L4 - Galactic Astronomy Part 2 Flashcards
Explain what ‘Cosmological Redshift’ is.
Stretching of photon due to expansion of the Universe, not the relative motion of the source (that’s the Doppler shift)
• Photon loses energy when travelling large distances
• Cosmological redshift is an indicator of the ratio of the size of the universe now and ‘back then’, not what is happening to the rate of expansion now.
• If light travels through many objects, multiple absorption and re-emission causes forest of redshifted lines, eg Lyman alpha forest.
Explain what ‘Cosmological Redshift’ is.
Stretching of photon due to expansion of the Universe, not the relative motion of the source (that’s the Doppler shift)
• Photon loses energy when travelling large distances
• Cosmological redshift is an indicator of the ratio of the size of the universe now and ‘back then’, not what is happening to the rate of expansion now.
• If light travels through many objects, multiple absorption and re-emission causes forest of redshifted lines, eg Lyman alpha forest.
Outline the concept of Active Galactic Nuclei (AGN)
Physical properties of quasars
High luminosity
Small emitting volume - small in size
Variability
Radio & optical & X-rays
Broad emission lines in spectrum => high velocity clouds
They are often bipolar… existence of radio lobes
The emitting region is small: ~0.6AU to 6AU for the disk => 50 light minutes
AGN is the manifestation of a supermassive black hole.
Matter is falling inwards towards the black hole
Due to dissipative processes, it is heated up
Radiates usually in the UV and optical.
Gravitational energy of the in-falling matter is converted to heat and light in the central engine.
Jets are often formed parallel to the spin axis of the disc or the spin axis of the black holes. Jets are source of the radio lobes in radioactive AGN. The jets produce radiation due to synchrotron processes and inverse-Compton scattering that takes place.
Quasars outshine host galaxy; Seyfert 1 - host is visible
Explain what the ‘Cosmic Microwave Background’ radiation is
Blackbody radiation with small fluctuations in the CMB.
Isotropic
Contains most of the radiation energy in the Universe.
Anisotropies:
a. Created before the universe became transparent to radiation, such as Baryon Acoustic Oscillations
b. Effects on photons after that time, due to interaction of photons with matter.
Dark matter explains
Rotation curves of galaxies, where the rotational speed of stars do not account for the visible mass
Gravitational lensing evidence. Some gravity lenses need to be more massive to produce the observed effect.
Cluster of galaxies, like the bullet cluster
CMB anisotropies need dark matter to be explained.
List the basic principles of the Lambda-CDM Cosmology Model
Assuming isotropy and homogeneity of spacetime
i. We live in an expanding universe.
ii. Spacetime seems to be flat (Euclidean) or slightly open.
iii. The expansion of the Universe is accelerating.
iv. The acceleration is caused by the cosmological constant, lambda, and is manifested by Dark Energy. The total fraction of dark energy in the Universe is ~68% (recently reduced following publication of the first Planck results).
v. There is a strange form of matter, Dark Matter, that fills ~26% of the Universe (again corrected since Planck data emerged). This is necessary to account for various observations. The DM particles, whatever they are, are not moving with relativistic speeds (DM is cold).
vi. There was a beginning, the Big Bang.
vii. Following the Big Bang, there was a period where the Universe expanded faster than the speed of light, the period of inflation.
Outline the concept of Active Galactic Nuclei (AGN)
Physical properties of quasars
High luminosity
Small emitting volume - small in size
Variability
Radio & optical & X-rays
Broad emission lines in spectrum => high velocity clouds
They are often bipolar… existence of radio lobes
The emitting region is small: ~0.6AU to 6AU for the disk => 50 light minutes
AGN is the manifestation of a supermassive black hole.
Matter is falling inwards towards the black hole
Due to dissipative processes, it is heated up
Radiates usually in the UV and optical.
Gravitational energy of the in-falling matter is converted to heat and light in the central engine.
Jets are often formed parallel to the spin axis of the disc or the spin axis of the black holes. Jets are source of the radio lobes in radioactive AGN. The jets produce radiation due to synchrotron processes and inverse-Compton scattering that takes place.
Quasars outshine host galaxy; Seyfert 1 - host is visible
Explain what the ‘Cosmic Microwave Background’ radiation is
Blackbody radiation with small fluctuations in the CMB.
Isotropic
Contains most of the radiation energy in the Universe.
Anisotropies:
a. Created before the universe became transparent to radiation, such as Baryon Acoustic Oscillations
b. Effects on photons after that time, due to interaction of photons with matter.
Dark matter explains
Rotation curves of galaxies, where the rotational speed of stars do not account for the visible mass
Gravitational lensing evidence. Some gravity lenses need to be more massive to produce the observed effect.
Cluster of galaxies, like the bullet cluster
CMB anisotropies need dark matter to be explained.
List the basic principles of the Lambda-CDM Cosmology Model
Assuming isotropy and homogeneity of spacetime
i. We live in an expanding universe.
ii. Spacetime seems to be flat (Euclidean) or slightly open.
iii. The expansion of the Universe is accelerating.
iv. The acceleration is caused by the cosmological constant, lambda, and is manifested by Dark Energy. The total fraction of dark energy in the Universe is ~68% (recently reduced following publication of the first Planck results).
v. There is a strange form of matter, Dark Matter, that fills ~26% of the Universe (again corrected since Planck data emerged). This is necessary to account for various observations. The DM particles, whatever they are, are not moving with relativistic speeds (DM is cold).
vi. There was a beginning, the Big Bang.
vii. Following the Big Bang, there was a period where the Universe expanded faster than the speed of light, the period of inflation.
