The Cosmic Microwave Background

The Cosmic Microwave Background Class 22 Prof J. Kenney June 26, 2018

The Cosmic Microwave Background Class 22 Prof J. Kenney November 28, 2016

Cosmic star formation history inf 10 4 3 2 1 0 z Peak of star formation in universe z=2-4, t ABB = 1.5-4 Byr Star formation begins in universe z~20, t ABB ~0.1 Byr

Emission of cosmic background radiation (380,000 yrs ABB) Era of Peak star, galaxy & black hole formation (~ 1-4 Byrs ABB) BIG BANG Cosmic Time NOW

Discovery of Cosmic Microwave Background (CMB) telescope used to discover CMB Radio horn at Bell Labs 1964 Penzias & Wilson with their telescope

Some TV static is CMB radiation!

Spectrum of cosmic microwave background Perfect blackbody! Peak of spectrum at λ = 1.1 millimeter

Spectra of stars NOT perfect blackbodies

Map of whole sky at millimeter wavelengths ( microwaves ) -- very nearly uniform! Where is center of Big Bang explosion?

Near isotropy & perfect blackbody spectrum of CMB indicate that CMB photons come from universe as a whole

Near isotropy & perfect blackbody spectrum of CMB indicate that CMB photons come from universe as a whole, not from individual, localized sources within it (like stars, galaxies, AGN)

CMB photons are: much older than those from stars and galaxies a relic from early universe, long before stars and galaxies existed, when universe was very smooth evidence of a Big Bang

Big Bang predicts CMB... why? if universe was once hot enough to ionize atoms, then collisions between electrons and nuclei would unavoidably make lots of photons (accelerating electrically-charged particles make EM waves) Photons inevitably produced in Big Bang predicted to cool due to expansion to T~3K by 13.7 Byr after Big Bang

How far could these photons have travelled through space? What happens to them as they go through space? What happens to these photons if they run into an atom in space?

How far could these photons have travelled through space? What happens to them as they go through space? What happens to these photons if they run into an atom in space? Think of 2 constituents of universe + how they interact: Atoms in space & Cosmic photons

A photon with a wavelength of 1 millimeter (10-3 m; radio) encounters an H atom. What happens? A. Nothing, the photon passes by B. The photon is absorbed by the atom C. The photon is scattered by the atom D. The photon and atom annihilate each other E. The atom ignores the photon

Energy level diagram of Hydrogen What does a photon with λ = 1mm do to an H atom?

Energy level diagram of Hydrogen What does a photon with λ = 1mm do to an H atom? NOTHING!

universe is now transparent to cosmic photons! zoom-in: Hydrogen atoms of gas cloud do not absorb cosmic photons since they are the wrong wavelength

observable universe universe is now transparent to cosmic photons! But it wasn t always like this

I thought that the Big Bang was hot! If the cosmic microwave background radiation is the radiation left over from the Big Bang, when did the universe cool down to about 3 K? A. one second after the Big Bang, when electronpositron pair production ceased B. three minutes after the start of the Big Bang, when primordial nuclear reactions ended C. 380,000 years after the Big Bang, when the universe became transparent to radiation D. 1 Byr after the big bang, after the peak of the quasar era E. just recently

why do CMB photons cool? expansion! photons are stretched out to longer wavelengths as space in universe expands galaxy A galaxy B time t 1 photon emitted λ 1 d(t 1 ) λ 2 time t 2 photon received d(t 2 ) for a photon emitted at time t 1 and detected at time t 2 λ 2 /λ 1 = 1 + z cos = d(t 2 )/d(t 1 )

Because of expansion, in the past not only were things closer together, but photons each had more energy so it was hotter in the past. Photons lose energy and cool continuously as universe expands. Think how expansion and cooling has changed interaction of CMB photons & matter If photons had more energy, how might interactions with matter change?

A photon with a wavelength of 0.1 microns (10-7 m; UV) encounters a H atom. What happens? A. Nothing, the photon passes by B. The photon is absorbed by the atom C. The photon is scattered by the atom D. The photon and atom annihilate each other E. The atom loves the photon and eats it right up

Energy level diagram of Hydrogen What energy and wavelength does a photon need to ionize an H atom?

Energy level diagram of Hydrogen What energy and wavelength does a photon need to ionize an H atom? E > 13.6eV λ < 0.1 µm

when the cosmic photons had shorter wavelengths (& higher energies) in the early universe, they interacted strongly with matter! They were absorbed or scattered by interactions with atoms and electrons. imagine running the expansion backwards cosmic photons NOW don t ionize atoms but AT MUCH EARLIER TIME cosmic photons had enough energy to ionize atoms this means the universe was opaque to cosmic photons!!

