The origin of the universe

Olbers’s Paradox Why is the sky dark at night? If the universe is infinite, then every line of sight should end on a star

Finite, and no edge

The Expanding Universe the galaxies are NOT moving through space. Space is expanding, carrying the galaxies along! Things that are smaller than galaxy clusters are not expanding!

Hubble’s data (1929) Riess et al (1996)

Georges LeMaitre George Gamow Ralph Alpher

Predictions of Big Bang Theory The Universe is homogeneous and isotropic (very smooth) But not too smooth… The ratio of H/He (about 75% H, 25% He) Trace abundances of D, 3 He, Li, Be The cosmic microwave background radiation

The Universe is Homogeneous and Isotropic Homogeneous: looks the same at all locations Not isotropic Isotropic: looks the same in all directions Not homogeneous

On the largest scales, Univese is homogeneous and isotropic!

Interactions among elementary particles of the Standard Model Matter particles Carriers of forces

The very early Universe: < seconds after Big Bang singularity: The Planck Epoch All for fundamental forces unified into one force realm of GR, string theory, and ??? to seconds: Grand Unification epoch gravitation separates from unified electroweak and strong force to seconds (???): Inflationary epoch universe expands faster than speed of light large-scale structure is established to seconds: electroweak epoch Universe cools off to K strong and electroweak forces separate triggers inflationary epoch (?) to seconds: quark epoch quark-gluon plasma

10 -6 to 1 second: hadron epoch quark-gluon plasma cools until hadrons (protons, neutrons) form T = 1 GeV hadrons and antihadrons annihilate each other (mostly) 1 to 10 seconds: lepton epoch leptons and antileptons annihilate each other T = 1 MeV 10 seconds to 380,000 years: the photon epoch Photons and electrons exist, continually recombining Universe still sufficiently hot to ionize H atoms 3-20 minutes: Nucleosynthesis 380,000 years: Recombination 380,000 to 150 million years: Dark ages 150 million years: Reionization

Big Bang Theory Expansion & Cooling The First Day LEPTON ERARADIATION ERAHADRONQUARK bound free 3q2q qq q q p,n,π…. Matter: (p) Anti-matter: 10 9 (p) (e ±,μ ±,γ….) qq ↔ E ¯ pp ↔ E ¯ pp → 2γ ¯ proton freeze-out p, n : 1 e ±, γ : 10 9 e + e − ↔ E electron freezout Time (s) e + e − → 2γ e +, e −, photons 2p2n → He 4 Photons dominate p, e − 1:1 p, He 4 12:1 (3:1) p, γ 1: day H fusion p:n → 1:1 7:1 ν decouple Temperature (K) ¯ n decay

The First Three Minutes: The Nucleus-building Era At t=3 minutes, T=1 billion K: Fusion of protons and the remaining free neutrons: * Formation of 2 H (Deuterium) & 4 He * End up with ~92% 1 H, 8% 4 He * Also end up with traces of 2 H, 3 He, Li, Be, B This is what the oldest stars are observed to be made of! Free neutrons decay into protons + electrons in about 10 minutes => p + e -

379,000 years old: First light escapes; Universe already has structure (light still arriving today) Early fluctuations become denser condensations of matter First stars form after ~150 million years (“reionization”) Galaxies and galaxy clusters form, according to the floorplan laid out at 379,000 years The Universe today: lots of stars and galaxies!

Observations of the Universe 4 He is extremely common: ~25% everywhere even oldest stars have ~24% He far too much to come from stars alone  It was made in the Big Bang, before stars existed! High temp & density  lower temp & density Like the core of a star  Radiated light like a star Expanding, cooling

The Universe cooled down to the temperature at which nuclei exist & nuclear fusion occurs!

Up to 1 second, thermal equilibrium: After 1 sec, expansion is faster than reaction: freeze-out of p/n = 6/ seconds: n decay: At about 100 seconds: Neutrons are safe in a nucleus: D formation, p/n = 7/ minutes: nucleosynthesis Net result leaves very little D (part in ~10 5 )

Calculations based on binding energies

10 -6 sec < t < 1 sec Pair-production of   e + + e -, high energies (  kT) maintain equilibrium: n + e +  p + e p + e -  n + e As T drops: (Average) photon below 2m e = x10 10 K e + -e - annihilate, too few left to drive n-p conversion  n,p can’t be maintained in equilibrium for T  K ~

Using K as “characteristic” T when equilibrium ends… N n / N p = e -1.3 MeV / kT = e -1.5  0.22 So: N n =.22/(1 +.22) = 0.18 and N p = 1/(1 +.22) = 0.82 i.e., 18 n’s for every 82 p’s when p-n ratio “set” t = 1 sec) 1 sec < t < 250 sec Enough high E photons (E > 2.2 MeV) to disintegrate deuterons  baryons only between t  3-20 minutes

4 min < t < min n + p  2 H +  (“stabilizes” n’s) Then: 2 H + (n,p)  3 H, 3 He  4 He BUT: 4 He wont accept more n, p as  n,  p = 0 Won’t work anyway: No stable nuclei with A = 5 Can we build on 4 He nuclei with larger nuclei than n, p?

