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Definition
| effect of finite speed of light, enables us to look into the past |
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Term
| Matter perturbations (deviations away from isotropy and homogeneity in density) evolution over time due to gravity |
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Definition
| Overdense regions become denser, underdense regions become less dense |
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| number of Collisions more frequent in the past |
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Definition
| Density of galaxies was larger in the past due to expansion of Universe over time |
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Term
| Hierarchical growth of galaxies |
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Definition
| smaller galaxies combine to form larger galaxies |
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| Potential outcomes of galaxy collisions |
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Definition
| two spirals forming an elliptical, a spiral getting spun up by a smaller galaxy merging |
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Term
| Tests of galaxy evolution through galaxy galaries |
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Definition
a) Over time the galaxy types go from irregular to more well formed spirals and ellipticals
b) Observations of more galaxies will lead to better tests of galaxy formation models |
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Term
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Definition
i) Many known from particle physics experiments in the 20th century
ii) Most important for us are electron, proton, neutron, and photon, but in early Universe others also existed
iii) Distinguished by their mass, forces that they are sensitive to |
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Term
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Definition
i) Electromagnetic - force mediated by photon. Attracts electrons to protons in the nucleus to form neutral atoms
ii) Weak - short ranged force that is important for neutron’s decay, fusion reactions in stars, and determining amounts of nuclei formed during nucleosynthesis
iii) Gravity - attractive force that keeps galaxies bound, governs the evolution of the expansion of the Universe
iv) Strong - force that binds quarks into protons and neutrons, also binds protons and neutrons into nucleus of atoms |
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Term
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Definition
| put enough energy in a region (for example by colliding two energetic particles), can create new particles. Basis for particle collider experiments that helped discover the known elementary particles and forces |
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Term
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Definition
| put a particle and its antimatter partner together, can annihilate into two energetic photons |
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Term
| Temperature of the Universe |
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Definition
i) As Universe expands, its temperature cools down
ii) Temperature should be thought of as the average energy of a group of particles. Example, temperature of a gas tells how much kinetic energy (the energy from particles moving at some speed) is in the gas particles.
iii) Since the Universe was much hotter in the past, once the temperature was higher than an elementary particle’s mass, that particle became part of the Universe’s plasma (e.g. for temperatures above 1016 K, top quarks were part of the content of the Universe). |
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Term
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Definition
Situations in science with an improbable observation (example:
two random numbers between -1 million and 1 million give a value of 2) |
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Term
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Definition
| There is no underlying cause, but is just an improbable event that occurred (like me winning the lottery). Best known example is Weinberg’s anthropic explanation of the cosmological constant |
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Term
| Underlying Reason explanations |
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Definition
| Some underlying mechanism makes the seemingly improbably event to be probable. Example: a psychic using a friend as a “stranger” to demonstrate his psychic powers. Examples in cosmology are explaining horizon and flatness problems with inflation. |
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Term
| Matter-Antimatter Asymmetry |
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Definition
i) A fine tuning problem since observed dominance of matter over anti-matter requires 1 billion + 1 proton for every 1 billion antiprotons.
ii) An example of a cosmological fine tuning problem which has inspired work in particle physics, leading to new theories and experiments to test each solution |
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Term
Cosmic Microwave Background (CMB) and Formation of Neutral Atoms at 400,000 years
after the Big Bang |
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Definition
i) Observed blackbody radiation with a temperature that is very close to 2.73 K in all directions
ii) Consequence of a hot Universe that formed neutral atoms, since before neutral atoms, the photons that were scattering around had a temperature. After neutral atoms form, the Universe was transparent and those photons could travel to us and be seen. The temperature of the photons cooled with the expansion of the Universe and thus the CMB gives evidence for a much hotter Universe in the past
iii) Evidence for isotropy: The temperature of the CMB is (after taking into account our motion through the Universe) equal in all directions to 1 part in 100,000.
iv) Anisotropies (the temperature deviations from 2.73 K) constrain the properties of the energy content of the Universe. |
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Term
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Definition
| By comparing anisotropies separated by a certain angle on sky, the temperature map of the CMB can be reduced into a power spectrum, detailing how likely temperature fluctuations are separated by a given angle |
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Term
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Definition
| Before neutral atoms formed, electrons, protons and pho- ton were interacting with each other, allowing waves of temperature (similar to sound waves being density waves of air). Since the speed of these waves are known and the time when neutral atoms formed is known, this predicts where the first peak of the power spectrum should be |
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Term
| Exact position of the first peak is sensitive to geometry of Universe, its location is consistent with the geometry being flat |
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Definition
| relative heights of peaks give additional information, e.g. this gives corroborat- ing information that there is matter which does not interact with photons (dark matter, see Chapter 14.1) |
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Term
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Definition
| formation of Primordial Abundance of Low Mass Nuclei in the first 5 minutes of the Universe |
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Term
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Definition
i) Due to the large amount of photons relative to protons, in the early Universe photons broke apart nuclei, so that only light mass nuclei (H, He, D, Li) get formed (heavier nuclei get formed in stars)
ii) Binding energy (the amount of energy it takes to free the neutrons and protons in a nucleus) determines which nuclei are more stable
iii) From binding energy, 4He is the most stable nuclei, so when the nuclear reactions can form helium-4, all of the neutrons will be captured in helium-4 nuclei
iv) Using the properties of the weak force, there are 1 neutrons to every 7 protons at this moment, leading to 1 helium-4 nuclei to every 12 protons. This means that the mass fraction in helium-4 is predicted to be 25%
v) By looking regions with low heavy nuclei (i.e. with little star processing of heavy elements), the primordial abundances can be measured. |
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Definition
| Speculative, but successful theory of accelerated expansion in the early Universe |
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Term
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Definition
| the puzzling fact that the CMB temperature is equal to one part in 100,000 in each direction, but that there is no time for the regions in each direction to have communicated (since CMB photons were emitted 400,000 years after the Big Bang, but the current age of the Universe is 14 Billions years) |
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Term
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Definition
| the geometry of an isotropic, homogeneous Universe can be flat, spherical or saddle. The CMB is very consistent with flat (see first peak) and this is unusual since a spherical or saddle geometry becomes more noticeable over time (you can see more of the Universe over time, revealing the geometry) |
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Term
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Definition
| a small region which had enough time to reach similar temper- ature, then rapidly expands and thus the regions on opposite sides of the sky are actually part of the same small region of near uniform temperature |
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Term
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Definition
| accelerated expansion causes the size of a spherical or saddle geometry Universe to get really large, so large that we can’t see enough of the Universe to detect its non-flat geometry even if it existed. |
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Term
| Inflation’s other predictions |
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Definition
i) Origin of perturbations away from isotropy, homogeneity: Since inflation is due to a scalar field which obeys quantum mechanics, there are quantum fluctuations causing the Universe to be hotter or colder in di↵erent areas, as well as overdense and underdense, leading to the fluctuations that we observe in the CMB and allowing the growth of structure as seen in galaxies
ii) Gravitational Waves: Inflation also predicts gravitational waves in the early Uni- verse, these indirectly a↵ect the polarization (direction of the EM waves) of the CMB and can be detected by detailed studies. Recently, a detection of this e↵ect looked like a confirmation but turned out to be due to polarized emission from dust in the galaxy. |
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Term
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Definition
| a way to break down a complicated calculation into smaller calculations, which can be argued and refined |
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