how the universe came to its present state

Question 55

Dark Matter and Dark Energy

Answer 55

• Current theories of gravity (both Newtonian and Einsteinian) predict that a particular graph should predict the rotation of stars- stars orbit the center of galaxies in much the same way that planets orbit the center of solar systems- but when rotation of stars is actually observed, the graph one draws is substantially different
  
  • Appears great deal of mass that should be in each galaxy is missing called dark matter as it cannot be seen
• When one uses general relativity to describe the expansion of the universe and inputs what is known about the current state of the universe, it appears that the expansion of the universe should be decreasing (distant galaxies should be redshifted by smaller amounts) however, observing the actual expansion of the universe reveals that it is in fact increasing
  
  • The source of the energy that could cause this, called dark energy is unknown
• Investigations of black holes, dark matter, and dark energy are ongoing