Lecture 26: Big Bang Nucleosynthesis Astronomy 1143Spring 2014

Key Ideas: Primordial Nucleosynthesis Shows that the Universe once had a temperature of millions K Amounts of primordial Deuterium & Helium & Lithium explained Nucleosynthesis did not proceed past Helium because heavier elements difficult to make and the Universe was cooling rapidly Shows that the dark matter cannot be made out of baryons Exact amount of D & He depends on density of baryons in the first ~15 minutes of the Universe

Big Bang Nucleosynthesis Predictions are that the Universe was very hot and very dense Any evidence for this? Fusion requires high temperatures and high densities

Most elements are made in stars There are over 100 elements in the Universe. The vast majority were made in stars because of high temperatures and densities Source of energy for stars, in general Older stars have compositions with more hydrogen, fewer metals than younger stars Thousands of supernovae contributed to the composition of the Earth But helium does not act like other elements…

Where Did Helium come from? Younger stars (and the Sun): 70% H, 28% He, & ~2% metals Metals come from earlier supernovae Metal-poor, older stars: 75% H, 25% He, & <0.01% metals Where did all the He in old stars come from? Difficult to make that much He in the 1 st generation If from the first stars, where are all the metals that would have formed along with it?

Where did the Li come from? Another result: early gas clouds invariably contain traces of lithium. Stars (even the oldest) contain Li in their atmospheres 3 grams of lithium for every 10,000 tons of hydrogen. Where did this lithium come from?

Omnipresent He and Li Stellar fusion occurs in the centers of stars and is released into the gas in the galaxy as the star dies We know stellar fusion is occurring because of the solar neutrinos, not because we can see the photons from the core He and Li everywhere we look implies that the conditions throughout the Universe were like the inside of a star

Ingredients for BBN Nuclear fusion – creation of heavier nuclei by sticking smaller nuclei together Ingredients: Protons Neutrons if there are still any around ½ life is ~ 15 minutes – BBN happens at ~ 3 minutes Neutrons made from protons & electrons at high temperatures, but that has stopped by BBN Bound in heavier nuclei Heavier nuclei that were just created

The Importance of Temperature Particles need to be moving; T must be > 0. Particles need to be moving fast Electric repulsion between positively charged can stop particles from getting close They need to be going fast enough that repulsion can’t slow particles enough In a gas, high speeds=high temperatures Fusion requires high temperatures Fusing helium needs higher T than fusing H

Fusing Charged Particles Protons (or 2 H, etc.) are conflicted They feel electric repulsion from other protons because they have like charges They feel strong force attraction to other protons (and neutrons) if they get close enough Trick is to get the protons (or other nuclei) close enough together to fuse Fusing p + n much easier than p+p

But not too high….. However, nuclei cannot stay together if temperatures get too high Break apart into protons & neutrons So even though fusion is happening, the nuclei don’t last Similarly, atoms can only form at lower temperatures

A stumbling block to making deuterium ( 2 H) in the early universe: very The early universe was very hot, and thus contained photons energetic enough to blast apart deuterium. 2 H + γ → p + n

Primordial Nucleosynthesis When the Universe was only 1 second old: Temperature: 10 Billion K Too hot for atomic nuclei to exist Only protons, neutrons, electrons, & photons 1 neutron for every ~7 protons General hot, dense soup of subatomic particles & photons. As it expanded, it cooled off

Primordial Deuterium Formation When the Universe was 2 minutes old: Temperature dropped to 1 Billion K Neutrons & protons fuse into Deuterium ( 2 H) Free neutrons go into making Deuterium nuclei or they decay Leftover protons stay free as Hydrogen nuclei Proportions: about 1 2 H for every 4 protons (H) Soup of mostly H and 2 H along with a mix of photons, electrons & other particles.

With both protons & neutrons present, deuterium ( 2 H, heavy hydrogen) formed by fusion: p + n → 2 H + γ proton neutron deuterium photon different This is different from how deuterium is made in stars.

