AST 248, Lecture 5 - Stony Brook University · AST 248, Lecture 5 James Lattimer Department of Physics & Astronomy 449 ESS Bldg. Stony Brook University February 17, 2020 The Search

AST 248, Lecture 5

James Lattimer

Department of Physics & Astronomy449 ESS Bldg.

Stony Brook University

February 17, 2020

The Search for Intelligent Life in the [email protected]

James Lattimer AST 248, Lecture 5

Galaxies

• Galaxies are self-gravitating systems with up to a trillions starsand diameters of 10–25 kpc.

• The observeduniverse has billionsof galaxies; ourGalaxy is known asthe Milky Way.

• Galaxies areclassified accordingto shape. It’sunknown if galaxiesevolved from oneclass into another.

• Our galaxy is abarred spiral.


Galaxies

• Galaxies aren’t randomly distributed but tend to cluster.Cluster sizes range from a few dozen members, as in our LocalGroup, to 10,000 like the Virgo Cluster.

• Galaxies began forming 12.5Gyr ago when the universe was1 Gyr old.

• Quasars may be young galaxies

• Galaxies are believed toconsume neighboring galaxies.The Milky Way has consumedseveral nearby dwarf galaxies;the Magellanic Clouds will beconsumed in the near future.

S. Harris


Structure of the Milky Way

• The Milky Way is a barred spiral galaxywith 2−4 ·1011 stars, 6 ·1011 M of starsand gas and 30 · 1011 M of dark matter.

• Major components include the nucleus(bulge), halo and disk (spiral arms.)

• The galactic center, as do most galaxies,contains a supermassive black hole(4.1 · 106 M, called Sagittarius A∗).

• The galaxy’s disk contains molecularclouds, young stars, and galactic clusters,largely concentrated in a ring around thebar and in 4 major spiral arms. Mostcurrent star formation occurs here. Thedisk diameter is about 100,000 lt.-yrs.

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• The Sun is located at theinner rim of the Orion Arm, 8kpc from the galactic center.

• The galaxy’s nucleus isbar-shaped, about 8 kpclong, and composed of oldred stars.

• The galactic halo is aspheroid of old stars andglobular clusters with adiameter estimated at 60kpc. The galaxy’s darkmatter is uniformlydistributed within the halo.

• Spiral arms are logarithmicspirals (r = aebθ), just like anautilus shell or a hurricane.

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Components of the Galaxy• The disk contains open

clusters; the halo containsglobular clusters.

• Molecular clouds are thebirthplaces of stars andcontain gas (atoms andmolecules) and dust grains.

• The material between stars(99% gas, 1% dust) comprisesthe interstellar medium (15%of visible mass of Galaxy).

• 75% by mass of gas is H, 23%is He. Many molecules areobserved, most complex onesare organic.

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Gas and Dust

• Dust is composed ofheavy elements and ices(H2O, CO2).

• Gas and dust obscurestarlight.

• Some gas reflects light,some radiates light andradio waves.

• Radio emissions are usedto map galactic motionsand structure.

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Observing Interstellar MoleculesSimple Hydrides, Oxides, Suldes, Halides, and Related Mole ulesH2 CO NH3 CS NaCl N2OHCl SiO SiH4 SiS AlClH2O SO2 CC H2S KClHNO OCS CH4 PN AlFNitriles, A etylene Derivatives, and Related Mole ulesHCN HC C|CN H3C|CC|CN H3C|CH2|CN H2C=CH2H3CCN H(CC)2|CN H3C|C CH H2C=CH|CN HCCHCCCO H(CC)3|CN H3C(CC)2| H HNC H3CNCCCCS H(CC)4|CN H3C(CC)2| CN? HN=C=O HCCNCHCCCHO H(CC)5|CN? HN=C=S CCO C5OCCC CCCCC C4Si NaCNAldehydes, Al ohols, Ethers, Ketones, Amides, and Related Mole ulesH2C=O H3COH HO|CH=O H2CNH H2C=C=OH2C=S H3C|CH2|OH H3C|O|CH=O H3CNH2 H2C=C=CH3C|CH=O H3CSH H3C|O|CH3 H2NCN H2C=C=C=C(CH3)2CO? NH2|CH=O NH2|CH2|C|O|OHCy li Mole ulesC3H2 SiC2 C3H PAHsIonsCH+ HCO+ HCNH+ H3O+ HCS+ HN+2CO+ HOCO+ HC3NH+ HOC+ H2D+ SO+Radi alsOH C3H CN HCO C2S HCOCH2CH C4H C3N NO NSNH C5H H2CCN SO SiCCH2 C6H H2CN MgCN SiNC2H C8H HCCN2 MgNC CP

