ACE Class Report: Dark Matter, Dark Energy: The dark side of the universe.
Begin: 2/29/2024
Finish: 5/16/2024
Title: Dark Matter, Dark Energy:
The dark side of the universe.
Instructor: Professor Sean Carroll
California Institute of Technology.
Why I choose to take this class:
To Delve into the unknown field in science and cosmology. Especially dark matter and dark energy.
What I learned from this class:
Fundamental Building Blocks: When the universe was young enough, it was opaque after Nucleosynthesis; you couldn’t see your hands in front of your face. When it was 400,000 years after the big bang, it became transparent after recombination. Scientist have a comprehensive picture of what the universe is made of: 5% ordinary matters, 25% unseen dark matter and 70% dark energy.
The smooth, expanding universe: Hubble’s result is that observers in any galaxy as moving away from themselves.
Space, Time and Gravity: Special Relativity replaces the absolute space and time of Newton with a single unified notion of spacetime. General Relativity gives spacetime a geometry that can warp and bent through time; we interpret the effect of spacetime’s geometry as the force of gravity.
Cosmology in Einstein’s Universe: The curvature of spacetime should be pretty much the same everywhere. Although the universe is homogeneous in space, it does seem to be evolving with time, in fact it is expanding. We invent a quantity called the scale factor, which tells us how much the universe has expanded between any two moments. General Relativity doesn’t just relate these numbers (Energy density, expansion rate and curvature of space) at each moment in time – it tells us how they change with time. it allows us to determine how scale factor evolves.
Galaxies and Clusters: In 1970 Vera Rubin conducted the measurement of Rotational curves of Galaxies. Observations of the speed at which matter is orbiting the center of the Galaxy as we move away from it in distance. Distant matter is moving much faster that it should be. The rotational speed remains approximately constant, including there is unseen matter giving rise to Extra gravity, we need dark matter it is a completely new kind of particle.
Gravitational Lensing: Part of the General Relativity is the idea that everything reacts to gravity in the same way, this includes light. A photon passing through a gravitational field will be deflected – Gravitational Lensing. We can use gravitational lensing as an alternate way to measure total mass.
Atoms and Particles: There are two basic types of particles: Boson and Fermions. Matter particles are known as Fermions, they take up space and you cannot pile a bunch of Fermions on top of each other. Force carrying particles are called Bosons, photons and gluons are examples. Bosons can be pile on top of each other like electric or magnetic field. Gravity is carried by particles called gravitons.
The Standard Model of Particles Physics: The standard Model and its particles were checked to ensure they are not a good candidate for dark matter. The Boson of the Standard Model comes in five types: electromagnetic force, strong nuclear force, weak nuclear force, gravity and Higgs Boson. The Neutrino almost has the right properties to be a good dark matter candidate – It is stable, neutral and weakly interacting; however their masses are too small, moves very quickly, zipping right out of galaxies rather than settling into them. Higgs Force was found in Cern 2018, it carried by a hypothetical particle called Higgs Boson. Dark Matter should be neutral (or it wouldn’t be dark). stable (or it would decay into something else) and slowly moving (or it wouldn’t settle into galaxies and clusters). There are no good dark Matter candidate within the Standard Model.
Relic Particles from the Big Bang: Protons do not decay into Neutron plus a Positron plus an electron Neutrino because it does not match energy conservation. The proton is lighter than the Neutron; therefore, it will never decay. Neutron has a lifetime of about 10 minutes. Heavy particles decay into light ones but not vice versa. A Neutron will decay into a Proton, an Electron and an Electron Anti-Neutrino. If there exists some particle that resemble the Neutrino (stable, weakly interactive, electrically neutral) but is substantially more massive, it could very easily have the appropriate relic density to account for Dark Matter today.
Primodal Nucleosynthesis: Nucleosynthesis – the process by which heavy elements are formed from lighter one. Primodal Nucleosynthesis occur when the universe was a few minutes old, and proton and Neutron were converted to Hydrogen, Deuterium, Helium and Lithium. Heavier elements are formed in Stars and Supernova explosions in the later universe. Nucleosynthesis provides a definitive measurement of the total number of Nucleons in the universe. provide a measurement of the total mass density of ordinary matter in the universe. The answer is: it’s not enough to account for dark matter. Ordinary matter makes up only 5%of the critical density, whereas the total matter density is about 30 percent of the critical density.
The Cosmic Microwave Background: After Nucleosynthesis, eventually the temperature dips low enough that electron can join with nuclei to form atoms, a process known recombination – that’s when the universe becomes transparent. We can observe radiation in the form of Cosmic Microwave Background (CMB). The variations in density lead, in turn, to variation in temperature, which we can observe today. CMB provides more evidence in favor of Dark Matter. CMB provides a measurement of the total number of Nucleons in the universe,
Dark Stars and Black Holes: Search for Massive Compact Halo Objects (Macho’s) through gravitational lensing known as microlensing – for stars who gravitational field is weak. Macho’s definitely exist but total amount of infrared from the frequency of microlensing seems to fall short of what is needed to account for the Dark Matter. We don’t have any plausible way to make enough of these kinds of black holes to account for dark matters.
How will this class contribute to my success upon release:
Scientific knowledge about the cosmology improves my analytical and critical thinking skills. Which would help me upon my release.
