Lecture 1

AST4320 - Cosmology and extragalactic astronomy

Introduction/Background

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Relevant information

Class times + day: Tuesday + Wednesday 12:15-14:00.

Group sessions: Monday 14:15-16:00, led by Max Gronke. Questions to help understand lectures, or to work out some details we skipped over in the lecture.

Exams: midterm: October 8 (30% of grade) final: December 11(times to be announced, 50% of the grade ) class presentations: weeks of Nov 17-18 & Nov 24-25 (20% of the grade)

No class: Sep 29-30 (week prior to midterm), Nov 10, Dec 1-2 (week prior to exam)

Contact information: mark.dijkstra@astro.uio.no (room 205), max.gronke@astro.uio.no

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Class Presentations

Presentations of 20-25 minutes by each student + 5 minutes of questions

Topic can be picked by students after discussion with me. List of suggested subjects:

Breakthroughs obtained with Hubble Space Telescope

The Hubble Deep Fields

Direct Collapse Black holes

Supermassive Stars

Constraints on Cosmological Magnetic Fields

Observational constraints on variation fundamental constants.

....

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Literature

Books: we’ll follow several books for different topics. Lecture slides + additional notes (for blackboard lectures) will be available online on course website.

Suggested reading material will be announced on the notes, or on slides.

Today: Chapter 1 of Loeb & Furlanetto “How did the first stars & galaxies form?”Chapter is available on website of Prof. Loeb: https://www.cfa.harvard.edu/~loeb/LF/

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2. Light separated from darkness

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How the Universe started & developed

Origin of Universe has been pondered upon for millenia

For example, book of Genesis

1. Universe was created

3. water separated from sky

4. continents separated from water

5. vegetation appeared

6. stars formed

7. life emerged 8. humans appeared

2. Light dominates Universal energy content

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How the Universe started & developed

Origin of Universe has been pondered upon for millenia

Current cosmological model

1. Universe was created in Big Bang

3. Matter dominates Universal energy content

4. stars (& planets) formed

5. life emerged

6. humans appeared

Cosmology is a now a mature field, in which we can explore this picture critically with observations.

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How the Universe started & developed

Importantly, finite light speed allows us to look back on younger Universe.

Cosmic Microwave Background (CMB) from Planck Satellite. Universe ~ 0.4 Myr old.

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How the Universe started & developed

Importantly, finite light speed allows us to look back on younger Universe.

Galaxy surveys. Universe ~ 1-14 Gyr old.

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How the Universe started & developed

Importantly, finite light speed allows us to look back on younger Universe.

Galaxy surveys. Universe ~ 1-14 Gyr old.

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How the Universe started & developed

We currently snapshots of the Universe at age tUni ~ 1-14 Gyr.

...and one at age tUni ~ 0.4Myr.

Homogenous plasma at ~ few thousand K

galaxies...

What happened in between? How did stars, galaxies and black holes form?

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How the Universe started & developed

We currently have an incomplete photo-album of the evolution of our Universe.

“The situation that astronomers face is similar to having a photo album containing the first ultrasound image of an unborn baby and some additional photos of that same person as a teenager and an adult” (Loeb, 2006) -

not just scaled up version

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How the Universe started & developed

Observational efforts are being developed to fill in our `missing pictures’, and o address how stars etc actually formed.

`JWST’ James Webb Space Telescopesuccessor to Hubble & Spitzer space telescopesTo be launched in 2018. Science goals include observing the first stars & galaxies.

`SKA’ Square Kilometer ArrayLow frequency radio `interferometer’. Construction in 2018. First observations in 2020. Science goals include observing intergalactic atomic HI directly in the young Universe

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Standard Cosmological Model

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Standard Cosmological Model

Einstein applied his `field equations’ from general relativity to Universe as a whole.

To simplify the analysis, Einstein assumed/asserted that the Universe was •isotropic (same in all directions)•homogeneous (same in every location)

Exercise: think of distributions which are isotropic, but not homogeneous and vice versa

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Einstein could not reproduce a static solution, and needed to introduce his cosmological constant. He later realized that this solution was not stable however.

At the time, it was thought that the entire Universe consisted of the Milky Way, which was/is not expanding.

Standard Cosmological Model

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Einstein could not reproduce a static solution, and needed to introduce his cosmological constant. He later realized that this solution was not stable however.

At the time, it was thought that the entire Universe consisted of the Milky Way (MW), which was/is not expanding.

Less than a decade later, Hubble discovered that `spiral nebulae’ - initially thought of as constituents on the MW - were moving away from us with velocity v = H0r

Hubble “constant”.