Before recombination: era of nuclei (or plasma) After recombination: era of atoms

Before recombination After recombination

Q: When did the Cosmic Background Radiation have enough energy to ionize all the H in the universe? (i.e., enough photons with λ<0.1 µm) log I Intensity blackbody spectra of CMB at z=0 and z=1100 T=3000 K CMB, z=1100 T=2.7 K CMB, z=0 0.1µm 1µm 10µm 100µm 1mm log λ these photons have enough energy to ionize H atoms A: When T(CMB)=3000 K

T=3000K is sufficient to ionize all H in the universe. WHY? Since there are 10 9 cosmic photons for each atom, There are enough photons in the high energy tail of a 3000K photon distribution to ionize all the H in the universe

what was redshift z when cosmic photons had enough energy to ionize H? λ obs / λ em = T then / T now for CMB photons but we also know: λ obs / λ em = 1 + z thus 1 + z = T then / T now for CMB photons since T then / T now = 1100, 1 + z = 1100 à z= 1099 1100 so cosmic photons originate from z=1100

what was redshift z when cosmic photons had enough energy to ionize H? temperature of CMB was T = 3000K when cosmic photons had enough energy to ionize H T then = 3000K T now = 2.7K -> T was 1100 x higher what was redshift z? (use Wiens law) λ max,em = 0.29 / T then λ max,obs = 0.29 / T now λ obs / λ em = T then / T now for CMB photons but we also know: λ obs / λ em = 1 + z thus 1 + z = T then / T now for CMB photons since T then / T now = 1100, 1 + z = 1100 à z= 1099 1100 so cosmic photons originate from z=1100

What time does at z=1100 correspond to? t = 380,000 yr after the Big Bang (ABB)

What time does at z=1100 correspond to? t = 380,000 yr after the Big Bang (ABB) How do we know this? We figure out what z had to be at time of decoupling in order for cosmic photons to ionize all H (what we just did ) then get t from z and the cosmological parameters (H o, Ω m, Ω Λ )

Before recombination Matter is in the form of plasma (p s & e s) Cosmic photons collide with p s and e s, scattering them changing energy & direction Atoms can form but are quickly destroyed (ionized) by cosmic photons Cosmic photons had enough energy to ionize atoms Universe opaque to cosmic photons Matter & energy (cosmic photons) tightly coupled

Universe AT z=1100, T=3000K, t ABB =380,000 yr Cosmic photons suddenly no longer have enough energy to ionize atoms (due to expansion) p s & e s combine to form atoms (not ionized) era of recombination (combination?) era of decoupling (of matter & cosmic photons) Moment when universe changed from being opaque to transparent (for cosmic photons) all cosmic photons we see now were created or last scattered (direction or energy changed) at this time

observable universe = cosmic light horizon universe is now transparent to cosmic photons!

Things we learn from CMB Universe was once much hotter and denser than now (evidence for BIG BANG)

Things we learn from CMB We are not in special location in universe Where is the center??? NO CENTER of expansion at any one location in space --> Big Bang occurred in all of space at the same time with same intensity (what Einstein says in GR)

Things we learn from CMB Universe was once much more uniform than now (almost) no substructure in universe at early times

Things we learn from CMB Universe was once much more uniform than now. So when did stars, planets, galaxies, clusters, black holes ("structure") begin to form? Growth of structure (stars, galaxies, clusters ) in universe began in earnest only after decoupling (when CMB photons were made)

Things we learn from CMB Evidence for dark energy & inflation

Things we learn from CMB How space of universe is curved Positive (spherical) Negative (saddle) Zero curvature (flat) Types of possible curvatures for universe in General Relativity

Things we learn from CMB Universe was once much hotter and denser than now (evidence for BIG BANG) Universe was once much more uniform than now When structure (stars, galaxies, clusters ) in universe began to form Evidence for dark energy Evidence for inflation We are not in special location in universe (BIG BANG occurred in all of space at same time) How space of universe is curved

Although CMB is pretty isotropic (uniform to 99.9%), there are 2 small but important anisotropies a. dipole anisotropy Doppler shift, caused by motion of earth through space (& wrt CMB)

Map of Cosmic Background Radiation over whole sky Map of Cosmic Background Radiation over whole sky

90% of average signal removed 90% of average signal removed

99% of average signal removed 90% of average signal removed

99.9% of average signal removed 99.9% of average signal removed a Dipole pattern remains!! a Dipole pattern remains!! (all of sky shown)