Spin- ip radiation from hydrogen atoms. The state where the spins ofthe proton and ele tron are aligned has a slightly higher energy than thestate where the spins are opposed. The energy hange between the twostates is E = h = h =, leading to a frequen y of = 1420 MHz or awavelength of = = = 21:1 m.James Lattimer AST 248, Lecture 5

Implications of ISM Organic Molecules

• Formed in harsh conditions, but formation aided by dust.

• Organic molecules seem to be preferred over other varieties.

• How complex can they get?

• Complexity should be greater incomets, planetary atmospheres,planetary surfaces.

• PolyAromatic Hydrocarbons (PAHs)and amino acids found in meteorites.

• Could living molecules or virusesform?

• Could primitive life travel in cometsor meteorites from planet-to-planetor star-to-star (panspermia)?


The Main Evidence for a Big Bang

• Hubble discovered in the 1920’sthat the universe is expanding.Galaxy redshifts (z = v/c)increase with distance: Hubble’sLaw: v = H0 D

Hubble’s constant is H ' 68−73km/sec/Mpc. Note relation toD = vt which gives the age ofthe universe ast = D/v = H−1

0 ≈ 14.4 Gyr.

Universe has not expanded at aconstant rate; still the estimatedage is about 13.8 Gyr.


Cepheid Variables and Hubble’s Law

1929 (Hubble)H0 = 500 km s−1 Mpc−1

2004H0 = 62 km s−1 Mpc−1

2001H0 = 71 km s−1 Mpc−1


• Cosmic Microwave Background(CMB) radiation with T = 2.7 K(Predicted by Alpher, Herman andGamow in 1948 and discovered byPenzias and Wilson in 1965, whoreceived 1978 Nobel Prize).

This originated at the decouplingera when the universe was300,000 yrs old and hot(T ≈ 3000 K).

• Cosmic nucleosynthesis of lightelements (He, Li, Be, B) whenthe universe was seconds old andhotter (T ≈ 109 K). Producedcomposition of 75% H and 25%He by mass, observed in oldeststars.

WMAP


S. Harris


Future of the UniverseDepends upon density d : Ω0 = d0/dc0

dc0 = 3H20/8πG ≈ 3 H atoms/m3 (critical density)

• Ω0 > 1 (d > dc): closed universe, bound, eventuallyrecontracts.

• Ω0 < 1 (d < dc): open universe, unbound, expands forever.

• Ω0 = 1 (d = dc): critical case, expansion stalls, but norecontraction.

Observed abundances of lightnuclei (1H

1, 2He4, 2He3, 1H2,

4Li7) agree with theory if thedensity of baryonic matter(neutrons, protons, nuclei) isabout 4% of critical(Ωbaryon ≈ 0.04). This is thesame as a baryon-to-photonratio about 10−6.


Evidence For Dark Matter

Measurements ofour Galaxy’s massand motions ofgalaxies in clustersimply large amountsof “missing mass”or dark matter(Ωdm ≈ 0.26) thatis not in the form ofbaryons. What it ismade of is unknown.Identifying it is nowa majorexperimental effort.

Nick Strobel www.astronomynotes.com


Evidence For Dark Energy

Moreover, observations ofI cosmic background

radiation inhomogeneitiesI distant Type I supernovae

imply thatI the universe’s expansion is

now accelerating, notdecelerating as gravityalone would cause,

I and the total Ω0 is 1, i.e.,the universe is critical.

