Physics: Cosmology Flashcards
Georges Lemaître
20th century Belgian cosmologist who formulated the modern big-bang theory, which holds that the universe began in a cataclysmic explosion of a small, primeval “super-atom.”
Fred Hoyle
20th century English astronomer who rejected the Big Bang theory, championing the Steady-State theory instead.
Steady-State Theory
The theory that the universe doesn’t have a single starting point but that new matter is continuously created to maintain a constant density as the universe expands.
Doppler Effect
The Doppler effect refers to the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the source of the wave. Was theorized in 1842 by the Austrian physicist Christian Doppler.
Red shift
The Doppler effect for light; objects moving further have larger wavelengths and appear redder whereas objects moving closer have shorter wavelengths and appear bluer.
What are the key pieces of evidence that the universe is expanding?
- Redshift of galaxies
- Cosmic Microwave Background Radiation
Hubble’s Law
The redshift of distant galaxies is proportional to their distance from us; v=H0*d
Radial Velocity
The component of an object’s velocity that is directed along the line of sight of the observer.
Recession Velocity
The apparent velocity at which a distant object (such as a galaxy) is moving away from an observer due to the expansion of the universe.
Cosmic Microwave Background Radiation
Radiation in the form of low-energy photons, which transitioned from high-energy photons when the universe began to expand and cool; consists of the uniform glow across the entire universe.
Planck Epoch
Period within the first 10^(-43) seconds after the big bang in which the effects of quantum mechanics and gravity were comparable.
Inflation
Period immediately after the big bang in which the fabric of space grew exponentially, thus allowing the matter contained within the singularity to expand; had the effect of making the distribution of matter and energy more uniform on large scales; occurred within 10^(-36) to 10^(-32) seconds.
Reheating
Period immediately after inflation, where the energy stored in the inflation period was converted into various particles (quarks, electrons, etc.), thus reheating the relatively cold and dilute inflation-universe.
Recombination
The period 370,000 years after the big bang, where the universe had expanded and cooled enough for protons and electrons to combine and form neutral hydrogen atoms. This transition made the universe transparent to radiation as photons could now travel freely through space.
Sagittarius A*
Black hole at the center of the Milky Way
The Local Group
Galaxy group which includes the Andromeda (largest), Milk Way (2nd largest), and 52 other galaxies.
Big Bounce Theory
The theory that the universe undergoes a repeating cycle of Big Bangs followed by contractions, resulting in a series of cosmic cycles.
Big Bang Theory
Theory that the universe began as an extremely hot and dense state approximately 13.8 billion years ago and has been expanding ever since.
The Planck Scale
The energy and length scales at which our current understanding of physics breaks down and quantum mechanics and gravity become comparable.
Accretion Disk
A swirling disk of gas and matter caused by gravitational attraction; rotational velocity of objects moving towards center increases as distance between them decreases (to preserve angular momentum), which has a tangential component, ultimately leading to the spiral.
Horizon Problem
Fact that temperature is constant around the universe, which can’t be explained without the cosmic inflation
Flatness Problem
Fact that the universe is relatively flat, which can’t be explained without the cosmic inflation.
Evidence of Black Holes
- High-velocity gas clouds (Accretion disks)
- High amounts of radiation (The high gravitational forces near the black hole cause the surrounding material to heat up (as gravitational potential energy is converted to kinetic energy) and emit radiation as it spirals inward; Detectable by using telescopes which are sensitive to X-rays since X-rays are high radiation)
Singularity
Point which is so dense that as you get closer to it, its gravitational pull becomes infinitely strong
