Lecture 26:Big Bang Nucleosynthesis

Astronomy 1143 Spring 2014

Key Ideas: Primordial NucleosynthesisShows 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 NucleosynthesisPredictions are that the Universe was very

hot and very denseAny evidence for this?

Fusion requires high temperatures and high densities

Most elements are made in starsThere are over 100 elements in the Universe.The vast majority were made in stars because

of high temperatures and densitiesSource of energy for stars, in generalOlder stars have compositions with more

hydrogen, fewer metals than younger starsThousands of supernovae contributed to the

composition of the EarthBut 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 1st 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

atmospheres3 grams of lithium for every 10,000 tons of

hydrogen.Where did this lithium come from?

Omnipresent He and LiStellar 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 BBNNuclear fusion – creation of heavier nuclei by

sticking smaller nuclei togetherIngredients:

• 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 TemperatureParticles 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 temperaturesFusion requires high temperaturesFusing helium needs higher T than fusing H

Fusing Charged Particles

Protons (or 2H, 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 highBreak apart into protons & neutronsSo even though fusion is happening, the

nuclei don’t lastSimilarly, atoms can only form at lower

temperatures

A stumbling block to making deuterium (2H) in the early universe:

The early universe was veryvery hot, and thus contained photons energetic enough to blast apart deuterium.

2H + γ → p + n

Primordial NucleosynthesisWhen 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 FormationWhen the Universe was 2 minutes old:

• Temperature dropped to 1 Billion K

Neutrons & protons fuse into Deuterium (2H) • Free neutrons go into making Deuterium nuclei

or they decay• Leftover protons stay free as Hydrogen nuclei• Proportions: about 1 2H for every 4 protons (H)

Soup of mostly H and 2H along with a mix of photons, electrons & other particles.

With both protons & neutrons present, deuterium (2H, heavy hydrogen) formed by fusion:

p + n → 2H + γproton

neutron

deuterium

photon

This is differentdifferent from how deuterium is made in stars.

One possible reactionHigh temperatures needed to overcome repulsion

Primordial Helium FormationMost of the 2H fuses to form 4He nuclei

• Other reactions make 3He and Li in very tiny quantities

When the Universe was 15 minutes old:• Much of the Deuterium turned into 4He• 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).

Why the abundance of heliumhelium? Because nucleosynthesis didn’t go much beyond helium.

4He + p → 5Li not a stable element

not a stable element4He + n → 5He

4He + 4He → 8Be not a stable element

Small amounts of stable lithium were made.

3H + 4He → 7Li + γtritium

helium

lithium

photon

However, by this time (t ≈ 15 minutes) the temperature dropped low enough that fusion ceased.

Synthesis of Lithium

Beyond HeliumWhy do stars succeed when the early Universe

failed?Helium can fuse to heavier elements –

@ 100 million KBut 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 olderUniverse – cools down as it gets older

Before BBN, there were about 2 neutrons for every 14 protons. (Some neutrons had already decayed.)

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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-meterRatio of D/H very sensitive to density of normal

matter during nucleosynthesisDeuterium will fuse easily with another particle if it

encounters oneTherefore 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 densitiesEven if measuring the overall density of baryons is hard, measuring the D/H ratios much easier!

Deuterium & Nature of Dark MatterWe know that the matter in the Universe is ~25%

of the critical density from the gravitational pullCounting up the stars/gas/dust gives us just ~4%

of the critical densityBut 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 UniverseIdea: make measurements at very high redshiftProblem: Not enough light for our telescopes to detect, but can see D in absorptionIdea: 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 LinesLight from distant quasars passes through

cool gas clouds between us and quasarCreates an absorption-line spectrumDeuterium absorbs light at slightly different

wavelengths than 1H 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 linesHe measurements are difficult to make in cold gas clouds (transitions aren’t very strong)Measurements made from hot nebula – emission-line spectraMore helium = more emission (+ some modeling)

He emission lines

Lithium absorption linesLithium measurements come from

absorption-line spectra of starsStellar atmospheres at the right temperature

for the electrons of Li to absorb visible lightBut, 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 StatusPredictions 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