Hubble also resolved individual stars in these `spiral nebulae’ which unambiguously determined their distances.

Hubble’s observations indicated that the Universe was expanding.

Standard Cosmological Model

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Einstein’s Cosmological PrinciplesEinstein’s cosmological principles of isotropy and homogeneity have been remarkably successful.

Isotropy

CMB looks the same in all directions to within 10-5

(also see galaxy distribution, X-ray background,...)

homogeneity

Isotropy without homogeneity possible, if inhomogeneity in spherical shells. This has been ruled out by galaxy surveys.

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Einstein’s Cosmological Principles

Observations thus indicate that the Universe is indeed the simplest way we could have imagined it to be.

Why? It has been demonstrated the `cosmic inflation’ - a brief period of accelerated expansion - naturally gives rise to the conditions postulated by the cosmological principle.

Why? It has been demonstrated the `cosmic inflation’ - a brief period of accelerated expansion - naturally gives rise to the conditions postulated by the cosmological principle.

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The Expanding Universe: The Past

Expansion of the Universe implies that it was denser in the past.

Evidence that the Universe was indeed denser & hotter in the past include.

CMB..

CMB has a blackbody spectrum, which implies matter and radiation are tightly coupled. This requires extremely dense & ionized (and therefore hot) gas.

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The Expanding Universe: The Past

Expansion of the Universe implies that it was denser in the past.

Evidence that the Universe was indeed denser & hotter in the past include.

Existence of certain nuclei, such as D, Li could only have been synthesized during conditions of Big Bang.

There are no known post-Big Bang processes which can produce significant amounts of deuterium.

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The Expanding Universe: The Future

Simple insight into future evolution of Universe (and structure formation in general!) can be obtained from Kirchhoff ’s theorem.

In spherically symmetric Universe, when considering sphere of matter inside of it, when we can ignore gravitational influence of everything inside the sphere.

If sphere behaves like the rest of the Universe (homogeneity), then we can deduce expansion history of Universe as a whole by examining its behavior.

Universe

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The Expanding Universe: The Future

Universe

Total energy of gas element per unit massblackboard

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The Expanding Universe: The Future

Universe

Total energy of gas element per unit mass

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The Expanding Universe: The Future

Total energy of shell of material < 0. Shell is gravitationally bound, and collapses back to R=0.

Total energy of shell of material = 0. Shell keeps expanding until it becomes stationary (v=0), at infinitely large R.

Total energy of shell of material > 0. Shell is not gravitationally bound, and continues to expand forever.

Future evolution of the Universe as a whole depends critically on its matter content.

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The Expanding Universe: The Future

Einstein’s field equations connects dynamics of matter to geometry of space.

How can we tell geometry?

Universe is `closed’ (positive curvature)

Universe is `flat’ (no curvature)

Universe is `open’ (negative curvature)

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The Expanding Universe: The Future

How can we tell geometry? From acoustic peaks in the CMB.

Physics of `primordial’ plasma is straightforward and well understood.

Any perturbation inside the plasma generates `soundwaves’ that travel at 0.57c.

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The Expanding Universe: The Future

How can we tell geometry? From acoustic peaks in the CMB.

Physics of `primordial’ plasma is straightforward and well understood.

Once the Universe cools enough to recombine (~0.4 Myr post Big-Bang), the sound waves practically `freeze’, and leave a pattern on the matter distribution that we can observe as acoustic peaks in the CMB.

Any perturbation inside the plasma generates `soundwaves’ that travel at 0.57c.

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The Expanding Universe: The Future

How can we tell geometry? From acoustic peaks in the CMB.

Powerspectrum is a more compact, complete representation of the CMB

Location of peaks provides direct measure on curvature of the Universe.

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The Expanding Universe: The Future

Universe is isotropic & homogeneous, and its geometry is (very close to) flat.

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The Flat Universe Description

The space-time (4D) line element is

For flat space

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The Flat Universe Description

The space-time (4D) line element is

For flat space

The scale factor, which quantifies the expansion of Universe

Comoving coordinates, coordinate system that moves together with expanding Universe. Natural to express observables like number density of galaxies in comoving coordinates.

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Expansion & Comoving Coordinates

Galaxies maintain their `comoving’ positions on the sphere, but the expansion of the sphere as a whole increases their separation (ignore that the geometry is not flat..)

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Expansion & Comoving Coordinates

Scale factor plays a key role in cosmology. Some useful relations (to be derived on board)

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Expansion & Comoving Coordinates

Scale factor plays a key role in cosmology. Some useful relations (to be derived on board) part I

Photon frequency

Photon wavelength

Energy NR-particle

Scale factor - redshift relationScale factor - redshift relation

Hubble constant (parameter)

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Expansion & Comoving Coordinates

Scale factor - redshift relation

Most distance known galaxy (spectroscopically confirmed) at z~8.7.