2 H + 4 He  6 Li +  and 6 Li + n  7 Li +  and 3 H + 4 He  7 Li +  but 7 Li + n  8 Li +   8 Be +  - + e 8 Be  2 4 He in sec! How about … ~

3 He + 4 He  7 Be +  but 7 Be + n  8 Be and then 8 Be  2 4 He (in sec!) or 7 Be + p  8 B +   8 Be +  + + e (in 0.5 sec) but then 8 Be  2 4 He (in sec!) or 4 He + 4 He  8 Be +  but then 8 Be  2 4 He (in sec!) Or …

BB nucleosynthesis “stops” at 4 He (& tiny amounts of others) Problems: Must somehow “jump over” A = 5 and 8 (Thank you, stars!)

What is the composition of the universe at t = 20 min ? Complication: free n’s aren’t stable n  p +  - + e with T ½ = 10.3 min So: 110 n’s & 690 p’s  55 4 He’s p’s % 4 He by mass = (4 x 55)/(4x55 + 1x580) =.275 i.e., the BB made universe 73% H and 27% He (by mass) As nucleosynthesis didn’t start until 4 m, some of the n’s “set” at t = 1 sec didn’t survive to be fused into 4 He N n (at 4 m ) = N n (at 1 s ) e - t ln2 / T ½ = 18 e - 4(.693)/10.3 = Synthesis starts with n’s for every = p’s ~

379,000 years later… Universe cooled enough to have H atoms = recombination of protons and electrons Atoms DO NOT absorb photons: light escapes! Space is expanding: optical wavelength photons redshifted to microwave Predicted by Gamow and Alpher Discovered by Penzias and Wilson (1968) Nobel went to P & W

Looking Back in Time: the Early Universe The more distant the objects we observe, the further back into the past we are looking.

we see a glowing wall of bright fog Prediction: The universe once glowed like a star. The early glow of the Universe should still be visible! expansion cooling BigBang dense hot Now thin cool atomic transparent us hot glowing fog Photons keep getting absorbed redshift z = 1000 microwaves orange light 3000 K 380,000 yr Ionized, foggy

The Cosmic Background Radiation R. Wilson & A. Penzias The radiation from the very early phase of the universe is still detectable today discovered in mid-1960s Blackbody radiation with a temperature of T = 2.73 K

The Cosmic Background Radiation (2) After recombination, photons can travel freely through space. Their wavelength is only stretched (red shifted) by cosmic expansion. Recombination: z = 1000; T = 3000 K This is what we can observe today as the cosmic background radiation!

Extremely uniform!!!

Our galaxy is here 3 billion light years (~20% to “the edge”) Sloan Digital Sky Survey: Univese is clumpy!

1990: Anisotropy discovered

The Universe’s Baby Picture: WMAP (Wilkinson Microwave Anisotropy Probe) Photons that were emitted when Universe was 379,000 years old. Fluctuations in the temperature (= structure) of the Universe appeared when it was very young

Many waves of different sizes, Directions & phases, all “superposed” Sound waves : red/blue = high/low gas & light pressure Water waves : high/low level of water surface

Temperature and density fluctuations are minimal: BUT IMPORTANT!

Very uniform and smooth: no stars or galaxies yet! (379,000 years) Smooth to 1/100,000 Patchiness due to not perfectly smooth distribution of matter (“sound waves”)

The History of the Universe Universe expands as time passes Universe cools down as time passes P+e=atoms Light can escape!

transparent Universe is ionized (still today) but transparent because it is very diffuse

Reionization After less than ~ 1 billion years, the first stars form. Formation of the first stars Ultraviolet radiation from the first stars re- ionizes gas in the early universe Reionization

Lyman-alpha and cosmology

Quasars all have similar power spectra HI cloud near quasar - safely assume that the light being absorbed is 1216A Many clouds between us and the quasar leads to a “forest” of Ly-alpha lines

Lyman-alpha Forest But what if there is so much HI that it blocks it completely? Extremely dense HI is only present in very early universe. This can only happen at very high redshift!

Gunn-Peterson Trough One of the few examples of a real prediction in astrophysics!! (1965,2001) Many clouds of HI between us and quasar at z = 6 or so Ly-alpha absorption causes a “forest” of lines “trough” predicted for when H is very dense (at very high redshift)

Gunn-Peterson Trough The discovery of the trough in a z = 6.28 quasar, and the absence of the trough in quasars detected at redshifts just below z = 6 presented strong evidence for the hydrogen in the universe having undergone a transition from neutral to ionized around z = 6.

From SDSS: Quasar spectra. Note the height of the spectral lines on the left side of the spectrum. The bottom image shows the first Gunn-Peterson trough ever discovered. Further away

Why is this cool? How much HI is out there, and how is it distributed? Ly-alpha regions trace out dark matter, because the H atoms are concentrated by DM’s gravity

The Cosmological Principle 1) Homogeneous: On the largest scales, the universe should have the same physical properties throughout Every region has the same density, expansion rate, luminous vs. dark matter 2) Isotropic: On the largest scales, the universe looks the same in any direction that one observes. You should see the same large- scale structure in any direction. 3) Universality: The laws of physics are the same everywhere in the universe.