Particles Flashcards
What is ‘Cosmology’?
the science of the origin and development of the universe
What are the 3 properties of the Cosmological Principle?
- Universality
- Homogeneity
- Isotropy
What is Universality?
- the law of physics are the same everywhere, even if conditions vary from place to place or time to time
What is Homogeneity?
- matter is evenly distributed throughout the Universe
What is Isotropy?
- no direction is special
- the Universe looks the same in all directions
What is the Cosmological Principle?
- viewed on a sufficiently large scale, the properties of the Universe are the same for all observers
What are the specific rules / principles to the Cosmological Principle?
- Homogeneity is true only in an average sense
- we do see structure at all distance scales, but you can’t tell “where you are” in the universe
- the Universe evolves, looking different at different times
What is the Big Bang model?
- approximately 13.7 billion years ago, the universe was very hot and dense.
- it has since expanded and cooled
What is the evidence for the Big Bang?
- it is dark at night
- the Universe is expanding
- we observe a cosmic microwave background
- it predicts the correct relic abundance of different elements in the Universe
- the formation of Galaxies and Large Scale structures
What is Olber’s paradox?
Hypothesis assumes that the universe is;
- static, eternal, infinite and uniformly filled with stars
PROBLEM - if this hypothesis was true, then the sky would be as bright as the surface of the star
- distant stars will be dimmer than near ones
- their light is spread out over a sphere of radius r, spread over an area 4 pi r(2)
- BUT if the stars are uniform, the number of stars a distance proportional to r from us also grows at r(2)
- these two factors should cancel out and the night time sky should be as bright as the surface of a star!
Possible (wrong) solutions for Olber’s paradox?
1) dust obscures the star light
- BUT if the dust kept absorbing the light, it would get hotter and hotter so eventually it would radiate the heat up so much that it would glow as brightly as a star
2) finite number of stars
- Kepler believed that the universe has a finite number of stars so there are ‘gaps’ where we see no stars
- it is true that the observable universe has a finite number of stars, but other observations disfavour Kepler’s static finite universe
What did Hubble discover in 1928?
- he observed that other galaxies are moving away from us
- this does not imply that we are at the centre of the universe since the galaxies are moving away from every point in the universe
What is Hubble’s Law?
v = H x d
- found that the speed the galaxy moves away from us is proportional to the distance it is from us
v = velocity things move away from us d = distance from us H = Hubble's constant
What is the Doppler Effect?
- a wave emitted from a moving source will have its apparent wavelength altered by the direction of the source
source is moving away = the wavelength will be longer
source is towards the observer = the wavelength will be shorter
What is Redshift?
- astronomers observe that distant objects emit light which appears more red than one would expect
- since red light has a longer wavelength than blue light, this tells us that these distant sources are moving away from us
Penzias and Wilson (1964)
- built a radio antenna intended for radio astronomy and satellite communications, but were plagued by an annoying background hiss
- it was isotropic, indicating a cosmic source and of a very particular frequency / wavelength
- previously called the “echo of the Big Bang”
What is the WMAP and what does it measure?
the Wilkinson Microwave Anisotropy Probe
- measures the Cosmic Microwave Background Radiation very precisely
What is the surface of last scattering?
- if you go far back enough in time, at some point the universe was so hot and dense that it was a sea of charged particles that absorbed and emitted light constantly
- it was opaque to photons
- after around 379,000 years the universe had cooled enough to form neutral atoms and then light could suddenly travel freely
- the universe became transparent
What is the CMBR?
- it is a perfect black body spectrum
- it is so cold (long wavelength) because it has been redshifted by the expansion of the universe
What is the mythic cosmology?
very early cosmologies (that is descriptions of the origin and development of the universe) were based on supernatural events and often supernatural creator beings
What is physical cosmology?
- science is based on observation and prediction
- we make observations, formulate theories, make predictions and then these predictions by experiment
Thales of Mileteus
- greek philosopher
- credited with the first every scientific cosmology
- described the Earth as a disk floating on a huge body of water
Pythagoras and Plato
- thought everything must be based on circles and spheres
Eudoxus
- created a system of 27 concentric spheres that carried heavenly bodies, with the Earth at the centre
- an attempt to explain retrograde motion
Aristotle
- agreed with Euxodus
- emphasised that the heavenly bodies were different from Earth
- they were made from “a perfect and unchanging celestial material”
- this reinforced the idea of Earth being special, at the centre
- knew that the Earth is a sphere from seeing the Earth’s shadow on the moon at a lunar eclipse
Heraleides of Athens
- believed that the Earth rotated on its axis
- Mercury and Venus go around the Sun which orbits the Earth
Aristarchus of Samos
- quoted by Archimedes as believing that the Earth goes around the Sun
Eratosthenes
- measured the circumference of the Earth in 240 BC
- estimated the distance from Syene to Alexandria
- measured a shadow of column in Alexandria at a time when the Sun was directly overhead in Syene
(360/7) x 820km = 42,000km
(the true value is roughly 40,000km)
Ptolemy
- had all heavenly motion based on circles, with planetary orbits centred on the Earth
- used epicycles to get retrograde motion
- like the earlier Greeks, he thought that the stars were on a celestial sphere, outside of the planets
Copernicus
- studied the astronomical theories and techniques developed by the Greeks and Arabs to explain the motions of the planets
- published his own system, with the Sun at the centre of the “universe”, in the last year of his life
What are the major features of Copernican theory?