One possible reaction High temperatures needed to overcome repulsion

Primordial Helium Formation Most of the 2 H fuses to form 4 He nuclei Other reactions make 3 He and Li in very tiny quantities When the Universe was 15 minutes old: Much of the Deuterium turned into 4 He Left with tiny traces of Deuterium and other light elements. The Universe cooled so much that fusion stopped (and it ran out of free neutrons).

helium Why the abundance of helium? Because nucleosynthesis didn’t go much beyond helium. 4 He + p → 5 Li not a stable element 4 He + n → 5 He 4 He + 4 He → 8 Be not a stable element

Small amounts of stable lithium were made. 3 H + 4 He → 7 Li + γ tritium helium lithium photon However, by this time (t ≈ 15 minutes) the temperature dropped low enough that fusion ceased. Synthesis of Lithium

Beyond Helium Why do stars succeed when the early Universe failed? Helium can fuse to heavier elements 100 million K But the Universe is cooling rapidly enough from 3-15 minutes that by the time He is formed, it is too cold to go farther Stars – heat up in center as they get older Universe – cools down as it gets older

Before BBN, there were about 2 neutrons for every 14 protons. (Some neutrons had already decayed.) n n p pp p pppp p p p p p p Predicting the amount of He

2 neutrons combine with 2 protons to form 1 stable helium nucleus, with 12 lonely protons (hydrogen nuclei) left over. helium nucleus

About 25% of the initial protons & neutrons (hence 25% of their mass) will be in helium: the rest will be hydrogen.

Inventory of the Universe

Deuterium as a density-meter Ratio of D/H very sensitive to density of normal matter during nucleosynthesis Deuterium will fuse easily with another particle if it encounters one Therefore if the density of nucleons in the early Universe is High then less deuterium will survive to the present day Low then more deuterium will survive to the present day

Ratio of D/H depends on density Just as nucleosynthesis is ending, for three different densities

Ratio of D/H depends on density Just after nucleosynthesis ended, for three different densities Even if measuring the overall density of baryons is hard, measuring the D/H ratios much easier!

Deuterium & Nature of Dark Matter We know that the matter in the Universe is ~25% of the critical density from the gravitational pull Counting up the stars/gas/dust gives us just ~4% of the critical density But could the dark matter be “hidden” normal matter? Black holes? White dwarfs? Brown dwarfs? Hot (-ish) gas? In other words, could the amount of normal matter in the Universe be ~30% matter-energy density?

Hot (-ish) Gas has been detected around the Milky Way Turns out that gas can have just the right temperature and density to emit very little light, and most of that is in UV, X-ray

No! Majority of Dark Matter CANNOT be baryons!!

Test of Big Bang We can get more sophisticated than just predicting He. If we determine the density of baryons using D, can we predict the correct amount of He isotopes and Li as well? YES!

Measuring Composition of the Early Universe Idea: make measurements at very high redshift Problem: Not enough light for our telescopes to detect, but can see D in absorption Idea: make measurements of gas that has been the least affected by pollution from planetary nebula, SNe, etc. Problem: pristine gas impossible to find. Do the best with very metal-poor dwarf galaxies.

Deuterium Absorption Lines Light from distant quasars passes through cool gas clouds between us and quasar Creates an absorption-line spectrum Deuterium absorbs light at slightly different wavelengths than 1 H because its energy levels are slightly different Measure the amount of Deuterium from the strength of the absorption (+ some modeling)

Deuterium Absorption Lines

Helium emission lines He measurements are difficult to make in cold gas clouds (transitions aren’t very strong) Measurements made from hot nebula – emission-line spectra More helium = more emission (+ some modeling)

He emission lines

Lithium absorption lines Lithium measurements come from absorption-line spectra of stars Stellar atmospheres at the right temperature for the electrons of Li to absorb visible light But, many stars can destroy Li as they go through life Determining primordial Li difficult Very clear that BBN made Li, but exact amount still the subject of dispute

Lithium in Metal-Poor Stars

Current Status Predictions of Primordial Nucleosynthesis agree well with current observations: Observations: Need refinement of the primordial abundances Very difficult observations to make Theory: Need to know average density of p & n light-element reaction rates need refinement