Cosmology Flashcards
When was the Big Bang?
14Ga
What is the Milky Way’s nearest galaxy. How far?
Andromeda Galaxy. 2 million light years away.
What is the look-back time?
The lookback time to an object is the difference between the age of the Universe now (at observation) and the age of the Universe at the time the photons were emitted (according to the object).
What is red shift and why does it happen?
Redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light) when two objects are moving away from each other. Light can be described like a wave, so light from a luminous object undergoes a Doppler-like shift if the source is moving relative to us.
What does Hubbles Law tell us?
Everything in space is moving away from us. The most distant galaxies have the largest red shifts, and the most distant galaxies are moving the fastest away from us. Vr = Hd (Vr = velocity of recession; H = Hubble Constant; d = distance between a galaxy and Earth)
What is Einstein’s theory of redshift?
Light from galaxies with a known distance to us is being redshifted even more than the Doppler effect predicts. This indicates that after emission, the light’s wavelength is stretched as space-time expands, and that the size of the change is based on the distance to the source. This behaviour, which is additional to Doppler redshift, seems to only be explained acceptably via the expansion of space. The universe/space is itself expanding.
What is Cosmic Microwave Background Radiation (CMBR)?
The CMBR is long-wavelength radiation which is everywhere and essentially constant in all directions. the Big Bang, which formed the CMBR, was everywhere, therefore the CMBR is seen everywhere. Gamma rays and X-rays released 400,000 years after the Big Bang have been red shifted (by ~1000x) and stretched over time to much longer microwave wavelengths which we now observe everywhere. CMBR is now ~2.7K but the clouds which initially emitted the radiation had a T of ~3000K.
What is the Hubble time? Why is this an oversimplification? What is a more valid expression for the age of the Universe?
The reciprocal of the Hubble constant. T = 1/H, because d/t (aka the radial velocity vr which = d/t) = Hd. This is an oversimplification because it assumes the expansion of the universe was (and is) linear and therefore unperturbed by any effects of matter (gravity) or energy present. It is only valid for an empty universe. A more valid expression is therefore T = 1/H . F where F is a correction factor which considers relevant effects (density parameters) which impact the rate of expansion through time.
What is the Big Crunch?
A model of Universe expansion which stops expanding due to gravity and then contracts to a final ‘Big Crunch’. This causes cycles of contraction (Big Crunch) and expansion (Big Bang). This yields the youngest universe age due to the strong gravity effects.
What is the empty universe model?
This model assumes a constant rate of expansion of the universe, as the universe is assumed to have a density of zero (no density parameters).
What is a closed universe?
If Ω (density parameter) is greater than 1, the Universe is closed and the will eventually halt its expansion and recollapse.
What is an open universe?
If Ω (density parameter) is less than 1, the Universe is open and will continue to expand forever. This is favoured by the ΛCDM (lambda cold dark matter) model. The expansion of the universe is also accelerating, which requires the presence of dark energy.
What is a flat universe?
If Ω (density parameter) is exactly equal to 1 then the Universe is flat and contains enough matter to halt the expansion but not enough to recollapse it.
Which two methods can be used to determine the age of the Universe?
- Using the Hubble time equation with a correction factor, F: T = 1/H x F
- Assess the chemical evolution of the Milky Way using the 238U and 232Th ratio and their decay constants in meteorites to yield a solar system age of 4.57Ga. This is then combined with the 238U/232Th ratio generated by element production (nucleosynthesis) in stars. Combined with our understanding of the rate of nucleosynthesis yields an age of 14.5 +- 2.6Ga for the Milky Way, which agrees with the Hubble time evidence.
What is a singularity in the Big Bang?
The Big Bang theory says that the universe came into being from a single, extremely hot and dense point (aka, a singularity) nearly 14Ga. It didn’t occur in an already existing space. Rather, it initiated the expansion—and cooling—of space itself.
What happened during recombination 375,000 years after the formation of the Universe?
The free electrons present in the gas cloud bonded with the positively charged H and He nuclei to form neutral atoms. This occurred at about 3000K.
What is the significance of recombination in turning the universe hot gas transparent?