This implies the existence ofyet another form ofmass-energy, so-called darkenergy, with Ωde ≈ 0.7 so thatΩde + Ωdm + Ωbaryon = Ω0 = 1.

Planck

-


Missing Mass

S. Harris

NASA/CXC/CIA/STSci/Magellan/Univ. of Ariz./ESO




Inflation as Solution to Big Bang ProblemsI Horizon: why is background radiation

uniform when it originates from regionsthat are now causally disconnected?

I Flatness: If Ω0 is not exactly 1, then Ωis much different from 1 in distant past.

I Smoothness: where did perturbationsin nearly uniform universe originate?Without these perturbations, stars andgalaxies would never have formed.

T. Knott, Wikipedia

About 10−36 seconds after birth, parts of the universemassively expanded like weak parts of inflating balloon.Expansion was so great, they now appear flat (Ω = 1). Allobservable matter was causally connected before inflation, souniform temperature maintained after inflation. Perturbationsneeded for star formation come from quantum fluctuations.


Fundamentals of Music – The Flute

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Flute Sound Spectrum

A modern flute with a B foot playing the note D5.

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locations between the Horizon lineswere in causal contact at that time

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CMB Temperature Fluctuates on Angular ScalesThe background radiation is not completely smooth but has inhomogen-eities of a few parts per million. Red (blue) areas have higher (lower)temperatures than average that confirm the universe’s composition.A Fourier transform reveals the powers of various harmonic frequencies.


CMB Fluctuation Spectrum

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These quantum fluctuationsgrow exponentially by inflationto form structures like stars and galaxies.

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History of the Universe

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Cosmic Coincidence

I P.A.M. Dirac pointed out in 1937 that ratio of electricalto gravitational forces in H atom is about 1039. This isthe same of the size of the observable universe to the sizeof the proton! Why? Must gravity be weakening in orderto keep these numbers the same, since the universe isexpanding?

I R. Dicke suggested the anthropic argument: The lifetimesof stars are controlled by the ratio of the electrical andgravitational forces, but the size of the universe isdictated by the time since the Big Bang. Therefore, thesize of the universe at the time WE can observe it issomehow controlled by the same ratio since a stellarlifetime is needed for biological evolution.


Other Examples of Fine-Tuning

I Excited state in C12 at exactly right level for triple-αprocess (He fusion) to synthesize C and get around theproblem that no stable elements of mass 5 or 8 exist.

I The strength of the nuclear weak force controls theneutrino flows that help ensure supernova explosions,which are necessary to create and eject heavy elementslike C, O, Si and Fe. Otherwise, black holes would alwaysform and swallow these heavy elements.

I The neutron star maximum mass is high enough thatbinary neutron stars do not immediately collapse into ablack hole following a merger. This allows the ejection ofsufficient mass to explain abundances of the r -processelements, including long-lived radioactive isotopes thatkeep the Earth’s core hot, permitting tectonic motions inthe Earth’s crust to persist for billions of years.


Cosmic Anthropic Principle

• We must take into account theselection effects imposed byour status as living observers.

• Fundamental constants ofnature G ,me ,mp,mn, e, ~, chave values that permitexistence of intelligence. Isthis chance or necessity?(Weak Anthropic Principle)

I Perhaps constants arerelated through a GrandUnified Theory

I Perhaps life is not sodependent uponfine-tuning of constants


• The Universe must have properties that allow life to develop atsome stage (Strong Anthropic Principle):

I Are observers necessaryfor the universe to exist?

I Are there multipleuniverses, so that there isa finite chance thatintelligence can develop inone of them?

• Suggests that intelligenceis rare and maybe unique.

• Carter deduces that if biological evolution and stellar evolutiontimes are not related, life is rare.• Will intelligence last forever once it comes into existence?

(Final Anthropic Principle)• Participatory Anthropic Principle: we participate in creation of

universe (Quantum Entanglement).James Lattimer AST 248, Lecture 5

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AST 248, Lecture 5 - Stony Brook University · AST 248, Lecture 5 James Lattimer Department of Physics & Astronomy 449 ESS Bldg. Stony Brook University February 17, 2020 The Search