- the heavenly motions are uniform, eternal and circular or compounded of several circles (epicycles)
- the epicycles were included to explain observational data
- in reality the planets’ orbits are elliptical not circular with the Sun at one focus
- the centre of the universe is near the Sun
- around the Sun in order are M, V, E and Moon, M, J, S and the fixed stars
- the Earth has three motions: daily motion, annual revolution and annual tilting of its axis
- the retrograde motion of the planets is explained by the Earth’s motion
- the distance from the Earth to the Sun is small compared to the distance to the stars
Brahe
- developed large instruments for making accurate celestial observations
- accumulated accurate data on position of stars and the positions of the Sun, Moon and Planets
- observed a supernova in 1572 and seeing it had no daily parallax, convinced himself it was a star, hence the celestial sphere is not immutable
- in his system, the planets go around the Sun while the Sun goes around the Earth
Galilei
Used a telescope and observed that:
- Jupiter has moons
- the moon is not perfect
- the Milky Way is made of stars
- Venus has phases, like the moon
- the moons rotating around Jupiter shows that there is no unique centre of rotation in the Universe
Kepler
- using Brahe’s instruments and records, Kepler found the first accurate mathematical laws that the planets obey
Kepler’s Laws
- the planets follow elliptical, not circular, orbits with the Sun at one of the foci
- a line joining a planet and the Sun sweeps out equal areas during equal intervals of time
(period) 2 is proportional to (average radius)3
Newton
- made many contributions to science
- main contribution to astronomy - the realisation that the law of gravity that makes things fall on Earth is the same law that governs planetary motion
Einstein
- most famous for his Special and General Theories of Relativity
General Theory of Relativity
Gravity is an effect caused by the curvature of space-time
- energy tells space-time how to curve
- the curvature tells matter how to accelerate
Some consequences:
- gravitational lensing
- binary pulses
- gravitational waves (infrared but not yet directly observed)
Special Theory of Relativity
Postulates:
- the laws of physics are the same in all inertial frames of reference
- the speed of light in vacuum is the same in all inertial frames of reference and is independent of the motion of the source
Consequences:
- nothing can go faster than light
- moving clocks tick more slowly
- moving objects are shrunk in the direction of motion
1916 - Harlow Shapley
studied globular clusters (groups of hundreds of thousands of stars) and realised that the Milky Way is much much bigger than previously thought
1924 - Hubble
- showed that a Cepheid variable star in Andromeda was way outside our own galaxy
- this showed that Andromeda, a “spiral nebula”, was actually itself a galaxy
1929 - Hubble
- demonstrated that the universe is expanding (Hubble’s law)
1948 - George Gamow
develops a model of big band nucleosynthesis
1964 - Penzias and Wilson
discover the Cosmic Microwave Background
What is the atom made up of? What is the neutron made up of?
ATOM- mostly empty space with electrons orbiting a very tiny nucleus
NUCLEUS - made up of even smaller protons and neutrons
What quarks are protons made of?
Up + Up + Down
What quarks are neutrons made of?
Up + Down + Down
The atom
- formed of an atomic nucleus with one or more electrons in orbit
- most of the space in an atom is taken up by the electron orbits
The electron
- the electron’s electric charge provides the basis for electricity and all modern electronics
The neutrino
- a very light, very weakly interacting particle which is difficult to observe
- produced in radioactive decays, such as Beta Decay
Three generations
- the universe has been made in triplicate
- the structure of quarks and leptons come in three almost identical ‘generation’ with each slightly heavier than the last
The Four Forces
- there are four forces of nature; Gravity, Electromagnetism and the Strong and Weak nuclear forces
- gravity is not included in the Standard model
- the matter particles (fermions) interact by exchanging force carriers (boson)
- each force is mediated by a different particle
What is Electromagnetism?
- the force felt by particles with an electric charge
- has an infinite range
- mediated by the photon
- atoms which have an equal number of protons and electrons are neutral
- the electromagnetic force between astrophysical objects is typically very small but it binds electrons and protons into atoms and atoms into molecules
What is the Strong Force?