Prior to recombination (free electrons + positive H and He -> neutral atoms), the free electrons rendered the hot gas very opaque, meaning photons could not travel very far before it was deflected by interaction with a free electron. Recombination (removal of free electrons to produce atoms) made the gas transparent and enabled radiation to travel through the Universe. Photons released during the recombination process manifest as the CMBR.
How long after the Big Bang did the first star form?
~100Ma
How long after the Big Bang did the first galaxies form?
~-670Ma
What are neutron capture reactions?
A nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Neutral neutrons enter nuclei easier than protons which are repelled electrostatically. Neutron capture plays a significant role in the cosmic nucleosynthesis of heavy elements. Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions (i.e., by nuclear fusion) but can be formed by neutron capture.
What is nuclear fusion?
A thermonuclear reaction in which two or more (USUALLY light) atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). Exothermic when produces atomic nuclei lighter than iron-56 or nickel-62. Supernova can produce enough energy to fuse nuclei into elements heavier than iron.
What is a galaxy?
A system bound by gravity, made up of stars, stellar remnants, interstellar dust and gas, and dark matter. They have 10^10 stars on average but range from dwarfs (10^8 stars) to supergiants (10^12 stars) each orbiting their centre of mass. Milky Way 10^11 stars. The oldest known galaxy formed 670Ma after the Big Bang.
What are the 3 types of galaxies?
Elliptical, spiral and irregular.
What are the characteristics of elliptical galaxies?
Elliptical appearance, little interstellar matter, low rates of star formation, dominated by older stars, larger, thought to have formed by the collision and merging of progenitor galaxies. The galaxy colour–magnitude diagram describes these as red non star-forming galaxies.
What are the characteristics of spiral galaxies?
A flat, rotating disc of stars and interstellar matter of which spiral arms are prominent components. Will continue to produce stars as long as dense molecular clouds are present in they spiral arms. A supermassive black hole at the very center of the central bulge. E.g. the Milky Way. The galaxy colour–magnitude diagram describes these as blue star-forming galaxies.
What names are used to describe the clustering of galaxies?
Galaxies lump together in groups to form clusters. Clusters of clusters are called superclusters. The Milky Way is part of the Local Group galaxy group (which contains more than 54 galaxies).
What are galaxy seeds?
Small irregularities in mass/density distribution which may have provided the nuclei around which galaxies and large-scale galaxy structures formed and grew.
How do galaxies form?
The leading hypothesis uses ‘structure formation’ theories to explain galaxy formation, where tiny quantum fluctuations exist in the aftermath of the Big Bang, causing heterogeneity in space, with galaxies clustering together with matter existing between.
What is (cold) dark matter?
~25% of matter in the Universe. Invisible and no one knows what it looks like. Does not emit or reflect light.
What role does dark matter play in galaxy formation?
Dark matter makes it possible for galaxies to form because, if we consider a Universe with only the visible matter (~5%), there is not enough mass of visible matter to form galaxies. With only visible matter, stars would not combine to form galaxies, instead they would spread out across the Universe. Dark matter provides the mass and gravity for galaxy formation.
How do we know dark matter exists?
The observation of large-scale galaxy structures (not gravitationally possible without); gravitational lensing (high concentrations of dark matter) bends light from distant galaxies around it to form arcs; the rotation of spiral galaxies (the visible matter does not behave according to Kepler’s Second Law).
What is dark energy?
Undetectable. Unmeasurable. Does not absorb reflect or emit light. ~70% of the Universe. Dark energy is thought to cause the accelerating rate of expansion of the Universe - but we don’t know what it is.
If the CMBR is irregular, what does this tell us about the Big Bang?
The differences in brightness, measured by the WMAP, are related to the temperature, density and pressure. We see small variations (or “anisotropes”) in this glow, at a range of less than about 1 part per 100,000. The low, but still there, anisotropy indicates that the Big Bang was not perfectly uniform - if it was, then the CMBR (which was emitted 370,000 after the Big Bang) would be perfectly uniform.
what is the density parameter?
the ratio of the average density of matter and energy in the Universe to the critical density (the density at which the Universe would stop expanding only after an infinite time). >1 would stop expanding and collapse back (crunch) vs. <1 infinite expansion