FULL RANGE 2.735 K temperature of CMB photons 0 K position in sky

FULL RANGE 2.735 K temperature of CMB photons zoom in 0 K position in sky

90% OF RANGE REMOVED 2.735 K temperature of CMB photons 2.46 K position in sky

90% OF RANGE REMOVED 2.735 K temperature of CMB photons zoom in 2.46 K position in sky

99% OF RANGE REMOVED 2.735 K temperature of CMB photons 2.708 K position in sky

99% OF RANGE REMOVED 2.735 K temperature of CMB photons zoom in 2.708 K position in sky

temperature of CMB photons 99.9% OF RANGE REMOVED 2.7366 K 2.7350 K 2.7334 K 2.7320 K 0 180 360 position in sky sinusoidal variation across sky

temperature of CMB photons 2.7366 K 99.9% OF RANGE REMOVED red regions 2.7350 K 2.7334 K blue regions 2.7320 K 0 180 360 position in sky sinusoidal variation across sky

2.7366 K 2.7334 K 99.9% of average signal removed 99.9% of average signal removed a Dipole pattern remains!! a Dipole pattern remains!! (all of sky shown)

toward Aquarius toward Leo 180 apart on sky (opposite sides of sky) 99.9% of average signal removed 99.9% of average signal removed a Dipole pattern remains!! a Dipole pattern remains!! (all of sky shown)

Doppler shift origin of the dipole pattern in the CMB in direction of Aquarius in direction of Leo

temperature of CMB photons 99.9% OF RANGE REMOVED 2.7366 K 2.7350 K 2.7334 K 2.7320 K 0 180 360 position in sky sinusoidal variation across sky exactly what is expected from Doppler shift

CMB spectrum slightly blueshifted toward Leo CMB spectrum slightly redshifted toward Aquarius log I Intensity Leo average Δλ Aquarius difference is exaggerated λ L λ av λ A log λ this is Doppler shift caused by motion of earth wrt CMB Δλ /λ avg = v / c = 0.0012 = 1.2x10-3 V earth,cmb = 0.0012c = 370 km/sec

Dipole Anisotropy in CMB arises from earth s motion caused by gravitational pull of different objects Earth around sun 30 km/s

Dipole Anisotropy in CMB arises from earth s motion caused by gravitational pull of different objects Earth around sun Sun around center of MW 30 km/s 220 km/s In different directions!

Dipole Anisotropy in CMB arises from earth s motion caused by gravitational pull of different objects Earth around sun Sun around center of MW MW toward Virgo cluster 30 km/s 220 km/s 350 km/s

Dipole Anisotropy in CMB arises from earth s motion caused by gravitational pull of different objects Earth around sun 30 km/s Sun around center of MW 220 km/s MW toward Virgo cluster 350 km/s MW+VC toward Great Attractor 620 km/s

Dipole Anisotropy in CMB arises from earth s motion caused by gravitational pull of different objects Earth around sun Sun around center of MW MW toward Virgo cluster 30 km/s 220 km/s 350 km/s MW+VC toward Great Attractor 620 km/s Pulls are in different directions so net motion is 370 km/s toward Leo (in between Virgo & Great Attractor)

Map of nearby galaxies showing Great Attractor (biggest nearby supercluster)

The density of nearby universe & The Great Attractor supercluster Red = more dense Blue = less dense Great Attractor is 150 MLY from earth

Dipole Anisotropy in CMB Perfect dipole pattern (sinusodial variation) across sky tells us this is due to our motion w.r.t. CMB

Dipole Anisotropy in CMB Perfect dipole pattern (sinusodial variation) across sky tells us this is due to our motion w.r.t. CMB Caused by our galaxy being pulled by gravity of nearby clusters & superclusters Provide us with a way to measure our motion with respect to the absolute frame of reference of expanding space itself

Final Exam: Friday June 29 at 2-5pm in Watson A48 What the Final Exam will emphasize: Classroom lectures 10-24 (starting FRI June 8 thru the end of the semester) Homeworks #5 - #10 Assigned Readings in Universe -- for lectures 10-24 There is some overlap with the first exam. Topically, you are responsible for everything starting with the Milky Way Galaxy. While material from the first few weeks of the course will not be emphasized on the exam, you are responsible for knowing things from these first few weeks that become relevant for later material, e.g., Newton's Laws, properties of light, electromagnetic spectrum, blackbody radiation, atoms, HR diagram, mass-lifetime relation for stars. Try the Practice Exam! (on canvas & class website)