- felt by particles with “colour” which are quarks and the gluon
- each quark has a colour change which is either red, green or blue
- mediated by the gluon
- since the gluon can throw other gluons, it turns out that the forces strength grows with distance
- this causes the quarks to be confined in bound states such as the proton which has one quark of each colour making it colour neutral
- the energy of the strong interaction is what gives you most of its mass
What is the Weak Force?
- felt by all the particles except the photon and the gluon
- mediated by the W and Z bosons
- provides the source of the sun’s energy via fusion and causes radioactive decays
- it’s weak because the W and Z bosons are very heavy which makes them hard to produce
in the Standard Model, the W and Z bosons get mass because of their interactions with the Higgs boson
What is Gravity?
- the weakest force and is NOT part of the Standard Model
- thought to be mediated by the gravitation though
- no negative mass / energy so all the ‘gravitational charge’ adds up
- large objects like stars and planets have huge masses but very small charges, so gravity is the force which governs their motion
What are antiparticles?
- corresponds to a particle
- are like particles in every way except they have the opposite values for all their properties, except for mass and energy
- an electron has a -1 charge but its antiparticle, the positron has +1
- some particles are their own antiparticle (eg the photon)
What happens when a particle and a antiparticle meet?
they annihilate each other, releasing all their energy in an explosion
E = mc(2)
- the energy is converted into bosons, usually light
What are Particle Physics Experiments?
- to see very small things, we need very small wavelengths / high energies
- to crate particles with large masses, we need large energies
- have to accelerate particles to very high energies, then collide them
- the resulting debris tells us about the fundamental particles
What is the Large Hadron Collider (LHC)?
- the largest collider in the world
- designed to test the Standard Model
- 2012 - reported the discovery of the Higgs Boson
What are the stages in the evolution of the Universe?
Planck GUT Electroweak Particle Nucleosynthesis Nuclei Atoms
What is the universe made up of?
- modern observational evidence shows that the matter which makes up everything, the fundamental particles of the Standard Model is not the full story
- normal matter only really makes up about 4.6% of the energy of the Universe
- the rest is made up of Dark Matter and Dark Energy
What is Gravitational Lensing?
- general relativity tells us that very large masses can bend light, so massive galaxies can act as gravitational lens
- most gravitational lensing is not so direct as this, but distortions of the light can tell us how massive a galaxy is and where its mass lies
- this supports the hypothesis of a dark matter halo around galaxies
What is the Bullet Cluster?
- actually two colliding clusters of galaxies
- can compare the visible matter, marked in pink, as observed by the Chandra X-Ray telescope and the ‘dark’ matter as determined by gravitational lensing, marked in blue
The Cosmic Microwave Background
- acoustic oscillations is the plasma, from just before the surface of the last scattering should show up in the CMBR
- since the dark matter is ‘dark’, it does not give off photons should it contributed differently to the CMBR
- the first peak tells us about the (charged) normal matter
- the second peak tells us about the (neutral) dark matter
- this is consistent with other measurements of the Dark Matter density
What is Dark Matter?
- we currently don’t know what it is
- BUT we can make educated guesses
Dark Matter - MACHOs
Massive Compact Halo Objects
- dark matter could be made of normal matter bound up so that it does not emit light
- these are called MACHOs and could be black holes, neutron stars, brown dwarfs or large Jupiter-large planets
- however this is disfavoured by data
- we simply don’t see enough of these
- they also cannot account for the baryon acoustic oscillations in the CMB
Dark Matter - WIMPs
Weakly Interacting Massive Particles
- this would be a stable particles which only feels the weak nuclear force
- so it doesn’t emit light (it is ‘dark’), is very heavy and is very hard t detect
- this is in much better agreement with the data which points to a particles with mass of a few hundred times the proton mass
Candidate particles:
- a heavy (sterile) neutrino
- an axion
- a supersymmetric particles, such as a neutralino
What is modified gravity?
- it may be possible to account for the effects of “Dark Matter” without introducing new matter at all, and instead modifying our laws of gravity (again)
- however this is very hard and there is no convincing theory
Searching for WIMPs
There are many ways to detect WIMP matter:
- direct production of WIMPs at particle collider such as the LHC
- direct detection of WIMPs that may be surrounding us right now
- indirect detection of WIMPs via their annihilation products, such as gamma rays or neutrinos
What is the Alpha Magnetic Spectrometer?
- one example of an indirect detection of WIMP dark matter is this spectrometer
- is installed on the International Space Station
- this looks for evidence of Dark Matter in cosmic rays
Dark Energy - early evidence
1998 - Schmidt & Riess
1999 - Perlmutter
- both separately observed a Type 1a supernovae and showed that the expansion of the universe is accelerating
- supernovae are standard candles so we can use their Redshift & intensity to determine how fast they are moving away from us
- the supernovae appeared dimmer than expected, indicating that they were further away and the universe’s expansion is speeding up.