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Expansion & Comoving Coordinates

Scale factor plays a key role in cosmology. Some useful relations part II

Hubble constant (parameter)

Time evolution Hubble parameter

Matter domination

Radiation domination

Vacuum domination

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Universe in Comoving Coordinatesfrom Loeb & Furlanetto

most distant galaxy

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Universe in Comoving Coordinatesfrom Loeb & Furlanetto

most distant galaxy

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Observables in Expanding Universe

Scale factor plays a key role in cosmology. Some useful relations part II

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Observables in Expanding Universe

Scale factor plays a key role in cosmology. Some useful relations

arcsec/kpc

Beyond a certain redshift (see assignment 1), the angular size of of an object of a fixed size increases with distance.

log luminosity distance

`Most of our information on astronomical objects are derived from the radiation we receive from it, or by the absorption it causes in the light of background sources.’ Mo, VdB, White

This information is encoded within the objects Spectral Energy Distribution (SED)

Measure flux (energy/time/area) in the frequency range

Often expressed as (AB)-magnitude:

(CGS units)

Observables in Expanding Universe

Absolute magnitude is magnitude of source if it were located 10 pc away.

Apparent magnitude of sun is mV~-26. Absolute magnitude....

of sun is MV~4.8.

Absolute Magnitude

Absolute Magnitude

Absolute magnitude is magnitude of source if it were located 10 pc away.

Apparent magnitude of sun is mV~-26. Absolute magnitude....

of sun is MV~4.8.

of a massive O-star is MV~-5

Absolute Magnitude

Absolute magnitude is magnitude of source if it were located 10 pc away.

Apparent magnitude of sun is mV~-26. Absolute magnitude....

of sun is MV~4.8.

of a massive O-star is MV~-5

of a (Type 1a) supernova explosion MV~-19.3

Absolute Magnitude

Absolute magnitude is magnitude of source if it were located 10 pc away.

Apparent magnitude of sun is mV~-26. Absolute magnitude....

of sun is MV~4.8.

of a massive O-star is MV~-5

of a (Type 1a) supernova explosion MV~-19.3

of our Milky way as a whole MV~-20

brightest elliptical galaxies MV~-23

Absolute Magnitude

Absolute magnitude is magnitude of source if it were located 10 pc away.

Apparent magnitude of sun is mV~-26. Absolute magnitude....

of sun is MV~4.8.

of a massive O-star is MV~-5

of a (Type 1a) supernova explosion MV~-19.3

of our Milky way as a whole MV~-20

brightest elliptical galaxies MV~-23

brightest quasars MV~-30

Absolute Magnitude

Absolute magnitude is magnitude of source if it were located 10 pc away.

Apparent magnitude of sun is mV~-26. Absolute magnitude....

of sun is MV~4.8.

of a massive O-star is MV~-5

of a (Type 1a) supernova explosion MV~-19.3

of our Milky way as a whole MV~-20

brightest elliptical galaxies MV~-23

brightest quasars MV~-30

gamma-ray bursts MV~-36

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Material Content of Universe

We know the Universe is geometrically flat, and that Omegatot=1

But...ordinary matter (baryons) only accounts for ~ 5% of the Universal energy density!

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Overwhelming Evidence for `Dark Components’

Galaxy rotation curves: evidence for dark matter

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Overwhelming Evidence for `Dark Components’Kinematics of galaxies inside galaxy clusters

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Overwhelming Evidence for `Dark Components’

Growth of structure, see next lectures....

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Overwhelming Evidence for `Dark Components’

S

Supernovae type 1a are thought to be `standard candles’ (luminosity is known and same everywhere)

The total flux we observe from a SN1a at a given z then depends on cosmological parameters only.

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Open Questions

How did (the first) stars, galaxies and black holes form?

Why do galaxies & stars the properties (e.g. mass), they have?

Origin of black holes. Is there a difference between stellar mass & super massive black holes?

How can we test our theoretical models?

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Some Topics to be Covered

Structure formation

`Virialization’ Gas cooling & condensation.

Supersonically driven star formation

Black holes & their formation

Non-linear collapse in the spherical top-hat model

Linear, Newtonian perturbation theory: quantitative describes growth of structure (Jeans stability)

Observational probes

Lyman-a Forest, gravitational lensing

Galaxy colors, spectra,IR, optical, UV, X-ray,...