Dark energy - other evidence
- observations by Schmidt & Riess and Perlmutter were later confirmed by the Hubble Space Telescope
- CMBR
- indicates that the universe is incredibly flat
- to achieve this, the universe needs to have a critical density, but without Dark Energy the density of the universe would be too low - Large-scale structure
- provides a standard ruler
- looking at these ‘rulers’ at different redshifts tells us the universe is accelerating - the Sachs-Wolfe effect
What is Dark Energy?
One candidate is the Cosmological Constant
- in 1915, when Einstein published his theory of General Relativity, the principle equation was:
Energy = curvature + Cosmological constant
- the Cosmological constant provides an outward presses making the universe expansion accelerate
What is Quintessence?
- a possible fifth fundamental force that could provide a repulsion that can account for the accelerating expansion
- models of quintessence usually involve a boson (like the Higgs boson) whose density varies over space and provides a negative pressure
- this boson (or a similar one) may also cause inflation, which was also an accelerating expansion
What is Modified Gravity?
- it may be possible to explain Dark Energy by modifying gravity (General Relativity) at large distances
What is entropy?
- the Second Law of Thermodynamics tells us that entropy (disorder) always increases
- Lord Kelvin pointed out that we can only ‘use’ energy for doing work for a fixed amount of time before it is all converted into heat
- the universe will reach thermal equilibrium and all (non-thermodynamic) motion will stop for ever
- this is known as the Heat Death of the Universe or sometimes The Big Freeze
What is the Big Crunch?
- the Big Bang tells us that billions of years ago the universe was hot and dense and then expanded
- how its size changes depends on the density
- if there were no Dark Energy and we had the critical density, the universe’s expansion would slow over time (slowed by gravity), but only slow to a stop after infinitely long
- a more dense universe would slow its expansion and collapse to a Big Crunch
- a less universe would continue expanding forever
- Dark energy changes this picture by making the expansion accelerate
What is the Big Rip?
- in some models of quintessence (Dark Energy), the outward acceleration becomes so strong that our horizon starts to shrink
- objects we could previously see become so far away that the light can no longer get to us
- eventually even the Sun would leave our horizon, and then atoms would be ripped apart by the acceleration
- the universe would effectively end since no particles would be in contact with each other
A Metastable Universe
- physical systems tend to settle into their lowest energy states
- sometimes the lowest energy state is blocked by a barrier
- we may currently not be in the lowest energy but be in a metastable state
- Quantum Mechanics tells us that systems can tunnel through barriers if given long enough
- the true vacuum may have entirely different physical constants, so the universe would cease to exists as we know it
The Multiverse
- String theories have may meta-stable vacua
- each of these would (potentially) have different physical constants
- as the universe cools, it chooses one of the meta-stable vacua
- however, regions far enough apart fall into different vacua
- the universe would then form lots of bubbles each containing a different “universe” inside, separated by energy walls
- the energy boundaries should be visible and since we don’t see them, this means the bubbles must be bigger than our own observable universe
- this solves the problems of fine tuning by using an anthropic principle
What is positive curvature?
curled up like a ball
What is negative curvature?
shaped like a saddle
What is zero curvature?
completely flat
What is Inflation?
- a proposed theory where the universe expanded very rapidly just after the Big Bang, increasing the volume of space by a factor of 10(78)
- this expansion would naturally make the universe flat, just as a balloon’s surface becomes flatter as you add more air
- then omega becomes very close to one just after the Big Bang, explaining why it is still approximately one now
What is the Horizon Problem?
- the CMB is incredibly uniform
- however it was created only 300,000 years after the Big Bang so regions of space separated by more than 300,000 light-years didn’t have time to talk to each other
- so how did they manage to be so exactly in thermal equilibrium?
- this is known as the Horizon problem
What is Inflation?
- inflation comes to the rescue for the Horizon Problem
- to be in such good thermal agreement, maybe these different regions were at some point in contact
- it causes the regions of space to grow faster than their horizons, so areas that were in causal contact at the start of inflation may no longer be at the end of inflation
What is the Large Scale Structure Problem?
- the structure of the universe (galaxies and clusters of galaxies) arise in the Big Bang Model from inhomogeneities in the early universe
- these may be the same fluctuations we see in the CMB, but we expect the (spatial) size of CMB fluctuations to be the size of the atom, not Mpc!
- it is possible that these inhomogeneities are indeed quantum fluctuations and are incredibly small but Inflation expands these tiny fluctuations into macroscopic sizes
What is the Exotic Relic problem?
- many theories for physics beyond the Standard Model at very high energies (eg GUTs) contain exotic relics
